Phototransformation of Vanillin in Artificial Snow by Direct Photolysis and Mediated by Nitrite

The photodegradation of vanillin, as a proxy of methoxyphenols emitted by biomass burning, was investigated in artificial snow at 243 K and in liquid water at room temperature. Nitrite (NO2-) was used as a photosensitizer of reactive oxygen and nitrogen species under UVA light, because of its key photochemical role in snowpacks and atmospheric ice/waters. In snow and in the absence of NO2-, slow direct photolysis of vanillin was observed due to back-reactions taking place in the quasi-liquid layer at the ice-grain surface. The addition of NO2- made the photodegradation of vanillin faster, because of the important contribution of photoproduced reactive nitrogen species in vanillin phototransformation. These species triggered both nitration and oligomerization of vanillin in irradiated snow, as the identified vanillin by-products showed. Conversely, in liquid water, direct photolysis was the main photodegradation pathway of vanillin, even in the presence of NO2-, which had negligible effects on vanillin photodegradation. The results outline the different role of iced and liquid water in the photochemical fate of vanillin in different environmental compartments.