Heterogeneous Uptake of Sulfur Dioxide On Aluminum and Magnesium Oxide Particles

Abstract

In this study, the heterogeneous uptake of SO2 on α-Al2O3 and MgO particles at 296 K was investigated. Transmission FT-IR experiments reveal that under dry conditions SO2 reacts with α-Al2O3 and MgO to form strongly adsorbed bisulfite, HSO3-(aq), and sulfite, SO32-(aq), on α-Al2O3 and sulfite on MgO. To quantify the uptake of SO2 under dry conditions, heterogeneous uptake coefficients were measured with a Knudsen cell reactor. The Knudsen cell data were modeled to account for gas diffusion in to the underlying layers and surface saturation. The initial uptake coefficient, γo, is calculated using the BET surface area of the powdered samples as diffusion of SO2 into the powder is found to readily occur. Values of γo were determined to be 9.5 ± 0.3 × 10-5 and 2.6 ± 0.2 × 10-4 for SO2 uptake on α-Al2O3 and MgO, respectively, at 296 K under dry conditions at a gas concentration near 1 × 1011 molecules/cm3. Additionally, water uptake on sulfite-coated α-Al2O3 and sulfite-coated MgO particles was investigated with FT-IR spectroscopy. In the presence of adsorbed water, some of the adsorbed sulfite converted to sulfate on MgO but not on α-Al2O3. Atmospheric implications of these studies on SO2 uptake on mineral dust aerosol are discussed.