The interplay between snow and polluted air masses in cold urban environments

The role of persistent snow covers in wintertime urban air pollution chemistry remains largely unexplored. The interactions of chemistry and transport processes are complex and the physicochemical structure of snow is uncertain. For instance, it is still unclear to what extent uptake and chemistry occur on ice, a disordered interface layer on the ice, or in brine pockets at grain boundaries. We use a process-based one-dimensional coupled atmosphere-snow model to gain initial insight into the interaction of snow with high concentrations of SO₂ and NO₂ in polluted wintertime Fairbanks, AK, USA. Snow can act as a reservoir for both gases, allowing for fluxes into the snow (during polluted periods) and out of the snow (during cleaner periods). The geometrical distribution of liquid on ice is varied to approximate the conceptual difference between the disordered ice interface and brine in localized pockets. The behavior of SO₂ is more sensitive to these differences, mostly due to its greater stickiness on ice and solubility in water compared to NO₂, which remains mostly in the snow interstitial air. Liquid-phase chemical processing of both compounds is almost insensitive to the distribution of the liquid phase in the snow and mostly determined by the volume of liquid. Our study highlights the value of comprehensive process-based modeling to further our understanding of snow chemistry. Our model platform can serve as a tool to inform and support future research efforts on improving our understanding of the liquid content of snow, chemical processing on ice surfaces, and, in general, the influence of snow on atmospheric chemistry.