Effects of ozone and relative humidity in secondary inorganic aerosol formation during haze events in Beijing, China

Abstract

Secondary inorganic aerosols (SIAs) are major components of PM2.5 in haze events in Beijing, China. Gas-phase oxidation and aqueous heterogeneous reactions play vital roles in the SIAs formation. Here we analyzed the sensitivity of oxidation ratios of sulfur (SOR) and nitrogen (NOR) to relative humidity (RH) and ozone (O3), based on comprehensive observations from January 15 to March 6, 2019. The observations reveal that the SOR and NOR increased generally with both O3 and RH in haze events. Further mechanism analyses reveal striking differences between the sensitivity of the SOR to O3 and RH and that of the NOR. The SOR is sensitive to O3 when RH is lower than 40%, but converts to RH when RH is higher than 40%, while the NOR is sensitive to both O3 and RH in all regimes. In haze event, aqueous heterogeneous reactions are enhanced due to increasing RH while atmospheric oxidation capacity decreases. The observations show that the SOR increased significantly in haze event but the NOR kept at a relatively stable level. These results suggest that aqueous heterogeneous reactions plays more important roles in sulfate formation than in nitrate formation.