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

The reaction probabilities, γ, of ClONO 2 and N 2 O 5 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 ClONO 2 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 N 2 O 5 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 HNO 3 ‐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 ClONO 2 on pure ice surfaces, a very rapid rate for the reaction ClONO 2 + HCl on NAT surfaces, and that pure ice surfaces will become “passivated” when coated with one monolayer of NAT crystal.