STRATOSPHERIC EFFECTS OF BROMINE ACTIVATION ON IN SULFATE AEROSOL

Heterogeneous reactions on/in stratospheric sulfate aerosol and polar stratospheric clouds are important in the photochemical balance of the stratosphere. Recent laboratory measurements indicate that heterogene ous reactions with bromine compounds are possible under stratospheric conditions. We have used a box photochemical model of the stratosphere with a detailed heterogeneous module to evaluate the effect of the he terogeneous reactions of the inorganic bromine for background and volc anic conditions at 60 degrees S and 50 mbar. For the conditions studie d the most important reaction is the hydrolysis of BrONO2 on the aeros ol, especially for volcanic conditions. It results in the indirect inc rease of the ClOx, fraction due to the suppression of NOx (thus reduci ng ClONO2) and the direct release of ClOx, due to an additional consum ption of HCl. The NOx decrease is pronounced going into winter and lea ds to a ClO activation to about 0.5 parts per billion by volume (ppbv) 1 to 2 weeks earlier than without the heterogeneous reactions of brom ine species. Due to the temperature sensitivity of the solubility of H Cl, the reaction HOBr + HCl(a) --> BrCl + H2O results in direct activa tion of ClO at temperatures below similar to 205 K. The enhanced proce ssing for the bromine reactions leads to an additional increase simila r to 30% in ozone depletion for volcanic conditions during the winter, The stratospheric effects of the reactions ClONO2 + HBr(a) --> BrCl HNO3, N2O5 + HBr(a) --> BrNO2 + HNO3 and HOBr + HBr(a) --> Br-2 + H2O are limited even under volcanic conditions due to slow gas phase prod uction rate of HBr.