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

Halfacre, John W.; Shepson, Paul B.; Pratt, Kerri A.

The mechanisms of molecular halogen production from frozen saline surfaces remain incompletely understood, limiting our ability to predict atmospheric oxidation and composition in 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 inline-formulaCl, inline-formulaBr, and inline-formulaI, leading to the release of inline-formulaCl2, inline-formulaBr2, inline-formulaI2, and IBr. At moderately acidic pH (buffered between 4.5 and 4.8), irradiation of ice containing OH precursors (either of hydrogen peroxide or nitrite ion) produced elevated amounts of inline-formulaI2. Subsequent addition of inline-formulaO3 produced additional inline-formulaI2, as well as small amounts of inline-formulaBr2. At lower pH (1.7–2.2) and in the presence of an OH precursor, rapid dark conversion of inline-formulaI to inline-formulaI2 occurred from reactions with hydrogen peroxide or nitrite, followed by substantial photochemical production of inline-formulaBr2 upon irradiation. Exposure to inline-formulaO3 under these low pH conditions also increased production of inline-formulaBr2 and inline-formulaI2; this likely results from direct inline-formulaO3 reactions with halides, as well as the production of gas-phase HOBr and HOI that subsequently diffuse to frozen solution to react with inline-formulaBr and inline-formulaI. Photochemical production of inline-formulaCl2 was only observed when the irradiated sample was composed of high-purity NaCl and hydrogen peroxide (acting as the OH precursor) at pH inline-formula= 1.8. Though condensed-phase OH was shown to produce inline-formulaCl2 in this study, kinetics calculations suggest that heterogeneous recycling chemistry may be equally or more important for inline-formulaCl2 production in the Arctic atmosphere. The condensed-phase OH-mediated halogen production mechanisms demonstrated here are consistent with those proposed from recent Arctic field observations of molecular halogen production from snowpacks. These reactions, even if slow, may be important for providing seed halogens to the Arctic atmosphere. Our results suggest the observed molecular halogen products are dependent on the relative concentrations of halides at the ice surface, as we only observe what diffuses to the air–surface interface.

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Halfacre, John W. / Shepson, Paul B. / Pratt, Kerri A.: pH-dependent production of molecular chlorine, bromine, and iodine from frozen saline surfaces. 2019. Copernicus Publications.

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