Stratospheric effects of bromine activation on/in sulfate aerosol

Abstract

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 heterogeneous 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 heterogeneous reactions of the inorganic bromine for background and volcanic conditions at 60°S and 50 mbar. For the conditions studied the most important reaction is the hydrolysis of BrONO 2 on the aerosol, especially for volcanic conditions. It results in the indirect increase of the ClO χ fraction due to the suppression of NO χ (thus reducing ClONO 2 ) and the direct release of ClO χ due to an additional consumption of HCl. The NO χ decrease is pronounced going into winter and leads 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 bromine species. Due to the temperature sensitivity of the solubility of HCl, the reaction HOBr + HCl(a) → BrCl + H 2 O results in direct activation of ClO at temperatures below ∼205 K. The enhanced processing for the bromine reactions leads to an additional increase ∼30% in ozone depletion for volcanic conditions during the winter. The stratospheric effects of the reactions ClONO 2 + HBr(a) → BrCl + HNO 3 , N 2 O 5 + HBr(a) → BrNO 2 + HNO 3 and HOBr + HBr(a) → Br 2 + H 2 O are limited even under volcanic conditions due to slow gas phase production rate of HBr.