A sink for SO2 in ice clouds

Introduction Conclusions References Tables Figures Back Close Print Version Interactive Discussion Abstract Introduction Conclusions References Tables Figures Back Close Print Version Interactive Discussion Abstract The heterogeneous reaction SO 2 + H 2 O 2 → H 2 SO 4 on ice at 228 K has been studied in a low temperature coated-wall flow tube. With H 2 O 2 in excess of SO 2 , the loss of SO 2 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 5 the reaction inhibit the dissociation of adsorbed SO 2. This lowers the surface concentrations of HSO − 3 , a participant in the rate-determining step of the oxidation mechanism. For a fixed SO 2 partial pressure of 1.4 × 10 −4 Pa, the reaction probabilities for SO 2 loss on a freshly exposed surface scale linearly with H 2 O 2 partial pressures between 2.7 × 10 −3 and 2.7 × 10 −2 Pa because the H 2 O 2 surface coverage is unsaturated in 10 this regime. Conversely, the reaction probabilities decrease as the partial pressure of SO 2 is raised from 2.7 × 10 −5 to 1.3 × 10 −3 Pa, for a fixed H 2 O 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 SO 2. It may also arise to some degree if there is competition between gas phase SO 2 and H 2 O 2 for adsorption sites. The reaction is sufficiently fast 15 that the lifetime of SO 2 within ice clouds could be controlled by this heterogeneous reaction and not by the gas-phase reaction with OH.