3-D Modeling of Bromine Chemistry and Its Impacts on Ozone and Mercury in the Arctic Boundary Layer

Gas-phase bromine radical chemistry is the main driver for the frequent and concurrent depletion of ozone and mercury (Hg) from surface air in polar regions during the spring. Sea ice and its overlying snow cover are broadly understood as the key elements in the production of reactive bromine in polar spring. However, a full characterization remains on how physicochemical states of snow and ice influence the release of bromine into the atmosphere. Uncertainties in the kinetics and reaction mechanisms of Hg redox chemistry add further complexity to an accurate assessment of the behavior of Hg during its depletion from air. Three-dimensional (3-D) models have been developed to simulate the impact of bromine chemistry on Hg oxidation at both global and arctic-basin regional scales. However, thus far, such models have relied upon simplified and indirect representations of the release of gaseous bromine from snow/ice, its photochemical transformation to radical species and eventual deposition from the polar atmosphere. Within Environment and Climate Change Canada's operational air-quality model, GEM-MACH, we have developed a process-oriented representation for the coupled bromine-ozone-mercury chemistry and the exchange of bromine, ozone and mercury species between air and snow/ice surface. The model is run at 15-km horizontal resolution in a limited-area domain of the Arctic and is capable of capturing the evolution of high BrO columns associated with synoptic weather disturbances during polar sunrise as can be seen from satellite. The concurrent depletion of ozone and Hg is simulated by consistent model formulations, where the release of reactive bromine from the frozen surfaces is facilitated by the presence of ozone in air. We will show and discuss the impact of using our process-oriented representation of bromine and Hg chemistry on the spatial and temporal patterns of deposition of oxidized Hg during depletion events and as seasonal averages.