Laboratory studies of bromide oxidation in the presence of ozone: Evidence for a glass-surface mediated reaction

The reaction of sodium bromide particles in the presence of ozone was studied in a flow system both under dark conditions and with 254 nm radiation. We found that there was significant formation of gaseous bromine (probably Br(2)) in the presence of ozone in the dark, and that bromide deposited to the walls of the Pyrex reaction flask was its source. The observed rate of gaseous bromine formation in these experiments was approximately 100-1000 times faster than expected based on the known rate constant for aqueous reaction of bromide with ozone. While the mechanism responsible for this enhanced reactivity was not identified, based on previous reports we suggest that the glass surface converted ozone to more reactive species, such as hydroxyl radical, which in turn oxidized bromide. In the presence of 254 nm radiation, rates of gaseous bromine collection were further enhanced, likely as a result of increased radical production in the system, and wall-deposited bromide was also the source of the gaseous bromine. In these `light' experiments, there was a significant decline in ozone mixing ratios, consistent with bromine radical chemistry. These results suggest the possibility that ozone reacting with internally mixed silicate/sea-salt particles might be a significant mechanism for the oxidation of particulate halides, and subsequent release of photoactive halogen species, in the marine boundary layer.