Activation of stratospheric chlorine by reactions in liquid sulphuric acid

Active chlorine release on H 2 SO 4 aerosol particles via the reaction of HOCl and HCl in solution is discussed. Based on current laboratory data, the process is shown to have an appreciable rate at temperatures below about 195 K, when the aerosol particles become sufficiently dilute to allow both HCl dissolution and fast ClONO 2 hydrolysis. Evolution of part‐per‐billion concentrations of Cl 2 could be significant in cases in which volcanically enhanced aerosol remains liquid until the frost point (≈ 190 K) as observed on several occasions in the Arctic stratosphere.