Chapter 3 Eddy Covariance Fluxes of Nitrogen Oxides at Harvard Forest Abstract

NOx deposition is important to both the biosphere and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants Tropospheric NO and NO2 rapidly interconvert in

a fast photochemical cycle but entirely different processes govern their removal rates at

the surface and their interactions with ecosystems Concentrations and vertical fluxes of

both must be measured in order to interpret the behavior of either species near the surface

and to quantify the net removal of NOx by deposition

We present eddy covariance flux measurements of NO and NO2 above a mixed

deciduous canopy at Harvard Forest in central Massachusetts April-November 2000 At

night NO2 is deposited at a rate that depends nonlinearly on NO2 concentration Eddy

flux observations of coupled downward NO and upward NO2 fluxes above the forest

during the day conform to the predicted behavior based on gradients of light and eddy

diffusivity through the canopy NO2 flux can be parameterized as a simple function of

light concentration and stomatal conductance The light-dependent term reflects

photochemical cycling of NO-NO2 and does not contribute to net NOx flux Net

deposition velocity of NOx was approximately 02 cm s-1 and showed little variation with

time of day or season

41

31 Introduction

Current estimates indicate that fossil fuel combustion and soil microbial emissions

are the largest sources of NOx to the global troposphere each accounting for

approximately 20 Tg N yr-1 followed by smaller contributions from biomass burning

lightning ammonia oxidation the ocean and the stratosphere [Aneja et al 2001] NO is

rapidly converted to NO2 via reaction with O3 (Reaction 31) with typical lifetimes

between 30 seconds and several hundred seconds NO2 can be photolyzed back to NO

(R32) regenerating O3 by the reaction of O(3P) with molecular oxygen (R33)

NO + O3 NO2 + O2 (R31)

NO2 + hν NO + O(3P) (R32)

O(3P) + O2 O3 (R33)

This cycle has no net effect on the concentration of O3

Oxidation of natural and anthropogenic hydrocarbon emissions produces

intermediate products such as the hydro-peroxy radical HO2 and higher molecular

weight organic peroxy radicals RO2 which provide alternative oxidants for NO (R34)

NO + RO2 NO2 + RO (R34)

In the presence of peroxy radicals a single NOx radical may cycle between NO and NO2

to produce multiple O3 molecules before oxidation to HNO3 occurs This process leads

to net O3 production via (R32) and (R33) and concentrations of NOx and hydrocarbons

together have a major effect on atmospheric levels of O3 Recent modeling work

suggests that for typical tropospheric conditions (R34) can influence the concentration

and flux gradients of NO NO2 and O3 as much as or more than (R31) - (R33) [eg Heal

et al 2001]

42

Removal of NOx from the atmosphere is believed to occur mostly after NOx has

been oxidized to nitric acid HNO3 via reaction with OH (R35)

NO2 + OH + M HNO3 + M (R35)

HNO3 + hν NO2 + OH (R36)

Also NO2 can react with NO3 to form N2O5 which can then hydrolyze to form HNO3 in

the absence of OH [Galmarini et al 1997] HNO3 is a highly soluble and surface-

reactive molecule with large wet and dry deposition rates but with a very long lifetime

against photolysis to regenerate NO2 (R36) Hence deposition is the dominant loss

pathway for HNO3 [Jacob 2000]

Deposition of NO2 directly to a surface can short-circuit O3 production bypassing

the slower mechanism of oxidation to HNO3 and limiting O3 production rates [Wesley

and Hicks 2000 Lerdau et al 2000 Hosker and Lindberg 1982] Deposition of NO2

may thus be very important even though it may be a small fraction of total NOy flux [see

Chapter 4 Lefer et al 1999 Munger et al 1996] Because measurements of nitrogen

oxide radicals NOx = NO + NO2 are prone to chemical interferences and artifacts there

have been few in-situ few studies of their interaction with natural plant canopies and

results to date have been inconclusive in both direction and size of fluxes [Hanson and

Lindberg 1991] Most of the non-urban troposphere sees low NOx concentrations often

less than 1 ppb much lower than used in typical chamber studies In fact leaf chamber

and other plant-level measurements of NO2 deposition made at higher concentrations

extrapolate linearly to predict NO2 emission at low ambient concentrations [eg Sparks et

al 2001] NO2 release by vegetation under such conditions would have the opposite

43

effect on troposphere O3 potentially elevating production rates in NOx-limited regions

over much of the globe

NO2 flux measurements over short (lt 1m) crops and grasses vary greatly in

direction and diurnal pattern Past studies have reported nearly constant deposition [Coe

and Gallagher 1992] variable deposition [Wesely et al 1982] morning deposition

followed by afternoon emission [Delaney et al 1986] and daytime emission with

nighttime deposition [Stocker et al 1993 Padro et al 1998] NO2 deposition to soil

litter bark and other non-foliar surfaces may be as large as through leaf stomata [Eugster

and Hesterberg 1996 Hanson and Lindberg 1991] There have been very few

measurements of NO NO2 and O3 fluxes above tall deciduous coniferous or mixed

canopies most have employed enclosed chamber measurements on selected branches

leaves or other forest components [Rondon et al 1993 Sparks et al 2001] These

measurements are consistent with the hypothesis that daytime NO2 deposition to foliar

surfaces is controlled by stomatal conductance and that deposition to the forest floor and

soils may be as important as deposition to leaves They also suggest the presence of a

compensation point for NO2 such that below ambient concentrations of 1 ppb NO2

leaves may cease to take up or begin to emit NO2

We present measurements here of eddy covariance fluxes of NO2 NO and O3 at

the Harvard Forest Environmental Measurement Site during the spring summer and fall

of 2000 The triad of fluxes has not previously been simultaneously observed above a tall

forest canopy with reliable eddy covariance techniques The observations reveal NOx

deposition behavior that is distinctly different from the conventional parameterizations

currently used in many tropospheric chemistry and transport models

44

32 Methods

321 Site Description

The Harvard Forest site in central Massachusetts (4254N 7218W elevation 340

m) is a 50- to 70-year old mixed deciduous forest consisting primarily of red oak and red

maple with scattered hemlock red pine and white pine stands The terrain is roughly

95 forested and moderately hilly closest paved roads are more than 1 km away small

towns greater than 10 km distant Dominant winds are from the northwest and southwest

comprising two distinctly different chemical and meteorological regimes Northwesterly

flows correspond to cooler drier and less polluted air masses whereas the southwesterly

regime transports warmer more humid and significantly more polluted air masses

[Moody et al 1998] Further descriptions of the atmospheric chemistry at the site can be

found in Munger et al [1996]

322 Instruments

Our goal was to measure simultaneously the fluxes of NO NO2 NOy and O3 by

eddy covariance In order to measure eddy covariance fluxes of NO2 a technique with

consistent linear response high sensitivity and lack of interferences is required

Methods used in past studies that do not meet these criteria may show spurious fluxes due

to correlations between efficiency interferences and vertical wind speed or may require

extrapolation from unrealistically high concentrations For example systems which

photolyze NO2 to NO and subsequently detect NO via chemiluminescence suffer from

fluctuating conversion efficiencies [eg Rondon et al 1993 Munger et al 1996] the

45

chemiluminescent reaction of NO2 with Luminol has an interference which depends on

ambient O3 concentrations which in turn co-vary with vertical wind [Coe et al 1992

Walton et al 1997] and hot molybdenum or gold surfaces convert not only NO2 but

other NOy species as well to NO [eg Wesely et al 1982] NO measurements are more

straightforward as no conversion step is necessary before chemilumenscent detection O3

measurements are also more routine Flux measurements of NOy require special

instrumentation but have been routine at Harvard Forest since 1990 [Munger et al

1996]

From late August to mid-October 2000 a chemiluminescence detector (29 m

sampling height) was configured to measure NO concentrations at 8 Hz At other times

the chemiluminescence detector measured profiles and concentrations of NO and NO2

which was converted to NO in a photolysis cell prior to detection We installed a new

tunable diode laser absorption spectrometer (TDLAS) on a nearby tower to measure NO2

(22 m sampling height) at 1 Hz For complete TDLAS details refer to Chapter 2

Measurements of O3 concentration and flux employ C2H4-chemiluminescence (29 m

sampling height) and UV absorbance instruments respectively Three-axis sonic

anemometers facing into the prevailing wind direction (west) at the sampling heights on

both towers provided the vertical and horizontal wind velocities and virtual temperatures

(8 Hz) needed to compute eddy covariance fluxes of heat momentum NO NO2 and O3

(Munger et al 1996 1998) The layout of the site instruments and towers is depicted

schematically in Figure 31

46

EMSTower

TDLASTower100 m

SonicChemilumNO amp NO2NOy O3T PPFD

h=29 m

h=22 mSonic TDLASHNO3 amp NO2

Figure 31 Layout of Harvard Forest field site The EMS tower supports a sonic

anemometer at a height of 29 m along with inlets for chemiluminescence NO and NO2

NOy O3 and other trace gas instruments Temperature water vapor photosynthetic

photon flux density (PPFD) and other quantities are also measured on the EMS tower

some at multiple heights through the canopy The TDLAS tower a scaffolding structure

approximately 100 m distant supported a sonic anemometer and the TDLAS inlet at the

22 m height The TDLAS instrument was housed in two weatherproof enclosures (optics

and electronics) on the tower itself in order to minimize the inlet length Buildings near

the bases of both towers house instrumentation pumps gas cylinders and provide

electrical and internet connections

47

Photosynthetic photon flux density (PPFD) consisting of wavelengths between

400 nm and 700 nm was measured continuously at 29 m on the EMS tower using a LI-

COR quantum sensor Although the UV wavelengths associated with NO2 photolysis are

outside of the photosynthetic wavelength band we have used the more routine PPFD

measurements as a proxy for overall light level including the UV bands associated with

NOx photochemistry The midday UV (295 to 385 nm wavelengths) profile between 0

and 30 m was measured on one occasion at Harvard Forest using an Eppley total UV

radiometer placed facing upwards to capture direct plus diffuse downwelling radiation

and then facing downwards to capture diffuse upwelling radiation at each height

323 Eddy covariance fluxes

We computed 30-minute fluxes from the covariance of detrended vertical wind

velocity (wprime) with fluctuations of detrended temperature (Tprime) or detrended trace gas

concentration (Cprime) Details of the analysis process and software can be found in

Chapter 2

Because the photochemical reactions (R31)-(R34) occur on timescale

comparable to and in some cases faster than the turbulent diffusion timescale in the

surface layer we must consider the system of concentrations and fluxes for NO NO2

and O3 together [Fitzjarrald and Lenschow 1983] Deposition or exchange velocities

cannot be inferred from the fluxes and concentrations of any one species Our daytime

results were complicated by the fact that the NO2 concentration and flux were measured

at a different height than the NO and O3 concentrations and fluxes Physical and

experimental limitations to the height of the TDLAS tower and placement of the

48

instrument prevented co-location of the inlets We have therefore placed the daytime

deposition rates within the context of a simple model of turbulent diffusion and chemical

reaction in the surface layer described below

33 Results

331 NO2 Concentration Analysis and Comparison

Concurrent photolysis-chemiluminescence (hereafter P-C) and TDLAS

measurements allowed us to compare concentration data for NO2 over a wide range of

conditions throughout the spring summer and fall of 2000 The P-C system is routinely

calibrated with standard additions of a small flow of NO2 in N2 (calibration tank NO2

concentration of 5 micromol mol-1) to the air to determine the photolysis efficiency plus

addition of NO in N2 to determine the chemiluminescence detector response The NO2

tanks have been periodically calibrated against the NO standard but their long-term

stability remains a potential source of uncertainty Towards the end of the TDLAS

measurement period the concentrations of the standard tanks were checked by titration of

NO standards and by comparison with a larger cohort of NO2 standards The standard

tanks were found to be very close to their original concentrations See Munger et al

[1996] for complete details of the P-C NOx calibration procedure including corrections

for the effect (R31) in the sampling tubing

The TDLAS relies on accurate knowledge of the following spectroscopic

parameters to determine concentration absorption cross sections light path length laser

mode purity frequency tuning rate function pressure and temperature and laser line

49

width Absorption cross sections come from the HITRAN database and typically have

uncertainties of less than 10 [Rothman et al 1998] The light path length is confirmed

in the lab using standard additions into the multi-pass cell of an inert gas such as N2O

with absorption features in the frequency region of the measurement This calibration

can be done in the laboratory because the multi-pass cell body and mirrors are locked into

position and do not move relative to one another during field deployment The laserrsquos

frequency is first established using a built-in monochromator and a sealed reference cell

with a large concentration of NO2 This same cell provides a signal for locking the

frequency during instrument operation The laserrsquos frequency tuning rate function is

independently determined using a calibrated etalon The tuning rate can change as the

diode ages or after having warmed from liquid nitrogen to room temperature but can be

readjusted in the field by matching to the known spectral features Pressure and

temperature are measured in the multi-pass cell in order to compensate for the pressure

broadening and temperature dependence of the absorption lines

Typical TDLAS laser linewidths (due to all contributions to spectroscopic

instrument distortion) are well-approximated by a Gaussian function and are usually

small compared to the pressure broadened width for molecular spectral lines at 01 to

025 Pa We checked the laser linewidth by taking measurements at lower pressures with

all of the above factors already determined The particular diode used to measure NO2 in

this field deployment had an unusually large linewidth (approximately 0004 cm-1) and

non-Gaussian line shape The cause could not be determined before the diode reached

the end of its useable lifetime

50

In order to correct for the excess linewidth we simultaneously measured the

concentration of ambient water vapor in the absorption cell using our NO2 diode

(assuming Gaussian distortion) and a second diode with a more typical laser linewidth of

lt 0001 cm-1 We repeated this two-diode measurement with tank NO2 in N2 These

checks were performed before during and after field deployment The concentration

correction factor for the excess non-Gaussian distortion of the NO2 diode varied between

160 and 165 with a mean of 163 Field data were fit in real-time and in post analysis

using the standard Gaussian broadening function and later corrected by this factor

NO2 concentration measurements by the P-C and TDLAS instruments overlapped

between 4 April and 29 August 2000 The time series in Figure 32 shows the hourly

average NO2 concentrations from the two instruments during a four-day period In

Figure 33 all hourly TDLAS and P-C NO2 concentration measurements are compared

An orthogonal distance fit to the data with errors at the 95th confidence interval yields a

slope of 11 plusmn 02 a y-intercept of ndash02 plusmn 10 nmol mol-1 and an R2 of 091 The

agreement between the measurement methods is very good given the spatial separation of

the inlets both horizontally and vertically the precision of each instrument and the

calibration uncertainties for both instruments (~10)

332 Eddy Covariance Flux Validation

Lagged correlations ltwprimeNO2primegt and copsectra for vertical wind speed (w) and NO2

exhibit the expected offset and smearing due to instrument lag time and response function

(Figure 34) as compared for example to w and virtual temperature ltwprimeTsprimegt The peak in

51

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

31 Introduction

Current estimates indicate that fossil fuel combustion and soil microbial emissions

are the largest sources of NOx to the global troposphere each accounting for

approximately 20 Tg N yr-1 followed by smaller contributions from biomass burning

lightning ammonia oxidation the ocean and the stratosphere [Aneja et al 2001] NO is

rapidly converted to NO2 via reaction with O3 (Reaction 31) with typical lifetimes

between 30 seconds and several hundred seconds NO2 can be photolyzed back to NO

(R32) regenerating O3 by the reaction of O(3P) with molecular oxygen (R33)

NO + O3 NO2 + O2 (R31)

NO2 + hν NO + O(3P) (R32)

O(3P) + O2 O3 (R33)

This cycle has no net effect on the concentration of O3

Oxidation of natural and anthropogenic hydrocarbon emissions produces

intermediate products such as the hydro-peroxy radical HO2 and higher molecular

weight organic peroxy radicals RO2 which provide alternative oxidants for NO (R34)

NO + RO2 NO2 + RO (R34)

In the presence of peroxy radicals a single NOx radical may cycle between NO and NO2

to produce multiple O3 molecules before oxidation to HNO3 occurs This process leads

to net O3 production via (R32) and (R33) and concentrations of NOx and hydrocarbons

together have a major effect on atmospheric levels of O3 Recent modeling work

suggests that for typical tropospheric conditions (R34) can influence the concentration

and flux gradients of NO NO2 and O3 as much as or more than (R31) - (R33) [eg Heal

et al 2001]

42

Removal of NOx from the atmosphere is believed to occur mostly after NOx has

been oxidized to nitric acid HNO3 via reaction with OH (R35)

NO2 + OH + M HNO3 + M (R35)

HNO3 + hν NO2 + OH (R36)

Also NO2 can react with NO3 to form N2O5 which can then hydrolyze to form HNO3 in

the absence of OH [Galmarini et al 1997] HNO3 is a highly soluble and surface-

reactive molecule with large wet and dry deposition rates but with a very long lifetime

against photolysis to regenerate NO2 (R36) Hence deposition is the dominant loss

pathway for HNO3 [Jacob 2000]

Deposition of NO2 directly to a surface can short-circuit O3 production bypassing

the slower mechanism of oxidation to HNO3 and limiting O3 production rates [Wesley

and Hicks 2000 Lerdau et al 2000 Hosker and Lindberg 1982] Deposition of NO2

may thus be very important even though it may be a small fraction of total NOy flux [see

Chapter 4 Lefer et al 1999 Munger et al 1996] Because measurements of nitrogen

oxide radicals NOx = NO + NO2 are prone to chemical interferences and artifacts there

have been few in-situ few studies of their interaction with natural plant canopies and

results to date have been inconclusive in both direction and size of fluxes [Hanson and

Lindberg 1991] Most of the non-urban troposphere sees low NOx concentrations often

less than 1 ppb much lower than used in typical chamber studies In fact leaf chamber

and other plant-level measurements of NO2 deposition made at higher concentrations

extrapolate linearly to predict NO2 emission at low ambient concentrations [eg Sparks et

al 2001] NO2 release by vegetation under such conditions would have the opposite

43

effect on troposphere O3 potentially elevating production rates in NOx-limited regions

over much of the globe

NO2 flux measurements over short (lt 1m) crops and grasses vary greatly in

direction and diurnal pattern Past studies have reported nearly constant deposition [Coe

and Gallagher 1992] variable deposition [Wesely et al 1982] morning deposition

followed by afternoon emission [Delaney et al 1986] and daytime emission with

nighttime deposition [Stocker et al 1993 Padro et al 1998] NO2 deposition to soil

litter bark and other non-foliar surfaces may be as large as through leaf stomata [Eugster

and Hesterberg 1996 Hanson and Lindberg 1991] There have been very few

measurements of NO NO2 and O3 fluxes above tall deciduous coniferous or mixed

canopies most have employed enclosed chamber measurements on selected branches

leaves or other forest components [Rondon et al 1993 Sparks et al 2001] These

measurements are consistent with the hypothesis that daytime NO2 deposition to foliar

surfaces is controlled by stomatal conductance and that deposition to the forest floor and

soils may be as important as deposition to leaves They also suggest the presence of a

compensation point for NO2 such that below ambient concentrations of 1 ppb NO2

leaves may cease to take up or begin to emit NO2

We present measurements here of eddy covariance fluxes of NO2 NO and O3 at

the Harvard Forest Environmental Measurement Site during the spring summer and fall

of 2000 The triad of fluxes has not previously been simultaneously observed above a tall

forest canopy with reliable eddy covariance techniques The observations reveal NOx

deposition behavior that is distinctly different from the conventional parameterizations

currently used in many tropospheric chemistry and transport models

44

32 Methods

321 Site Description

The Harvard Forest site in central Massachusetts (4254N 7218W elevation 340

m) is a 50- to 70-year old mixed deciduous forest consisting primarily of red oak and red

maple with scattered hemlock red pine and white pine stands The terrain is roughly

