Impacts of short-term mitigation measures on PM 2.5 and radiative effects: a case study at a regional background site near Beijing, China

Abstract. An intensive measurement campaign was conducted in a regional background site near Beijing during the 19th National Congress of the Communist Party of China (NCCPC) to investigate the effectiveness of short-term mitigation measures on PM 2.5 and aerosol direct radiative forcing (DRF). Average mass concentration of PM 2.5 and its major chemical composition are decreased by 20.6–43.1 % during the NCCPC control period compared with the non-control period. When considering days with the stable meteorological conditions, larger reduction of PM 2.5 is found compared with that for all days. Further, a positive matrix factorization receptor model shows that the mass concentrations of PM 2.5 from traffic-related emissions, biomass burning, industry processes, and mineral dust are reduced by 38.5–77.8 % during the NCCPC control period compared with the non-control period. However, there is no significant difference in PM 2.5 from coal burning between these two periods, and an increasing trend of PM 2.5 mass from secondary inorganic aerosol is found during the NCCPC control period. Two pollution episodes were occurred subsequently after the NCCPC control period. One is dominated by secondary inorganic aerosol, and the WRF-Chem model shows that the Beijing-Tianjin-Hebei (BTH) region contributes 73.6 % of PM 2.5 mass; the other is mainly caused by biomass burning, and the BTH region contributes 46.9 % of PM 2.5 mass. Calculations based on a revised IMPROVE method show that organic matter (OM) is the largest contributor to the light extinction coefficient (b ext ) during the non-control period while NH 4 NO 3 is the dominant contributor during the NCCPC control period. The Tropospheric Ultraviolet and Visible radiation model reveals that the average DRF values at the Earth's surface are −14.0 and −19.3 W m -2 during the NCCPC control and non-control periods, respectively, and the reduction ratios of DRF due to the decrease in PM 2.5 components vary from 22.7–46.7 % during the NCCPC control period. Our study would further provide valuable information and dataset to help controlling the air pollution and alleviating the cooling effects of aerosols at the surface in Beijing.