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

Other literature type English OPEN
Clegg, S. M. ; Abbatt, J. P. D. (2001)

The heterogeneous reaction SO<sub>2</sub> + H<sub>2</sub>O<sub>2</sub> <img border="0" src="/img/rarrow.gif" width="24" height="9">&nbsp; H<sub>2</sub>SO<sub>4</sub> on ice at 228 K has been studied in a low temperature coated-wall flow tube. With H<sub>2</sub>O<sub>2</sub> in excess of SO<sub>2</sub>, the loss of SO<sub>2</sub> 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 SO<sub>2</sub>. This lowers the surface concentrations of HSO<sub>3</sub><sup>-</sup>, a participant in the rate-determining step of the oxidation mechanism. For a fixed SO<sub>2</sub> partial pressure of 1.4 x 10<sup>-4</sup> Pa, the reaction probabilities for SO<sub>2</sub> loss on a freshly exposed surface scale linearly with H<sub>2</sub>O<sub>2</sub> partial pressures between 2.7 x 10<sup>-3</sup> and 2.7 x 10<sup>-2</sup> Pa because the H<sub>2</sub>O<sub>2</sub> surface coverage is unsaturated in this regime. Conversely, the reaction probabilities decrease as the partial pressure of SO<sub>2</sub> is raised from 2.7 x 10<sup>-5</sup> to 1.3 x 10<sup>-3</sup> Pa, for a fixed H<sub>2</sub>O<sub>2</sub> partial pressure of 8.7 x 10<sup>-3 </sup>Pa. This is expected if the rate determining step for the mechanism involves HSO<sub>3</sub><sup>-</sup> rather than SO<sub>2</sub>. It may also arise to some degree if there is competition between gas phase SO<sub>2</sub> and H<sub>2</sub>O<sub>2</sub> for adsorption sites. The reaction is sufficiently fast that the lifetime of SO<sub>2</sub> within ice clouds could be controlled by this heterogeneous reaction and not by the gas-phase reaction with OH.
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