95 forested and moderately hilly closest paved roads are more than 1 km away small

towns greater than 10 km distant Dominant winds are from the northwest and southwest

comprising two distinctly different chemical and meteorological regimes Northwesterly

flows correspond to cooler drier and less polluted air masses whereas the southwesterly

regime transports warmer more humid and significantly more polluted air masses

[Moody et al 1998] Further descriptions of the atmospheric chemistry at the site can be

found in Munger et al [1996]

322 Instruments

Our goal was to measure simultaneously the fluxes of NO NO2 NOy and O3 by

eddy covariance In order to measure eddy covariance fluxes of NO2 a technique with

consistent linear response high sensitivity and lack of interferences is required

Methods used in past studies that do not meet these criteria may show spurious fluxes due

to correlations between efficiency interferences and vertical wind speed or may require

extrapolation from unrealistically high concentrations For example systems which

photolyze NO2 to NO and subsequently detect NO via chemiluminescence suffer from

fluctuating conversion efficiencies [eg Rondon et al 1993 Munger et al 1996] the

45

chemiluminescent reaction of NO2 with Luminol has an interference which depends on

ambient O3 concentrations which in turn co-vary with vertical wind [Coe et al 1992

Walton et al 1997] and hot molybdenum or gold surfaces convert not only NO2 but

other NOy species as well to NO [eg Wesely et al 1982] NO measurements are more

straightforward as no conversion step is necessary before chemilumenscent detection O3

measurements are also more routine Flux measurements of NOy require special

instrumentation but have been routine at Harvard Forest since 1990 [Munger et al

1996]

From late August to mid-October 2000 a chemiluminescence detector (29 m

sampling height) was configured to measure NO concentrations at 8 Hz At other times

the chemiluminescence detector measured profiles and concentrations of NO and NO2

which was converted to NO in a photolysis cell prior to detection We installed a new

tunable diode laser absorption spectrometer (TDLAS) on a nearby tower to measure NO2

(22 m sampling height) at 1 Hz For complete TDLAS details refer to Chapter 2

Measurements of O3 concentration and flux employ C2H4-chemiluminescence (29 m

sampling height) and UV absorbance instruments respectively Three-axis sonic

anemometers facing into the prevailing wind direction (west) at the sampling heights on

both towers provided the vertical and horizontal wind velocities and virtual temperatures

(8 Hz) needed to compute eddy covariance fluxes of heat momentum NO NO2 and O3

(Munger et al 1996 1998) The layout of the site instruments and towers is depicted

schematically in Figure 31

46

EMSTower

TDLASTower100 m

SonicChemilumNO amp NO2NOy O3T PPFD

h=29 m

h=22 mSonic TDLASHNO3 amp NO2

Figure 31 Layout of Harvard Forest field site The EMS tower supports a sonic

anemometer at a height of 29 m along with inlets for chemiluminescence NO and NO2

NOy O3 and other trace gas instruments Temperature water vapor photosynthetic

photon flux density (PPFD) and other quantities are also measured on the EMS tower

some at multiple heights through the canopy The TDLAS tower a scaffolding structure

approximately 100 m distant supported a sonic anemometer and the TDLAS inlet at the

22 m height The TDLAS instrument was housed in two weatherproof enclosures (optics

and electronics) on the tower itself in order to minimize the inlet length Buildings near

the bases of both towers house instrumentation pumps gas cylinders and provide

electrical and internet connections

47

Photosynthetic photon flux density (PPFD) consisting of wavelengths between

400 nm and 700 nm was measured continuously at 29 m on the EMS tower using a LI-

COR quantum sensor Although the UV wavelengths associated with NO2 photolysis are

outside of the photosynthetic wavelength band we have used the more routine PPFD

measurements as a proxy for overall light level including the UV bands associated with

NOx photochemistry The midday UV (295 to 385 nm wavelengths) profile between 0

and 30 m was measured on one occasion at Harvard Forest using an Eppley total UV

radiometer placed facing upwards to capture direct plus diffuse downwelling radiation

and then facing downwards to capture diffuse upwelling radiation at each height

323 Eddy covariance fluxes

We computed 30-minute fluxes from the covariance of detrended vertical wind

velocity (wprime) with fluctuations of detrended temperature (Tprime) or detrended trace gas

concentration (Cprime) Details of the analysis process and software can be found in

Chapter 2

Because the photochemical reactions (R31)-(R34) occur on timescale

comparable to and in some cases faster than the turbulent diffusion timescale in the

surface layer we must consider the system of concentrations and fluxes for NO NO2

and O3 together [Fitzjarrald and Lenschow 1983] Deposition or exchange velocities

cannot be inferred from the fluxes and concentrations of any one species Our daytime

results were complicated by the fact that the NO2 concentration and flux were measured

at a different height than the NO and O3 concentrations and fluxes Physical and

experimental limitations to the height of the TDLAS tower and placement of the

48

instrument prevented co-location of the inlets We have therefore placed the daytime

deposition rates within the context of a simple model of turbulent diffusion and chemical

reaction in the surface layer described below

33 Results

331 NO2 Concentration Analysis and Comparison

Concurrent photolysis-chemiluminescence (hereafter P-C) and TDLAS

measurements allowed us to compare concentration data for NO2 over a wide range of

conditions throughout the spring summer and fall of 2000 The P-C system is routinely

calibrated with standard additions of a small flow of NO2 in N2 (calibration tank NO2

concentration of 5 micromol mol-1) to the air to determine the photolysis efficiency plus

addition of NO in N2 to determine the chemiluminescence detector response The NO2

tanks have been periodically calibrated against the NO standard but their long-term

stability remains a potential source of uncertainty Towards the end of the TDLAS

measurement period the concentrations of the standard tanks were checked by titration of

NO standards and by comparison with a larger cohort of NO2 standards The standard

tanks were found to be very close to their original concentrations See Munger et al

[1996] for complete details of the P-C NOx calibration procedure including corrections

for the effect (R31) in the sampling tubing

The TDLAS relies on accurate knowledge of the following spectroscopic

parameters to determine concentration absorption cross sections light path length laser

mode purity frequency tuning rate function pressure and temperature and laser line

49

width Absorption cross sections come from the HITRAN database and typically have

uncertainties of less than 10 [Rothman et al 1998] The light path length is confirmed

in the lab using standard additions into the multi-pass cell of an inert gas such as N2O

with absorption features in the frequency region of the measurement This calibration

can be done in the laboratory because the multi-pass cell body and mirrors are locked into

position and do not move relative to one another during field deployment The laserrsquos

frequency is first established using a built-in monochromator and a sealed reference cell

with a large concentration of NO2 This same cell provides a signal for locking the

frequency during instrument operation The laserrsquos frequency tuning rate function is

independently determined using a calibrated etalon The tuning rate can change as the

diode ages or after having warmed from liquid nitrogen to room temperature but can be

readjusted in the field by matching to the known spectral features Pressure and

temperature are measured in the multi-pass cell in order to compensate for the pressure

broadening and temperature dependence of the absorption lines

Typical TDLAS laser linewidths (due to all contributions to spectroscopic

instrument distortion) are well-approximated by a Gaussian function and are usually

small compared to the pressure broadened width for molecular spectral lines at 01 to

025 Pa We checked the laser linewidth by taking measurements at lower pressures with

all of the above factors already determined The particular diode used to measure NO2 in

this field deployment had an unusually large linewidth (approximately 0004 cm-1) and

non-Gaussian line shape The cause could not be determined before the diode reached

the end of its useable lifetime

50

In order to correct for the excess linewidth we simultaneously measured the

concentration of ambient water vapor in the absorption cell using our NO2 diode

(assuming Gaussian distortion) and a second diode with a more typical laser linewidth of

lt 0001 cm-1 We repeated this two-diode measurement with tank NO2 in N2 These

checks were performed before during and after field deployment The concentration

correction factor for the excess non-Gaussian distortion of the NO2 diode varied between

160 and 165 with a mean of 163 Field data were fit in real-time and in post analysis

using the standard Gaussian broadening function and later corrected by this factor

NO2 concentration measurements by the P-C and TDLAS instruments overlapped

between 4 April and 29 August 2000 The time series in Figure 32 shows the hourly

average NO2 concentrations from the two instruments during a four-day period In

Figure 33 all hourly TDLAS and P-C NO2 concentration measurements are compared

An orthogonal distance fit to the data with errors at the 95th confidence interval yields a

slope of 11 plusmn 02 a y-intercept of ndash02 plusmn 10 nmol mol-1 and an R2 of 091 The

agreement between the measurement methods is very good given the spatial separation of

the inlets both horizontally and vertically the precision of each instrument and the

calibration uncertainties for both instruments (~10)

332 Eddy Covariance Flux Validation

Lagged correlations ltwprimeNO2primegt and copsectra for vertical wind speed (w) and NO2

exhibit the expected offset and smearing due to instrument lag time and response function

(Figure 34) as compared for example to w and virtual temperature ltwprimeTsprimegt The peak in

51

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Removal of NOx from the atmosphere is believed to occur mostly after NOx has

been oxidized to nitric acid HNO3 via reaction with OH (R35)

NO2 + OH + M HNO3 + M (R35)

HNO3 + hν NO2 + OH (R36)

Also NO2 can react with NO3 to form N2O5 which can then hydrolyze to form HNO3 in

the absence of OH [Galmarini et al 1997] HNO3 is a highly soluble and surface-

reactive molecule with large wet and dry deposition rates but with a very long lifetime

against photolysis to regenerate NO2 (R36) Hence deposition is the dominant loss

pathway for HNO3 [Jacob 2000]

Deposition of NO2 directly to a surface can short-circuit O3 production bypassing

the slower mechanism of oxidation to HNO3 and limiting O3 production rates [Wesley

and Hicks 2000 Lerdau et al 2000 Hosker and Lindberg 1982] Deposition of NO2

may thus be very important even though it may be a small fraction of total NOy flux [see

Chapter 4 Lefer et al 1999 Munger et al 1996] Because measurements of nitrogen

oxide radicals NOx = NO + NO2 are prone to chemical interferences and artifacts there

have been few in-situ few studies of their interaction with natural plant canopies and

results to date have been inconclusive in both direction and size of fluxes [Hanson and

Lindberg 1991] Most of the non-urban troposphere sees low NOx concentrations often

less than 1 ppb much lower than used in typical chamber studies In fact leaf chamber

and other plant-level measurements of NO2 deposition made at higher concentrations

extrapolate linearly to predict NO2 emission at low ambient concentrations [eg Sparks et

al 2001] NO2 release by vegetation under such conditions would have the opposite

43

effect on troposphere O3 potentially elevating production rates in NOx-limited regions

over much of the globe

NO2 flux measurements over short (lt 1m) crops and grasses vary greatly in

direction and diurnal pattern Past studies have reported nearly constant deposition [Coe

and Gallagher 1992] variable deposition [Wesely et al 1982] morning deposition

followed by afternoon emission [Delaney et al 1986] and daytime emission with

nighttime deposition [Stocker et al 1993 Padro et al 1998] NO2 deposition to soil

litter bark and other non-foliar surfaces may be as large as through leaf stomata [Eugster

and Hesterberg 1996 Hanson and Lindberg 1991] There have been very few

measurements of NO NO2 and O3 fluxes above tall deciduous coniferous or mixed

canopies most have employed enclosed chamber measurements on selected branches

leaves or other forest components [Rondon et al 1993 Sparks et al 2001] These

measurements are consistent with the hypothesis that daytime NO2 deposition to foliar

surfaces is controlled by stomatal conductance and that deposition to the forest floor and

soils may be as important as deposition to leaves They also suggest the presence of a

compensation point for NO2 such that below ambient concentrations of 1 ppb NO2

leaves may cease to take up or begin to emit NO2

We present measurements here of eddy covariance fluxes of NO2 NO and O3 at

the Harvard Forest Environmental Measurement Site during the spring summer and fall

of 2000 The triad of fluxes has not previously been simultaneously observed above a tall

forest canopy with reliable eddy covariance techniques The observations reveal NOx

deposition behavior that is distinctly different from the conventional parameterizations

currently used in many tropospheric chemistry and transport models

44

32 Methods

321 Site Description

The Harvard Forest site in central Massachusetts (4254N 7218W elevation 340

m) is a 50- to 70-year old mixed deciduous forest consisting primarily of red oak and red

maple with scattered hemlock red pine and white pine stands The terrain is roughly

95 forested and moderately hilly closest paved roads are more than 1 km away small

towns greater than 10 km distant Dominant winds are from the northwest and southwest

comprising two distinctly different chemical and meteorological regimes Northwesterly

flows correspond to cooler drier and less polluted air masses whereas the southwesterly

regime transports warmer more humid and significantly more polluted air masses

[Moody et al 1998] Further descriptions of the atmospheric chemistry at the site can be

found in Munger et al [1996]

322 Instruments

Our goal was to measure simultaneously the fluxes of NO NO2 NOy and O3 by

eddy covariance In order to measure eddy covariance fluxes of NO2 a technique with

consistent linear response high sensitivity and lack of interferences is required

Methods used in past studies that do not meet these criteria may show spurious fluxes due

to correlations between efficiency interferences and vertical wind speed or may require

extrapolation from unrealistically high concentrations For example systems which

photolyze NO2 to NO and subsequently detect NO via chemiluminescence suffer from

fluctuating conversion efficiencies [eg Rondon et al 1993 Munger et al 1996] the

45

chemiluminescent reaction of NO2 with Luminol has an interference which depends on

ambient O3 concentrations which in turn co-vary with vertical wind [Coe et al 1992

Walton et al 1997] and hot molybdenum or gold surfaces convert not only NO2 but

other NOy species as well to NO [eg Wesely et al 1982] NO measurements are more

straightforward as no conversion step is necessary before chemilumenscent detection O3

measurements are also more routine Flux measurements of NOy require special

instrumentation but have been routine at Harvard Forest since 1990 [Munger et al

1996]

From late August to mid-October 2000 a chemiluminescence detector (29 m

sampling height) was configured to measure NO concentrations at 8 Hz At other times

the chemiluminescence detector measured profiles and concentrations of NO and NO2

which was converted to NO in a photolysis cell prior to detection We installed a new

tunable diode laser absorption spectrometer (TDLAS) on a nearby tower to measure NO2

(22 m sampling height) at 1 Hz For complete TDLAS details refer to Chapter 2

Measurements of O3 concentration and flux employ C2H4-chemiluminescence (29 m

sampling height) and UV absorbance instruments respectively Three-axis sonic

anemometers facing into the prevailing wind direction (west) at the sampling heights on

both towers provided the vertical and horizontal wind velocities and virtual temperatures

(8 Hz) needed to compute eddy covariance fluxes of heat momentum NO NO2 and O3

(Munger et al 1996 1998) The layout of the site instruments and towers is depicted

schematically in Figure 31

46

EMSTower

TDLASTower100 m

SonicChemilumNO amp NO2NOy O3T PPFD

h=29 m

h=22 mSonic TDLASHNO3 amp NO2

Figure 31 Layout of Harvard Forest field site The EMS tower supports a sonic

anemometer at a height of 29 m along with inlets for chemiluminescence NO and NO2

NOy O3 and other trace gas instruments Temperature water vapor photosynthetic

photon flux density (PPFD) and other quantities are also measured on the EMS tower

some at multiple heights through the canopy The TDLAS tower a scaffolding structure

approximately 100 m distant supported a sonic anemometer and the TDLAS inlet at the

22 m height The TDLAS instrument was housed in two weatherproof enclosures (optics

and electronics) on the tower itself in order to minimize the inlet length Buildings near

the bases of both towers house instrumentation pumps gas cylinders and provide

electrical and internet connections

47

Photosynthetic photon flux density (PPFD) consisting of wavelengths between

400 nm and 700 nm was measured continuously at 29 m on the EMS tower using a LI-

COR quantum sensor Although the UV wavelengths associated with NO2 photolysis are

outside of the photosynthetic wavelength band we have used the more routine PPFD

measurements as a proxy for overall light level including the UV bands associated with

NOx photochemistry The midday UV (295 to 385 nm wavelengths) profile between 0

and 30 m was measured on one occasion at Harvard Forest using an Eppley total UV

radiometer placed facing upwards to capture direct plus diffuse downwelling radiation

and then facing downwards to capture diffuse upwelling radiation at each height

323 Eddy covariance fluxes

We computed 30-minute fluxes from the covariance of detrended vertical wind

velocity (wprime) with fluctuations of detrended temperature (Tprime) or detrended trace gas

concentration (Cprime) Details of the analysis process and software can be found in

Chapter 2

Because the photochemical reactions (R31)-(R34) occur on timescale

comparable to and in some cases faster than the turbulent diffusion timescale in the

surface layer we must consider the system of concentrations and fluxes for NO NO2

and O3 together [Fitzjarrald and Lenschow 1983] Deposition or exchange velocities

cannot be inferred from the fluxes and concentrations of any one species Our daytime

results were complicated by the fact that the NO2 concentration and flux were measured

at a different height than the NO and O3 concentrations and fluxes Physical and

experimental limitations to the height of the TDLAS tower and placement of the

48

instrument prevented co-location of the inlets We have therefore placed the daytime

deposition rates within the context of a simple model of turbulent diffusion and chemical

reaction in the surface layer described below

33 Results

331 NO2 Concentration Analysis and Comparison

Concurrent photolysis-chemiluminescence (hereafter P-C) and TDLAS

measurements allowed us to compare concentration data for NO2 over a wide range of

conditions throughout the spring summer and fall of 2000 The P-C system is routinely

calibrated with standard additions of a small flow of NO2 in N2 (calibration tank NO2

concentration of 5 micromol mol-1) to the air to determine the photolysis efficiency plus

addition of NO in N2 to determine the chemiluminescence detector response The NO2

tanks have been periodically calibrated against the NO standard but their long-term

stability remains a potential source of uncertainty Towards the end of the TDLAS

measurement period the concentrations of the standard tanks were checked by titration of

