Heterogeneous oxidation of sulfur dioxide by ozone on the surface of sodium chloride and its mixtures with other components

Abstract

The heterogeneous oxidation of SO2 by O3 on NaCl particles has been studied using diffuse reflectance infrared Fourier transform spectroscopy. The formation of sulfite and sulfate on the surface was identified, and the roles of O3 and water in the oxidation processes were determined. The results showed that in the presence of O3, SO2 could be oxidized to sulfate on the surface of NaCl particles. The reaction is first order in O3 and zero order in SO2. The initial reactive uptake coefficient for SO2 [(0.6–9.8) × 1014 molecule cm−3] oxidation by O3 [(1.2–12) × 1014 molecule cm−3] was determined to be (4.8–0.7) × 10−8 using the Brunauer‐Emmett‐Teller area as the reactive area and (9.8–1.4) × 10−5 using the geometric area at 40% relative humidity. A three‐stage mechanism that involves the adsorption of O3 results in an alkalescent surface, the adsorption of SO2 followed by O3 oxidation is proposed, and the adsorption of O3 on the NaCl surface is the rate‐determining step. The proposed mechanism can well explain the experiment results. Furthermore, the surface oxidation on mixtures of NaCl with other components such as CaCO3, Al2O3, TiO2, MgCl2 6H2O, MgO, elemental carbon, and soot were studied. The reactivity of mixtures can be predicted from the reactivity of the single component with each component weighted by its abundance in the mixture. The catalytic and basic additives could enhance the production of sulfate on the NaCl surface.