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Production and Release of Molecular Bromine and Chlorine from the Arctic Coastal Snowpack

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journal contribution
posted on 12.04.2017, 00:00 by K. D. Custard, A. R. W. Raso, P. B. Shepson, R. M. Staebler, K. A. Pratt
Atmospheric bromine and chlorine atoms have a significant influence on the pathways of atmospheric chemical species processing. The photolysis of molecular halogens and subsequent reactions with ozone, mercury, and hydrocarbons are common occurrences in the Arctic boundary layer during spring, following polar sunrise. While it was recently determined that Br2 is released from the sunlit surface snowpack, the source(s) and mechanisms of Cl2 and BrCl production have remained unknown. Current efforts to model Arctic atmospheric composition are limited by the lack of knowledge of the sources and emission rates of these species. Here, we present the first simultaneous direct measurements of Br2, Cl2, and BrCl in snowpack interstitial air, as well as the first measured emission rates of Br2 and Cl2 out of the snowpack into the atmosphere. Using chemical ionization mass spectrometry, Br2, Cl2, and BrCl were observed to be produced within the tundra surface snowpack near Utqiaġvik, AK, during Feb 2014, following both artificial and natural irradiation, consistent with a photolytic production mechanism. Maximum Cl2 and Br2 fluxes from the snowpack to the overlying atmosphere were quantified and reached maxima at mid-day during peak radiation. In-snowpack Br2 and BrCl production was enhanced, with Cl2 production reduced, at air temperatures below the eutectic point for the formation of NaCl·2H2O, suggesting limited chloride availability, as compared to production at air temperatures above this eutectic point. These new observations improves the ability of the community to simulate Arctic boundary layer composition and pollutant fate.