NO standards and by comparison with a larger cohort of NO2 standards The standard

tanks were found to be very close to their original concentrations See Munger et al

[1996] for complete details of the P-C NOx calibration procedure including corrections

for the effect (R31) in the sampling tubing

The TDLAS relies on accurate knowledge of the following spectroscopic

parameters to determine concentration absorption cross sections light path length laser

mode purity frequency tuning rate function pressure and temperature and laser line

49

width Absorption cross sections come from the HITRAN database and typically have

uncertainties of less than 10 [Rothman et al 1998] The light path length is confirmed

in the lab using standard additions into the multi-pass cell of an inert gas such as N2O

with absorption features in the frequency region of the measurement This calibration

can be done in the laboratory because the multi-pass cell body and mirrors are locked into

position and do not move relative to one another during field deployment The laserrsquos

frequency is first established using a built-in monochromator and a sealed reference cell

with a large concentration of NO2 This same cell provides a signal for locking the

frequency during instrument operation The laserrsquos frequency tuning rate function is

independently determined using a calibrated etalon The tuning rate can change as the

diode ages or after having warmed from liquid nitrogen to room temperature but can be

readjusted in the field by matching to the known spectral features Pressure and

temperature are measured in the multi-pass cell in order to compensate for the pressure

broadening and temperature dependence of the absorption lines

Typical TDLAS laser linewidths (due to all contributions to spectroscopic

instrument distortion) are well-approximated by a Gaussian function and are usually

small compared to the pressure broadened width for molecular spectral lines at 01 to

025 Pa We checked the laser linewidth by taking measurements at lower pressures with

all of the above factors already determined The particular diode used to measure NO2 in

this field deployment had an unusually large linewidth (approximately 0004 cm-1) and

non-Gaussian line shape The cause could not be determined before the diode reached

the end of its useable lifetime

50

In order to correct for the excess linewidth we simultaneously measured the

concentration of ambient water vapor in the absorption cell using our NO2 diode

(assuming Gaussian distortion) and a second diode with a more typical laser linewidth of

lt 0001 cm-1 We repeated this two-diode measurement with tank NO2 in N2 These

checks were performed before during and after field deployment The concentration

correction factor for the excess non-Gaussian distortion of the NO2 diode varied between

160 and 165 with a mean of 163 Field data were fit in real-time and in post analysis

using the standard Gaussian broadening function and later corrected by this factor

NO2 concentration measurements by the P-C and TDLAS instruments overlapped

between 4 April and 29 August 2000 The time series in Figure 32 shows the hourly

average NO2 concentrations from the two instruments during a four-day period In

Figure 33 all hourly TDLAS and P-C NO2 concentration measurements are compared

An orthogonal distance fit to the data with errors at the 95th confidence interval yields a

slope of 11 plusmn 02 a y-intercept of ndash02 plusmn 10 nmol mol-1 and an R2 of 091 The

agreement between the measurement methods is very good given the spatial separation of

the inlets both horizontally and vertically the precision of each instrument and the

calibration uncertainties for both instruments (~10)

332 Eddy Covariance Flux Validation

Lagged correlations ltwprimeNO2primegt and copsectra for vertical wind speed (w) and NO2

exhibit the expected offset and smearing due to instrument lag time and response function

(Figure 34) as compared for example to w and virtual temperature ltwprimeTsprimegt The peak in

51

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

effect on troposphere O3 potentially elevating production rates in NOx-limited regions

over much of the globe

NO2 flux measurements over short (lt 1m) crops and grasses vary greatly in

direction and diurnal pattern Past studies have reported nearly constant deposition [Coe

and Gallagher 1992] variable deposition [Wesely et al 1982] morning deposition

followed by afternoon emission [Delaney et al 1986] and daytime emission with

nighttime deposition [Stocker et al 1993 Padro et al 1998] NO2 deposition to soil

litter bark and other non-foliar surfaces may be as large as through leaf stomata [Eugster

and Hesterberg 1996 Hanson and Lindberg 1991] There have been very few

measurements of NO NO2 and O3 fluxes above tall deciduous coniferous or mixed

canopies most have employed enclosed chamber measurements on selected branches

leaves or other forest components [Rondon et al 1993 Sparks et al 2001] These

measurements are consistent with the hypothesis that daytime NO2 deposition to foliar

surfaces is controlled by stomatal conductance and that deposition to the forest floor and

soils may be as important as deposition to leaves They also suggest the presence of a

compensation point for NO2 such that below ambient concentrations of 1 ppb NO2

leaves may cease to take up or begin to emit NO2

We present measurements here of eddy covariance fluxes of NO2 NO and O3 at

the Harvard Forest Environmental Measurement Site during the spring summer and fall

of 2000 The triad of fluxes has not previously been simultaneously observed above a tall

forest canopy with reliable eddy covariance techniques The observations reveal NOx

deposition behavior that is distinctly different from the conventional parameterizations

currently used in many tropospheric chemistry and transport models

44

32 Methods

321 Site Description

The Harvard Forest site in central Massachusetts (4254N 7218W elevation 340

m) is a 50- to 70-year old mixed deciduous forest consisting primarily of red oak and red

maple with scattered hemlock red pine and white pine stands The terrain is roughly

95 forested and moderately hilly closest paved roads are more than 1 km away small

towns greater than 10 km distant Dominant winds are from the northwest and southwest

comprising two distinctly different chemical and meteorological regimes Northwesterly

flows correspond to cooler drier and less polluted air masses whereas the southwesterly

regime transports warmer more humid and significantly more polluted air masses

[Moody et al 1998] Further descriptions of the atmospheric chemistry at the site can be

found in Munger et al [1996]

322 Instruments

Our goal was to measure simultaneously the fluxes of NO NO2 NOy and O3 by

eddy covariance In order to measure eddy covariance fluxes of NO2 a technique with

consistent linear response high sensitivity and lack of interferences is required

Methods used in past studies that do not meet these criteria may show spurious fluxes due

to correlations between efficiency interferences and vertical wind speed or may require

extrapolation from unrealistically high concentrations For example systems which

photolyze NO2 to NO and subsequently detect NO via chemiluminescence suffer from

fluctuating conversion efficiencies [eg Rondon et al 1993 Munger et al 1996] the

45

chemiluminescent reaction of NO2 with Luminol has an interference which depends on

ambient O3 concentrations which in turn co-vary with vertical wind [Coe et al 1992

Walton et al 1997] and hot molybdenum or gold surfaces convert not only NO2 but

other NOy species as well to NO [eg Wesely et al 1982] NO measurements are more

straightforward as no conversion step is necessary before chemilumenscent detection O3

measurements are also more routine Flux measurements of NOy require special

instrumentation but have been routine at Harvard Forest since 1990 [Munger et al

1996]

From late August to mid-October 2000 a chemiluminescence detector (29 m

sampling height) was configured to measure NO concentrations at 8 Hz At other times

the chemiluminescence detector measured profiles and concentrations of NO and NO2

which was converted to NO in a photolysis cell prior to detection We installed a new

tunable diode laser absorption spectrometer (TDLAS) on a nearby tower to measure NO2

(22 m sampling height) at 1 Hz For complete TDLAS details refer to Chapter 2

Measurements of O3 concentration and flux employ C2H4-chemiluminescence (29 m

sampling height) and UV absorbance instruments respectively Three-axis sonic

anemometers facing into the prevailing wind direction (west) at the sampling heights on

both towers provided the vertical and horizontal wind velocities and virtual temperatures

(8 Hz) needed to compute eddy covariance fluxes of heat momentum NO NO2 and O3

(Munger et al 1996 1998) The layout of the site instruments and towers is depicted

schematically in Figure 31

46

EMSTower

TDLASTower100 m

SonicChemilumNO amp NO2NOy O3T PPFD

h=29 m

h=22 mSonic TDLASHNO3 amp NO2

Figure 31 Layout of Harvard Forest field site The EMS tower supports a sonic

anemometer at a height of 29 m along with inlets for chemiluminescence NO and NO2

NOy O3 and other trace gas instruments Temperature water vapor photosynthetic

photon flux density (PPFD) and other quantities are also measured on the EMS tower

some at multiple heights through the canopy The TDLAS tower a scaffolding structure

approximately 100 m distant supported a sonic anemometer and the TDLAS inlet at the

22 m height The TDLAS instrument was housed in two weatherproof enclosures (optics

and electronics) on the tower itself in order to minimize the inlet length Buildings near

the bases of both towers house instrumentation pumps gas cylinders and provide

electrical and internet connections

47

Photosynthetic photon flux density (PPFD) consisting of wavelengths between

400 nm and 700 nm was measured continuously at 29 m on the EMS tower using a LI-

COR quantum sensor Although the UV wavelengths associated with NO2 photolysis are

outside of the photosynthetic wavelength band we have used the more routine PPFD

measurements as a proxy for overall light level including the UV bands associated with

NOx photochemistry The midday UV (295 to 385 nm wavelengths) profile between 0

and 30 m was measured on one occasion at Harvard Forest using an Eppley total UV

radiometer placed facing upwards to capture direct plus diffuse downwelling radiation

and then facing downwards to capture diffuse upwelling radiation at each height

323 Eddy covariance fluxes

We computed 30-minute fluxes from the covariance of detrended vertical wind

velocity (wprime) with fluctuations of detrended temperature (Tprime) or detrended trace gas

concentration (Cprime) Details of the analysis process and software can be found in

Chapter 2

Because the photochemical reactions (R31)-(R34) occur on timescale

comparable to and in some cases faster than the turbulent diffusion timescale in the

surface layer we must consider the system of concentrations and fluxes for NO NO2

and O3 together [Fitzjarrald and Lenschow 1983] Deposition or exchange velocities

cannot be inferred from the fluxes and concentrations of any one species Our daytime

results were complicated by the fact that the NO2 concentration and flux were measured

at a different height than the NO and O3 concentrations and fluxes Physical and

experimental limitations to the height of the TDLAS tower and placement of the

48

instrument prevented co-location of the inlets We have therefore placed the daytime

deposition rates within the context of a simple model of turbulent diffusion and chemical

reaction in the surface layer described below

33 Results

331 NO2 Concentration Analysis and Comparison

Concurrent photolysis-chemiluminescence (hereafter P-C) and TDLAS

measurements allowed us to compare concentration data for NO2 over a wide range of

conditions throughout the spring summer and fall of 2000 The P-C system is routinely

calibrated with standard additions of a small flow of NO2 in N2 (calibration tank NO2

concentration of 5 micromol mol-1) to the air to determine the photolysis efficiency plus

addition of NO in N2 to determine the chemiluminescence detector response The NO2

tanks have been periodically calibrated against the NO standard but their long-term

stability remains a potential source of uncertainty Towards the end of the TDLAS

measurement period the concentrations of the standard tanks were checked by titration of

NO standards and by comparison with a larger cohort of NO2 standards The standard

tanks were found to be very close to their original concentrations See Munger et al

[1996] for complete details of the P-C NOx calibration procedure including corrections

for the effect (R31) in the sampling tubing

The TDLAS relies on accurate knowledge of the following spectroscopic

parameters to determine concentration absorption cross sections light path length laser

mode purity frequency tuning rate function pressure and temperature and laser line

49

width Absorption cross sections come from the HITRAN database and typically have

uncertainties of less than 10 [Rothman et al 1998] The light path length is confirmed

in the lab using standard additions into the multi-pass cell of an inert gas such as N2O

with absorption features in the frequency region of the measurement This calibration

can be done in the laboratory because the multi-pass cell body and mirrors are locked into

position and do not move relative to one another during field deployment The laserrsquos

frequency is first established using a built-in monochromator and a sealed reference cell

with a large concentration of NO2 This same cell provides a signal for locking the

frequency during instrument operation The laserrsquos frequency tuning rate function is

independently determined using a calibrated etalon The tuning rate can change as the

diode ages or after having warmed from liquid nitrogen to room temperature but can be

readjusted in the field by matching to the known spectral features Pressure and

temperature are measured in the multi-pass cell in order to compensate for the pressure

broadening and temperature dependence of the absorption lines

Typical TDLAS laser linewidths (due to all contributions to spectroscopic

instrument distortion) are well-approximated by a Gaussian function and are usually

small compared to the pressure broadened width for molecular spectral lines at 01 to

025 Pa We checked the laser linewidth by taking measurements at lower pressures with

all of the above factors already determined The particular diode used to measure NO2 in

this field deployment had an unusually large linewidth (approximately 0004 cm-1) and

non-Gaussian line shape The cause could not be determined before the diode reached

the end of its useable lifetime

50

In order to correct for the excess linewidth we simultaneously measured the

concentration of ambient water vapor in the absorption cell using our NO2 diode

(assuming Gaussian distortion) and a second diode with a more typical laser linewidth of

lt 0001 cm-1 We repeated this two-diode measurement with tank NO2 in N2 These

checks were performed before during and after field deployment The concentration

correction factor for the excess non-Gaussian distortion of the NO2 diode varied between

160 and 165 with a mean of 163 Field data were fit in real-time and in post analysis

using the standard Gaussian broadening function and later corrected by this factor

NO2 concentration measurements by the P-C and TDLAS instruments overlapped

between 4 April and 29 August 2000 The time series in Figure 32 shows the hourly

average NO2 concentrations from the two instruments during a four-day period In

Figure 33 all hourly TDLAS and P-C NO2 concentration measurements are compared

An orthogonal distance fit to the data with errors at the 95th confidence interval yields a

slope of 11 plusmn 02 a y-intercept of ndash02 plusmn 10 nmol mol-1 and an R2 of 091 The

agreement between the measurement methods is very good given the spatial separation of

the inlets both horizontally and vertically the precision of each instrument and the

calibration uncertainties for both instruments (~10)

332 Eddy Covariance Flux Validation

Lagged correlations ltwprimeNO2primegt and copsectra for vertical wind speed (w) and NO2

exhibit the expected offset and smearing due to instrument lag time and response function

(Figure 34) as compared for example to w and virtual temperature ltwprimeTsprimegt The peak in

51

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

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Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

32 Methods

321 Site Description

The Harvard Forest site in central Massachusetts (4254N 7218W elevation 340

m) is a 50- to 70-year old mixed deciduous forest consisting primarily of red oak and red

maple with scattered hemlock red pine and white pine stands The terrain is roughly

95 forested and moderately hilly closest paved roads are more than 1 km away small

towns greater than 10 km distant Dominant winds are from the northwest and southwest

comprising two distinctly different chemical and meteorological regimes Northwesterly

flows correspond to cooler drier and less polluted air masses whereas the southwesterly

regime transports warmer more humid and significantly more polluted air masses

[Moody et al 1998] Further descriptions of the atmospheric chemistry at the site can be

found in Munger et al [1996]

322 Instruments

Our goal was to measure simultaneously the fluxes of NO NO2 NOy and O3 by

eddy covariance In order to measure eddy covariance fluxes of NO2 a technique with

consistent linear response high sensitivity and lack of interferences is required

Methods used in past studies that do not meet these criteria may show spurious fluxes due

to correlations between efficiency interferences and vertical wind speed or may require

extrapolation from unrealistically high concentrations For example systems which

photolyze NO2 to NO and subsequently detect NO via chemiluminescence suffer from

fluctuating conversion efficiencies [eg Rondon et al 1993 Munger et al 1996] the

45

chemiluminescent reaction of NO2 with Luminol has an interference which depends on

ambient O3 concentrations which in turn co-vary with vertical wind [Coe et al 1992

Walton et al 1997] and hot molybdenum or gold surfaces convert not only NO2 but

other NOy species as well to NO [eg Wesely et al 1982] NO measurements are more

straightforward as no conversion step is necessary before chemilumenscent detection O3

measurements are also more routine Flux measurements of NOy require special

instrumentation but have been routine at Harvard Forest since 1990 [Munger et al

1996]

From late August to mid-October 2000 a chemiluminescence detector (29 m

sampling height) was configured to measure NO concentrations at 8 Hz At other times

the chemiluminescence detector measured profiles and concentrations of NO and NO2

which was converted to NO in a photolysis cell prior to detection We installed a new

tunable diode laser absorption spectrometer (TDLAS) on a nearby tower to measure NO2

(22 m sampling height) at 1 Hz For complete TDLAS details refer to Chapter 2

Measurements of O3 concentration and flux employ C2H4-chemiluminescence (29 m

sampling height) and UV absorbance instruments respectively Three-axis sonic

anemometers facing into the prevailing wind direction (west) at the sampling heights on

both towers provided the vertical and horizontal wind velocities and virtual temperatures

(8 Hz) needed to compute eddy covariance fluxes of heat momentum NO NO2 and O3

(Munger et al 1996 1998) The layout of the site instruments and towers is depicted

schematically in Figure 31

46

EMSTower

TDLASTower100 m

SonicChemilumNO amp NO2NOy O3T PPFD

h=29 m

h=22 mSonic TDLASHNO3 amp NO2

Figure 31 Layout of Harvard Forest field site The EMS tower supports a sonic

anemometer at a height of 29 m along with inlets for chemiluminescence NO and NO2

NOy O3 and other trace gas instruments Temperature water vapor photosynthetic

photon flux density (PPFD) and other quantities are also measured on the EMS tower

some at multiple heights through the canopy The TDLAS tower a scaffolding structure

approximately 100 m distant supported a sonic anemometer and the TDLAS inlet at the

22 m height The TDLAS instrument was housed in two weatherproof enclosures (optics

and electronics) on the tower itself in order to minimize the inlet length Buildings near

the bases of both towers house instrumentation pumps gas cylinders and provide

electrical and internet connections

47

Photosynthetic photon flux density (PPFD) consisting of wavelengths between

400 nm and 700 nm was measured continuously at 29 m on the EMS tower using a LI-

COR quantum sensor Although the UV wavelengths associated with NO2 photolysis are

outside of the photosynthetic wavelength band we have used the more routine PPFD

measurements as a proxy for overall light level including the UV bands associated with

NOx photochemistry The midday UV (295 to 385 nm wavelengths) profile between 0

and 30 m was measured on one occasion at Harvard Forest using an Eppley total UV

radiometer placed facing upwards to capture direct plus diffuse downwelling radiation

and then facing downwards to capture diffuse upwelling radiation at each height

323 Eddy covariance fluxes

We computed 30-minute fluxes from the covariance of detrended vertical wind

velocity (wprime) with fluctuations of detrended temperature (Tprime) or detrended trace gas

concentration (Cprime) Details of the analysis process and software can be found in

