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Heterogeneous/multiphase oxidation of SO₂ by NO₂ on solid or aqueous particles is thought to be a potentially important source of sulfate in the atmosphere, for example, during heavily polluted episodes (haze), but the reaction mechanism and rate are uncertain. In this study, in order to assess the importance of the direct oxidation of SO₂ by NO₂ we investigated the heterogeneous/multiphase reaction of SO₂ with NO₂ on individual CaCO₃ particles in N₂ using Micro-Raman spectroscopy. In the SO₂ ∕ NO₂ ∕ H₂O ∕ N₂ gas mixture, the CaCO₃ solid particle was first converted to the Ca(NO₃)₂ droplet by the reaction with NO₂ and the deliquescence of Ca(NO₃)₂, and then NO₂ oxidized SO₂ in the Ca(NO₃)₂ droplet forming CaSO₄, which appeared as needle-shaped crystals. Sulfate was mainly formed after the complete conversion of CaCO₃ to Ca(NO₃)₂, that is, during the multiphase oxidation of SO₂ by NO₂. The precipitation of CaSO₄ from the droplet solution promoted sulfate formation. The reactive uptake coefficient of SO₂ for sulfate formation is on the order of 10⁻⁸, and RH enhanced the uptake coefficient. We estimate that the direct multiphase oxidation of SO₂ by NO₂ is not an important source of sulfate in the ambient atmosphere compared with the SO₂ oxidation by OH in the gas phase and is not as important as other aqueous-phase pathways, such as the reactions of SO₂ with H₂O₂, O₃, and O₂, with or without transition metals.