Simulating halogen radical chemistry and Br propagation during ozone depletion events in Barrow, Alaska

The springtime depletion of tropospheric ozone in the Arctic is believed to be caused by active halogen photochemistry resulting from halogen atom precursors present on snow, ice, or aerosol surfaces. The role of bromine in driving ozone depletion events (ODEs) has been generally accepted from numerous field studies that have observed high concentrations of BrO and filterable bromide during this time. The presence of chlorine in the Arctic has been recognized, but much less is known about the role of chlorine radicals in ozone depletion chemistry. Iodine monoxide has yet to be successfully detected in the High Arctic, although there have been indications of active iodine chemistry through observed enhancements in filterable iodide and probable detection of IO. Despite decades of research, significant uncertainty remains regarding the chemical mechanisms associated with the bromine-catalyzed depletion of ozone, as well as the complex interactions that occur in the polar boundary layer due to halogen chemistry. We developed a 0-D, multiphase, photochemical model to investigate the chemistry of bromine, chlorine and iodine relating to the occurrence of ODEs. Our model is highly constrained to time-varying observations of O3, Cl2, Br2, OVOCs, and VOCs from the 2009 Ocean-Atmosphere-Sea Ice-Snowpack (OASIS) campaign in Barrow, Alaska. We investigated a 7-day period in late March to determine the contribution of Br, Cl, and potential contribution of I to ozone depletion and the interactions occurring between these three halogens under the chemical conditions observed. We find that while Br accounts for the majority of ozone depletion, iodine is more efficient on a per molecule basis and that both chlorine and iodine serve to enhance the Br-induced depletion of ozone through synergistic effects. Though Cl does not directly contribute significantly to ozone depletion, chlorine impacts bromine chemistry through ClO and RO2, which in turn impact BrOx propagation, and by impacting the HOx and NOx partitioning. A significant finding of this work is the relatively small gas-phase Br chain length, suggesting that the heterogeneous recycling/production of Brx (via Br2 and BrCl production) is critically important to the evolution of ODEs, and that estimations of ozone depletion rate based on gas-phase propagation reactions may significantly underestimate the rate and timescale of ozone depletion. Because of the importance of these surface processes in driving ODEs, the changing conditions of the Arctic could have significant impacts on the tropospheric chemistry and radiative properties of the polar boundary layer.