Chapter 2

Because the photochemical reactions (R31)-(R34) occur on timescale

comparable to and in some cases faster than the turbulent diffusion timescale in the

surface layer we must consider the system of concentrations and fluxes for NO NO2

and O3 together [Fitzjarrald and Lenschow 1983] Deposition or exchange velocities

cannot be inferred from the fluxes and concentrations of any one species Our daytime

results were complicated by the fact that the NO2 concentration and flux were measured

at a different height than the NO and O3 concentrations and fluxes Physical and

experimental limitations to the height of the TDLAS tower and placement of the

48

instrument prevented co-location of the inlets We have therefore placed the daytime

deposition rates within the context of a simple model of turbulent diffusion and chemical

reaction in the surface layer described below

33 Results

331 NO2 Concentration Analysis and Comparison

Concurrent photolysis-chemiluminescence (hereafter P-C) and TDLAS

measurements allowed us to compare concentration data for NO2 over a wide range of

conditions throughout the spring summer and fall of 2000 The P-C system is routinely

calibrated with standard additions of a small flow of NO2 in N2 (calibration tank NO2

concentration of 5 micromol mol-1) to the air to determine the photolysis efficiency plus

addition of NO in N2 to determine the chemiluminescence detector response The NO2

tanks have been periodically calibrated against the NO standard but their long-term

stability remains a potential source of uncertainty Towards the end of the TDLAS

measurement period the concentrations of the standard tanks were checked by titration of

NO standards and by comparison with a larger cohort of NO2 standards The standard

tanks were found to be very close to their original concentrations See Munger et al

[1996] for complete details of the P-C NOx calibration procedure including corrections

for the effect (R31) in the sampling tubing

The TDLAS relies on accurate knowledge of the following spectroscopic

parameters to determine concentration absorption cross sections light path length laser

mode purity frequency tuning rate function pressure and temperature and laser line

49

width Absorption cross sections come from the HITRAN database and typically have

uncertainties of less than 10 [Rothman et al 1998] The light path length is confirmed

in the lab using standard additions into the multi-pass cell of an inert gas such as N2O

with absorption features in the frequency region of the measurement This calibration

can be done in the laboratory because the multi-pass cell body and mirrors are locked into

position and do not move relative to one another during field deployment The laserrsquos

frequency is first established using a built-in monochromator and a sealed reference cell

with a large concentration of NO2 This same cell provides a signal for locking the

frequency during instrument operation The laserrsquos frequency tuning rate function is

independently determined using a calibrated etalon The tuning rate can change as the

diode ages or after having warmed from liquid nitrogen to room temperature but can be

readjusted in the field by matching to the known spectral features Pressure and

temperature are measured in the multi-pass cell in order to compensate for the pressure

broadening and temperature dependence of the absorption lines

Typical TDLAS laser linewidths (due to all contributions to spectroscopic

instrument distortion) are well-approximated by a Gaussian function and are usually

small compared to the pressure broadened width for molecular spectral lines at 01 to

025 Pa We checked the laser linewidth by taking measurements at lower pressures with

all of the above factors already determined The particular diode used to measure NO2 in

this field deployment had an unusually large linewidth (approximately 0004 cm-1) and

non-Gaussian line shape The cause could not be determined before the diode reached

the end of its useable lifetime

50

In order to correct for the excess linewidth we simultaneously measured the

concentration of ambient water vapor in the absorption cell using our NO2 diode

(assuming Gaussian distortion) and a second diode with a more typical laser linewidth of

lt 0001 cm-1 We repeated this two-diode measurement with tank NO2 in N2 These

checks were performed before during and after field deployment The concentration

correction factor for the excess non-Gaussian distortion of the NO2 diode varied between

160 and 165 with a mean of 163 Field data were fit in real-time and in post analysis

using the standard Gaussian broadening function and later corrected by this factor

NO2 concentration measurements by the P-C and TDLAS instruments overlapped

between 4 April and 29 August 2000 The time series in Figure 32 shows the hourly

average NO2 concentrations from the two instruments during a four-day period In

Figure 33 all hourly TDLAS and P-C NO2 concentration measurements are compared

An orthogonal distance fit to the data with errors at the 95th confidence interval yields a

slope of 11 plusmn 02 a y-intercept of ndash02 plusmn 10 nmol mol-1 and an R2 of 091 The

agreement between the measurement methods is very good given the spatial separation of

the inlets both horizontally and vertically the precision of each instrument and the

calibration uncertainties for both instruments (~10)

332 Eddy Covariance Flux Validation

Lagged correlations ltwprimeNO2primegt and copsectra for vertical wind speed (w) and NO2

exhibit the expected offset and smearing due to instrument lag time and response function

(Figure 34) as compared for example to w and virtual temperature ltwprimeTsprimegt The peak in

51

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

chemiluminescent reaction of NO2 with Luminol has an interference which depends on

ambient O3 concentrations which in turn co-vary with vertical wind [Coe et al 1992

Walton et al 1997] and hot molybdenum or gold surfaces convert not only NO2 but

other NOy species as well to NO [eg Wesely et al 1982] NO measurements are more

straightforward as no conversion step is necessary before chemilumenscent detection O3

measurements are also more routine Flux measurements of NOy require special

instrumentation but have been routine at Harvard Forest since 1990 [Munger et al

1996]

From late August to mid-October 2000 a chemiluminescence detector (29 m

sampling height) was configured to measure NO concentrations at 8 Hz At other times

the chemiluminescence detector measured profiles and concentrations of NO and NO2

which was converted to NO in a photolysis cell prior to detection We installed a new

tunable diode laser absorption spectrometer (TDLAS) on a nearby tower to measure NO2

(22 m sampling height) at 1 Hz For complete TDLAS details refer to Chapter 2

Measurements of O3 concentration and flux employ C2H4-chemiluminescence (29 m

sampling height) and UV absorbance instruments respectively Three-axis sonic

anemometers facing into the prevailing wind direction (west) at the sampling heights on

both towers provided the vertical and horizontal wind velocities and virtual temperatures

(8 Hz) needed to compute eddy covariance fluxes of heat momentum NO NO2 and O3

(Munger et al 1996 1998) The layout of the site instruments and towers is depicted

schematically in Figure 31

46

EMSTower

TDLASTower100 m

SonicChemilumNO amp NO2NOy O3T PPFD

h=29 m

h=22 mSonic TDLASHNO3 amp NO2

Figure 31 Layout of Harvard Forest field site The EMS tower supports a sonic

anemometer at a height of 29 m along with inlets for chemiluminescence NO and NO2

NOy O3 and other trace gas instruments Temperature water vapor photosynthetic

photon flux density (PPFD) and other quantities are also measured on the EMS tower

some at multiple heights through the canopy The TDLAS tower a scaffolding structure

approximately 100 m distant supported a sonic anemometer and the TDLAS inlet at the

22 m height The TDLAS instrument was housed in two weatherproof enclosures (optics

and electronics) on the tower itself in order to minimize the inlet length Buildings near

the bases of both towers house instrumentation pumps gas cylinders and provide

electrical and internet connections

47

Photosynthetic photon flux density (PPFD) consisting of wavelengths between

400 nm and 700 nm was measured continuously at 29 m on the EMS tower using a LI-

COR quantum sensor Although the UV wavelengths associated with NO2 photolysis are

outside of the photosynthetic wavelength band we have used the more routine PPFD

measurements as a proxy for overall light level including the UV bands associated with

NOx photochemistry The midday UV (295 to 385 nm wavelengths) profile between 0

and 30 m was measured on one occasion at Harvard Forest using an Eppley total UV

radiometer placed facing upwards to capture direct plus diffuse downwelling radiation

and then facing downwards to capture diffuse upwelling radiation at each height

323 Eddy covariance fluxes

We computed 30-minute fluxes from the covariance of detrended vertical wind

velocity (wprime) with fluctuations of detrended temperature (Tprime) or detrended trace gas

concentration (Cprime) Details of the analysis process and software can be found in

Chapter 2

Because the photochemical reactions (R31)-(R34) occur on timescale

comparable to and in some cases faster than the turbulent diffusion timescale in the

surface layer we must consider the system of concentrations and fluxes for NO NO2

and O3 together [Fitzjarrald and Lenschow 1983] Deposition or exchange velocities

cannot be inferred from the fluxes and concentrations of any one species Our daytime

results were complicated by the fact that the NO2 concentration and flux were measured

at a different height than the NO and O3 concentrations and fluxes Physical and

experimental limitations to the height of the TDLAS tower and placement of the

48

instrument prevented co-location of the inlets We have therefore placed the daytime

deposition rates within the context of a simple model of turbulent diffusion and chemical

reaction in the surface layer described below

33 Results

331 NO2 Concentration Analysis and Comparison

Concurrent photolysis-chemiluminescence (hereafter P-C) and TDLAS

measurements allowed us to compare concentration data for NO2 over a wide range of

conditions throughout the spring summer and fall of 2000 The P-C system is routinely

calibrated with standard additions of a small flow of NO2 in N2 (calibration tank NO2

concentration of 5 micromol mol-1) to the air to determine the photolysis efficiency plus

addition of NO in N2 to determine the chemiluminescence detector response The NO2

tanks have been periodically calibrated against the NO standard but their long-term

stability remains a potential source of uncertainty Towards the end of the TDLAS

measurement period the concentrations of the standard tanks were checked by titration of

NO standards and by comparison with a larger cohort of NO2 standards The standard

tanks were found to be very close to their original concentrations See Munger et al

[1996] for complete details of the P-C NOx calibration procedure including corrections

for the effect (R31) in the sampling tubing

The TDLAS relies on accurate knowledge of the following spectroscopic

parameters to determine concentration absorption cross sections light path length laser

mode purity frequency tuning rate function pressure and temperature and laser line

49

width Absorption cross sections come from the HITRAN database and typically have

uncertainties of less than 10 [Rothman et al 1998] The light path length is confirmed

in the lab using standard additions into the multi-pass cell of an inert gas such as N2O

with absorption features in the frequency region of the measurement This calibration

can be done in the laboratory because the multi-pass cell body and mirrors are locked into

position and do not move relative to one another during field deployment The laserrsquos

frequency is first established using a built-in monochromator and a sealed reference cell

with a large concentration of NO2 This same cell provides a signal for locking the

frequency during instrument operation The laserrsquos frequency tuning rate function is

independently determined using a calibrated etalon The tuning rate can change as the

diode ages or after having warmed from liquid nitrogen to room temperature but can be

readjusted in the field by matching to the known spectral features Pressure and

temperature are measured in the multi-pass cell in order to compensate for the pressure

broadening and temperature dependence of the absorption lines

Typical TDLAS laser linewidths (due to all contributions to spectroscopic

instrument distortion) are well-approximated by a Gaussian function and are usually

small compared to the pressure broadened width for molecular spectral lines at 01 to

025 Pa We checked the laser linewidth by taking measurements at lower pressures with

all of the above factors already determined The particular diode used to measure NO2 in

this field deployment had an unusually large linewidth (approximately 0004 cm-1) and

non-Gaussian line shape The cause could not be determined before the diode reached

the end of its useable lifetime

50

In order to correct for the excess linewidth we simultaneously measured the

concentration of ambient water vapor in the absorption cell using our NO2 diode

(assuming Gaussian distortion) and a second diode with a more typical laser linewidth of

lt 0001 cm-1 We repeated this two-diode measurement with tank NO2 in N2 These

checks were performed before during and after field deployment The concentration

correction factor for the excess non-Gaussian distortion of the NO2 diode varied between

160 and 165 with a mean of 163 Field data were fit in real-time and in post analysis

using the standard Gaussian broadening function and later corrected by this factor

NO2 concentration measurements by the P-C and TDLAS instruments overlapped

between 4 April and 29 August 2000 The time series in Figure 32 shows the hourly

average NO2 concentrations from the two instruments during a four-day period In

Figure 33 all hourly TDLAS and P-C NO2 concentration measurements are compared

An orthogonal distance fit to the data with errors at the 95th confidence interval yields a

slope of 11 plusmn 02 a y-intercept of ndash02 plusmn 10 nmol mol-1 and an R2 of 091 The

agreement between the measurement methods is very good given the spatial separation of

the inlets both horizontally and vertically the precision of each instrument and the

calibration uncertainties for both instruments (~10)

332 Eddy Covariance Flux Validation

Lagged correlations ltwprimeNO2primegt and copsectra for vertical wind speed (w) and NO2

exhibit the expected offset and smearing due to instrument lag time and response function

(Figure 34) as compared for example to w and virtual temperature ltwprimeTsprimegt The peak in

51

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

EMSTower

TDLASTower100 m

SonicChemilumNO amp NO2NOy O3T PPFD

h=29 m

h=22 mSonic TDLASHNO3 amp NO2

Figure 31 Layout of Harvard Forest field site The EMS tower supports a sonic

anemometer at a height of 29 m along with inlets for chemiluminescence NO and NO2

NOy O3 and other trace gas instruments Temperature water vapor photosynthetic

photon flux density (PPFD) and other quantities are also measured on the EMS tower

some at multiple heights through the canopy The TDLAS tower a scaffolding structure

approximately 100 m distant supported a sonic anemometer and the TDLAS inlet at the

22 m height The TDLAS instrument was housed in two weatherproof enclosures (optics

and electronics) on the tower itself in order to minimize the inlet length Buildings near

the bases of both towers house instrumentation pumps gas cylinders and provide

electrical and internet connections

47

Photosynthetic photon flux density (PPFD) consisting of wavelengths between

400 nm and 700 nm was measured continuously at 29 m on the EMS tower using a LI-

COR quantum sensor Although the UV wavelengths associated with NO2 photolysis are

outside of the photosynthetic wavelength band we have used the more routine PPFD

measurements as a proxy for overall light level including the UV bands associated with

NOx photochemistry The midday UV (295 to 385 nm wavelengths) profile between 0

and 30 m was measured on one occasion at Harvard Forest using an Eppley total UV

radiometer placed facing upwards to capture direct plus diffuse downwelling radiation

and then facing downwards to capture diffuse upwelling radiation at each height

323 Eddy covariance fluxes

We computed 30-minute fluxes from the covariance of detrended vertical wind

velocity (wprime) with fluctuations of detrended temperature (Tprime) or detrended trace gas

concentration (Cprime) Details of the analysis process and software can be found in

Chapter 2

Because the photochemical reactions (R31)-(R34) occur on timescale

comparable to and in some cases faster than the turbulent diffusion timescale in the

surface layer we must consider the system of concentrations and fluxes for NO NO2

and O3 together [Fitzjarrald and Lenschow 1983] Deposition or exchange velocities

cannot be inferred from the fluxes and concentrations of any one species Our daytime

results were complicated by the fact that the NO2 concentration and flux were measured

at a different height than the NO and O3 concentrations and fluxes Physical and

experimental limitations to the height of the TDLAS tower and placement of the

48

instrument prevented co-location of the inlets We have therefore placed the daytime

deposition rates within the context of a simple model of turbulent diffusion and chemical

reaction in the surface layer described below

33 Results

331 NO2 Concentration Analysis and Comparison

Concurrent photolysis-chemiluminescence (hereafter P-C) and TDLAS

measurements allowed us to compare concentration data for NO2 over a wide range of

conditions throughout the spring summer and fall of 2000 The P-C system is routinely

calibrated with standard additions of a small flow of NO2 in N2 (calibration tank NO2

concentration of 5 micromol mol-1) to the air to determine the photolysis efficiency plus

addition of NO in N2 to determine the chemiluminescence detector response The NO2

tanks have been periodically calibrated against the NO standard but their long-term

stability remains a potential source of uncertainty Towards the end of the TDLAS

measurement period the concentrations of the standard tanks were checked by titration of

NO standards and by comparison with a larger cohort of NO2 standards The standard

tanks were found to be very close to their original concentrations See Munger et al

[1996] for complete details of the P-C NOx calibration procedure including corrections

for the effect (R31) in the sampling tubing

The TDLAS relies on accurate knowledge of the following spectroscopic

parameters to determine concentration absorption cross sections light path length laser

mode purity frequency tuning rate function pressure and temperature and laser line

49

width Absorption cross sections come from the HITRAN database and typically have

uncertainties of less than 10 [Rothman et al 1998] The light path length is confirmed

in the lab using standard additions into the multi-pass cell of an inert gas such as N2O

with absorption features in the frequency region of the measurement This calibration

can be done in the laboratory because the multi-pass cell body and mirrors are locked into

position and do not move relative to one another during field deployment The laserrsquos

frequency is first established using a built-in monochromator and a sealed reference cell

with a large concentration of NO2 This same cell provides a signal for locking the

frequency during instrument operation The laserrsquos frequency tuning rate function is

independently determined using a calibrated etalon The tuning rate can change as the

diode ages or after having warmed from liquid nitrogen to room temperature but can be

readjusted in the field by matching to the known spectral features Pressure and

temperature are measured in the multi-pass cell in order to compensate for the pressure

broadening and temperature dependence of the absorption lines

Typical TDLAS laser linewidths (due to all contributions to spectroscopic

instrument distortion) are well-approximated by a Gaussian function and are usually

small compared to the pressure broadened width for molecular spectral lines at 01 to

025 Pa We checked the laser linewidth by taking measurements at lower pressures with

all of the above factors already determined The particular diode used to measure NO2 in

this field deployment had an unusually large linewidth (approximately 0004 cm-1) and

non-Gaussian line shape The cause could not be determined before the diode reached

the end of its useable lifetime

50

In order to correct for the excess linewidth we simultaneously measured the

concentration of ambient water vapor in the absorption cell using our NO2 diode

(assuming Gaussian distortion) and a second diode with a more typical laser linewidth of

lt 0001 cm-1 We repeated this two-diode measurement with tank NO2 in N2 These

checks were performed before during and after field deployment The concentration

correction factor for the excess non-Gaussian distortion of the NO2 diode varied between

160 and 165 with a mean of 163 Field data were fit in real-time and in post analysis

using the standard Gaussian broadening function and later corrected by this factor

NO2 concentration measurements by the P-C and TDLAS instruments overlapped

between 4 April and 29 August 2000 The time series in Figure 32 shows the hourly

average NO2 concentrations from the two instruments during a four-day period In

Figure 33 all hourly TDLAS and P-C NO2 concentration measurements are compared

An orthogonal distance fit to the data with errors at the 95th confidence interval yields a

slope of 11 plusmn 02 a y-intercept of ndash02 plusmn 10 nmol mol-1 and an R2 of 091 The

agreement between the measurement methods is very good given the spatial separation of

the inlets both horizontally and vertically the precision of each instrument and the

calibration uncertainties for both instruments (~10)

332 Eddy Covariance Flux Validation

Lagged correlations ltwprimeNO2primegt and copsectra for vertical wind speed (w) and NO2

exhibit the expected offset and smearing due to instrument lag time and response function

(Figure 34) as compared for example to w and virtual temperature ltwprimeTsprimegt The peak in

51

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Photosynthetic photon flux density (PPFD) consisting of wavelengths between

400 nm and 700 nm was measured continuously at 29 m on the EMS tower using a LI-

COR quantum sensor Although the UV wavelengths associated with NO2 photolysis are

outside of the photosynthetic wavelength band we have used the more routine PPFD

measurements as a proxy for overall light level including the UV bands associated with

