Reaction of BrONO2 with H2O on submicron sulfuric acid aerosol and the implications for the lower stratosphere

Uptake of BrONO2 onto submicron sulfuric acid aerosol was studied in a cylindrical flow reactor coupled to a chemical ionization mass spectrometer. On aerosol particles of similar to 0.1 mu m radius, the reaction of BrONO2 with H2O to produce HOBr and HNO3 is very efficient, with the reaction probability gamma approximate to 0.8 for H2SO4-content less than or equal to 70 wt%. The equilibrium constant for the gas phase process, BrONO2 + H2O reversible arrow HOBr + HNO3, was measured and the heats of formation of the bromine compounds were deduced. The conversion of HOBr back into BrONO2 via reaction with HNO3 can be neglected in the stratosphere. The heterogeneous hydrolysis of BrONO2, followed by photolysis of the HOBr product, is a major source of HOx in the lower stratosphere. The role of bromine as an activator of chlorine is also discussed.