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In order to fully understand the inorganic water-soluble ions and their sources in the atmospheric fine particles PM2.5 in Beijing after coal-to-gas conversion, 41 PM2.5 samples were continuously collected in Haidian District of Beijing during the winter heating period in 2017.The concentrations of 10 kinds of water-soluble ions(Na+, NH4+, K+, Ca2+, Mg2+, F-, Cl-, NO2-, NO3-, SO42-)were analyzed by ion chromatography.The results show that the daily average mass concentration of PM2.5 in Beijing during the sampling period is (94.28±52.49) μg/m3.Compared with the winter of 2016, the average daily PM2.5 concentration in Beijing in 2017 decreased by 29 μg/m3, with a decrease of 28.2%, and the number of pollution days decreased significantly.During the pollution period, NO3-, SO42-, and NH4+in the particulate matter were significantly higher than the clean period.The ratio of the concentration of ρ(NO3-)/ρ(SO42-) is greater than 1, and the contribution of the mobile source is relatively large, reflecting that the implementation of the "coal to gas" policy has contributed to the reduction of sulfate emissions from fixed sources such as coal.Time-of-flight secondary ion mass spectrometry(TOF-SIMS) was combined with mass high resolution mode and positive and negative ion imaging mode to characterize PM2.5.The results show that NH4+tends to exist in the form of molecular ammonium sulfate or ammonium hydrogen sulfate during the pollution period.