The heterogeneous reactions of NO₂ with CaCO₃–(NH₄)₂SO₄ mixtures were markedly dependent on RH. Calcium nitrate was formed under both dry and wet conditions; bassanite, gypsum, and koktaite were produced depending on RH. The heterogeneous uptake of NO₂ on the CaCO₃–(NH₄)₂SO₄ mixtures was similar to that on pure CaCO₃ particles under the dry condition, whereas the mixtures exhibited a promotive effect on the heterogeneous uptake of NO₂ and the formation of nitrate, especially at medium RHs.