ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-5237-2012 Temperature dependent halogen activation by N<sub>2</sub>O<sub>5</sub> reactions on halide-doped ice surfaces Lopez-Hilfiker F. D. 1 Constantin K. 1 Kercher J. P. 1 2 Thornton J. A. 1 Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA now at: Department of Chemistry, Hiram College, Hiram, Ohio 44234, USA 15 06 2012 12 11 5237 5247 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/5237/2012/acp-12-5237-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/5237/2012/acp-12-5237-2012.pdf

We examined the reaction of N<sub>2</sub>O<sub>5</sub> on frozen halide salt solutions as a function of temperature and composition using a coated wall flow tube technique coupled to a chemical ionization mass spectrometer (CIMS). The molar yield of photo-labile halogen compounds was near unity for almost all conditions studied, with the observed reaction products being nitryl chloride (ClNO<sub>2</sub>) and/or molecular bromine (Br<sub>2</sub>). The relative yield of ClNO<sub>2</sub> and Br<sub>2</sub> depended on the ratio of bromide to chloride ions in the solutions used to form the ice. At a bromide to chloride ion molar ratio greater than 1/30 in the starting solution, Br<sub>2</sub> was the dominant product otherwise ClNO<sub>2</sub> was primarily produced on these near pH-neutral brines. We demonstrate that the competition between chlorine and bromine activation is a function of the ice/brine temperature presumably due to the preferential precipitation of NaCl hydrates from the brine below 250 K. Our results provide new experimental confirmation that the chemical environment of the brine layer changes with temperature and that these changes can directly affect multiphase chemistry. These findings have implications for modeling air-snow-ice interactions in polar regions and likely in polluted mid-latitude regions during winter as well.

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