Heterogeneous Uptake of Sulfur Dioxide On Aluminum and Magnesium Oxide Particles

Abstract

In this study, the heterogeneous uptake of SO 2 on α-Al 2 O 3 and MgO particles at 296 K was investigated. Transmission FT-IR experiments reveal that under dry conditions SO 2 reacts with α-Al 2 O 3 and MgO to form strongly adsorbed bisulfite, HSO 3 - (aq), and sulfite, SO 3 2- (aq), on α-Al 2 O 3 and sulfite on MgO. To quantify the uptake of SO 2 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, γ o, is calculated using the BET surface area of the powdered samples as diffusion of SO 2 into the powder is found to readily occur. Values of γ o were determined to be 9.5 ± 0.3 × 10 -5 and 2.6 ± 0.2 × 10 -4 for SO 2 uptake on α-Al 2 O 3 and MgO, respectively, at 296 K under dry conditions at a gas concentration near 1 × 10 11 molecules/cm 3 . Additionally, water uptake on sulfite-coated α-Al 2 O 3 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-Al 2 O 3 . Atmospheric implications of these studies on SO 2 uptake on mineral dust aerosol are discussed.