Heterogeneous uptake of ClONO₂ and N₂O₅ by sulfuric acid solutions

Abstract

A droplet train apparatus has been used to measure the heterogeneous reactive uptake of gaseous N 2 O 5 and ClONO 2 by concentrated sulfuric acid solutions. H 2 SO 4 concentrations in the range of 39 to 69 wt% were investigated between 229 and 260 K. Uptake rates normalized to the gas‐liquid collision frequency, γ 0 , for N 2 O 5 ranged from 0.086 to 0.16, decreasing moderately with increasing temperature and decreasing H 2 SO 4 concentration. Uptake rates for ClONO 2 , measured over a slightly narrower concentration range of 39–59 wt% H 2 SO 4 , ranged between 0.0037 and 0.056, decreasing moderately with H 2 SO 4 temperature but significantly with increasing concentration. Results are compared with measurements from other laboratories using different experimental techniques. In general, the data from the different groups agree well. A phenomenological model is presented which addresses the solubility, diffusion, and chemical reactivity of XNO 3 (X=Cl, NO 2 ) in sulfuric acid solutions and accounts for the dependence of the observed uptake rates on H 2 SO 4 concentration and temperature. Two XNO 3 hydrolysis pathways are proposed, one involving direct reaction with H 2 O and the other involving participation of H + ions to promote bond dissociation. Differences between the concentration dependencies of γ 0 for ClONO 2 and N 2 O 5 can be ascribed largely to different rates of acid‐catalyzed hydrolysis. The implications of these results for the effects of lower stratospheric sulfuric acid aerosols on ozone depletion chemistry are discussed.