Kinetics and mechanism of heterogeneous oxidation of sulfur dioxide by ozone on surface of calcium carbonate

Sulfate particles play a key role in the air quality and the global climate, but the heterogeneous formation mechanism of sulfates on surfaces of atmospheric particles is not well established. Carbonates, which act as a reactive component in mineral dust due to their special chemical properties, may contribute significantly to the sulfate formation by heterogeneous processes. This paper presents a study on the oxidation of SO2 by O-3 on CaCO3 particles. Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), the formation of sulfite and sulfate on the surface was identified, and the roles of O-3 and water in oxidation processes were determined. The results showed that in the presence of O-3, SO2 can be oxidized to sulfate on the surface of CaCO3 particles. The reaction is first order in SO2 and zero order in O-3. The reactive uptake coefficient for SO2 [(0.6 - 9.8) x 10(14) molecule cm(-3)] oxidation by O-3 [(1.2 - 12) x 10(14) molecule cm(-3)] was determined to be (1.4 +/- 0.3) x 10(-7) using the BET area as the reactive area and (7.7 +/- 1.6) x 10(-4) using the geometric area. A two-stage mechanism that involves adsorption of SO2 followed by O-3 oxidation is proposed and the adsorption of SO2 on the CaCO3 surface is the rate-determining step. The proposed mechanism can well explain the experiment results. The atmospheric implications were explored based on a box model calculation. It was found that the heterogeneous reaction might be an important pathway for sulfate formation in the atmosphere.