Oxidation of SO₂ by H₂O₂ on ice surfaces at 228 K: a sink for SO₂ in ice clouds

Abstract

Abstract. The heterogeneous reaction SO2 + H2O2   H2SO4 on ice at 228 K has been studied in a low temperature coated-wall flow tube. With H2O2 in excess of SO2, the loss of SO2 on an ice surface is time dependent with the reaction most efficient on a freshly exposed surface. The deactivation of the surface arises because the protons formed in the reaction inhibit the dissociation of adsorbed SO2. This lowers the surface concentrations of HSO3-, a participant in the rate-determining step of the oxidation mechanism. For a fixed SO2 partial pressure of 1.4 x 10-4 Pa, the reaction probabilities for SO2 loss on a freshly exposed surface scale linearly with H2O2 partial pressures between 2.7 x 10-3 and 2.7 x 10-2 Pa because the H2O2 surface coverage is unsaturated in this regime. Conversely, the reaction probabilities decrease as the partial pressure of SO2 is raised from 2.7 x 10-5 to 1.3 x 10-3 Pa, for a fixed H2O2 partial pressure of 8.7 x 10-3 Pa. This is expected if the rate determining step for the mechanism involves HSO3- rather than SO2. It may also arise to some degree if there is competition between gas phase SO2 and H2O2 for adsorption sites. The reaction is sufficiently fast that the lifetime of SO2 within ice clouds could be controlled by this heterogeneous reaction and not by the gas-phase reaction with OH.