Synergistic reaction between SO2 and NO2 on mineral oxides: a potential formation pathway of sulfate aerosol

Sulfate is one of the most important aerosols in the atmosphere. A new sulfate formation pathway via synergistic reactions between SO2 and NO2 on mineral oxides was proposed. The heterogeneous reactions of SO2 and NO2 on CaO, alpha-Fe2O3, ZnO, MgO, alpha-Al2O3, TiO2, and SiO2 were investigated by in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (in situ DRIFTS) at ambient temperature. Formation of sulfate from adsorbed SO2 was promoted by the coexisting NO2, while surface N2O4 was observed as the crucial oxidant for the oxidation of surface sulfite. This process was significantly promoted by the presence of O-2. The synergistic effect between SO2 and NO2 was not observed on other mineral particles (such as CaCO3 and CaSO4) probably due to the lack of the surface reactive oxygen sites. The synergistic reaction between SO2 and NO2 on mineral oxides resulted in the formation of internal mixtures of sulfate, nitrate, and mineral oxides. The change of mixture state will affect the physicochemical properties of atmospheric particles and therefore further influence their environmental and climate effects.