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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/acp-2018-757
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-757
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 22 Aug 2018

Research article | 22 Aug 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

pH-Dependent production of molecular chlorine, bromine, and iodine from frozen saline surfaces

John W. Halfacre1, Paul B. Shepson2,3, and Kerri A. Pratt4 John W. Halfacre et al.
  • 1Department of Chemistry, Indiana University Southeast, New Albany, IN, USA
  • 2Department of Chemistry, Purdue University, West Lafayette, IN, USA
  • 3Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
  • 4Department of Chemistry and Earth & Environmental Sciences, University of Michigan, Ann Arbor, MI, USA

Abstract. The mechanisms of molecular halogen productions from frozen saline surfaces remain incompletely understood, limiting our ability to predict atmospheric oxidation and composition in the polar regions. In this laboratory study, condensed-phase hydroxyl radicals (OH) were photochemically generated in frozen saltwater solutions that mimicked the ionic composition of ocean water. These hydroxyl radicals were found to oxidize Cl, Br, and I, leading to the release of Cl2, Br2, and I2. This finding is consistent with mechanisms proposed from recent Arctic field studies for observed snowpack molecular halogen production. At moderately acidic pre-freezing pH (buffered between 4.5–4.8), irradiation of ice surfaces containing OH-precursors produced elevated amounts of I2. Subsequent addition of O3 produced additional I2, as well as low amounts of Br2. At lower pH (1.7–2.2), substantial photochemical production of Br2 was observed, following rapid dark conversion of I to I2 via reactions with hydrogen peroxide or nitrite. Exposure to O3 under these low pH conditions also increased production of Br2 and I2, possibly through surfaced-based reactions with O3, or the production and heterogeneous recycling of gas-phase HOBr and HOI. Our results suggest the observed products are dependent on the relative concentrations of halides at the ice surface. Finally, photochemical production of Cl2 was only observed when the irradiated sample was composed of high-purity NaCl and hydrogen peroxide (acting as the OH precursor) at low pH (~1.8). While OH was shown to produce Cl2 in this study, kinetics calculations suggest that heterogeneous recycling chemistry may be equally or more important for Cl2 production in the Arctic atmosphere.

John W. Halfacre et al.
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Short summary
In this study, we found that a chemical called hydroxyl radical can help create chlorine, bromine, and iodine (i.e., halogens) from acidic frozen imitation seawater. Even more halogens are created if we also add ozone. This result helps our understanding of how halogens are released from the frozen Arctic ice and snow into the atmosphere, where they alter how well the atmosphere cleans itself of pollution.
In this study, we found that a chemical called hydroxyl radical can help create chlorine,...
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