Release of gaseous bromine from the photolysis of nitrate and hydrogen peroxide in simulated sea-salt solutions

We have carried out a series of laboratory experiments to investigate the oxidation of bromide (Br-) by hydroxyl radical ((OH)-O-center dot) in solutions used to mimic sea-salt particles. Aqueous halide solutions with nitrate or hydrogen peroxide (HOOH) as a photochemical source of (OH)-O-center dot were illuminated with 313 nm light and the resulting gaseous bromine (Br*(g)) was collected. While illumination of these solutions nearly always formed gaseous bromine (predominantly Br-2 based on modeling results), there was no evidence for the release of gaseous chlorine. The rate of Br*(g) release increased (up to a plateau value) with increasing concentrations of bromide and was enhanced at lower pH values for both nitrate and HOOH solutions. Increased ionic strength in nitrate solutions inhibited Br*(g) release and the extent of inhibition was dependent upon the salt used. In HOOH solutions, however, no ionic strength effects were observed and the presence of Cl- strongly enhanced Br*(g) release. Overall, for conditions typical of aged, deliquesced, sea-salt particles, the efficiencies of gaseous bromine release, expressed as mole of Br*(g) released per mole of (OH)-O-center dot photochemically formed, were typically 20-30%. Using these reaction efficiencies, we calculated the Br-2(g) release rate from aged, ambient sea-salt particles due to (OH)-O-center dot oxidation to be approximately 0.07 pptv h(-1) with the main contributions from nitrate photolysis and partitioning of gas-phase (OH)-O-center dot into the particle. While our solution conditions are simplified compared to ambient particles, this estimated rate of Br-2 release is high enough to suggest that (OH)-O-center dot-mediated reactions in sea-salt particles could be a significant source of reactive bromine to the marine boundary layer. (c) 2006 Elsevier Ltd. All rights reserved.