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

Abstract

Abstract.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 (NO 2 ) 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 NO 2 in the Dome C snowpack were calculated as 5. using the TUV-snow radiative-transfer model. Depending upon the snowpack stratigraphy, a minimum of 85 % of the NO 2 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.

The impact of parameterising light penetration into snow on the photochemical production of NO<sub><i>x</i></sub> and OH radicals in snow

“…the albedo of the surface underlying the snow does not affect the calculation of the actinic flux within the snowpack -the radiation decreases exponentially according to Beer-Lambert law (France et al, 2011, define semi-infinite as 3-4 e-folding depths). …”

“…hydroxyl radical (OH), within the snowpack (e.g. Mauldin et al, 2001;Chen et al, 2004;Sjostedt et al, 2005;France et al, 2011). The porous structure of snowpacks allows the exchange of gases and particles with the atmosphere.…”

The effect of measurement geometry on recording solar radiation attenuation in snowpack (e-folding depth) using fibre-optic probes

“…All measurements were taken within $2 hours of each other to minimize changes in snowpack temperature and changes in conditions due to snow metamorphism. Further details of the technique, analysis and location are provided by France and others (2011a).…”

“…A snow pit $1.5 m  1 m  1 m was dug in this snowpack, and the in-snow irradiance measured using a custom-built six-spectrometer instrument described in detail by France and others (2011a). Each spectrometer was connected to a fibre-optic probe, and each fibre optic has a Lambertian diffusing optic fitted.…”

“…Values of e-folding depths are frequently reported in the characterization of snow optical properties with typical values of 10-20 cm in the ultraviolet/visible wavelength range for Antarctic plateau near-surface snow (France and others, 2011a). It has been inferred by Warren and others (2006) that the measurement of e-folding depth may depend upon the geometry of the measurement.…”