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