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| Titel |
Modeling the Physical Multi-Phase Interactions of HNO3 Between Snow and Air on the Antarctic Plateau (Dome C) and coast (Halley) |
| VerfasserIn |
Hoi Ga Chan, Markus M. Frey, Martin D. King |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250138595
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| Publikation (Nr.) |
 EGU/EGU2017-1658.pdf |
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| Zusammenfassung |
| Nitrogen oxides (NOx = NO + NO2) emissions from nitrate (NO3−) photolysis in snow
affect the oxidising capacity of the lower troposphere especially in remote regions of the high
latitudes with low pollution levels. The porous structure of snowpack allows the exchange of
gases with the atmosphere driven by physicochemical processes, and hence, snow can act as
both source and sink of atmospheric chemical trace gases. Current models are limited by poor
process understanding and often require tuning parameters. Here, two multi-phase physical
models were developed from first principles constrained by observed atmospheric
nitrate, HNO3, to describe the air-snow interaction of nitrate. Similar to most of the
previous approaches, the first model assumes that below a threshold temperature,
To, the air-snow grain interface is pure ice and above To, a disordered interface
(DI) emerges assumed to be covering the entire grain surface. The second model
assumes that Air-Ice interactions dominate over the entire temperature range below
melting and that only above the eutectic temperature, liquid is present in the form
of micropockets in grooves. The models 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, 123∘33′E, 3233 m a.s.l.) and at a relatively warm site on the Antarctica coast
(Halley, 75∘35′S, 26∘39′E, 35 m a.s.l). The first model agrees reasonably well
with observations at Dome C (Cv(RMSE) = 1.34), but performs poorly at Halley
(Cv(RMSE) = 89.28) while the second model reproduces with good agreement
observations at both sites without any tuning (Cv(RMSE) = 0.84 at both sites). It is
therefore suggested that air-snow interactions of nitrate in the winter are determined by
non-equilibrium surface adsorption and co-condensation on ice coupled with solid-state
diffusion inside the grain. In summer, however, the air-snow exchange of nitrate is
mainly driven by solvation into liquid micropockets following Henry’s law with
contributions to total NO3− concentrations 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 HNO3 are sensitive to total solute concentration and pH. In conclusion, the
second model can be used to predict nitrate concentration in surface snow over
the entire range of environ- mental conditions typical for Antarctica and forms a
basis for parameterisations in regional or global atmospheric chemistry models. |
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