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

Abstract

Uptake of BrONO 2 onto submicron sulfuric acid aerosol was studied in a cylindrical flow reactor coupled to a chemical ionization mass spectrometer. On aerosol particles of ∼0.1 μm radius, the reaction of BrONO 2 with H 2 O to produce HOBr and HNO 3 is very efficient, with the reaction probability γ ≈ 0.8 for H 2 SO 4 ‐content ≤ 70 wt%. The equilibrium constant for the gas phase process, BrONO 2 + H 2 O ⇄ HOBr + HNO 3 , was measured and the heats of formation of the bromine compounds were deduced. The conversion of HOBr back into BrONO 2 via reaction with HNO 3 can be neglected in the stratosphere. The heterogeneous hydrolysis of BrONO 2 , followed by photolysis of the HOBr product, is a major source of HO x in the lower stratosphere. The role of bromine as an activator of chlorine is also discussed.