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Release of gaseous bromine from the surface snowpack and the boundary-layer depletion of ozone and mercury in the springtime Arctic: Findings from 1-D and 3-D model simulations


Atmospheric boundary layer (ABL) and its underlying snow cover constitute a multiphase system for reactive compounds to foster their photochemical interactions and mass exchange occurring at the timescale of hours to days. The buildup of reactants can be contained in the shallow ABL under calm weather, whereas, under weather disturbances, the air mass can be transported horizontally afar and lifted vertically above the ABL. To address scientific questions concerning the behavior of bromine, ozone and mercury in the Arctic environment at various spatial and temporal scales of interest, we have developed a suite of models at different complexities in their process representations. PHANTAS is a 1-D multiphase reaction-diffusion model that resolves the different physical and transport properties across the ABL and the porous snowpack while using a detailed chemical mechanism common to both model regimes. This model demonstrates the roles of ozone, sunlight and snow acidity in the production and release of reactive gaseous bromine from the saline snowpack. However, it currently employs an uncertain assumption that the liquid-like layer hosts the reactions on the surface of snow grains, which will likely need to be re-designed in the future with the progress of our understanding of chemistry on the ice surface. 3-D models are better suited to studying the movement of air masses within which the snow-sourced bromine compounds build up and decay under changing meteorological conditions. Environment and Climate Change Canada’s air quality model, GEM-MACH, has been extended with bromine and mercury chemistry along with the parameterization of air-snow bromine exchange. The model runs, performed at 15-km horizontal resolution across the Arctic region, capture the temporal variability of the ozone and mercury concentrations measured at the surface and the spatiotemporal evolution of BrO vertical column densities seen from satellites reasonably well. Observational data from field platforms and satellites are the key tools for evaluating and improving the process representations in such models.