Heterogeneous uptake of sulfur dioxide on aluminum and magnesium oxide particles

In this study, the heterogeneous uptake of SO2 on alpha -Al2O3 and MgO part icles at 296 K was investigated. Transmission FT-IR experiments reveal that under dry conditions SO2 reacts with alpha -Al2O3 and MgO to form strongly adsorbed bisulfite, HSO3-(aq), and sulfite, SO32-(aq), on alpha -Al2O3 and sulfite on MgO. To quantify the uptake of SO2 under dry conditions, hetero geneous uptake coefficients were measured with a Knudsen cell reactor. The Knudsen cell data were modeled to account for gas diffusion in to the under lying layers and surface saturation. The initial uptake coefficient, gamma (o), is calculated using the BET surface area of the powdered samples as di ffusion of SO2 into the powder is found to readily occur. Values of gamma ( o) were determined to be 9.5 +/- 0.3 x 10(-5) and 2.6 +/- 0.2 x 10(-4) for SO2 uptake on alpha -Al2O3 and MgO2 respectively, at 296 K under dry condit ions at a gas concentration near 1 x 10(11) molecules/cm(3). Additionally, water uptake on sulfite-coated alpha -Al2O3 and sulfite-coated MgO particle s was investigated with FT-IR spectroscopy. In the presence of adsorbed wat er, some of the adsorbed sulfite converted to sulfate on MgO but not on alp ha -Al2O3 Atmospheric implications of these studies on SO2 uptake on minera l dust aerosol are discussed.