This paper considers the effect of heterogeneous bromine reactions on stratospheric photochemistry. We have considered reactions on both sulfate aerosols and on polar stratospheric clouds (PSCs). It is shown that the hydrolysis of BrONO2 on sulfate aerosols enhances the HOBr concentration, which in turn enhances the OH and HO2 concentrations, thereby reducing the HCl lifetime and concentration. The hydrolysis of BrONO2 leads to a nighttime production of HOBr, making HOBr a major nighttime bromine reservoir. The photolysis of HOBr gives a rapid increase in the OH and HO2 concentration at dawn, as was recently observed by Salawitch et al. . The increase in the OH and HO2 concentration, and the decrease in the HCl concentration, leads to additional ozone depletion at all latitudes and for all season. At temperatures below 210 K the bulk phase reaction of HOBr with HCl in sulfate aerosols becomes important. The most important heterogeneous bromine reactions on polar stratospheric clouds are the mixed halogen reactions of HCl with HOBr and BrONO2 and of HBr with HOCl and ClONO2.
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