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PM2.5 samples were collected in Hangzhou, eastern China, during the winter and summer of 2016 and 2018. Concentrations of water-soluble ions, elemental carbon (EC), and organic carbon (OC) in PM2.5 were determined. By comparing these data with previous observations, we analyzed the concentrations, variational trends, and main sources of PM2.5 and its dominant chemical components in Hangzhou since the implementation of the Air Pollution Prevention and Control Action Plan. The results indicate that the concentrations of PM2.5 and its main components have shown a declining trend since 2013, which exemplifies the significant effect of the Action Plan on the reduction of PM2.5 pollution in Hangzhou. The contribution of secondary inorganic aerosol (SIA) to PM2.5 in winter showed an increasing trend, and the percentage of SIA in PM2.5 was significantly higher in Hangzhou than in other Chinese cities reported in recent years, which showed that secondary pollution was prominent in Hangzhou. During winter 2018 and 2016, sulfate decreased by 40% and 14%, respectively, compared to that in the same period in 2013. This indicates that the impact of coal combustion emission on PM2.5 pollution has been significantly reduced in Hangzhou since the implementation of the Action Plan. The ratio of nitrate to sulfate increased in winter (winter 2018, 1.87; winter 2016, 1.44; winter 2013, 0.89), indicating that the relative contribution of motor vehicle emissions to PM2.5 is increasing in Hangzhou. The poor correlation and high ratio of OC and EC indicated that there was strong secondary organic carbon formation in summer in Hangzhou. [ABSTRACT FROM AUTHOR]