Abstract: Abstract. Stable Isotope Ratios Of Nitrate Preserved In Deep Ice Cores Are Expected To Provide Unique And Valuable Information Regarding Paleoatmospheric Processes. However, Due To The Post-depositional Loss Of Nitrate In Snow, This Information May Be Erased Or Significantly Modified By Physical Or Photochemical Processes Before Preservation In Ice. We Investigated The Role Of Solar Uv Photolysis In The Post-depositional Modification Of Nitrate Mass And Stable Isotope Ratios At Dome C, Antarctica, During The Austral Summer Of 2011/2012. Two 30 Cm Snow Pits Were Filled With Homogenized Drifted Snow From The Vicinity Of The Base. One Of These Pits Was Covered With A Plexiglass Plate That Transmits Solar Uv Radiation, While The Other Was Covered With A Different Plexiglass Plate Having A Low Uv Transmittance. Samples Were Then Collected From Each Pit At A 2–5 Cm Depth Resolution And A 10-day Frequency. At The End Of The Season, A Comparable Nitrate Mass Loss Was Observed In Both Pits For The Top-level Samples (0–7 Cm) Attributed To Mixing With The Surrounding Snow. After Excluding Samples Impacted By The Mixing Process, We Derived An Average Apparent Nitrogen Isotopic Fractionation (15ϵapp) Of −67.8 ± 12 ‰ For The Snow Nitrate Exposed To Solar Uv Using The Nitrate Stable Isotope Ratios And Concentration Measurements. For The Control Samples In Which Solar Uv Was Blocked, An Apparent Average 15ϵapp Value Of −12.0 ± 1.7 ‰ Was Derived. This Difference Strongly Suggests That Solar Uv Photolysis Plays A Dominant Role In Driving The Isotopic Fractionation Of Nitrate In Snow. We Have Estimated A Purely Photolytic Nitrogen Isotopic Fractionation (15ϵphoto) Of −55.8 ± 12.0 ‰ From The Difference In The Derived Apparent Isotopic Fractionations Of The Two Experimental Fields, As Both Pits Were Exposed To Similar Physical Processes Except Exposure To Solar Uv. This Value Is In Close Agreement With The 15ϵphoto Value Of −47.9 ± 6.8 ‰ Derived In A Laboratory Experiment Simulated For Dome C Conditions (Berhanu Et Al., 2014). We Have Also Observed An Insensitivity Of 15ϵ With Depth In The Snowpack Under The Given Experimental Setup. This Is Due To The Uniform Attenuation Of Incoming Solar Uv By Snow, As 15ϵ Is Strongly Dependent On The Spectral Distribution Of The Incoming Light Flux. Together With Earlier Work, The Results Presented Here Represent A Strong Body Of Evidence That Solar Uv Photolysis Is The Most Relevant Post-depositional Process Modifying The Stable Isotope Ratios Of Snow Nitrate At Low-accumulation Sites, Where Many Deep Ice Cores Are Drilled. Nevertheless, Modeling The Loss Of Nitrate In Snow Is Still Required Before A Robust Interpretation Of Ice Core Records Can Be Provided.

Abstract: 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 gG. Theoretical calculations show that the albedo depends on the ratio B/(1-gG) and the asymptotic flux extinction coefficient depends on the product B(1-gG). To understand the influence of grain shape, the values of B and gG are calculated for a variety of simple geometric shapes using ray tracing simulations. The results show that B and (1-gG) 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.