1 Purdue University, West Lafayette IN, USA 2 Meteorological Service of Canada, Toronto, Ontario Canada3 World Meteorological Organization, Geneva, SW4 LGGE, Grenoble, FR5 British Antarctic Survey, Cambridge, UK

Implications of Photochemistry Involving Organic Compounds in

Sunlit Snowpacks

Paul B. Shepson1, Amanda M. Grannas1, Terra M. Dassau1, Ann Louise Sumner1

Jan W. Bottenheim2, Leonard A. Barrie3, Florent Dominé4, and Eric W. Wolff5

Alert

Summit

Ny-Alesund

Barrow

Geographic North Pole

PSE '98 Ocean Snow Sampling Site

Magnetic North Pole

Importance of HCHO in the Troposphere

Radical source:

HCHO + h CO + H2

H• + •CHO

H• + O2 HO2•

•CHO + O2 HO2• + CO

2HO2• + 2NO• 2•OH + 2NO2

•

HCHO + h + 2O2 + 2NO• 2 •OH + 2NO2

• + 2CO + H2

Net:

This is relatively more important at the Poles, where absolute humidities are low, so that O3 photolysis is ineffective.

Role of Aldehydes in Ozone DestructionRadical Source

HCHO + h •H + HO2•

HO2• + BrO• HOBr + O2

HOBr(aq) + Br - + H+ Br2(aq) Br2(g) + h 2 •Br

Bromine Radical Sink

•Br + O3 BrO• + O2

BrO• + BrO• 2 •Br + O2

•Br + HCHO HBr + HCO•

•Br + CH3CHO HBr + CH3CO•

Gas-Phase FormaldehydePolar Sunrise Experiment 1998

Day of Year

50 60 70 80 90 100 110

[HC

HO

], pp

t

0

50

100

150

200

250

300

350

400

450

500

550

X Axis 2

35860 35880 35900

[Ozo

ne],

ppb

0

10

20

30

40

50

60

50 60 70 80 90 100 110

J HC

HO, s

-1

0.05.0e-61.0e-51.5e-52.0e-52.5e-5

JHCHO

[HCHO], O3 = 0

[HCHO], low O3

[HCHO], O3 > 40 ppb

[Ozone]

The HCHO lifetime changes from ~3 months in the dark, to 0.5 days in sunlight

Sumner and Shepson, Nature, 398, 230-233, 1999.

StainlessSteel Open

Tube

TeflonProbe with

Filter

Snow

Air

¼ PFA tubing

30 m heated inlet line to GC/MS

probe supportSnowpack Measurements:

Snowpack Interstitial Air Measurements

Vertical HCHO Profiles

Distance from air/snow Interface, m

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

[HC

HO

], p

pt

0

100

200

300

400

500

600

700

800Air/Snow Interface

Day 068 (Twilight)

Day 097 (Sunlight)

Day 106 (Sunlight)

Gradient implies a flux out of the snowpack

A. L. Sumner and P. B. Shepson, Nature, 398, 230-233, 1999.

Hutterli et al., GRL, 26, 1691-1694, 1999.

Hutterli et al. have discussed that firn air HCHO can be explained as aresult of temperature-dependent adsorption/desorption from snow grains.

But Physical processes (metamorphism, T-dependent adsorption/desorption)may also be very important!

Summit Formaldehyde DataJune, 1999

Time of Day

12:00 14:00 16:00 18:00 20:00 22:00 00:00

[HC

HO

], p

pb

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Firn

Te

mp

era

ture

, o C

-13

-12

-11

-10

-9

-8

Ra

dia

tion

, vo

lts

0

2

4

6

8

10

Ambient, 63 cmShaded Snowpack Air, -10 cmUnshaded Snowpack Air, -10 cmSnowpack Temperature, -10 cmAmbient Radiation

Dassau et al., in press, JGR, 2002.

Snow Irradiation Experiment

Decimal Day50.0 50.5 51.0 51.5 52.0 52.5

[HC

HO

], p

pt

0

100

200

300

400

500

600

700

T, o C

-38

-37

-36

-35

-34

-33

-32

-31

-30

-29

-28

NO

2, s

no

w p

ile

0

100

200

300

400

500

HCHO, ambientHCHO, Pile - darkHCHO, Pile-litT, base of pile

Lamp onNO2, pile

Lamp off

Snow phase measurements, FarTx

Before After

[HC

HO

] snow

x10-7

M

0

1

2

3

4

2/18 2/19

Lampon

Carbonyl Compound Diel Cycles

HC

HO

(ppt)

100

200

300

400

Ozo

ne

(ppb)

10

20

30

40

50

CH

3C

HO

(ppt)

0

50

100

150

200

Ozo

ne

(ppb)

1020304050

Date

Ace

tone

(ppt)

200

400

600

Ozo

ne

(ppb)

10203040

Ambient Air @ +1 mSnowpack Air @ -2 cmOzone

HCHO

CH3CHO

CH3C(O)CH3

03/28 03/30 04/01 04/03 04/05

Radia

tion

(kW

m-2)

0.0

0.1

0.2

0.3

Snow

Tem

p(C

elc

ius)

-40

-35

-30

-25

Snow

Tem

p(C

elc

ius)

04/17 04/18 04/19 04/20 04/21

Radia

tion

(kW

m-2)

0.0

0.1

0.2

0.3

0.4

-33

-30

-27

-24

Radiation Snowpack Temp @ -6 cm

ALERT2000

Grannas et al., Atmos. Environ.36, 2733-2742, 2002.

