Development of a mechanism for nitrate photochemistry in snow

A reaction mechanism to reproduce photochemical processes in the snow is reported. We developed a box model to represent snow chemistry. Constrained by laboratory experiments carried out with artificial snow, we deduced first a reaction mechanism for N-containing species including 13 reactions. An optimization tool was developed to adjust systematically unknown photolysis rates of nitrate and nitrite (NO2−) and transfer rates of nitrogen oxides from the snow to the gas phase resulting in an optimum fit with respect to the experimental data. Further experiments with natural snow samples are presented, indicating that NO2− concentrations were much lower than in the artificial snow experiments. These observations were used to extend the reaction mechanism into a more general scheme including hydrogen peroxide (H2O2) and formaldehyde (HCHO) chemistry leading to a set of 18 reactions. The simulations indicate the importance of H2O2 and HCHO as either a source or sink of hydroxyl radicals in the snow photochemistry mechanism. The addition of H2O2 and HCHO in the mechanism allows the reproduction of the observed low NO2− concentration.