Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 1 1 2001 10.5194/acpd-1-77-2001 http://www.atmos-chem-phys-discuss.net/1/77/2001/ http://www.atmos-chem-phys-discuss.net/1/77/2001/acpd-1-77-2001.html http://www.atmos-chem-phys-discuss.net/1/77/2001/acpd-1-77-2001.pdf 77 92 2001-09-03 Oxidation of SO₂ by H₂O₂ on ice surfaces at 228 K: a sink for SO₂ in ice clouds S. M. Clegg J. P. D. Abbatt 1Department of Chemistry, University of Toronto, 80 St. George St., Toronto, M5S 3H6 Ontario, Canada 2now at: Combustion Research Facility, Sandia National Laboratory, P.O. Box 969, MS 9055, Livermore, CA 94551-0969, USA The heterogeneous reaction SO₂ + H₂O₂ to 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 H₂O₂ partial pressure of 8.7 × 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.