Large diel cycles observedfor carbonyl compounds; not well correlated with snowpackTemperature

Average Concentration Difference[snow] - [ambient]

[HC

HO

] sno

w -

[H

CH

O] a

mbi

ent

(ppt

)0

20

40

60

80

Time of Day

[CH

3CH

O] sn

ow -

[C

H3C

HO

] ambi

ent

(ppt

)

-5

0

5

10

15

20

25

HCHOMarch 28-April 3, 2000

Nambient = 155Nsnow = 380

CH3CHO

April 17-April 20, 2000Nambient = 43

Nsnow = 40

00:00 04:00 08:00 12:00 16:00 20:00 00:00[Ace

tone

] snow

- [A

ceto

ne] am

bien

t

(ppt

)

-40

-20

0

20

40

60

AcetoneApril 17-April 20, 2000

Nambient = 43

Nsnow = 40

Snowpack CH3CHO Vertical Profiles

Snowpack Depth, cm-50 -40 -30 -20 -10 0

[sno

wpa

ck a

ir]/[

+ 1

met

er]

0

2

4

6

8

10

DarkTwilightSunriseMay+1 meter

PSE2000

Guimbaud et al., Atmos. Environ., 36, 2743-2752, 2002.

But, our previous measurements all focused on observations of the gas phase, in equilibrium (or not) with the snow.

Are these species really produced in snow?

Laboratory Experiments

LN2

ZeroAir

1 mm sieve

0.25 mm sieve

Water(+ nitrate, DOM) ٭٭ ٭ ٭ ٭

٭ ٭٭ ٭٭ ٭٭

DNPH cartridge In –10°F Freezer

snowCoolant

RecirculatingPump

hDetect carbonyls

generated in snowvia DNPH

derivitization andUV-vis detection

UncharacterizedCleavage Products

Refractory DOM

Humic and fulvic substances

Labile DOM

carbonyls BIOTA

hv, [O]

hv

hv [O]

carbonyls

Marine Boundary Layer

SNOW

hv

NO3- hv NO2 + O-

O- + H2O OH- + OH

NO3- hv NO2

- + O

NO2- + H+ HONO

HONO NO + OHhvhv, [O]

[O] = OH, 1O2, HO2, O3, RO2, etc

Mopper & Stahovec, 1986Mopper et al, 1991Kieber et al, 1990Matsuda et al, 1992Sumner & Shepson, 1999Honrath et al, 2000

So, what is happening?

O + Br- Br2

What could generate carbonyl compounds in snow?

NO3- NO2 + O-

OH (aq)O- + H+

hv NOx shown to be produced insnow via nitrate photolysis (Honrath et al., 1999, 2000)

Could OH reaction with organic matterproduce carbonyl compounds?

Possible Mechanism???

RO

CH3O

CH CH – CH2OH

•OH

RO

CH3O

CH CH – CH2O•

RO CH CH •

CH3O

+ HCHO

+ H2O

RHO

CH3O

•OH

RHO

•CH2O

RHO

•

+ HCHO

Organic material derived from plant matter (lignin)

O

RR R

R

O*

RR

OH

ALKENE + CARBONYL. .

Norrish type IIPhotofragmentation

Carbonyl-cleavage(Norrish type I)

Riemer et al, Marine Chemistry, 2000, 71, 177-198.

O

RR RC

O

. +R

.

ALKENE + ALDEHYDE

h

h

h

Photo-oxidation mechanism

Note: Ethene and propene production observed in snow at Summit, Greenland!!! Carbonyl compound production observed in snow at Alert, Canada!!!

Implications for snow/ice core composition?

If indeed carbonyl compounds within snow/ice can be produced from DOM oxidation, the snow/ice core composition would to some extent reflect not atmospheric radical (i.e. OH) and VOC (e.g. CH4) concentrations, but variability in transport and production/mobilization of biogenic organic matter (e.g. forest fires), and deposition of other reactants/precursors, such as HNO3.

The extent to which NOx is remobilized may depend on snowpack acidity, as there may be competition between:

NO3- + h NO2

- + O(3P)NO2

- + H+ HONO (followed by volatilization)

NO2- + h NO + O(3P)

NO2- + oxidants NO3

-

Conclusions

No, seriously, I really think that carbonyl compounds can be photochemically produced in sunlit snowpacks!

There are significant consequences for the snow-covered boundary layer, and likely for ice cores, for photochemically rective species.

We need to:

•better understand the DOM content of snow!

•be able to quantitatively understand snow (surface?) phase photochemistry and kinetics

AcknowledgmentsAcknowledgmentsAcknowledgmentsAcknowledgments•NSF•CFS Alert and Environment Canada, Jan Bottenheim, Len Barrie, Al Gallant, John Deary•Jack Dibb, Richard Honrath, Aaron Swanson

•Purdue’s Amy Instrumentation Facility

•NSF•CFS Alert and Environment Canada, Jan Bottenheim, Len Barrie, Al Gallant, John Deary•Jack Dibb, Richard Honrath, Aaron Swanson

•Purdue’s Amy Instrumentation Facility