Formation of Bromine from Deliquesced Sodium Bromide Aerosol in the Presence of Ozone and UV Light

The formation of gaseous bromine has been reported at both high and midlatitudes and is believed to originate from the oxidation of bromide ions in sea salt deposited on ice or in particles. However, the mechanism of its formation is not well understood. We report laboratory studies of oxidation of deliquesced sodium bromide particles in a 561 L aerosol chamber in the absence and presence of UV radiation as well as kinetics modeling of this system. Sodium bromide particles were generated in air at relative humidities at or above 65% RH. Addition of gaseous ozone followed by 254 nm irradiation to produce OH resulted in formation of BrO radicals and Br₂. A comprehensive kinetics model, which couples gas and aqueous phase chemistry as well as mass transfer of species, was used to understand the rate-limiting steps responsible for bromine formation under different experimental conditions. A sensitivity analysis on the model shows that the rate-limiting steps can vary when the available particle surface area and volume change, changing the availability of bromide in the system. The role of interfacial processes and atmospheric implications will be discussed.