Snow optical properties at Dome C, (Concordia), Antarctica: implications for snow emissions and snow chemistry of reactive nitrogen
Measurements of e-folding depth, nadir reflectivity
and stratigraphy of the snowpack around Concordia station
(Dome C, 75.10 S, 123.31 E) were undertaken to determine
wavelength dependent coefficients (350 nm to 550 nm)
for light scattering and absorption and to calculate potential fluxes (depth-integrated production rates) of nitrogen dioxide (NO2) from the snowpack due to nitrate photolysis within the snowpack. The stratigraphy of the top 80 cm of Dome C snowpack generally consists of three main layers:- a surface of soft windpack (not ubiquitous), a hard windpack, and a hoar-like layer beneath the windpack(s). The e-folding depths are 10 cm for the two windpack layers and 20 cm for the hoar-like layer for solar radiation at a wavelength of 400 nm; about a factor 2–4 larger than previous model estimates for South Pole. The absorption cross-section due to impurities in each snowpack layer are consistent with a combination of absorption due to black carbon and HULIS (HUmic LIke Substances), with amounts of 1–2 ng g−1 of black carbon for the surface snow layers. Depth-integrated photochemical
production rates of NO2 in the Dome C snowpack were calculated as 5.3×1012 moleculesm−2 s−1, 2.3×1012 moleculesm−2 s−1 and 8×1011 moleculesm−2 s−1 for clear
skies and solar zenith angles of 60, 70 and 80 respectively using the TUV-snow radiative-transfer model. Depending upon the snowpack stratigraphy, a minimum of 85% of the NO2 may originate from the top 20 cm of the Dome C snowpack. It is found that on a multi-annual time-scale photolysis can remove up to 80% of nitrate from surface snow, confirming independent isotopic evidence that photolysis is an important driver of nitrate loss occurring in the EAIS (East Antarctic Ice Sheet) snowpack. However, the model cannot completely account for the total observed nitrate loss of 90–95% or the shape of the observed nitrate concentration depth profile. A more complete model will need to include also physical processes such as evaporation, re-deposition or diffusion between the quasi-liquid layer on snow grains and firn air to account for the discrepancies.
Authors: France, J.L., King, M.D., Frey, Markus, Erbland, J., Picard, G., Preunkert, S., MacArthur, A., Savarino, J.