Uptake of SO2 on HOBr-Ice Surfaces

Uptake of SO2 on HOBr-treated ice surfaces has been studied using a flow reactor coupled with a differentially pumped quadrupole mass spectrometer at 190-240 K. The initial uptake coefficient was determined as a function of HOBr surface coverage, θHOBr, on the ice. The uptake coefficients increase as the HOBr coverage increases. The uptake coefficient can be expressed as γt=kh θHOBr, where kh=5.7×10-20 molecules-1cm-2 at 191 K, and kh=5.5×10-21 molecules-1cm-2 at 210 K, θ HOBr is in the range of 8×1013-1.2×1015 molecules.cm-2. The effect of temperature on the uptake coefficients of SO2 by the HOBr-treated ice films was also studied. The activation energy Ea of SO2 on HOBr-ice surfaces is approximately -76±11 kJ/mol in the 190-215 K range. Other effects on the uptake coefficient of SO2 for the HOBr-treated ice surfaces were also investigated, and kinetic results were interpreted in terms of the Eley-Rideal mechanism. In addition, SO2 uptake coefficient on HOCl-ice surfaces was investigated. This study suggests that the uptake of SO2 on ice/snow surfaces is enhanced by the presence of HOBr or HOCl near the ice surface. The implication for atmospheric chemistry is that HOBr-ice surfaces may provide an efficient pathway for oxidation of S(IV) in the boundary layer, similar to S(IV) oxidation by HOBr in the aqueous phase.