Characteristics of Water-Soluble Inorganic Ions in PM(2.5) in Typical Urban Areas of Beijing, China

[Image: see text] Following the implementation of “coal-to-gas conversion” policy in the Haidian District of Beijing during summer, the present comparative study was performed employing 41 PM(2.5) samples as precursors to analyze the characteristics of water-soluble inorganic ions. The concentrations of water-soluble inorganic ions in PM(2.5) were analyzed by ion chromatography, and the occurrence form of ions was characterized via time-of-flight secondary ion mass spectrometry (TOF-SIMS). Results revealed that the daily average mass concentration of PM(2.5) in Beijing during the sampling period was 94.28 ± 52.49 μg/m(3). As compared to the winter of 2016, the average daily PM(2.5) concentration in Beijing decreased by 29 μg/m(3) in 2017 (28.2% decrease), with a remarkable decline in the number of days with pollution. During the pollution period, the concentrations of NO(3)(–), SO(4)(2–), and NH(4)(+) were significantly higher in PM(2.5) as compared to the cleaning period. The ratio of the concentrations of [NO(3)(–)]/[SO(4)(2–)] was greater than 1, and the contribution from mobile sources was relatively large, indicating that the implementation of the “coal-to-gas conversion” policy in Beijing has led to the reduction of SO(4)(2–) emissions from fixed sources, such as coal. Furthermore, TOF-SIMS analysis results showed that NH(4)(+) tended to exist in the form of molecular ammonium sulfate or ammonium hydrogen sulfate during the period of pollution.