Surface-Atmosphere fluxesSurface-Atmosphere fluxes

Alex GuentherAlex Guenther

Atmospheric Chemistry DivisionAtmospheric Chemistry Division National Center for Atmospheric ResearchNational Center for Atmospheric Research

Boulder CO, USA Boulder CO, USA

OutlineOutline• IntroductionIntroduction• Major cyclesMajor cycles• Recent scientific advances and Recent scientific advances and challengeschallenges

1. IntroductionWhat is in the atmosphere? How did it get there? How does it leave?

What is in the Atmosphere?What is in the Atmosphere?

N2 (78.084%), O2 (20.948%), Ar (0.934%), CO2 (0.039%), Ne (0.0018%), He (0.000524%), CH4 (0.00018%), H2 (0.000055%), N2O (0.000032%), Halogens (0.0000003%), CFCsH2O, O3, CO, non-methane VOC, NOy, NH3, NO3, NH4, OH, HO2, H2O2, CH2O, SO2, CH3SCH3, CS2, OCS, H2S, SO4, HCN

Well mixed

Variable

What is in the atmosphere?What is in the atmosphere?

• 1950s: Atmosphere is 99.999% composed of N2, O2, CO2, H2O, He, Ar, Ne. All are inert! (no chemistry). O3 in the stratosphere. Trace CH4, N2O

• 1960s: Recognized that reactive compounds in the atmosphere were important even at extremely low levels.

• 1970s: Regional air quality becomes a major research topic.

• 1980s: Global atmospheric chemistry becomes a major research topic.

Earth

Cosmos

Where does the atmosphere come from?Where does the atmosphere come from?

1. Original atmosphere2. Dead planet3. Living planet4. Anthropocene

Organic aerosol

processes

Photo-oxidant

processes

Cloud processes

Global Biogeochemical Cycles

Carbon Cycle

Nitrogen Cycle

Water & Energy Cycles

Ozone and N deposition

NO/NH3 emission

CO2H2O NOy

NH3

Precipitation and solar radiation

Latent and sensible heat

Biological particles and VOC emissions

Air Quality:ozone and particles

Weather/Climate:Temperature,

sunshine, precipitation

Ecosystem Health:Productivity,

diversity, water availability

AnthropogenicNatural

How do we measure surface exchange?How do we measure surface exchange?

• Eddy covariance: The flux is related to the product of fluctuations in vertical wind and concentration. This is the only direct measurement.

• Gradient: The flux is related to vertical concentration gradient.

• Mass balance (Inverse Model): The flux is related to a concentration or concentration change.

01020

0 10 20 30 40 50 60 70C (

g m

-3) A

-1.50

1.53

0 10 20 30 40 50 60 70

w (

m s

-1) B

-606

12

0 10 20 30 40 50 60 70

w'C

'

(g

m-2

s-1

) C

Time (seconds)

Eddy Covariance Flux DataEddy Covariance Flux DataConcentration and wind speed measurements above a forest canopy Sampling rate = 10 Hz

The flux of a trace gas is calculated as the covariance between the instantaneous deviation of the vertical wind velocity (w’) and the instantaneous deviation of the trace gas (c’) for time periods between 30 min and an hour.

Concentration

Vertical wind speed

Flux

roughness sublayer

ConcentrationProfile

HE

IGH

TSurface layer gradientsSurface layer gradients

Flux = K dC/dz K: eddy diffusivity coefficient

dz: vertical height difference

dC: concentration difference

inertial sublayerdC

dz

Enclosure measurements

0

zi

MIXED LAYER Conc.Profile

0

HE

IGH

T

Emission (deposition) rate is related to the increase (decrease) in mass

Static: change with time

Dynamic: difference between inflow and outflow

Boundary Layer Budget

Imaginary box

May need to consider

- chemical loss/production

- horizontal advection

- non-stationary

Mass Balance Budgets

2. The CyclesFrom the earth surface to the atmosphere and back again

Chapter 5. Trace Gas Exchanges and Biogeochemical Cycles. In: Atmospheric Chemistry and Global Change (1999). Brasseur et al. (editors).

