The kinetics and mechanism of SO₂oxidation by O₃on mineral dust

The oxidation of SO2 by O3 on mineral dust was studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Formation of sulfate was observed on the surface. A two-step mechanism that involves physisorbed SO2 followed by oxidation is presented. The formation rate was determined to be first order with respect to SO2 and zero order in O3. The reactive uptake coefficient, γ, was determined from the infrared absorbance, that was calibrated by ion chromatography, and from the geometric or the BET surface area. γSO2 was independent of the SO2 concentration and was determined to be in the order of 10−3 using the geometric surface area, or 10−7 using the BET surface area for [SO2] = 2.2 × 1012 to 2.0 × 1013 and [O3] = 5.6 × 1012 (in units of molecule cm−3). γO3 depended linearly on the O3 concentration and varied from 10−2 to 10−4 using the geometric area or 10−6 to 10−8 using the BET area for [O3] = 1.9 × 1012 to 5.5 × 1013 and [SO2] = 5.4 × 1012 (in units of molecule cm−3). In all experiments surface saturation was observed with an amount of 2 × 1019 sulfate ions g−1 on the mineral dust sample. In the presence of water vapor regeneration of active sites was observed. After several exposures to water vapor corresponding to 80% relative humidity and successive SO2 and O3 treatments the amount of formed sulfate covering the surface was increased by 47% compared to the dry experiments.