OH-Initiated Release of Br2 from Sea-Salt Particles

The formation of reactive halogen species such as Br₂ in sea-salt particles has significant effects upon the chemistry of ozone, sulfur dioxide, and mercury in the marine boundary layer (MBL). Past work has shown that an autocatalytic cycle involving gaseous HOBr reacting with particulate bromide can be an important source of Br₂. Our goal in this work was to use a newly developed experimental technique to examine whether hydroxyl radical (úOH) can also lead to Br₂ formation. To examine this question we monitored the formation of reactive halogen species (X, X₂^-, X₂, and HOX, where X = Br or Cl) in illuminated aqueous halide solutions containing HOOH as a photochemical source of OH. There are two key results from our experiments. First, we have found that the reaction of dibromide radical anion (Br₂^-) with hydroperoxyl radical (HO₂) produces Br₂. This is in agreement with early studies of this reaction, but is in contrast to most current numerical models, which list Br^- as the product. Allowing this reaction to produce Br₂ rather than Br^- greatly enhances the rate of OH-initiated Br₂ release. Our second major finding is that oxidized chloride species (such as Cl₂^-) formed from the reaction of OH with Cl^- can react with bromide to ultimately form Br₂. A key intermediate in some of these mixed halide radical reactions appears to be BrCl^- (bromide chloride radical anion), a species that has been largely overlooked in the past. Putting these new findings into a model of sea-salt aerosol chemistry indicates that hydroxyl radical oxidation of bromide leads to a pH-dependent release of Br₂ from the particles. The rates of Br₂ release from this OH-initiated chemistry are comparable to the release rates due to the HOBr(g) autocatalytic cycle, and are large enough to influence chemistry in the MBL.