Snow optical properties at Dome C (Concordia), Antarctica; implications for snow emissions and snow chemistry of reactive nitrogen

摘要

Measurements of e-folding depth, nadir reflectivity and stratigraphy of thesnowpack around Concordia station (Dome C, 75.10° S, 123.31° E) wereundertaken 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 thesnowpack due to nitrate photolysis within the snowpack. The stratigraphy ofthe top 80 cm of Dome C snowpack generally consists of three main layers:- asurface of soft windpack (not ubiquitous), a hard windpack, and a hoar-likelayer beneath the windpack(s). The e-folding depths are ~10 cm for thetwo windpack layers and ~20 cm for the hoar-like layer for solarradiation at a wavelength of 400 nm; about a factor 2–4 larger than previousmodel estimates for South Pole. The absorption cross-section due toimpurities in each snowpack layer are consistent with a combination ofabsorption due to black carbon and HULIS (HUmic LIke Substances), withamounts of 1–2 ng g?1 of black carbon for the surface snow layers.Depth-integrated photochemical production rates of NO₂ in the Dome Csnowpack were calculated as 5.3 × 1012 molecules m?2 s?1,2.3 × 1012 molecules m?2 s−1 and 8 × 1011 molecules m?2 s−1 for clear skies and solar zenithangles of 60°, 70° and 80° respectivelyusing the TUV-snow radiative-transfer model. Depending upon the snowpackstratigraphy, a minimum of 85% of the NO₂ may originate from the top20 cm of the Dome C snowpack. It is found that on a multi-annual time-scalephotolysis can remove up to 80% of nitrate from surface snow, confirmingindependent isotopic evidence that photolysis is an important driver ofnitrate loss occurring in the EAIS (East Antarctic Ice Sheet) snowpack.However, the model cannot completely account for the total observed nitrateloss of 90–95 % or the shape of the observed nitrate concentration depth profile. A morecomplete model will need to include also physical processes such asevaporation, re-deposition or diffusion between the quasi-liquid layer onsnow grains and firn air to account for the discrepancies.