Water Cycle: source of OH in the atmosphereWater Cycle: source of OH in the atmosphere

Separating evapotranspiration into evaporation and transpiration components is an active area of research

Atmospheric Chemistry and Global Change (1999). Brasseur et al. (editors).

THE NITROGEN CYCLETHE NITROGEN CYCLE

ATMOSPHERE

N2 NO

HNO3

NH3/NH4+ NO3

-

orgNBIOSPHERE

LITHOSPHERE

combustionlightning

oxidation

deposition

assimilation

decay

nitrification

denitri-fication

biofixation

burial weathering

fixation

SOIL/OCEAN

Daniel Jacob 2008

Atmospheric ammonia sources and sinks (Tg per year)

SourcesDomestic animals: 21Human excrement: 2.6Industry: 0.2Fertilizer losses: 9Fossil fuel combustion: 0.1Biomass Burning: 5.7Soil: 6Wild animals: 0.1Ocean: 8.2

SinksWet precipitation (land): 11Wet precipitation (ocean): 10Dry deposition (land): 11Dry deposition (ocean): 5Reaction with OH: 3

From Brasseur et al. 1999

Does it add up?Sources: 52.9 TgSinks: 40 TgThis is good agreement considering the uncertainties of factors of 2 or more

Atmospheric NOx sources and sinks (Tg per year)

SourcesAircraft: 0.5Fossil fuel combustion: 20Biomass Burning: 12Soil: 20Lightning: 8NH3 oxidation: 3Stratosphere: 0.1Ocean: <1

SinksWet precipitation (land): 19Wet precipitation (ocean): 8Dry deposition: 11

From Brasseur et al. 1999

Does it add up?Sources: 64 TgSinks: 43 TgThis is good agreement considering the uncertainties of factors of 2 or more

Vegetation and soils 0.4 to 1.2

Tg of H2S, DMS, OCS, CS2, DMDS

The Sulfur CycleAtmosphere

Wet deposition 50-75 Tg of SO2, SO4

SO2, SO4

Dry deposition50-75 Tg of SO2, SO4

H2S, DMS, OCS, CS2, DMDS

Anthropogenic 88-92 Tg of SO2,

sulfates

Volcanoes 7-10 Tg of H2S, SO2,

OCS

Ocean 10-40 Tg of

DMS, OCS, CS2, H2S

Biomass burning 2-4 Tg of H2S, SO2, OCS

Vegetation and soils

VOC, CH4, CO2, CO

The Carbon CycleAtmosphere

Wet precipitation

CO2

Dry deposition and

photosynthesis

VOC, CH4, CO

Anthropogenic VOC, CH4, CO2,

COOcean

VOC, CH4, CO2, CO

Biomass burning VOC, CH4, CO2, CO

Carbon Emissions: MethaneCarbon Emissions: Methane

There are hundreds of BVOCs emitted from Vegetation

flowers~100’s of VOCs

cell wallsMeOH, HCHO

phytohormonese.g. ethylene,

DMNTcell membranes

fatty acid peroxidationwound-induced OVOCs

resin ducts / glandsterpenoid VOCs

cytoplasm/chloroplastC1-C3 metabolites

chloroplastterpenoid VOCs

Vegetation and soils

HalogensAtmosphere Br-, I-, Cl-

Dry deposition and soil microbe

uptake

CH3Cl, CH3Br, CH3I

Anthropogenic

Ocean

Biomass burning

3. Surface-atmosphere exchange: Recent scientific advances and challenges

How will biogenic VOC emissions respond to future How will biogenic VOC emissions respond to future changes in landcover, temperature and CO2? changes in landcover, temperature and CO2?

