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

Abstract

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, α-Fe₂O₃, ZnO, MgO, α-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 O₂. 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.