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| Titel |
Air–snow exchange of nitrate: a modelling approach to investigate physicochemical processes in surface snow at Dome C, Antarctica |
| VerfasserIn |
Josué Bock, Joel Savarino, Ghislain Picard |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250122067
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| Publikation (Nr.) |
 EGU/EGU2016-1000.pdf |
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| Zusammenfassung |
| Snowpack is a multiphase (photo)chemical reactor that strongly influences the air
composition in polar and snow-covered regions. Snowpack plays a special role in the nitrogen
cycle, as it has been shown that nitrate undergoes numerous recycling stages (including
photolysis) in the snow before being permanently buried in the firn. However, the current
understanding of these physicochemical processes remains very poor. Several modelling
studies have attempted to reproduce (photo)chemical reactions inside snow grains, but these
required strong assumptions to characterise snow reactive properties, which are not well
defined.
Physical processes such as adsorption, solid state diffusion and co-condensation also
affect snow chemical composition. We developed a model including a physically based
parameterisation of these air–snow exchange processes for nitrate. This modelling study
divides into two distinct parts: firstly, surface concentration of nitrate adsorbed onto snow is
calculated using existing isotherm parametrisation. Secondly, bulk concentration of
nitrate in solid solution into the ice matrix is modelled. In this second approach,
solid state diffusion drives the evolution of nitrate concentration inside a layered
spherical snow grain. A physically-based parameterisation defining the concentration
at the air–snow interface was developed to account for the the co-condensation
process.
The model uses as input a one-year long time series of atmospheric nitrate concentration
measured at Dome C, Antarctica. The modelled nitrate concentration in surface snow is
compared to field measurements. We show that on the one hand, the adsorption of nitric acid
on the surface of the snow grains fails to fit the observed variations. During winter and spring,
the modelled adsorbed concentration of nitrate is 2.5 and 8.3-fold higher than the measured
one, respectively. A strong diurnal variation driven by the temperature cycle and a peak
occurring in early spring are two other major features that do not match the measurements.
On the other hand, the combination of bulk diffusion and co-condensation incorporation
processes allows a good reproduction of the measurements (correlation coefficient r=0.95),
especially with a correct amplitude and timing of summer peak concentration of nitrate in
snow. During wintertime, nitrate concentration in surface snow is mainly driven by
thermodynamic equilibrium, whilst the peak observed in summer is explained by the kinetic
process of co-condensation.
For the first time, this study elucidates the air–snow transfer function of nitrate, based
on a process-resolving modelling approach. This study also demonstrates that the
co-condensation is the most important process to explain nitrate incorporation in snow
subject to temperature gradients. |
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