Investigation of the photochemical decomposition of nitrate, hydrogen peroxide, and formaldehyde in artificial snow

Species like nitrate (NO3-), hydrogen peroxide (H2O2), and formaldehyde (HCHO) are ubiquitous trace compounds in snow. Photochemical reactions of these compounds in the snow can have important implications for the composition of the atmospheric boundary layer in snow-covered regions and for the interpretation of concentration profiles in snow and ice regarding the composition of the past atmosphere. Therefore, we performed laboratory experiments to investigate such reactions in artificially produced snow samples. Artificial snow samples allow to execute experiments under defined and reproducible conditions and to investigate single reactions. All reactions were carried out under comparable experimental conditions and indicated that the photolysis of H2O2 and NO3- occurred equally fast, while the photolysis of HCHO was considerably slower. Moreover, the photolysis of HCHO was only observed if initial concentrations were much higher than found in natural snow samples. These results indicate that the H2O2 and NO3- reactions are possibly equally important in natural snow covers regarding the formation of OH radicals, while the photolysis of HCHO is probably negligible. Nitrite (NO2-) was observed as one of the products of the NO3- photolysis; however, it was itself photolyzed at a higher rate than NO3-. After a certain photolysis period (>= 18 h) the NO3- and NO2- concentrations in the snow remained constant at a level of 10% of the initial nitrogen content. This is probably due to a recycling of the anions from nitrogen oxides in the gas phase of the reaction cells indicating that the chemical reactions occur in or near the surface layer of the snow crystals. (c) 2005 Elsevier B.V. All rights reserved.