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 SO 2 by O 3 on CaCO 3 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, SO 2can be oxidized to sulfate on the surface of CaCO 3 particles. The reaction is first order in SO 2 and zero order in O 3. The reactive uptake coefficient for SO 2 [(0.6–9.8)×10 14 molecule cm -3] oxidation by O 3 [(1.2–12)×10 14 molecule cm -3] was determined to be (1.4±0.3)×10 -7 using the BET area as the reactive area and (7.7±1.6)×10 -4 using the geometric area. A two-stage mechanism that involves adsorption of SO 2 followed by O 3 oxidation is proposed and the adsorption of SO 2 on the CaCO 3 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.