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SUMMARY:Modeling the Physical Multi-Phase Interactions of HNO3 Between Sno
 w and Air on the Antarctic Plateau (Dome C) and coast (Halley) - Hoi Ga Ch
 an\, British Antarctic Survey
DTSTART:20170306T141500Z
DTEND:20170306T151500Z
UID:TALK69523@talks.cam.ac.uk
CONTACT:Michelle McCrystall
DESCRIPTION:Nitrogen oxides (NOx = NO + NO2) emissions from nitrate photol
 ysis in snow affect the oxidising capacity of the lower troposphere especi
 ally in remote regions of the high latitudes with low pollution levels. \n
 \nThe porous structure of snowpack allows the exchange of gases with the a
 tmosphere driven by physicochemical processes\, and hence\, snow can act a
 s both source and sink of atmospheric chemical trace gases. Current models
  are limited by poor process understanding and often require tuning parame
 ters. Here\, two multi-phase physical models were developed from first pri
 nciples constrained by observed atmospheric nitrate\, HNO3\, to describe t
 he air-snow interaction of nitrate. \n\nSimilar to most of the previous ap
 proaches\, the first model assumes that below a threshold temperature\, To
 \, the air-snow grain interface is pure ice and above To\, a disordered in
 terface (DI) emerges assumed to be covering the entire grain surface. The 
 second model assumes that Air-Ice interactions dominate over the entire te
 mperature range below melting and that only above the eutectic temperature
 \, liquid is present in the form of micropockets in grooves. \n\nThe model
 s are validated with available year-round observations of nitrate in snow 
 and air at a cold site on the Antarctica Plateau (Dome C\, 75◦06′S\,12
 3◦33′E\, 3233 m a.s.l.) and at a relatively warm site on the Antarctic
 a coast (Halley\, 75◦35′S\,26◦39′E\, 35 m a.s.l). \n\nOur study su
 ggests that air-snow interactions of nitrate in the winter are determined 
 by non-equilibrium surface adsorption and co-condensation on ice coupled w
 ith solid-state diffusion inside the grain. In summer\, however\, the air-
 snow exchange of nitrate is mainly driven by solvation into liquid micropo
 ckets following Henry’s law with contributions to total nitrate concentr
 ations of 75% and 80% at Dome C and Halley respectively. It is also found 
 that liquid volume of the snow grain and air-micropocket partitioning of H
 NO3 are sensitive to total solute concentration and pH. In conclusion\, th
 e second model can be used to predict nitrate concentration in surface sno
 w over the entire range of environmental conditions typical for Antarctica
  and forms a basis for parameterisations in regional or global atmospheric
  chemistry models.
LOCATION:Pfizer Lecture Theatre\, Department of Chemistry
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