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

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 × 10-4 Pa, the reaction probabilities for SO2 loss on a freshly exposed surface scale linearly with H2O2 partial pressures between 2.7 × 10-3 and 2.7 × 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 × 10 -5 to 1.3 × 10-3 Pa, for a fixed H2O 2 partial pressure of 8.7 × 10-3 Pa. This is expected if the rate determining step for the mechanism involves HSO 3- rather than SO2. It may also arise to some degree if there is competition between gas phase SO2 and H 2O2 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. © European Geophysical Society 2001.