Enhancement of secondary aerosol formation by reduced anthropogenic emissions during Spring Festival 2019 and enlightenment for regional PM 2.5 control in Beijing

Abstract. A comprehensive field experiment measuring aerosol chemical and physical properties at a suburban site in Beijing around the 2019 Spring Festival was carried out to investigate the impact of reduced anthropogenic emissions on aerosol formation. Sharply reduced sulfur dioxide (SO2) and nitrogen dioxide (NO2) concentrations during the festival holiday resulted in an unexpected increase in the surface ozone (O3) concentration, leading to enhancement of the atmospheric oxidation capacity. Simultaneously, the reduced anthropogenic emissions resulted in massive decreases in particle number concentration at all sizes and the mass concentrations of organics and black carbon. However, the mass concentrations of inorganics (especially sulfate) decreased weakly. Detailed analyses of the sulfur oxidation ratio and the nitrogen oxidation ratio suggest that sulfate formation during the holiday could be promoted by enhanced nocturnal aqueous-phase chemical reactions between SO2 and O3 under moderate relative humidity (RH) conditions (40 %