Characteristics of PM2.5 pollution in Beijing after the improvement of air quality

Following the implementation of the strictest clean air policies to date in Beijing, the physicochemical characteristics and sources of PM2.5have changed over the past few years. To improve pollution reduction policies and subsequent air quality further, it is necessary to explore the changes in PM2.5over time. In this study, over one year(2017–2018) field study based on filter sampling(TH-150 C; Wuhan Tianhong, China) was conducted in Fengtai District, Beijing, revealed that the annual average PM2.5concentration(64.8 ± 43.1 μg/m 3) was significantly lower than in previous years and the highest PM2.5concentration occurred in spring(84.4 ± 59.9 μg/m 3). Secondary nitrate was the largest source and accounted for 25.7% of the measured PM2.5. Vehicular emission, the second largest source(17.6%), deserves more attention when considering the increase in the number of motor vehicles and its contribution to gaseous pollutants. In addition, the contribution from coal combustion to PM2.5decreased significantly. During weekends, the contribution from EC and NO 3-increased whereas the contributions from SO42-, OM, and trace elements decreased, compared with weekdays. During the period of residential heating, PM2.5mass decreased by 23.1%, compared with non-heating period, while the contributions from coal combustion and vehicular emission, and related species increased. With the aggravation of pollution, the contribution of vehicular emission and secondary sulfate increased and then decreased, while the contribution of NO 3-and secondary nitrate continued to increase, and accounted for 34.0% and 57.5% of the PM2.5during the heavily polluted days, respectively.