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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...
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Published in: | Atmospheric chemistry and physics 2019-04, Vol.19 (7), p.4917-4931 |
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Main Authors: | , , |
Format: | Article |
Language: | English |
Subjects: | |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | 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 Cl−, Br−, and
I−, leading to the release of Cl2, Br2, I2, 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 I2. Subsequent addition of O3
produced additional I2, as well as small amounts of Br2. At lower
pH (1.7–2.2) and in the presence of an OH precursor, rapid dark conversion
of I− to I2 occurred from reactions with hydrogen peroxide or
nitrite, followed by substantial photochemical production of Br2 upon
irradiation. Exposure to O3 under these low pH conditions also
increased production of Br2 and I2; this likely results from
direct O3 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 Cl2 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 Cl2 in this study, kinetics
calculations suggest that heterogeneous recycling chemistry may be equally
or more important for Cl2 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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ISSN: | 1680-7324 1680-7316 1680-7324 |
DOI: | 10.5194/acp-19-4917-2019 |