• Landcover, temperature and CO2 are changing

• Biogenic VOC (BVOC) emissions are very sensitive to these changes

• But it is difficult to even predict the sign of future changes in BVOC emissions

NCAR CCSM Future Landcover NCAR CCSM Future Landcover Change PredictionsChange Predictions

Mix Shrub/Grass 1461%

Bare Sparse Veg. 1317%

Dryland Crop. 267%

Urban 205%

Snow or Ice -100%

Mixed Tundra -100%

Wooden Tundra -100%

Wooded Wetland -100%

Evergrn. Broadlf. -100%

Current Future (2100)

Percent land cover changes

USDA predictions of tree species composition changes in the eastern U.S.

USDA climate change tree atlas• Current estimates are based on observations (FIA dist. Data). Future is

based on 2x CO2 equil. climate vars from 3 GCMs (PCM, GFDL, HAD)

• Provides future state level estimates of 135 tree species for eastern U.S.

Large increase in oak trees which have very high isoprene emissions

Landcover change could result in a large regional increases and decreases in U.S. isoprene emissions

High = 5600

Low = -5900 (Future Isoprene – Current Isoprene Emission factors g m-2 h-1)

The overall impact is a large decrease in U.S. average isoprene emission factor

(~800 g m-2 h-1)

This is mostly due to a predicted decrease in broadleaf tree coverage

High = 0%

Low = 30%

Broadleaf tree change

1.5

2

2.5

3

30 35 40 45Temperature (oC)

Iso

pre

ne

em

iss

ion

ac

tivi

ty

Short-term response

Short-term and Long-term response

BVOC emissions will increase with increasing temperatures

but we don’t know if the response will be similar to what is observed for short-term variations or if there will be an additional long-term component

Guenther et al. 2006

Decreasing emissions are expected for increasing CO2

but the magnitude is uncertain and there may be indirect CO2 effects (increasing LAI, changing species composition)

Heald et al. 2008

As a result of these uncertainties:Different models have substantially different predictions

of future changes in biogenic VOC emissions

Year 2050 BVOC – Year 2000 BVOC (g/m2/day)

Weaver et al. 2009

These differences have a large impact on predicted future ozone and particles

Hallquist et al., ACP, 2009

SOA: 134 TgC/yr

Why do recent “state-of-the-art” estimates of secondary organic aerosol (SOA) production differ by a factor of 5?

Goldstein and Galbally, ES&T, 2007

large uncertainty in estimates of Volatile Organic Carbon (VOC) deposition

Resistance Model

CBAd RRRv

1

CA

CUC

CS

CLC

CG

RA

RB

RSRM

RL

URSLRML

RAG

RGS

: RC

CC

Add CvF

for estimating dry deposition

Aerodynamic resistance (turbulent diffusion)

Boundary layer resistance (molecular diffusion)

Canopy resistance

We evaluated model performance for oxyVOC with measurements at a wide range of field sites

Our field flux measurements indicated that model Rc for oxygenated VOC is too high.

traditional model

modified model

Why are we underestimating VOC deposition?

The models assume that oVOC deposition

is just a physical process

FL0 growth chamber experiments with

Populus trichocarpa x deltoides

Stomata ~20-30 μm

FL0 growth chamber experiments with Populus

trichocarpa x deltoides

We suspected that the high deposition rates were due to a biological process.

Exposure Experiments

pre-fum fum post- fum pre-fum fum post- fum

acetaldehydemethyl vinyl ketone

acetaldehydemethyl vinyl ketone

O3 fumigation MVK fumigation

0.1

1.0

10.0

100.0

MsrA AAO2 ALDH2 SOD APX ACS ACO1 p450 DHQ WRKY

2-f

old

ing ozone

mvk

wound

significance level

qPCR (quantitative polymerase chain reaction)qPCR (quantitative polymerase chain reaction)

ROS

conversion of carbonyls (AAO2, ALDH2)and oxidative stress repair (MsrA)

a-carbonic acid synthase (ACS)carboxylic acid oxidase (ACO1)

biotic and a-biotic stress markers This tells us

that the plants turned on these genes to actively take up oVOC

Global increase in dry deposition: ~36%Global decrease in wet deposition: ~7%

Change in oVOC dry deposition when we put the new model in NCAR/MOZART model

This has a significant impact on regional atmospheric chemistry

Any Questions?Any Questions?