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1 Administration, Department of Chemistry, Faculty of Science, Københavns Universitet 2 Centre National de la Recherche Scientifique, Aix-Marseille Université 3 Department of Chemistry, Faculty of Science, Københavns Universitet 4 University of Grenoble 5 Academia Sinica, Institute of Astronomy & Astrophysics 6 Department of Chemistry, Faculty of Science, Københavns Universitet 7 Administration, Department of Chemistry, Faculty of Science, Københavns Universitet
II. Isotopic effects and wavelength dependence
Atmospheric nitrate is preserved in Antarctic snow firn and ice. However, at low snow accumulation sites, post-depositional processes induced by sunlight obscure its interpretation. The goal of these studies (see also Paper I by Meusinger et al. [" Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry," J. Chem. Phys. 140, 244305 (2014)]) is to characterize nitrate photochemistry and improve the interpretation of the nitrate ice core record. Naturally occurring stable isotopes in nitrate (15N, 17O, and 18O) provide additional information concerning post-depositional processes. Here, we present results from studies of the wavelength-dependent isotope effects from photolysis of nitrate in a matrix of natural snow. Snow from Dome C, Antarctica was irradiated in selected wavelength regions using a Xe UV lamp and filters. The irradiated snow was sampled and analyzed for nitrate concentration and isotopic composition (δ 15N, δ 18O, and Δ 17O). From these measurements an average photolytic isotopic fractionation of 15ε = (- 15 ± 1.2)‰ was found for broadband Xe lamp photolysis. These results are due in part to excitation of the intense absorption band of nitrate around 200 nm in addition to the weaker band centered at 305 nm followed by photodissociation. An experiment with a filter blocking wavelengths shorter than 320 nm, approximating the actinic flux spectrum at Dome C, yielded a photolytic isotopic fractionation of 15ε = (- 47.9 ± 6.8)‰, in good agreement with fractionations determined by previous studies for the East Antarctic Plateau which range from - 40 to - 74.3‰. We describe a new semi-empirical zero point energy shift model used to derive the absorption cross sections of 14NO3 - and 15NO 3 - in snow at a chosen temperature. The nitrogen isotopic fractionations obtained by applying this model under the experimental temperature as well as considering the shift in width and center well reproduced the values obtained in the laboratory study. These cross sections can be used in isotopic models to reproduce the stable isotopic composition of nitrate found in Antarctic snow profiles. © 2014 Author(s).
Journal of Chemical Physics, 2014, Vol 140, Issue 24
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