Snowpack photochemistry - focused on the PAN(Peroxyacetyl Nitrate)

in Summit Greenland

Saewung Kim

19 April 2004

Is (Ant)Arctic region photochemically DEAD?

Probably YES - too remote - low solar intensity

Synthetic image(Apr. 7 10am EST)

It seems still alive!

Species Predicted by gas phase chemistry

Actual Measurement

HCHO 70 pptv 200 pptv

CH3CHO 40 pptv 80 pptv

NOx 1 pptv 25 pptv

HONO 1 pptv 20 pptv

HOx 0.9 pptv 3.7 pptv(Domine and Shepson, 2002)

Summit, Greenland region

-Measurement data could not be explained by gas phase only prediction-The difference of each species seems non-linear the difference is not just come from the numerical calculation. We can assume some other chemical mechanism would be going on the snowpack region.

What is the invisible hand?

HETEROGENEOUS chemistry insideof the snowpack!

(Domine and Shepson, 2002)

Snowpack photochemistry

Red arrow : Inside of snowpackGreen arrow : Gas phase Chem.

-Nitrate photo dissociationNO3

-+h NO2- + O (3P)

h NO2- + O-

-O- ChannelO- + H+ OHNO2

- + H+ HONONO2

- + h NO + O-

-O(3P) ChannelO(3P) + O2 O3

O(3P),OH,O3 + DOM Carbonyl Compound

(Domine and Shepson, 2002)

Is this mechanism really big deal?? I

Sure!! in two aspects

First nearly 50% of N Hem.is covered by snow(Robinson et al., 1993)

The global budget of photochemically active species would be significantly affected

Is this mechanism really big deal?? II

If ion species is perturbed by the heterogeneous mechanism, what we understand of the paleo-environment reconstructed by icecore interpretation would be wrong.

Ice core research homepagehttp://www.secretsoftheice.org/icecore/

Importance of PAN in arctic environment I

-PAN in the snowpack environment will be studied

◉ PAN is the product of peroxy radical came from VOC and NOx CH3CH(O)OO + NO2+M CH3CH(O)OONO2+M

CxHy NOx HNO3

PAN

PAN

NOx HNO3

Long-range transportat low T

Deposition

ThermalDecompo.

NOx Source Region Remote Region

CH3CHO + OH -> CH3CO + H2OCH3CO + O2 + M -> CH3C(O)OO + M

CH3C(O)OO+NO2+M -> PAN + M

PAN -> CH3C(O)OO + NO2

Source of nitrate ion in the snowapck HNO3 : Easily soluble. But concentration is too low to explain the high concentration of nitrate ion in the snowpack.

Then what is the most abundant reactive nitrogen species (NOy) in arctic environment - PAN

PAN is most abundant species in the Arctic region, so the mechanism for dissolution to snow of PAN have been studied but the processes are not clearly understood (Ford et al, 2002)

Simple box-model for PAN in the Summit Greenland region-Understand of PAN budget in the Arctic region-How much of PAN could contribute for nitrate in the snow

Importance of PAN in arctic environment II

Box model-step I No icepack chemistry

Snowpack

Boundary layer

PAN(133pptv)

Long range transport (T)

Thermal decomposition(lifetime 76day at 255K)

Adsorption to snow(lifetime 1.3 day at 255k)

Wang et al(1998)- global Chem-Transport model

July August

133ppt 132ppt

Steady-State Assumption

d[PAN]/dt = (Long range transport) – (Thermal decomp.) – ( Ads. to snow) = 0

(Long range transport) = 1.1910-3 ppt/sec

Box model-step II With icepack chemistry

Snowpack

Boundary layer

PAN(75pptv)

Long range transport (T)

TD

ADS

Production from snowpack2.0 10-2ppt/sec

Missing Sink

Ford et al(2002)-Measured PAN: 74pptv-Production from snowpack : 2.0X10-2 ppt/sec

d[PAN]/dt = (Long range transport) – (Thermal decomp.) – ( Ads. to snow) +(Production from snowpack) -(Missing sink) = 0

(Deposition) = 1.1910-3 ppt/sec

Deposition velocity of PAN based onbox-model calculation

Snowpack

Boundary layer = 100m(Helmig et al,2002)

-Deposition velocity : how fast molecules will be deposited at given flux and concentration.

(column height) = (boundary layer height)=100 m

Calculated result = 3 cm/sec

Huge value compared with HNO3 deposition velocity 0.5 cm/sec(Seinfeld and Pandis, 1997)

Summary and conclusion

-Snow covered region in high latitude could be important role in global perspective photochemistry

-PAN is the most abundant species in Summit Greenland region, and also important NOy species in other (Ant) Arctic environment.

-PAN could be role of the dominate nitrate source in Greenland region from the box model.

-More study is needed specific physical-chemical mechanism of PAN dissolution on ice.