Abstract. Even though the specific surface area (SSA) and the snow area index (SAI) of snow are crucial variables to determine the chemical and climatic impact of the snow cover, few data are available on the subject. We propose here a novel method to measure snow SSA and SAI. It is based on the measurement of the hemispherical infrared reflectance of snow samples using the DUFISSS instrument (DUal Frequency Integrating Sphere for Snow SSA measurement). DUFISSS uses the 1310 or 1550 nm radiation of laser diodes, an integrating sphere 15 cm in diameter, and InGaAs photodiodes. For SSA<60 m 2 kg −1 , we use the 1310 nm radiation, reflectance is between 15 and 50% and the accuracy of SSA determination is 10%. For SSA>60 m 2 kg −1 , snow is usually of low density (typically 30 to 100 kg m −3 ), resulting in insufficient optical depth and 1310 nm radiation reaches the bottom of the sample, causing artifacts. The 1550 nm radiation is therefore used for SSA>60 m 2 kg −1 . Reflectance is then in the range 5 to 12% and the accuracy on SSA is 12%. We propose empirical equations to determine SSA from reflectance at both wavelengths, with that for 1310 nm taking into account the snow density. DUFISSS has been used to measure the SSA of snow and the SAI of snowpacks in polar and Alpine regions.

Abstract. The energy budget and the photochemistry of a snowpack depend greatly on the penetration of solar radiation in snow. Below the snow surface, spectral irradiance decreases exponentially with depth with a decay constant called the asymptotic flux extinction coefficient. As with the albedo of the snowpack, the asymptotic flux extinction coefficient depends on snow grain shape. While representing snow by a collection of spherical particles has been successful in the numerical computation of albedo, such a description poorly explains the decrease of irradiance in snow with depth. Here we explore the limits of the spherical representation. Under the assumption of geometric optics and weak absorption by snow, the grain shape can be simply described by two parameters: the absorption enhancement parameter B and the geometric asymmetry factor g G . Theoretical calculations show that the albedo depends on the ratio B/(1−g G ) and the asymptotic flux extinction coefficient depends on the product B(1 − g G ). To understand the influence of grain shape, the values of B and g G are calculated for a variety of simple geometric shapes using ray tracing simulations. The results show that B and (1 − g G ) generally covary so that the asymptotic flux extinction coefficient exhibits larger sensitivity to the grain shape than albedo. In particular it is found that spherical grains propagate light deeper than any other investigated shape. In a second step, we developed a method to estimate B from optical measurements in snow. A multi-layer, two-stream, radiative transfer model, with explicit grain shape dependence, is used to retrieve values of the B parameter of snow by comparing the model to joint measurements of reflectance and irradiance profiles. Such measurements were performed in Antarctica and in the Alps yielding estimates of B between 0.8 and 2.0. In addition, values of B were estimated from various measurements found in the literature, leading to a wider range of values (1.0-9.9) which may be partially explained by the limited accuracy of the data. This work highlights the large variety of snow microstructure and experimentally demonstrates that spherical grains, with B = 1.25, are inappropriate to model irradiance profiles in snow, an important result that should be considered in further studies dedicated to subsurface absorption of short-wave radiation and snow photochemistry.

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.