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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 O₃ 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 O₃ and water in oxidation processes were determined. The results showed that in the presence of O₃, SO₂can be oxidized to sulfate on the surface of CaCO₃ particles. The reaction is first order in SO₂ and zero order in O₃. The 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 (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 SO₂ followed by O₃ 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.