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

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 ere found to oxidize Cl-, Br-, and I-, leading to the release of Cl-2, Br-2, I-2, 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 I-2. Subsequent addition of O-3 produced additional I-2, as well as small amounts of Br-2. At lower pH (1.7-2.2) and in the presence of an OH precursor, rapid dark conversion of I- to I-2 occurred from reactions with hydrogen peroxide or nitrite, followed by substantial photochemical production of Br-2 upon irradiation. Exposure to O-3 under these low pH conditions also increased production of Br-2 and I-2; this likely results from direct O-3 reactions with halides, as well as the production of gas-phase HOBr and HOI that subsequently diffuse to frozen solution to react with Br- and I-. Photochemical production of Cl-2 was only observed when the irradiated sample was composed of high-purity NaCl and hydrogen peroxide (acting as the OH precursor) at pH = 1.8. Though condensed-phase OH was shown to produce Cl-2 in this study, kinetics calculations suggest that heterogeneous recycling chemistry may be equally or more important for Cl-2 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.