Heterogeneous reactions of sulfur dioxide on typical mineral particles

The heterogeneous reaction of SO₂ on Al₂O ₃, CaO, TiO₂, MgO, FeOOH, Fe₂O₃, MnO₂, and SiO₂, as well as authentic aerosol sample, was investigated by using a White Cell coupled with in situ-FTIR and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Simultaneous observations of reactants and products were performed to obtain full information on the mechanism and kinetics of the reactions. SO₂ was irreversibly adsorbed to form surface sulfite (SO₃^2-), bisulfite (HSO₃^-), and sulfate (SO₄^2-). The reactivity order of these particles is the following: FeOOH > Al ₂O₃ > mixture > MgO > Fe₂O₃ > SiO₂. Field-collected aerosol showed significant activity for the oxidation of SO₂. The uptake coefficient of SO₂ on Al₂O₃ with different acidity varied in the order of basic Al₂O₃ > neutral Al₂O₃ > acidic Al₂O₃. The surface-active oxygen and hydroxyl might be the key factors for the conversion of SO₂ to SO₄ ^2-. The faster reaction rate could be achieved with greater surface area on particles with the same mass. On the basis of the same surface area Fe₂O₃ could be most reactive in the reaction with SO ₂ compared with all other particles. The apparent rate constants were determined to be 1.35 × 10^-2 and 9.4 × 10^-3 for uptake on Al₂O₃ and MgO, respectively, which are the same as the results of other scientists.