[Comparative PM_2.5 Pollution Characteristics and Sources in the Autumn and Winter of 2016-2021 in Handan City]

In 2018, the State Council issued the Three-year Action Plan for Winning the Blue Sky Defense War （Blue Sky Defense War）. To study the characteristics and sources of PM_2.5 pollution in the early stage （first stage）, middle stage （second stage）, and late stage （third stage） of the Blue Sky Defense War in Handan City in autumn and winter, PM_2.5 samples were collected in the autumn and winter from 2016 to 2021. Based on the concentration data of eight water-soluble ions, EC, and OC the source analysis was performed using the positive definite matrix factorization model, backward trajectory, and potential source contribution factor （PSCF） analysis. The results showed that in recent years, the annual and autumn-winter concentrations of PM_2.5 showed an overall decreasing trend, and the concentrations of water-soluble ions and total carbon decreased stage by stage during the three stages of sampling. The concentration in the third stage （52.70 μg·m^-3 and 17.17 μg·m^-3） decreased by 34.31% and 63.38% compared with that in the first stage （80.23 μg·m^-3 and 46.89 μg·m^-3）, respectively. The concentration of SNA in the three stages accounted for 46.79%, 48.86%, and 43.22% of the total concentration of PM_2.5, respectively. The proportion of SOC in PM_2.5 was 12.43%, 14.05%, and 6.81%, all of which first increased and then decreased. The mean of SOR in the three stages was 0.34, 0.37, and 0.36, respectively, and NOR was 0.23, 0.28, and 0.28, indicating that the secondary transformation degree was the highest during the Blue Sky Defense War. From the perspective of pollutant sources, the main sources of PM_2.5 in Handan City were secondary, combustion sources, industrial sources, traffic sources, and dust sources. During the Blue Sky Defense War, the primary emissions decreased （mainly combustion sources and industrial sources）； afterward, the end of the primary emissions increased, of which the contribution rate of industrial sources increased from 12.17% to 20.20%. Moreover, the backward trajectory clustering analysis showed that the contribution rate of secondary sources in the second stage was significantly higher than that in other stages, especially the airflow from Shandong and Shanxi. The analysis of potential sources showed that the regions with high WPSCF values in the three stages were mainly concentrated in the south of Hebei and the north of Henan. The difference was that the potential source areas in the third stage shifted from the central part of Shanxi to the south compared with the first stage. This work confirms that the environmental benefits of the Blue Sky Defense War are obvious. With the development of the Blue Sky Defense War, the contribution of secondary components is prominent. After the Blue Sky Defense War, the proportion of secondary components decreased, the influence of primary components increased, and the control of direct emissions should be strengthened.