Heterogeneous reactions of NO₂ with CaCO₃–(NH₄)₂SO₄ mixtures at different relative humidities

Abstract

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.