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Laboratory study of nitrate photolysis in Antarctic snow: quantum yield, domain of photolysis, isotope effects and wavelength dependence

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Carl Meusinger1, Tesfaye A. Berhanu2, Joseph Erbland2, Joel Savarino2 and Matthew S. Johnson1 #1 Department of Chemistry, University of Copenhagen, Copenhagen, Denmark #2 LGGE (UMR5183), CNRS /Univ. Grenoble Alpes, Grenoble, France

Post-depositional processes alter nitrate concentration and nitrate isotopic composition in the top layers of snow at sites with low snow accumulation rates, such as Dome C, Antarctica. Available nitrate ice core records can provide input for studying past atmospheres and climate if such processes are understood. It has been shown that photolysis of nitrate in the snowpack plays a major role in nitrate loss. For unclear reasons the range of reported quantum yields for the main reaction spans orders of magnitude, constituting the largest uncertainty in models of snowpack NOx emissions. Here a laboratory study is presented that uses snow from Dome C and minimizes effects of desorption and recombination by flushing the snow with pure N2 at water vapor equilibrium during irradiation with UV light from a Xenon lamp. A selection of UV filters allowed examination of the effects of the 200 and 300 nm absorption bands of nitrate and to emulate actinic fluxes similar to those in Dome C. Irradiated snow was sampled in 1 cm sections and analyzed for nitrate concentration and isotopic composition (δ15N, δ18O and Δ17O); the actinic flux was measured at similar sections in the snow. The quantum yield was observed to decrease with increasing exposure to UV radiation. Observed values for the quantum yield lie in the middle of the range of previously reported values. The superposition of photolysis in two photochemical domains of nitrate in snow is proposed: one of photolabile nitrate and one of trapped or buried nitrate. The difference lies in the ability of reaction products to escape the snow crystal, versus undergoing secondary (recombination) chemistry. Modeled NOx emissions may be increased significantly due to the observed quantum yield in this study influencing predicted boundary layer chemistry significantly including ozone concentrations. For the tested snow, the quantum yield changes from 0.44 to 0.05 within what corresponds to weeks of UV exposure in Antarctica. An average photolytic isotopic fractionation of 15ε = -15 ± 1.2 ‰ was found for the experiments without a wavelength filter. These results are ascribed to excitation of the intense absorption band of nitrate around 200 nm. An experiment with a filter blocking wavelengths shorter than 320 nm, approximating the actinic flux spectrum at Dome C, showed a photolytic fractionation constant of 15ε = -47.9 ± 6.8 ‰ in good agreement with the fractionation determined for the East Antarctic Plateau ranging from -40 to -74.3 ‰. The isotopic fractionations obtained from this study are compared to theoretical estimates derived by applying the zero point energy shift model coupled with measured actinic fluxes at each depth. The results confirm that the photolytic fractionation of nitrate isotopes in snow is very sensitive to the actinic flux spectrum and indicate limitations in previous laboratory studies. This work demonstrates that the spectrum of the excitation source is a key parameter determining nitrogen isotope fractionation in the photolysis of nitrate in snow.

This talk is part of the BAS Chemistry & Past Climate Seminars series.

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