NOx photochemistry The midday UV (295 to 385 nm wavelengths) profile between 0

and 30 m was measured on one occasion at Harvard Forest using an Eppley total UV

radiometer placed facing upwards to capture direct plus diffuse downwelling radiation

and then facing downwards to capture diffuse upwelling radiation at each height

323 Eddy covariance fluxes

We computed 30-minute fluxes from the covariance of detrended vertical wind

velocity (wprime) with fluctuations of detrended temperature (Tprime) or detrended trace gas

concentration (Cprime) Details of the analysis process and software can be found in

Chapter 2

Because the photochemical reactions (R31)-(R34) occur on timescale

comparable to and in some cases faster than the turbulent diffusion timescale in the

surface layer we must consider the system of concentrations and fluxes for NO NO2

and O3 together [Fitzjarrald and Lenschow 1983] Deposition or exchange velocities

cannot be inferred from the fluxes and concentrations of any one species Our daytime

results were complicated by the fact that the NO2 concentration and flux were measured

at a different height than the NO and O3 concentrations and fluxes Physical and

experimental limitations to the height of the TDLAS tower and placement of the

48

instrument prevented co-location of the inlets We have therefore placed the daytime

deposition rates within the context of a simple model of turbulent diffusion and chemical

reaction in the surface layer described below

33 Results

331 NO2 Concentration Analysis and Comparison

Concurrent photolysis-chemiluminescence (hereafter P-C) and TDLAS

measurements allowed us to compare concentration data for NO2 over a wide range of

conditions throughout the spring summer and fall of 2000 The P-C system is routinely

calibrated with standard additions of a small flow of NO2 in N2 (calibration tank NO2

concentration of 5 micromol mol-1) to the air to determine the photolysis efficiency plus

addition of NO in N2 to determine the chemiluminescence detector response The NO2

tanks have been periodically calibrated against the NO standard but their long-term

stability remains a potential source of uncertainty Towards the end of the TDLAS

measurement period the concentrations of the standard tanks were checked by titration of

NO standards and by comparison with a larger cohort of NO2 standards The standard

tanks were found to be very close to their original concentrations See Munger et al

[1996] for complete details of the P-C NOx calibration procedure including corrections

for the effect (R31) in the sampling tubing

The TDLAS relies on accurate knowledge of the following spectroscopic

parameters to determine concentration absorption cross sections light path length laser

mode purity frequency tuning rate function pressure and temperature and laser line

49

width Absorption cross sections come from the HITRAN database and typically have

uncertainties of less than 10 [Rothman et al 1998] The light path length is confirmed

in the lab using standard additions into the multi-pass cell of an inert gas such as N2O

with absorption features in the frequency region of the measurement This calibration

can be done in the laboratory because the multi-pass cell body and mirrors are locked into

position and do not move relative to one another during field deployment The laserrsquos

frequency is first established using a built-in monochromator and a sealed reference cell

with a large concentration of NO2 This same cell provides a signal for locking the

frequency during instrument operation The laserrsquos frequency tuning rate function is

independently determined using a calibrated etalon The tuning rate can change as the

diode ages or after having warmed from liquid nitrogen to room temperature but can be

readjusted in the field by matching to the known spectral features Pressure and

temperature are measured in the multi-pass cell in order to compensate for the pressure

broadening and temperature dependence of the absorption lines

Typical TDLAS laser linewidths (due to all contributions to spectroscopic

instrument distortion) are well-approximated by a Gaussian function and are usually

small compared to the pressure broadened width for molecular spectral lines at 01 to

025 Pa We checked the laser linewidth by taking measurements at lower pressures with

all of the above factors already determined The particular diode used to measure NO2 in

this field deployment had an unusually large linewidth (approximately 0004 cm-1) and

non-Gaussian line shape The cause could not be determined before the diode reached

the end of its useable lifetime

50

In order to correct for the excess linewidth we simultaneously measured the

concentration of ambient water vapor in the absorption cell using our NO2 diode

(assuming Gaussian distortion) and a second diode with a more typical laser linewidth of

lt 0001 cm-1 We repeated this two-diode measurement with tank NO2 in N2 These

checks were performed before during and after field deployment The concentration

correction factor for the excess non-Gaussian distortion of the NO2 diode varied between

160 and 165 with a mean of 163 Field data were fit in real-time and in post analysis

using the standard Gaussian broadening function and later corrected by this factor

NO2 concentration measurements by the P-C and TDLAS instruments overlapped

between 4 April and 29 August 2000 The time series in Figure 32 shows the hourly

average NO2 concentrations from the two instruments during a four-day period In

Figure 33 all hourly TDLAS and P-C NO2 concentration measurements are compared

An orthogonal distance fit to the data with errors at the 95th confidence interval yields a

slope of 11 plusmn 02 a y-intercept of ndash02 plusmn 10 nmol mol-1 and an R2 of 091 The

agreement between the measurement methods is very good given the spatial separation of

the inlets both horizontally and vertically the precision of each instrument and the

calibration uncertainties for both instruments (~10)

332 Eddy Covariance Flux Validation

Lagged correlations ltwprimeNO2primegt and copsectra for vertical wind speed (w) and NO2

exhibit the expected offset and smearing due to instrument lag time and response function

(Figure 34) as compared for example to w and virtual temperature ltwprimeTsprimegt The peak in

51

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

instrument prevented co-location of the inlets We have therefore placed the daytime

deposition rates within the context of a simple model of turbulent diffusion and chemical

reaction in the surface layer described below

33 Results

331 NO2 Concentration Analysis and Comparison

Concurrent photolysis-chemiluminescence (hereafter P-C) and TDLAS

measurements allowed us to compare concentration data for NO2 over a wide range of

conditions throughout the spring summer and fall of 2000 The P-C system is routinely

calibrated with standard additions of a small flow of NO2 in N2 (calibration tank NO2

concentration of 5 micromol mol-1) to the air to determine the photolysis efficiency plus

addition of NO in N2 to determine the chemiluminescence detector response The NO2

tanks have been periodically calibrated against the NO standard but their long-term

stability remains a potential source of uncertainty Towards the end of the TDLAS

measurement period the concentrations of the standard tanks were checked by titration of

NO standards and by comparison with a larger cohort of NO2 standards The standard

tanks were found to be very close to their original concentrations See Munger et al

[1996] for complete details of the P-C NOx calibration procedure including corrections

for the effect (R31) in the sampling tubing

The TDLAS relies on accurate knowledge of the following spectroscopic

parameters to determine concentration absorption cross sections light path length laser

mode purity frequency tuning rate function pressure and temperature and laser line

49

width Absorption cross sections come from the HITRAN database and typically have

uncertainties of less than 10 [Rothman et al 1998] The light path length is confirmed

in the lab using standard additions into the multi-pass cell of an inert gas such as N2O

with absorption features in the frequency region of the measurement This calibration

can be done in the laboratory because the multi-pass cell body and mirrors are locked into

position and do not move relative to one another during field deployment The laserrsquos

frequency is first established using a built-in monochromator and a sealed reference cell

with a large concentration of NO2 This same cell provides a signal for locking the

frequency during instrument operation The laserrsquos frequency tuning rate function is

independently determined using a calibrated etalon The tuning rate can change as the

diode ages or after having warmed from liquid nitrogen to room temperature but can be

readjusted in the field by matching to the known spectral features Pressure and

temperature are measured in the multi-pass cell in order to compensate for the pressure

broadening and temperature dependence of the absorption lines

Typical TDLAS laser linewidths (due to all contributions to spectroscopic

instrument distortion) are well-approximated by a Gaussian function and are usually

small compared to the pressure broadened width for molecular spectral lines at 01 to

025 Pa We checked the laser linewidth by taking measurements at lower pressures with

all of the above factors already determined The particular diode used to measure NO2 in

this field deployment had an unusually large linewidth (approximately 0004 cm-1) and

non-Gaussian line shape The cause could not be determined before the diode reached

the end of its useable lifetime

50

In order to correct for the excess linewidth we simultaneously measured the

concentration of ambient water vapor in the absorption cell using our NO2 diode

(assuming Gaussian distortion) and a second diode with a more typical laser linewidth of

lt 0001 cm-1 We repeated this two-diode measurement with tank NO2 in N2 These

checks were performed before during and after field deployment The concentration

correction factor for the excess non-Gaussian distortion of the NO2 diode varied between

160 and 165 with a mean of 163 Field data were fit in real-time and in post analysis

using the standard Gaussian broadening function and later corrected by this factor

NO2 concentration measurements by the P-C and TDLAS instruments overlapped

between 4 April and 29 August 2000 The time series in Figure 32 shows the hourly

average NO2 concentrations from the two instruments during a four-day period In

Figure 33 all hourly TDLAS and P-C NO2 concentration measurements are compared

An orthogonal distance fit to the data with errors at the 95th confidence interval yields a

slope of 11 plusmn 02 a y-intercept of ndash02 plusmn 10 nmol mol-1 and an R2 of 091 The

agreement between the measurement methods is very good given the spatial separation of

the inlets both horizontally and vertically the precision of each instrument and the

calibration uncertainties for both instruments (~10)

332 Eddy Covariance Flux Validation

Lagged correlations ltwprimeNO2primegt and copsectra for vertical wind speed (w) and NO2

exhibit the expected offset and smearing due to instrument lag time and response function

(Figure 34) as compared for example to w and virtual temperature ltwprimeTsprimegt The peak in

51

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

width Absorption cross sections come from the HITRAN database and typically have

uncertainties of less than 10 [Rothman et al 1998] The light path length is confirmed

in the lab using standard additions into the multi-pass cell of an inert gas such as N2O

with absorption features in the frequency region of the measurement This calibration

can be done in the laboratory because the multi-pass cell body and mirrors are locked into

position and do not move relative to one another during field deployment The laserrsquos

frequency is first established using a built-in monochromator and a sealed reference cell

with a large concentration of NO2 This same cell provides a signal for locking the

frequency during instrument operation The laserrsquos frequency tuning rate function is

independently determined using a calibrated etalon The tuning rate can change as the

diode ages or after having warmed from liquid nitrogen to room temperature but can be

readjusted in the field by matching to the known spectral features Pressure and

temperature are measured in the multi-pass cell in order to compensate for the pressure

broadening and temperature dependence of the absorption lines

Typical TDLAS laser linewidths (due to all contributions to spectroscopic

instrument distortion) are well-approximated by a Gaussian function and are usually

small compared to the pressure broadened width for molecular spectral lines at 01 to

025 Pa We checked the laser linewidth by taking measurements at lower pressures with

all of the above factors already determined The particular diode used to measure NO2 in

this field deployment had an unusually large linewidth (approximately 0004 cm-1) and

non-Gaussian line shape The cause could not be determined before the diode reached

the end of its useable lifetime

50

In order to correct for the excess linewidth we simultaneously measured the

concentration of ambient water vapor in the absorption cell using our NO2 diode

(assuming Gaussian distortion) and a second diode with a more typical laser linewidth of

lt 0001 cm-1 We repeated this two-diode measurement with tank NO2 in N2 These

checks were performed before during and after field deployment The concentration

correction factor for the excess non-Gaussian distortion of the NO2 diode varied between

160 and 165 with a mean of 163 Field data were fit in real-time and in post analysis

using the standard Gaussian broadening function and later corrected by this factor

NO2 concentration measurements by the P-C and TDLAS instruments overlapped

between 4 April and 29 August 2000 The time series in Figure 32 shows the hourly

average NO2 concentrations from the two instruments during a four-day period In

Figure 33 all hourly TDLAS and P-C NO2 concentration measurements are compared

An orthogonal distance fit to the data with errors at the 95th confidence interval yields a

slope of 11 plusmn 02 a y-intercept of ndash02 plusmn 10 nmol mol-1 and an R2 of 091 The

agreement between the measurement methods is very good given the spatial separation of

the inlets both horizontally and vertically the precision of each instrument and the

calibration uncertainties for both instruments (~10)

332 Eddy Covariance Flux Validation

Lagged correlations ltwprimeNO2primegt and copsectra for vertical wind speed (w) and NO2

exhibit the expected offset and smearing due to instrument lag time and response function

(Figure 34) as compared for example to w and virtual temperature ltwprimeTsprimegt The peak in

51

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

In order to correct for the excess linewidth we simultaneously measured the

concentration of ambient water vapor in the absorption cell using our NO2 diode

(assuming Gaussian distortion) and a second diode with a more typical laser linewidth of

lt 0001 cm-1 We repeated this two-diode measurement with tank NO2 in N2 These

checks were performed before during and after field deployment The concentration

correction factor for the excess non-Gaussian distortion of the NO2 diode varied between

160 and 165 with a mean of 163 Field data were fit in real-time and in post analysis

using the standard Gaussian broadening function and later corrected by this factor

NO2 concentration measurements by the P-C and TDLAS instruments overlapped

between 4 April and 29 August 2000 The time series in Figure 32 shows the hourly

average NO2 concentrations from the two instruments during a four-day period In

Figure 33 all hourly TDLAS and P-C NO2 concentration measurements are compared

An orthogonal distance fit to the data with errors at the 95th confidence interval yields a

slope of 11 plusmn 02 a y-intercept of ndash02 plusmn 10 nmol mol-1 and an R2 of 091 The

agreement between the measurement methods is very good given the spatial separation of

the inlets both horizontally and vertically the precision of each instrument and the

calibration uncertainties for both instruments (~10)

332 Eddy Covariance Flux Validation

Lagged correlations ltwprimeNO2primegt and copsectra for vertical wind speed (w) and NO2

exhibit the expected offset and smearing due to instrument lag time and response function

(Figure 34) as compared for example to w and virtual temperature ltwprimeTsprimegt The peak in

51

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

14

12

10

8

6

4

2

0

[NO

2] (n

mol

mol

-1)

214213212211

Day of Year 2000

TDLAS [NO2] P-C [NO2]

Figure 32 Time series of hourly NO2 concentration measurements by the TDLAS and

P-C instruments at Harvard Forest

52

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

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• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

0 10 20 30

010

2030

11

TDLA

S [N

O2]

(nm

ol m

ol-1

)

Photolysis-Chemiluminescence [NO2] (nmol mol-1)

Hourly NO2 Comparison days 95-232 2000

Figure 33 Hourly NO2 concentrations obtained by the TDLAS and the photolysis-

chemiluminescence detector at Harvard Forest The two instruments operated from

separate towers roughly 200 m apart at 22 m (TDLAS) and 29 m (P-C) sampling heights

Orthogonal distance fit with errors at the 95th confidence interval (y=a+bx)

a = -02 plusmn 10 b = 11 plusmn 02 R2 = 091

53

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

r(wt)

00

01

02

03

04

05

00

01

02

03

04

05 Raw W

Smeared W

Daytime Lagged Correlations Sept-Oct 2000

Delay Time (s)

r(wn

o2)

-60 -40 -20 0 20 40 60

-00

10

010

03-0

01

001

003

Nor

mal

ized

Cos

pect

ra

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wT

wu

Daytime Cospectra Sept-Oct 2000

Freq (Hz)

0005 0050 0500

-02

00

02

04

06

08

10

-02

00

02

04

06

08

10 wNO2

wT smear 1wT smear 2wT smear 3

Figure 34 Average daytime lagged correlations and cospectra for (wT) and (wNO2)

r(wNO2) is lagged compared to r(wT) due to transit time in the inlet and the peak is

rounded due to an exponential instrument response function with time constant 1 second

The cospectrum of w with NO2 confirms that the response function is as expected when

compared with cospectra of w with T smeared by 1 s 2 s and 3 s exponentials

54

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

the lagged correlation r(wprime NO2prime) is shifted several seconds due to transit time through

the inlet to the TDLAS measurement cell The peak of r(wprimeNO2prime) is also rounded

compared to the sharp peak of r(wprimeTprime) because the instrument has a 1-second exponential

time response curve for NO2 (see Chapter 2) In order to correct for the instrument

response function the 8 Hz virtual temperature and w data were smeared with a 1-second

exponential and used to compute a heat flux for each 30-minute flux interval The ratio

of smeared heat flux to unaltered heat flux provided an estimate of the missing high-

frequency flux removed by the instrument response function [Goulden et al 1996

Munger et al 1996 1998] The corrections were typically 20 or smaller The

normalized cospectra in Figure 34 show that the NO2 flux begins to decline at

frequencies above 01 Hz unlike the heat flux which retains spectral information beyond

05 Hz The heat flux cospectrum computed with the 1-second smeared temperature data

has a shape similar to that of the NO2 cospectrum

333 Hourly Data

Hourly concentrations and fluxes of NO NO2 O3 and supporting trace gas and

meteorological measurements were recorded on a 24-hour basis at Harvard Forest during

the spring summer and fall of 2000 Eddy covariance NO2 fluxes span April through

November and eddy covariance NO fluxes were measured from late August to early

October All instruments experienced data dropouts ranging in length from hours to days

55

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

in length at various times during the measurement period due to equipment and computer

failures power outages software problems and routine maintenance

A typical Harvard Forest NOx and O3 time series is shown in Figure 35

Photochemical production of NO is apparent during the day as is conversion to NO2 at

night O3 concentrations and fluxes at the site plotted on the right-hand axes are

typically an order of magnitude greater than those of NOx Coupled fluxes of NO2

(upward) and NO (downward) arise during the day from photochemical cycling and

turbulent transport in the presence of the light gradient imposed by the forest canopy

Higher irradiance above the canopy favors production of NO (NO2+hν NO+O) lower

light below favors conversion back to NO2 (NO+ O3 NO2+ O2) Although the daytime

fluxes of NO2 appear to be greater in magnitude than the corresponding NO fluxes the

imbalance is mostly due to the difference in measurement height of NO (29 m) and NO2

(22 m) as we examine further in the next section Median diel cycles for NO NO2 and

O3 for the entire measurement period April-November 2000 segregated by wind sector

are shown in Figure 36

56

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

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Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

4

3

2

1

0

[NO

] [N

O2]

(nm

ol m

ol-1

)

287286285284283282281Day of Year 2000

8

4

0

-4

FNO

FN

O2 (

micromol

m-2

hr-1

)

-100-50050100

FO3 (microm

ol m-2 hr -1)

50403020100

[O3 ] (nm

ol mol -1)

O3 NO2 NO

Figure 35 Time series of hourly NO (solid lines left axes) NO2 (dashed lines left axes)

and O3 (solid lines right axes) concentrations (upper panels) and fluxes (lower panels)

Oct 7-12 2000 NO2 measurements were taken at a sampling height of 22 m NO and O3

at 29 m on a nearby tower

57

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

02

46

80

24

68

NW

Hour

0 6 12 18

-4-2

02

4

02

46

80

24

68

SW

Hour

0 6 12 18

-4-2

02

4

[NO][NO2][O3]10

FNOFNO2FO310

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

Figure 36 Diel cycles of median concentrations (upper panels) and fluxes (lower

panels) for the Northwest (2700-450 left panels) and Southwest (1800-2700 right panels)

wind sectors at Harvard Forest April-November 2000 for NO NO2 and O310 NO and

O3 were sampled at a height of 29 m and NO2 at 22 m Vertical bars indicate 25th and

