The kinetics and mechanism of SO2oxidation by O3 on mineral dust

Abstract

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. γ_SO2 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). γ_O3 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.