Changes in sources and formation mechanisms of carbonaceous aerosols driven by short-term air pollution controls in Megacity Xi'an, China

Carbonaceous aerosols have a significant impact on global climate and human health. Controlling carbonaceous aerosol pollution is a considerable challenge due to the complex sources and the unintelligible secondary formation processes. The 14th National Games of the People's Republic of China were successfully held in Xi'an in 2021, which imposed strict emission controls on air pollution sources for the whole society, providing an experimental opportunity to study the effects of air pollution control measures on carbonaceous aerosols. Based on a combination of atmospheric observation studies and the Weather Research and Forecasting model coupled with online chemistry (WRF-Chem) simulations, we explore the impact of air pollution control on carbonaceous aerosol pollution characteristics, sources, and formation mechanisms. After removing the meteorological conditions, the anthropogenic abatement measures significantly improve regional ambient air quality with the air quality index reduced by 25 % and carbonaceous aerosol concentrations reduced by 30 %. These improvements are mainly attributed to the effective control of primary sources, resulting in the concentrations of primary organic carbon (POC) decreased by 33 %, while only a 22 % decrease in secondary organic carbon (SOC) concentrations. According to the source analysis, carbonaceous aerosols in the control period are mainly from secondary formation and biomass burning. The emission reductions weaken the titration effect of air pollutants on atmospheric oxidants and intensify the near-ground columnar UV radiation, thus promoting the generation of atmospheric oxidants. Consequently, air pollution control changes the main pathway of SOC formation from aqueous-phase reactions to atmospheric oxidation reactions, resulting in a poor control effect on SOC. The aging of organic carbon from the biomass burning source after long-distance transport also contributes significantly to the SOC concentrations. Our results confirm the effectiveness in air pollution controls on carbonaceous aerosol pollution and also highlight that the control of SOC precursors needs to be further optimized to reduce atmospheric oxidation.