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In this work, the heterogeneous reactions of NO₂ with CaCO₃–(NH₄)₂SO₄ mixtures with a series of weight percentage (wt %) of (NH₄)₂SO₄ were investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at different relative humidity (RH) values. For comparison, the heterogeneous reactions of NO₂ with pure CaCO₃ particles and pure (NH₄)₂SO₄ particles, as well as the reaction of CaCO₃ with (NH₄)₂SO₄ particles, were also studied. The results indicated that NO₂ did not show any significant uptake on (NH₄)₂SO₄ particles, and it reacted with CaCO₃ particles to form calcium nitrate under both dry and wet conditions. The heterogeneous reactions of NO₂ with CaCO₃–(NH₄)₂SO₄ mixtures were markedly dependent on RH. Calcium nitrate was formed from the heterogeneous reactions at all the RHs investigated, whereas CaSO₄  ⋅  0.5H₂O (bassanite), CaSO₄  ⋅  2H₂O (gypsum), and (NH₄)₂Ca(SO₄)₂  ⋅  H₂O (koktaite) were produced depending on RH. Under the dry condition, the heterogeneous uptake of NO₂ on the mixtures was similar to that on CaCO₃ particles with neglectable effects from (NH₄)₂SO₄; the duration of initial stages and the NO₃⁻ mass concentrations had a negative linear relation with the mass fraction of (NH₄)₂SO₄ in the mixtures. Under wet conditions, the chemical interaction of (NH₄)₂SO₄ with Ca(NO₃)₂ enhances the nitrate formation, especially at medium RHs, while the coagulation of (NH₄)₂SO₄ with CaCO₃ exhibits an increasing inhibiting effects with increasing RH at the same time. In addition, the heterogeneous uptake of NO₂ on the mixtures of CaCO₃ and (NH₄)₂SO₄ was found to favor the formation of bassanite and gypsum due to the decomposition of CaCO₃ and the coagulation of Ca²⁺ and SO₄²⁻. A possible reaction mechanism was proposed and the atmospheric implications were discussed.