Mechanism for intense photochemical NOx fluxes from ice surfaces

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Photolysis of snow-bound nitrates has been identified as the source for intense photochemical NOx fluxes that have been observed to emanate for the sunlit snow pack in Antarctica, Greenland, the Canadian arctic and many other permanently or seasonally snow covered regions. Numerical models have failed to provide an adequate interpretation for this phenomenon which controls the oxidative potential of the polar boundary layer. This highlights our poor understanding of the mechanisms at the molecular level and the lack of appropriate physical parameters to describe the heterogeneous photolysis rates of nitrates in ice. Furthermore, there are several indications that there might be a considerable bias favouring photoproducts from nitrates adsorbed or segregated to the surface of snow grains. We propose to investigate the factors that control the photolysis rate at ice surfaces and quantify their surface specificity. Using thin ice film surrogates, we will investigate the most important physical parameters in an attempt to provide a molecular-level mechanistic description of this phenomenon. This will enable a quantitative evaluation of the photochemical fluxes and improve the interpretation of climate proxies from ice cores. These quantitative laboratory-based investigations are important and timely at the dawn of the International Polar Year 2007-2008.