Trends in heterogeneous aqueous reaction in continuous haze episodes in suburban Shanghai: An in-depth case study

Heterogeneous aqueous reaction plays important roles in the enhanced formation of secondary aerosols during haze. However, its occurrence in haze episodes remains poorly understood. In this study, the trends in heterogeneous aqueous reaction in continuous haze episodes were investigated by an in-depth case analysis. The highly time-resolved measurements of water-soluble inorganic ions of PM_2.5 were conducted in a suburban of Shanghai, China, and continuous haze episodes, which occurred from Feb. 18 to Feb. 28, were selected as studied cases. Results showed that fine particle pollution in Baoshan was serious. High concentrations of secondary inorganic aerosol ions and the higher sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) on haze days indicated enhanced conversions from SO₂ and NO_x to their corresponding particulate phases. The high-nitrate haze episode and the high-sulfate haze episode were identified. Further simulations revealed that the PM_2.5 particles had strong acidity during the high-nitrate and high-sulfate haze episodes whether they were calculated by E-AIM 4 or by ISORROPIA II. It was found that particulate liquid water was more sensitive to nitrate than sulfate, and played significant roles in the heterogeneous aqueous reactions of NO₂ and secondary nitrate formation during haze episodes, especially in the high-sulfate haze episode. Further analysis indicated that the high-nitrate haze episode favoured the occurrence of heterogeneous aqueous phase oxidation of SO₂, and the more water was in the particles, the more SO₂ was converted to sulfate aerosols. This work provides an important field measurement-based evidence for understanding the important contributions of the heterogeneous aqueous reactions to secondary aerosol pollution and the tendencies of heterogeneous aqueous reactions in the formation of secondary sulfate and nitrate aerosols in suburban Shanghai.