Air–snow exchange of nitrate: a modelling approach to investigate physicochemical processes in surface snow at Dome C, Antarctica
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 bei...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-10-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/12531/2016/acp-16-12531-2016.pdf |
| Summary: | 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 ice. 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 have relied on strong assumptions to characterise snow reactive
properties, which are not well defined. Air–snow exchange processes such as
adsorption, solid-state diffusion, or co-condensation also affect snow
chemical composition. Here, we present a physically based model of these
processes for nitrate. Using as input a 1-year-long time series of
atmospheric nitrate concentration measured at Dome C, Antarctica, our model
reproduces with good agreement the nitrate measurements in the surface snow.
By investigating the relative importance of the main exchange processes, this
study shows that, on the one hand, the combination of bulk diffusion and
co-condensation allows a good reproduction of the measurements (correlation
coefficient <i>r</i> = 0.95), with a correct amplitude and timing of summer peak
concentration of nitrate in snow. During winter, 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. On
the other hand, the adsorption of nitric acid on the surface of the snow
grains, constrained by an already existing parameterisation for the isotherm,
fails to fit the observed variations. During winter and spring, the modelled
concentration of adsorbed nitrate is respectively 2.5 and 8.3-fold higher
than the measured one. 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. This study clearly demonstrates that
co-condensation is the most important process to explain nitrate
incorporation in snow undergoing temperature gradient metamorphism. The
parameterisation developed for this process can now be used as a foundation
piece in snowpack models to predict the inter-relationship between snow
physical evolution and snow nitrate chemistry. |
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| ISSN: | 1680-7316 1680-7324 |