27th quartiles for NO and NO2 measurements NO2 concentration and nighttime

deposition are enhanced under southwesterly conditions as are O3 and the morning NO

maximum

58

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

34 Discussion

In the following sections we examine relationships of fluxes and concentrations

for various conditions and species The generalized exchange velocity for a species

(fluxconcentration) allows fluxes to be compared in a normalized form In order to

remain consistent with the flux sign convention used herein we define positive exchange

velocity at the sampling height as upward (in the direction of emission) and negative

exchange velocity as downward (in the direction of deposition) regardless of whether the

process represents net emission or deposition to the surface To facilitate the calculation

of exchange velocity in units of cm s-1 we express flux in units of concentration times

velocity (nmol mol-1 cm s-1) which is also the actual quantity measured by the

instrumentation The conversion to SI flux units is 1 nmol mol-1 cm s-1 = 1606 micromol m-2

hr-1 (STP) Temperature and pressure corrections to this conversion have been taken into

account in the following analyses

341 Nighttime Deposition of NO2

At night NO concentration and flux fall to near zero photochemical cycling

ceases and the NO2 flux should capture total NOx exchange The observed nighttime

NO2 flux was generally small Figure 37 shows that nighttime NO2 flux depends

quadratically on [NO2] A polynomial regression of the form FNO2 = F0 + V0 [NO2] + a

[NO2]2 gives the results in Table 31

The quadratic dependence in the regression is largely driven by the two nights

during the sampling period when the median concentration of NO2 was above 10 nmol

mol-1 excluding these nights the parameter a is not statistically significant

59

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

0 5 10 15 20 25 30

-20

-15

-10

-50

5 Hourly Data (fitted)Nightly Medians +

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)FNO2(night) = F0 + V0 [NO2] + a [NO2]2

F0 = 0V0 = -008 plusmn 003 (cm s-1)a = -0013 plusmn 0001 (nmol-1 mol cm s-1)

R2 = 063

Figure 37 Nighttime hourly (dots) and median nightly (pluses) NO2 flux vs

concentration with results of least-squares fit on the hourly data (curve) The flux is

expressed in units of concentration times velocity (nmol mol-1 cm s-1) in order to simplify

the interpretation of the coefficients in the least-squares fit Pressure and temperature

corrections have been taken into account in the conversion from density to mixing ratio

60

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Table 31 Polynomial regression results for nighttime FNO2 = F0 + V0 [NO2] + a [NO2]2

F0 plusmn std error (nmol mol-1 cm s-1)

V0 plusmn std error (cm s-1)

a plusmn std error (nmol-1 mol cm s-1) R2

-02 plusmn 01 (p-value=007)

-001 plusmn 005 (p-value=08)

-0015 plusmn 0002 (p-valuelt1E-4) 060

All Hourly Data

Constrained to 0 -008 plusmn 003

(p-value=001)

-0013 plusmn 0001 (p-valueltlt1E-4) 060

-02 plusmn 01 (p-value=01)

-002 plusmn 01

(p-value=09)

-0014 plusmn 0002 (p-value=03) 003

[NO2]le10 nmol mol-1

Constrained to 0 -016 plusmn 006

(p-value=0005)

-0002 plusmn 001 (p-value=09) 003

-02 plusmn 02 (p-value=03)

-002 plusmn 006

(p-value=07)

-0015 plusmn 0002 (p-valuelt1E-4) 063

[NO2] ge1 nmol mol-1

Constrained to 0 -007 plusmn 003 (p-value=0003)

-0013 plusmn 0002 (p-valuelt1E-4) 052

61

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

A statistically significant non-zero F0 term cannot be interpreted as a physically

meaningful result NO2 cannot deposit if its concentration is zero The polynomial

regression does not take into account potential uncertainty in [NO2] which could

introduce a bias at low values where the uncertainty in the measurement is larger than the

absolute value By excluding [NO2] lt 1 nmol mol-1 from the regression we obtain an F0

which is not significantly different from zero It is therefore useful to constrain F0 to

zero V0 is frequently indistinguishable from zero If the quadratic dependence is in fact

valid over the full range of NO2 concentrations this result implies that the nighttime

deposition velocity of NO2 increases from approximately 02 cm s-1 at [NO2]=1 nmol

mol-1 to 05 cm s-1 at [NO2]=30 nmol mol-1 The data do not support the existence of a

compensation point for NO2 at night on average NO2 continues to deposit to not emit

from the forest even at low concentrations Note that soil emission fluxes of NO are

very low at Harvard Forest less than 09 micromol m-2 hr-1 [Munger et al 1996] so the

effects of freshly emitted NO conversion to NO2 are insignificant

The downward NO2 flux measured above the forest canopy at night may arise

from direct NO2 deposition to leaves litter bark and soil but not via leaf stomata

because they are largely closed in the absence of sunlight Eugster and Hesterberg [1996]

found that NO2 deposition to a litter meadow at night encountered less resistance than O3

deposition to the same surfaces Other studies have also found significant deposition

velocities for NO2 to soil litter and bark [Hanson et al 1991 and references therein]

Non-stomatal NO2 deposition could proceed via formation and heterogeneous

hydrolysis of N2O5 on forest surfaces below the sensor height as in (R37) ndash (R39) with

formation of NO3 (R37) the rate limiting step [Jacob 2000]

62

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

NO2 + O3 NO3 + O2 (R37)

NO3 + NO2 + M N2O5 + M (R38)

N2O5 + H2O 2HNO3 (R39)

Since HNO3 deposits rapidly it is unlikely that much (if any) of the HNO3 produced

heterogeneously from N2O5 hydrolysis would escape from the forest in the gas phase

Recent field observations of N2O5 and NO3 at night have shown that concentrations of

N2O5 build up over the course of the night that the NO3 to N2O5 ratio closely follows the

theoretical temperature-dependent equilibrium and that accumulated N2O5 is photolyzed

at dawn to recycle NO2 [Brown et al 2001 a and b] Thus only a portion of any NO2

flux due to (R37) ndash (R39) corresponds to net loss of NOx from the atmosphere

We can estimate the maximum downward NO2 flux due to N2O5 hydrolysis by

assuming a negligible vertical NO2 concentration gradient below the sensor 100

conversion of NO3 to N2O5 and 100 hydrolysis of N2O5 below the sensor height

H=22 m These upper limits are compared to the measured fluxes in Figure 38 for

nightly median NO2 concentrations less than 10 nmol mol-1 (Eq 31)

Max FNO2 (N2O5 hydrolysis) = 2middotHmiddotP(NO3) = 2 Hmiddotk7[NO2][O3] (Eq 31)

The maximum NO2 deposition attributable to N2O5 hydrolysis depends linearly on

[NO2] a linear regression of the computed flux against concentration yields a deposition

velocity of 0048plusmn0008 cm s-1 (p-value lt 1E-4) R2 = 05 a factor of 4-10 lower than the

observed deposition velocities Noisy data prevent definitive identification of the shape

of the observed dependence at concentrations below 10 nmol mol-1 but we infer that the

N2O5 hydrolysis estimate underpredicts the observed NO2 deposition flux At higher

NO2 concentrations the N2O5 hydrolysis estimate vastly underpredicts observed NO2

63

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

[NO2] (nmol mol-1)

NO

2Fl

ux (n

mol

mol

-1cm

s-1

)

Max N2O5 hydrolysisNightly Medians +

0 2 4 6 8 10

-2-1

01

Figure 38 Nightly median NO2 flux vs concentration for concentrations below 10 nmol

mol-1 (pluses) The maximum contribution to NO2 deposition by N2O5 hydrolysis on

forest surfaces (boxes) was estimated by calculating the production rate of NO3 from NO

+ O3 (the vertical NO2 concentration gradient is negligible) 100 conversion of NO3 to

N2O5 and 100 N2O5 hydrolysis on forest surfaces below 22 m The linear regression of

maximum NO2 deposition due to N2O5 hydrolysis vs [NO2] indicates an upper limit

deposition velocity of 0048plusmn0008 cm s-1 due to this process (R2 = 05 p-valuelt1E-4)

64

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

deposition (not shown) Although N2O5 hydrolysis very likely occurs on forest surfaces

it appears to account for a small fraction (lt30) of the observed nighttime NO2

deposition

The quadratic dependence of nighttime NO2 flux on concentration could also arise

from heterogeneous hydrolysis of NO2 and production of HONO below sensor height

Observations at urban and rural sites have shown that NOx can be converted to HONO on

aerosols [Notholt et al 1992 Andreacutes-Hernaacutendez et al 1996] Although NO2 is known

to react on hydrated surfaces the mechanism and kinetics of the process are not fully

understood It appears that the most likely heterogeneous NO2 hydrolysis reaction is

(R310) [Goodman et al 1999]

2NO2(g)+ H2O(a) HONO(g) + HNO3(a) (R310)

Laboratory studies have shown first-order kinetics in NO2 for (R310) at parts-per-million

(micromol mol-1) concentrations with NO2 adsorption as the rate-limiting step but the low-

concentration kinetics are unknown The atmospheric mechanism may involve N2O4 as a

key intermediate on the surface [Barney and Finlayson-Pitts 2000] As in the N2O5

hydrolysis mechanism the aqueous-phase HNO3 produced in (R310) is not likely to be

released to the gas phase Harrison et al [1994 1996] observed upward HONO fluxes

over vegetated surfaces at NO2 concentrations above 10 ppb along with a quadratic

dependence of [HONO] on [NO2] If the observed NO2 flux at Harvard Forest is the

result of (R310) then approximately half might be released as gas-phase HONO and

would not contribute to net loss of NOx from the atmosphere The amount of

heterogeneous nighttime HONO formation at the surface is important to daytime HOx

65

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

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Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

chemistry because at sunrise HONO is photolyzed to deliver a burst of OH and NO to

the surface layer

The evidence for direct NO2 deposition to soil litter and bark rests on only a few

studies under ambient conditions where surfaces were likely to be hydrated and none of

which had simultaneous gas-phase HONO measurements [Eugster and Hesterberg 1996

Hanson et al 1991] Thus it is possible that a portion of reported NO2 deposition to

various surfaces proceeds via NO2 hydrolysis in which case half of the deposited NO2

would remain on the surface as HNO3 and half might re-emerge as gas-phase HONO

Nighttime N2O5 hydrolysis leads to aqueous-phase HNO3 production so that this process

is also likely to contribute to some net atmospheric NOx loss Clearly individual

measurements of HONO N2O5 and NO3 concentrations and fluxes are crucial to

unraveling the nighttime NOx budget and its impact on tropospheric HOx and O3

chemistry At Harvard Forest it appears likely that heterogeneous N2O5 and NO2

hydrolysis both contribute to the observed nighttime NO2 flux though we cannot rule

out additional processes

Nighttime concentrations and fluxes of NO2 and NOy (Figure 39) show that the

measured NO2 flux accounts for between zero and 24 of total NOy deposition with a

notable dependence on wind direction The NOy sensor detects NOx HNO3 HONO

PANs and additional species If a portion of the downward NO2 flux is balanced by an

upward HONO flux then additional NOy flux is unattributed For the northwest wind

sector where the flux budget appears to be closed within the noise of the measurements

the fractional impact on the flux budget would be larger than for southwesterly conditions

where unmeasured species account for much of the NOy deposition

66

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

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Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

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102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

02

46

810

12

SW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

NOyNO2+HNO3NO2

FNO2FNO2+FHNO3FNOy

Flux

(microm

ol m

-2hr

-1)

Con

cent

ratio

n (n

mol

mol

-1)

02

46

810

12

NW

Hour

19 20 21 22 23 0 1 2 3 4

-15

-10

-50

Figure 39 Nighttime medians for the northwest (left) and southwest (right) wind

sectors April-November 2000 Upper panels Concentrations of NO2 NO2+HNO3 and

NOy (with vertical bars spanning 25th and 75th quartiles) Lower panels Fluxes of NO2

NO2+HNO3 and NOy (with 25th and 75th quartiles) For northwesterly conditions NO2

accounts for 48 to 65 of nighttime NOy concentration and 0 to 24 of NOy flux For

southwesterly conditions NO2 accounts for 35 to 58 of nighttime NOy concentration

and 0 to 13 of nighttime NOy flux

67

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Most atmospheric models base their NO2 deposition parameterizations on the

modified Wesely [1989] scheme in which the surface resistance for NO2 prohibits

transfer except to lush vegetation exposed to sunlight where leaf stomata are open [eg

Bey et al 2001] Thus NO2 deposition velocity in these models falls to near zero at night

over vegetation regardless of the concentration of NO2 We observed nighttime NO2

deposition to the forest dependent on concentration and we infer that the behavior of

models departs significantly from the observed processes

Figure 310 compares observed nighttime NO2 deposition velocity to 24-hour

NO2 deposition from the Harvard GEOS-CHEM model on a monthly basis for Harvard

Forest [A Fiore personal communication] The nighttime NO2 deposition velocity of the

data is approximately half the modelrsquos 24-hour mean during the months when the canopy

is fully developed Since we believe that NO2 also deposits during the day (see below)

the modelrsquos 24-hour average deposition flux in summer may be approximately correct

although the day-night variation and dependence on NO2 concentration are incorrect

The model has no deposition when trees are bare but our data clearly indicate that

deposition continues at these times This discrepancy suggests a potentially large

underestimate of NOx deposition to forested landscapes during dormant months

342 Daytime Fluxes of NO NO2 O3 and NOy

During the day (R31) ndash (R34) establish a dynamic balance between NO NO2

and O3 while deposition of NO2 and O3 onto the forest canopy and sub-canopy

68

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

05

04

03

02

01

00

NO

2 Dep

ositi

on V

eloc

ity (c

m s

-1)

1110987654321

Month

GEOS-CHEM Harvard Forest Inland Box24-Hour Monthly Means 1995 1997 OBSERVED Harvard Forest TDLASNighttime Monthly Median 2000

Figure 310 Comparison of 24-hour monthly mean NO2 deposition in the GEOS-CHEM

model Harvard Forest box 1995 and 1997 to the observed nighttime monthly median

NO2 deposition measured at Harvard Forest in 2000 Unlike the observations the modelrsquos

nighttime deposition velocity of NO2 drops to near zero and contributes insignificantly to

total NOx deposition

69

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

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Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

surfaces continues The light gradient through the canopy (see Figure 311) drives (R32)

at a faster rate above than below shifting the balance of NOx toward NO above and NO2

below the canopy height (~20 m at this site) Theoretical studies have predicted that this

photochemical effect should produce coupled upward fluxes of NO2 and downward

fluxes of NO above the forest canopy [eg Gao et al 1993] These studies have

employed large suites of chemical reactions soil NO emissions canopy isoprene

emissions and leaf-level resistance models for NO2 HNO3 O3 and H2O2 deposition

We show below that a simpler approach captures much of the observed behavior of NOx

fluxes at Harvard Forest

Dominant features of the observed daytime NO and NO2 fluxes shown in Figures

35 and 36 are their opposite signs dependence on light and apparent imbalance

Upward NO2 flux at midday is reproducibly several times larger than downward NO flux

A simple model of canopy photochemical NOx cycling reveals that this apparent

imbalance is expected based on the difference in the height of the sampling inlets for the

two gases above the canopy For a trace gas species X continuity requires that a change

in concentration d[X]dt be due to the sum of chemical production PX chemical loss ndashLX

and flux - Φ

d[X]dt = Px ndash Lx ndash Φ (Eq 32)

For NOx photochemical cycling (R31)-(R33) we simplify Eq 32 by assuming steady

state d[X]dt = 0 ignoring horizontal transport and approximating the vertical flux as Φz

= ndashKc(z) d[X(z)]dz where Kc is a vertical exchange coefficient (ldquoeddy diffusivityrdquo) We

write the production and loss terms as

131

][]][[2 τ

NONOOkLP NONO === (Eq 33)

70

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

25

20

15

10

5

0

heig

ht (m

)

302520151050

295-385 nm Total UV (W m-2)

Harvard Forest 04 Oct 2001Local Noon Clear Sky

EMS Profile UV TDLAS Point UV

Figure 311 Midday profile of total UV radiation at Harvard Forest At each height the

Eppley total UV radiometer was placed horizontally facing upward to measure total

(direct plus diffuse) downwelling radiation and then facing downward to measure total

upwelling radiation The points indicate the sum of total downwelling and total

upwelling UV radiation Horizontal bars show observed variability in total downwelling

+ upwelling UV irradiance at each height within approximately one meter of the

measurement tower

71

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

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Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

2

2222

][][

τNO

NOjPL NONO === (Eq 34)

We scaled Kc(z) from Gao et al [1993] to the height of the Harvard Forest

canopy 20 m and approximated the effect of the light gradient from Figure 311 on the

lifetime of NO2 τ2(z) as shown in Figure 312 based on measurements made manually

with an Eppley UV photometer We estimated τ1asymp70s based on median k1(T) and [O3]

for the site the vertical gradient for O3 is small between 20 and 30 m making the

constant lifetime a reasonable approximation We then solved Eq 32 for NO and NO2 to

derive a simple model of vertical transport and photochemical cycling of NOx shown in

Figure 313

This calculation clarifies the cause of the imbalance between daytime NO and

NO2 fluxes observed in the time series and diel cycles in Figures 34 and 35 Because

the concentrations and fluxes of the two species were measured at separate heights 22 m

for NO2 and 29 m for NO they sampled different points along the concentration and flux

gradients induced by reduced light conditions below the canopy The larger observed

upward NO2 flux is expected based solely on photochemical cycling at a sampling point

closer to the canopy than that of NO

In addition to NO and NO2 fluxes corresponding to photochemical cycling

between the two NO2 is likely to interact with the vegetation via stomata during daylight

hours Canopy surface conductance gc is readily calculated using the Penman-Monteith

equation [Shuttleworth et al 1984] and the measured water vapor flux at Harvard Forest

Deposition velocities for species known to deposit through stomata (ie O3) correlate

strongly with gc [Munger et al 1996] As shown in Figure 314 daytime gc and the O3

72

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

01 05 50 500

020

4060

8010

0

0 1000 2000 30000 1000 2000 3000

020

4060

8010

0

Eddy Diffusivity K (m2 s-1) τ2 (s)

heig

ht (m

)

Figure 312 Simple NOx photochemical canopy model inputs Eddy diffusivities are

scaled from Gao et al [1993] to the Harvard Forest canopy height of 20 m τ2 the

lifetime of NO2 against photolysis (reaction 2 NO2 + hν NO + O) scales inversely

with UV radiation τ2 was estimated for typical conditions of [O3] [NO][NO2] and

daytime UV profile under full-canopy conditions at the site The simple canopy model

also accepts an estimate of τ1 the lifetime of NO against reaction with O3 (reaction 31)

