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

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.
Competing Interests: The authors declare no competing financial interest.
(© 2022 The Authors. Published by American Chemical Society.)