1 purdue university, west lafayette in, usa 2 meteorological service of canada, toronto, ontario...
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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