73

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Concentration (nmol mol-1)

heig

ht (m

)

00 02 04 06 08 10

020

4060

8010

0

NO NO2

Normalized NOx = NO+NO2 = 1 nmol mol-1

Flux (nmol mol-1 cm s-1)

-2 -1 0 1 2-2 -1 0 1 2

020

4060

8010

0

NO NO2

Figure 313 Simple NOx photochemical canopy model outputs Left panel

concentrations of NO (dashed) and NO2 (solid) right fluxes of NO (dashed) and NO2

(solid) Symbols indicate measurement heights for NO (29 m) and NO2 (22 m) at

Harvard Forest The model solves the continuity equation for NO concentration at 200

levels ddz(-Kc(dNOdz)) = PNO ndash LNO where PNO = [NO]τ1 LNO = [NO]τ2 and zero

net deposition or emission of NOx is allowed NOx = NO + NO2 is normalized to 1 ppb

τ1 = 70s in this example Due to the measurement height difference observed upward

NO2 flux due to photochemical cycling alone should be substantially larger than observed

downward NO flux attributable to the same process

74

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

V(O

3) (c

m s

-1)

g c(c

m s

-1)

PPFD (mmol m-2 s-1)

00 05 10 15

-05

00

05

10

15

20

00 05 10 15

-3-2

-10

12

Figure 314 Daytime (including dawn and dusk) canopy surface conductance gc and O3

exchange velocity V(O3) vs above-canopy photosynthetic photon flux density (PPFD)

during the summer of 2000 at Harvard Forest Dots are hourly values Filled squares

represent quantile-binned means (quantiles of PPFD were used to define bins and all gc

or V(O3) for data hours falling within the bins were averaged) Both quantities are

indicators of stomatal conductance and increase in absolute value with PPFD up to a

maximum level

75

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

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Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

exchange velocity are both related to photosynthetic photon flux density (PPFD) The

dependence is nonlinear absolute values of gc and V(O3) increase with PPFD up to

approximately 1 mmol m-2 s-1 and level off at higher PPFD values Note that PPFD is the

only continuously-monitored measure of light intensity at Harvard Forest (as with many

ecological and trace-gas monitoring stations) and is therefore used in the discussion

below as a proxy of photochemically-active UV wavelengths at the site despite the fact

that PPFD measures only the photosynthetically-active wavelength band (400-700 nm)

For the time period when eddy covariance fluxes of both NO and NO2 were

measured (29 August-8 October 2000) Figure 315 shows that the daytime (as defined by

light level PPFD gt 001 mmol m-2 s-1) upward NO2 flux was strongly correlated with the

downward NO flux We can account for more of the variance in daytime FNO2 by using

in addition to a photochemical NOx cycling term βmiddotFNO additional terms representing a

constant deposition velocity V0middot [NO2] and a stomatal flux γmiddotgc (Table 32)

FNO2 = F0 + βmiddotFNO + V0middot [NO2] + γmiddotgc (Eq 35)

The relationship between FNO2 and FNO is significant in all of the fits as shown by the

low p-values and consistent values of β Note that an upward NO2 flux contribution

could be included as either a constant (zero order) flux term F0 or as a function of

stomatal conductance γ when F0 is constrained to zero For the F0=0 cases R2 has been

calculated as follows in order to omit the origin

)var()var(1

2

2

FNOresidualR minus= (Eq 36)

The constant deposition velocity term V0 is significant when the stomatal term is also

included in the fit A flux term dependent on the square of the NO2 concentration

a[NO2]2 (not shown in Table 32) is only marginally significant regardless of which other

terms are included in the regression

76

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

NO

2Fl

ux (micro

mol

m-2

hr-1

)

- NO Flux (micromol m-2 hr-1)

0 2 4 6 8 10 12

-10

010

2030

40

Figure 315 Relationship between daytime hourly FNO2 and ndashFNO 29 August to 8

October 2000 FNO2 = (05plusmn01) - (35plusmn01)middotFNO R2 = 079

77

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

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103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Table 32 Regressions of daytime FNO2 (as defined by light level see above covered

hours 500 to 2000 EST) in terms of linear combinations of βmiddotFNO V0middot[NO2] and γmiddotgc

F0 is the intercept for each fit representing flux at [NO2] = FNO = gc = 0 Flux units of

(nmol mol-1 cm s-1) are used Note that FNO is largely negative (downward) so that a

negative value of β represents a positive flux term

F0 plusmn std error

(nmol mol-1 cm s-1) β plusmn std error

(dimensionless) V0 plusmn std error

(cm s-1) γ plusmn std error (nmol mol-1) R2

05plusmn01 (p-valuelt1E-4)

-35plusmn01 (p-valuelt1E-4) NA NA 079

constrained to 0 -37plusmn01 (p-valuelt1E-4) NA NA 079

07plusmn02 (p-valuelt1E-4)

-38plusmn02 (p-valuelt1E-4)

-016plusmn008 (p-value=004) NA 084

constrained to 0 -37plusmn02 (p-valuelt1E-4)

003plusmn007 (p-value=07) NA 083

01plusmn03 (p-value=06 )

-35plusmn01 (p-valuelt1E-4) NA 10plusmn04

(p-value=002) 083

constrained to 0 -35plusmn01 (p-valuelt1E-4) NA 12plusmn02

(p-valuelt1E-4) 083

04plusmn03 (p-value=02)

-38plusmn02 (p-valuelt1E-4)

-018plusmn009 (p-value=005)

09plusmn05 (p-value=008) 087

constrained to 0 -38plusmn02 (p-valuelt1E-4)

-012plusmn008 (p-value=01)

13plusmn03 (p-valuelt1E-4) 087

78

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

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Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

The stable statistically significant relationship between the daytime fluxes of NO

and NO2 appears to indicate that although the species were measured on nearby towers at

different heights their photochemical cycling is coherent on an hourly basis This

process clearly dominates the behavior of NOx fluxes near the forest canopy but

represents neither net deposition nor net emission The next largest term appears to be a

constant deposition velocity downward flux of NO2 Finally there is a small positive

NO2 flux component which is equally well-fit as a constant flux or a stomatal

conductance-dependent term We performed additional regressions (not shown) using a

stomatal term of the form γprimemiddotgcmiddot([NO2]-[NO2]c) where [NO2]c is the compensation point

concentration below which vegetation emits NO2 While marginally significant values of

γprime could be obtained for [NO2]c values fixed between 1 and 10 nmol mol-1 the data did

not contain enough information to determine the compensation point independently In

2000 median summerfall daytime [NO2] was 11 nmol mol-1 (75th percentile=23 nmol

mol-1) Thus the vegetation at Harvard Forest usually experiences NO2 concentrations at

or below the 05 to 15 nmol mol-1 range of previously observed compensation points for

coniferous and tropical trees in chamber studies [eg Sparks et al 2001 Rondoacuten et al

1993] It is not possible to definitively attribute the small positive flux (γmiddotgc) to a

stomatal process the term could also indicate a chemical or other process which reduces

the deposition velocity under daytime conditions

To investigate the behavior of the NO2 flux beyond the limited period of NO flux

measurements we use above-canopy PPFD as a proxy for photochemical activity for our

data when NO fluxes were not measured Figure 316 shows daytime exchange

velocities of NO and NO2 plotted against PPFD For clarity hourly points are omitted

79

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

00 05 10 15

-10

12

34

NO2 (h=22m) leaves onNO2 (h=22m) leaves offNO (h=29m) leaves on

Exch

ange

Vel

ocity

(cm

s-1

)

PPFD (mmol m-2 s-1)

Figure 316 Daytime emission velocities of NO2 (leaves on solid triangles

leaves off open triangles) and NO (leaves on solid circles no data for leaves off

periods) plotted against photosynthetic photon flux density (PPFD) Symbols are

medians binned by percentiles of above-canopy PPFD For clarity hourly points are not

shown V(NO2) data span spring through fall 2000 while V(NO) data were collected late

August to early October 2000 only Linear fits to binned median points (exchange

velocities in units of cm s-1 PPFD in units of mmol m-2 s-1) reported in Table 33

80

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

and only the PPFD quantile-binned medians are plotted For each species we determine

quantiles of PPFD for daytime hours with data These quantiles define for each trace gas

species the measurement hours whose median values are shown The coupling of the bi-

directional NOx fluxes becomes more intense as the difference in light intensity from

above- to below-canopy increases during periods of high above-canopy illumination

cycling ceases as light intensity and therefore light gradient declines NO2 flux data are

segregated into full-canopy (leaves on) conditions from early June to mid-October and

leafless periods in April-May and October-November Linear regressions to the

percentile-binned medians of PPFD (V = VPPFD=0 + bmiddotPPFD) yield the following

relationships for the daytime data shown in Table 33

Before leaf-out and after leaf-fall the coefficient of PPFD in the NO2 regression

is smaller than for full-canopy conditions This expected since the leaf area index at

Harvard Forest at mid-summer is approximately 34 [Munger et al 1996] whereas the

area index for bare stems and twigs is only 09 [Barford et al 2001] The light gradient

for leaves-off conditions is smaller resulting in smaller concentration gradients and

reduced coupled NOx fluxes It is also likely that NO2 emission by the leaves contributes

to the increased slope of VNO2 vs PPFD when the canopy is developed

NO is not expected to deposit to surfaces in large amounts though there is very

little experimental evidence [Hanson et al 1991 Wesely and Hicks 2000] Soil NO

emission at Harvard Forest is small less than 09 micromol m-2 hr-1 [Munger et al 1996]

making NO unlikely to escape from the canopy before conversion to NO2 Thus the NO

flux at Harvard Forest is dominated by photochemical cycling and can be parameterized

as a function of PPFD and [NO] Coefficients for least squares fits to hourly daytime

FNO data (measurement height = 29m 29 Aug-8 Oct 2000) are shown in Table 34

81

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Table 33 Linear regression of daytime percentile-binned median VNO2 and VNO

against of PPFD

VPPFD=0 plusmn std error

cm s-1 b plusmn std error

cm s-1 (mmol m-2 s-1)-1 R2

VNO2 leaves on -04 plusmn 01 (p-value=002)

27 plusmn 02 (p-valuelt1E-4) 097

VNO2 leaves off -01 plusmn 01 (p-value=03)

15 plusmn 02 (p-valuelt1E-4) 090

VNO leaves on -042 plusmn 007 (p-value=4E-4)

-05plusmn01 (p-value=7E-4) 078

82

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

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Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

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Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

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103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Table 34 Regressions results for daytime FNO = F0_NO + V0_NO [NO] + bprime PPFD [NO]

F0_NO plusmn std error

(nmol mol-1 cm s-1)

bprime plusmn std error cm s-1 (mmol m-2 s-1)-1

V0_NO plusmn std error (cm s-1) R2

-008plusmn001 (p-valuelt1E-4)

-110plusmn002 (p-valuelt1E-4) NA 091

Constrained to 0

-114plusmn002 (p-valuelt1E-4)

NA 091

-006plusmn001

(p-valuelt1E-4)

-092plusmn004

(p-valuelt1E-4)

-018plusmn004

(p-valuelt1E-4) 091

Constrained to 0

-087plusmn004 (p-valuelt1E-4)

-025plusmn003

(p-valuelt1E-4) 091

83

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

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Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

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Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

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Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

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103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

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Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

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Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

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• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

To treat the larger daytime NO2 flux dataset we combine a terms representing the

light dependence shown in Figure 316 constant deposition velocity and stomatal

conductance Because gc was calculated assuming the presence of leaves we use the

measured deposition velocity of O3 Vd(O3) = -V(O3) as a proxy for the stomatal opening

from early spring through late fall Table 35 shows the results for the regressions using

these terms

A quadratic term amiddot[NO2]2 is not statistically significant when included because

we have few measurements with sufficiently high NO2 concentrations We interpret the

term (b+f)middotPPFDmiddot[NO2] as approximately accounting for the NO2 flux that arises from

photochemical cycling with NO (b+f) agrees well with the coefficients found in Table

33 As in the limited dataset regressions using FNO instead of PPFD (Table 32) V0

indicates an exchange velocity for NO2 on the order of -03 cm s-1 (downward) Given

the robust appearance of this term with different fit parameters for both limited and full

datasets we find it likely that V0 represents a roughly constant deposition velocity

process independent of NOx photochemical cycling during the day Though statistical

significance is low an upward flux term associated with stomatal conductance or related

variables also appears in Table 35

343 Parameterization of NOx Fluxes

Both nighttime and daytime NOx fluxes can be combined into a single

parameterization based on the above discussion This framework adopts the hypothesis

that photochemical NO-NO2 cycling can be largely accounted for by a simple light-

dependent term and that net deposition may then be estimated from additional terms

representing constant exchange velocity concentration-dependent exchange velocity and

84

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

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Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Table 35 Multi-factor regression using all hourly daytime FNO2 data following the

functional form FNO2 = (b+f)middotPPFDmiddot[NO2] + V0 [NO2] + ΓmiddotVd(O3) The intercepts for all

fits have been constrained to zero The parameter f is a canopy flag set to zero for full-

canopy conditions so may be considered a correction to the PPFD term for leaves-off

periods in early spring and late fall

b plusmn std error cm s-1 (mmol m-2 s-1)-1

f plusmn std error cm s-1 (mmol m-2 s-1)-1

= 0 for leaves on V0 plusmn std error

(cm s-1) Γ plusmn std error (nmol mol-1) R2

196plusmn007 (p-valuelt1E-4)

-13plusmn01 (p-valuelt1E-4) NA NA 055

240plusmn009

(p-valueltlt1E-4)

-132plusmn009 (p-valuelt1E-4)

-035plusmn005 (p-valuelt1E-4) NA 060

196plusmn008 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4) NA 01plusmn01

(p-value=04) 062

229plusmn009 (p-valuelt1E-4)

-11plusmn01 (p-valuelt1E-4)

-030plusmn005 (p-valuelt1E-4)

03plusmn01 (p-value=003) 065

85

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

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Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

stomatal conductance-related exchange processes Thus for the height of 22 m relative to

the 20 m canopy the flux of NO2 day and night is given by the simple function of NO2

concentration PPFD and O3 deposition velocity

FNO2 = V0 middot [NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) (Eq 37)

where f is added to the coefficient of PPFD to account for the observed difference

between full-canopy and leaves-off conditions

NO is not present in significant quantities at night and FNO appears to be driven

by a simple dependence on PPFD during the day We include a constant deposition

velocity term as well since evidence of NO deposition to surfaces is weak and

inconclusive At a measurement height of 29 m the flux of NO is given by

FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] (Eq 38)

The net fluxes of NO and NO2 may be determined by omitting the photochemical terms

from Eq 37 and 38 Since we expect the deposition velocity of NO to be small

FNOx(net)asympFNO2(net)

FNOx(net) asymp V0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) (Eq 39)

Multi-factor regressions for Eq 37 and Eq 38 were performed to determine the

coefficients V0 a b f V0_NO and brsquo using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 The

results are presented in Table 36 along with the range of results from the separate

daytime and nighttime analyses discussed earlier

86

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Table 36 Multi-factor regressions using all available hourly observations of [NO2]

[NO] PPFD Vd(O3) FNO2 and FNO at Harvard Forest April-November 2000 Note

that Vd(O3)=deposition velocity positive values indicate downward O3 exchange

velocity

z = 22m FNO2 = V0 middot [NO2] + a middot [NO2]2 + (b+f)middotPPFDmiddot[NO2] + Γ middot Vd(O3)

coeff plusmn std err p-value

April ndash November 2000 (R2 = 060)

Range of results from separate day and night analyses

V0 (cm s-1) -021plusmn004 lt1E-4 -03 to -001

a (nmol-1 mol cm s-1) -0002plusmn0004dagger 06 -0013 to ndash0002

b (cm s-1 (mmol m-2 s-1)-1) 221plusmn007 lt1E-4 19 to 24

f (cm s-1 (mmol m-2 s-1)-1) =0 for leaves-on -110plusmn009 lt1E-4 -13 to -10

Γ (nmol mol-1) 021plusmn009 002 01 to 03

z = 29m FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO]

29 August ndash 8 Oct 2000 (R2 = 092)

V0_NO (cm s-1) -025plusmn002 lt1E-4 -087 to -042

bprime (cm s-1 (mmol m-2 s-1)-1) -087plusmn003 lt1E-4 -114 to -05

daggerThe more statistically robust nighttime a =-0013 plusmn 0001 (Table 31) is recommended

87

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Nearly all of the coefficients fall within the ranges determined previously except

V0 and V0_NO which differ only slightly from the earlier values As illustrated in Figures

317 and 318 Eq 37 and Eq 38 with the multi-factor regression coefficients in Table

36 are good predictors of the measured fluxes over their entire dynamic ranges and for

all sampled conditions

The diel variations of fluxes and residuals from this parameterization (Figure

319) reveal several interesting features The parameterization captures the overall diel

behavior of both NO and NO2 fluxes It also tends to underestimate the upward flux of

[NO2] during the day for northwesterly flow conditions and to slightly overestimate

during daytime southwesterly flows The disagreement rarely exceeds the 25th and 75th

quantiles of the measured fluxes The dominant flow regimes at Harvard Forest differ in

a number of key respects that may affect this result Northwesterly conditions tend to

bring lower concentrations of reactive nitrogen species cooler temperatures drier

weather and clearer skies than southwest winds [Moody et al 1998] It is possible that

one or more of these factors is contributing to the wind-sector difference in the residuals

One might expect that the residuals would depend on additional factors not taken

into consideration For example mixing and therefore deposition depends on the friction

velocity u (m s-1) As shown in figure 320 the residual NO2 flux tends upward only at

the highest u quantiles during the day and has no significant trend at night Similarly

the temperature-dependent rate for (R31) k1(T)[O3] has not been included yet we find

that the NO2 flux residuals exhibit no statistically significant trend with k1(T)[O3] (Figure

321) Although included as terms in the paratmeterization similar residual analyses are

shown for V(O3) during the day (Figure 321) and for [NO2] during the day and night

(Figure 322) The distinctly different daytime and nighttime behavior of the total NO2

flux is apparent again in Figure 322 as are the small residuals overall

88

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Res

idua

l (M

es ndash

Para

m)

FN

O2

(nm

ol m

ol-1

cm s

-1)

Parameterized FNO2 (nmol mol-1 cm s-1)

Mea

sure

d FN

O2

(nm

ol m

ol-1

cm s

-1)

FNO2 = V0middot[NO2] + amiddot[NO2]2 + (b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3)

Daytime +Nighttime

11

-20 -10 0 10 20 30

-20

-10

010

2030

Quantiles of Standard Normal

-2 0 2

-10

-50

510

Figure 317 Upper Measured vs parameterized NO2 flux shown with 11 line daytime

(+) and nighttime () The functional form the parameterization is shown at the top

