Work

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

Year: 2019

Type: article

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.

Source: Atmospheric chemistry and physics

DOI: 10.5194/acp-19-4917-2019

Authors: John W. Halfacre, Paul B. Shepson, Kerri A. Pratt

Institutions: Indiana University Southeast, Purdue University West Lafayette, Stony Brook University, University of Michigan

Language: English

Topic: Atmospheric chemistry and aerosols

Subfield: Atmospheric Science

Field: Earth and Planetary Sciences

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $1,725

Funders: National Science Foundation, Purdue University