The reaction probabilities of ClONO2 and N2O5 on polar stratospheric cloud materials

Abstract

The reaction probabilities, γ, of ClONO2 and N2O5 on ice and nitric acid trihydrate (NAT) surfaces were determined, using reactant concentrations that are typical of the lower stratosphere, by measuring the first‐order reactant loss rate coefficients over the substrate located on the walls of a fast‐flow reactor. Reactants were detected using chemical ionization mass spectrometry, a sensitive technique which allows the use of low reactant concentrations. The reaction probabilities obtained for ClONO2 are: 0.3 (+0.7−0.1) on pure ice, 0.006 (±30%) over a NAT surface, and 0.3 (+0.7−0.1) on an HCl‐doped NAT surface. Those for N2O5 are: 0.024 (±30%) on pure ice, 0.0006 (±50%) on a NAT surface, and 0.0032 (±30%) on an HCl‐doped NAT surface. We observed that an ice surface will be converted into a less reactive HNO3‐doped ice surface in a relatively short time, and we present arguments that this surface consisted of a NAT layer. The large differences between our results and previous measurements for some of these γ can be attributed to the relatively large reactant concentrations used in the previous work. The major implications of this work for polar stratospheric chlorine activation are: an efficient loss of ClONO2 on pure ice surfaces, a very rapid rate for the reaction ClONO2 + HCl on NAT surfaces, and that pure ice surfaces will become “passivated” when coated with one monolayer of NAT crystal.