Lower Residuals of FNO2 vs quantiles of the standard normal The residuals are

normally-distributed with extended upper and lower wings

89

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

-10 -8 -6 -4 -2 0 2

-10

-8-6

-4-2

02

Quantiles of Standard Normal

-2 0 2

-10

-05

00

05

10

15

Mea

sure

d FN

O (n

mol

mol

-1cm

s-1

)

Res

idua

l (M

es ndash

Para

m)

FN

O (n

mol

mol

-1cm

s-1)

Parameterized FNO (nmol mol-1 cm s-1)

FNO = V0_NOmiddot[NO] + b middotPPFDmiddot[NO]

11

Figure 318 Same as Figure 317 for FNO

90

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Fl

ux (n

mol

mol

-1cm

s-1

)R

esid

ual (

nmol

mol

-1cm

s-1

)Measured FNO (h=22m)Measured FNO2 (h=22m)Parameterized FNOParameterized FNO2

Meas-Param FNOMeas-Param FNO2

-20

24

-20

24

NW

0 5 10 15 20

-2-1

01

2-2

-10

12

-20

24

-20

24

SW

0 5 10 15 20

-2-1

01

2-2

-10

12

Hour

Figure 319 Top panels Median diel measured (closed symbols solid lines) and

parameterized (open symbols dashed lines) fluxes of NO2 and NO Vertical bars show

the 25th and 75th quantiles for the measurements Lower panels Median diel residual

(measured-parameterized) FNO2 and FNO Northwest wind conditions are shown on the

left southwest on the right The parameterizations capture the daytime behavior of NO

flux due to the photochemical gradient and eddy diffusivity effects For NO2 under

northwesterly conditions the daytime residual is positive indicating that the

parameterization underestimates upward flux of NO2 For southwesterly conditions

daytime NO2 flux residuals are more variable and tend in the opposite direction

91

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

u (m s-1) u (m s-1)

u (m s-1) u (m s-1)

-10

-50

510

00 05 10 15

-10

-50

510

-10

-50

510

00 05 10 15

-10

-50

510

00 02 04 06 08 10 12 14

-20

2

00 02 04 06 08 10 12 14

-20

2

Figure 320 From top to bottom measured and parameterized NO2 fluxes residuals

(measured-parameterized) and expanded-axes quantile-binned mean residuals (plusmn

standard deviation) vs u shown for daytime on the left and nighttime on the right

92

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

DAY k1(T)[O3] Day V(O3)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

k1(T)[O3] (s-1) V(O3) (cm s-1)

-10

010

2030

001 002 003 004

-10

-50

510

-10

010

2030

-003 -002 -001 00 001

-10

-50

510

0005 0010 0015 0020 0025 0030

-3-2

-10

12

-0015 -0010 -0005 00

-3-2

-10

12

Figure 321 Same as Figure 321 for daytime k1(T)[O3] (left) and daytime V(O3) (right)

93

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

DAY NIGHT

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

FNO

2(n

mol

mol

-1cm

s-1

)R

esid

ual F

NO

2(n

mol

mol

-1cm

s-1

)

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

Measured FNO2

Parameterized FNO2

Res

idua

l FN

O2

(nm

ol m

ol-1

cm s

-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

[NO2] (nmol mol-1) [NO2] (nmol mol-1)

-10

010

2030

0 5 10 15 20

-10

-50

510

-20

-15

-10

-50

510

0 5 10 15 20

-10

-50

510

0 2 4 6 8

-3-2

-10

12

0 2 4 6 8 10 12

-20

24

Figure 322 Same as Figure 321 for [NO2] during the daytime (left) and nighttime

(right)

94

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

The lack of robust trends in the NO2 flux residuals indicates that within the

limitations of the dataset Eq 37 captures the crucial aspects of observed NO2 flux

behavior within and above the tall forest canopy under a wide range of conditions

Similarly Eq 38 accounts for 92 of the observed variance in the measured NO fluxes

under a somewhat narrower set of conditions (the sampling period for NO eddy

covariance flux measurements was relatively short) Figure 323 shows the resulting net

NOx flux (Eq 39 Table 36) and exchange velocity VNOx(net)=FNOx(net)[NO2] on a

diel basis for the sampling period April-November 2000 For the rural low-NOx

conditions at Harvard Forest we find that the flux of NOx is downward at all hours with

a deposition velocity of approximately 02 cm s-1

35 Conclusions

We installed a Tunable Diode Laser Absorption Spectrometer (TDLAS) to

measure continuous NO2 concentrations and eddy covariance fluxes above the canopy at

Harvard Forest during the spring summer and fall of 2000 An existing

chemiluminescence NO detector was configured to measure above-canopy eddy

covariance fluxes for a shorter period in the late summer and early fall A photolysis-

chemiluminescence (P-C) NO2 instrument also measured concentrations for a portion of

the sampling period Hourly average TDLAS and P-C NO2 concentrations agreed very

well under all conditions The agreement between the two instruments in the field and

the independent spectroscopic calibration of the TDLAS validated the long-term

measurements of the tank-calibrated P-C system

95

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

0 6 12 18

-10

-08

-06

-04

-02

00

-14

-12

-10

-08

-06

-04

-02

00

Net

FN

Ox

(nm

ol m

ol-1

cm s

-1)

(microm

ol m

-2hr

-1)

Net

VN

Ox

(cm

s-1

)

Hour

median| 25th amp 75th percentiles

0 6 12 18

-024

-022

-020

-018

Figure 323 Diel cycle of parameterized net NOx exchange velocity (upper panel) and

flux (lower panel) April-November 2000 FNOx(net) asymp FNO2(net) = V0middot[NO2] +

amiddot[NO2]2 + ΓmiddotVd(O3) VNOx(net) = FNO2(net)[NO2]

96

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

At night in the absence of significant quantities of NO we observed a consistent

downward flux of NO2 which depended quadratically on the concentration of NO2

although the dependence was apparent mainly on two nights when [NO2] was above 10

nmol mol-1 The nighttime NO2 deposition velocity varied between 02 cm s-1 at [NO2]=1

nmol mol-1 and 05 cm s-1 at [NO2]=30 nmol mol-1 These fluxes could come from direct

NO2 deposition to non-stomatal surfaces in the forest such as soil litter bark and the

outer surfaces of leaves area-normalized deposition velocities to such surfaces in

chamber measurements have varied from less than 01 to 05 cm s-1 for NO2

concentrations between 40 and 80 nmol mol-1 [Hanson and Lindberg 1991] The rate of

heterogeneous N2O5 hydrolysis resulting in aqueous-phase HNO3 (unlikely to re-enter the

gas phase) is insufficient to explain the observed nighttime NO2 deposition rates The

quadratic flux dependence and similarity to the behavior of HONO observed at other sites

suggests that heterogeneous NO2 hydrolysis may also play a significant role in NO2

deposition [Harrison and Kitto 1994 Harrison et al 1996 Barney and Finlayson-Pitts

2000] Half of any NO2 flux due to heterogeneous NO2 hydrolysis would likely escape

from the surfaces as gas-phase HONO potentially impacting the daytime HOx and NOx

budgets when photolyzed at sunrise

During the day we observed coupled fluxes of NO2 (upward) and NO

(downward) due to photochemical cycling driven by canopy-induced light and eddy

diffusivity gradients The imbalance in the observed fluxes corresponded to the

difference in height of the sampling levels relative to the canopy The exchange

velocities of the two species in their respective directions increased with higher above-

canopy light levels as measured by photosynthetic photon flux density (PPFD) due to the

97

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

stronger light gradient through the canopy under brighter conditions Upward NO2

velocity was smaller in early spring and late fall when there were no leaves on the trees

corresponding to a weaker light gradient condition (NO fluxes were only measured

during full-canopy conditions) and possibly to the absence of stomata-dependent

emission of NO2 We believe that these coupled fluxes are the first to be reported from a

mature tall-canopy site using reliable eddy-covariance appropriate sampling techniques

All of the NO and NO2 flux data day and night were well explained by a simple

parameterization consisting of two equations FNO2 = V0 middot [NO2] + amiddot[NO2]2 +

(b+f)middotPPFDmiddot[NO2] + ΓmiddotVd(O3) and FNO = V0_NO middot [NO] + bprime middot PPFD middot [NO] The net flux

of NOx is approximately equal to the flux of NO2 omitting the PPFD-dependent term

FNOxasympV0 middot [NO2] + amiddot[NO2]2 + ΓmiddotVd(O3) V0 and V0_NO are small constant deposition

velocity terms b and bprime are coefficients of light-dependent coupled fluxes for full-canopy

conditions and f adjusts the NO2 light-dependent term for no-leaves conditions The

deposition velocity of O3 was used as a proxy for stomatal exchange the coefficient Γ

was consistently positive Stomatal conductance correlates with a number of different

variables (light temperature humidity) complicating the interpretation of the final term

It may indicate a stomatal emission of NO2 under low-NOx conditions typical at Harvard

Forest but could also correspond to a chemical or other process which slows NO2

deposition during the day The quadratic term amiddot[NO2]2 was particularly significant at

night during a few high-[NO2] events Multi-factor regressions fitting the hourly data

with these equations yield R2 of 061 for FNO2 and 092 for FNO the coefficients

summarized in Table 36 agree with analyses of segregated daytime and nighttime data

We note that the coefficients are relevant to the specific measurement heights of NO (29

98

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

m) and NO2 (22 m) at Harvard Forest relative to the forest canopy height (20 m)

However the mechanism should be adaptable for more general application

During the measurement period net NO2 deposition velocity (omitting the PPFD-

dependent term associated with photochemical cycling of NOx) -(V0 + amiddot[NO2] +

ΓmiddotVd(O3)[NO2]) was between 015 and 03 cm s-1 The mean 02 cm s-1 varied by less

than 002 cm s-1 between day and night and between full-canopy and leafless conditions

NO2 deposition velocity measurements to deciduous forest canopies are scarce this result

falls just outside of the range of reported values for coniferous forests 01 to 15 cm s-1

[Hanson and Lindberg 1991 Rondoacuten et al 1993 Joss and Graber 1996] and short

crops such as alfalfa soy and oats 007 to 125 cm s-1 [Hanson and Lindberg 1991]

The only estimates of NO deposition velocity are over grass ~01 to 02 cm s-1 [Hanson

and Lindberg 1991] other observations indicate NO emission from various species

though soil emissions could confound these results [Wildt et al 1997] This is quite

close to the ~025 cm s-1 NO deposition velocity at Harvard Forest (-V0_NO) but we

cannot resolve the debate as to whether NO deposits at all [Wesely and Hicks 2000]

Concentration and flux measurements of HONO N2O5 and NO3 (along with NO

and NO2) are required to better understand the nighttime chemistry and deposition of

NOx Recently-developed continuous analyzers for HONO [Harrison et al 1996] and N-

2O5 and NO3 [Brown et al 2001] may make these measurements possible in the coming

years In addition the use of flux chambers with a TDLAS or other instrument (sub-

nmol mol-1 sensitivity and species-specific detection method) on leaf litter and soil

samples in the lab and field may facilitate direct detection of NO2 compensation points

for various species and quantify the importance of different natural surfaces

99

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

NOx deposition matters to both the ecosystem and the atmosphere the form of

nitrogen entering plants affects nutrient loading acid stress and toxicity and the form of

nitrogen leaving the atmosphere affects O3 production HOx chemistry and long-range

transport of anthropogenic pollutants We are hopeful that future research endeavors will

approach the problem from the perspective of both ecosystem and atmosphere at scales

ranging from the individual leaf to the vegetated landscape using reliable species-

specific and sensitive measurement techniques

References

Andreacutes-Hernaacutendez J Notholt J Hjorth and O Schrems A DOAS study on the origin of initrous acid at urban and non-urban sites Atmos Environ 30 175-180 1996

Aneja V P P A Roelle G C Murray J Southerland J W Erisman D Fowler W A H Asman and N Patni Atmospheric nitrogen compounds II emissions transport transformation deposition and assessment Atmos Environ 35 1903-1911 2001

Barford C E Hammond Pyle L Hutyra S Wofsy Ecological Measurements to compliment eddy-flux measurements at Harvard Forest [Online data archive] httpwww-asharvardedudatanigec-datahtml 2001

Barnet W S and B J Finlayson-Pitts Enhancement of N2O4 on porous glass at room temperature A key intermediate in the heterogeneous hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Barney W S and B J Finlayson-Pitts Enhancement of N2O4 on Porous Glass at Room Temperature A Key Intermediate in the Heterogeneous Hydrolysis of NO2 J Phys Chem A 104 171-175 2000

Bey I D J Jacob R Yantosca J logan B Field A Fiore Q Li H Liu L Mickley M Schultz Global modeling of tropospheric chemistry with assimilated meteorology Model description and evaluation in press J Geophys Res 2001

Brown S H Stark S J Ciciora and A R Ravishankara In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy Geophys Res Lett 28 3237-3230 2001 (a)

100

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Brown S H Stark S J Ciciora and A R Ravishankara In-situ Detection of Atmospheric NO3 and N2O5 via Cavity Ring-Down Spectroscopy EOS Trans AGU 82 Fall Meeting Suppl A41B-0045 2001 (b)

Coe H and M W Gallagher Measurements of dry deposition of NO2 to a Dutch heathland using the eddy-correlation technique Q J R Meteorol Soc118 767-786 1992

Delaney A C D R Fitzjarrald D H Lenschow R Pearson Jr G J Wendel B Woodruff Direct Measurements of Nitrogen Oxides and Ozone Fluxes over Grassland J Atmos Chem 4 429-444 1986

Eugster W and R Hesterberg Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the Swiss plateau Atmos Environ 30 1247-1254 1996

Fitzjarrald D R and D H Lenschow Mean concentration and flux profiles for chemically reactive species in the atmospheric surface layer Atmos Environ 17 2505-2512 1983

Galmarini S P G Duynkerke J Vila-Guerau De Arellano Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer J Appl Meteorology 36 943-957 1997

Gao W M L Wesely P V Doskey Numerical Modeling of the Turbulent Diffusion and Chemistry of NOx O3 Isoprene and Other Reactive Trace Gases in and Above a Forest Canopy J Geophys Res 98 18339-18353 1993

Gao W and M L Wesely Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer J Appl Meteorology 33 835-847 1994

Goodman A L G M Underwood and V H Grassian Heterogeneous Reaction of NO2 Characterization of Gas-Phase and Adsorbed Products from the Reaction 2NO2(g) + H2O(a) HONO(g) + HNO3(a) on Hydrated Silica Particles J Phys Chem A 103 7217-7223 1999

Goulden ML JW Munger S-M Fan B C Daube and S C Wofsy Measurements of carbon sequestration by long-term eddy covariance Methods and a critical evaluation of accuracy Global Change Biology 2 169-182 1996

Harrison R M A-M N Kitto Evidence for a surface source of atmospheric nitrous acid Atmos Environ 28 1089-1094 1994

Harrison R M J D Peak G M Collins Tropospheric cycle of nitrous acid J Geophys Res 101 14429-14439 1996

101

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

Heal M R B B B Booth J N Cape K J Hargreaves The influence of simplified peroxy radical chemistry on the interpretation of NO2-NO-O3 surface exchange Atmos Environ 35 1687-1696 2001

Horii C V M S Zahniser D D Nelson J B McManus S C Wofsy Nitric Acid and Nitrogen Dioxide Flux Measurements a New Application of Tunable Diode Laser Absorption Spectroscopy Proc of SPIE 3758 152-161 1999

Hosker R P Jr and S E Lindberg Review Atmospheric deposition and plant assimilation of gases and particles Atmos Environ 16 889-910 1982

Hanson P and S E Lindberg Dry deposition of reactive nitrogen compounds a review of leaf canopy and non-foliar measurements Atmos Environ 25A 1615-1634 1991

Jacob D J Heterogeneous chemistry and tropospheric ozone Atmos Environ 34 2131-2159 2000

Joss U and W K Graber Profiles and Simulated Exchange of H2O O3 NO2 Between the Atmosphere and the HartX Scots Pine Plantation Theor Appl Climatol 53 157-172 1996

Lefer B L and R W Talbot Nitric acid and ammonia at a rural northeaster US site J Geophys Res 104 1645-1661 1999

Lerdau M T J W Munger D J Jacob The NO2 Flux Conumdrum Science289 2291-2293 2000

Moody J L J W Munger A H Goldstein D J Jacob and S C Wofsy Harvard Forest regional-scale air mass composition by Patterns in Atmospheric Transport History (PATH) J Geophys Res 103 13181-13194 1998

Munger J W S C Wofsy P S Bakwin S-M Fan M L Goulden B C Daube and A H Goldstein Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland 1 Measurements and mechanisms J Geophys Res 101 12639-12657 1996

Munger J W S-M Fan P S Bakwin M L Goulden A H Goldstein A S Colman and S C Wofsy Regional budgets for nitrogen oxides from continental sources Variations of rates for oxidation and deposition with season and distance from source regions J Geophys Res 103 8355-8368 1998

Notholt J J Hjorth and F Raes Formation of HNO2 on aerosol surfaces during foggy periods in the presence of NO and NO2 Atmos Environ 26A 211-217 1992

Padro J Zhang L and W J Massman An analysis of measurements and modlling of air-surface exchange of NO-NO2-O3 over grass Atmos Environ 32 1365-1375 1998

102

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results

103

Rondon A C Johansson L Granat Dry Deposition of Nitrogen Dioxide and Ozone to Coniferous Forests J Geophys Res 98 5159-5172 1993

Rothman LS et al The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation) 1996 Edition J Quant Spectrosc Radiat Transfer 60 665-710 1998

Sparks J P R K Monson K L Sparks M Lerdau Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest implications for tropospheric chemistry Oecologia 127 214-221 2001

Stocker D W D H Stedman K F Zeller W J Massman D G Fox Fluxes of Nitrogen Oxides and Ozone Measured by Eddy Correlation Over a Shortgrass Prairie J Geophys Res 98 12619-12630 1993

Walton S M W Gallagher T W Choularton J Duyzer Ozone and NO2 Exchange to Fruit Orchards Atmos Environ 31 2767-2776 1997

Wesely M L J A Eastman D H Stedman E D Yalvac An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux Atmos Environ 16 815-820 1982

Wesely M L Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models Atmos Environ 23 1293-1304 1989

Wesley M L and B B Hicks A review of the current status of knowledge on dry deposition Atmos Environ 34 2261-2282 2000

Wildt J D Kley A Rockel P Rockel H J Schneider Emission of NO from several higher plant species J Geophys Res 102 5919-5927 1997

• April ndash November 2000
• G (nmol mol-1)
• 29 August ndash 8 Oct 2000
• References
• Chapter3_NOx_Part2pdf
• • 34 Discussion
  • • Chapter3_NOx_Part1pdf
    • • 32 Methods
      • 33 Results