The importance of heterogeneous bromine chemistry in the lower stratosphere

Abstract

Heterogeneous conversion of bromine reservoirs (BrONO2, HOBr) on the surface of aerosol particles in the lower stratosphere has limited effect on the concentrations of reactive bromine radicals (Br, BrO). However, the heterogeneous hydrolysis of BrONO2 on sulfate aerosol particles, which produces HOBr, leads to the enhancement of the abundance of hydroxyl radicals, and consequently modifies the partitioning between inorganic chlorine compounds. Further, because of the enhanced abundance of HOBr, the heterogeneous reaction between HOBr and HCl(s) converts significant amounts of inactive chlorine (HCl) into reactive chlorine (ClO), especially during post‐volcanic periods. These processes could partially explain the unexpected high ClO/HCl and ClONO2/HCl concentration ratios at 20 km reported after the eruption of Mt. Pinatubo (1991), and could have contributed to the substantial ozone depletion observed after the large volcanic eruptions of the 1980's and 1990's. Our model simulations suggest that, owing to the heterogeneous hydrolysis of BrONO2 and the reaction HOBr+HCl(s) on sulfate aerosols, the increase in total bromine loading from 1960 (12 pptv) to 1990 (21 pptv) has led to enhanced concentrations of ClO, and consequently to an ozone depletion of 2–3% in the lower stratosphere at mid and high latitudes under post‐volcanic conditions.