Characteristics of Water-Soluble Inorganic Ions in PM2.5 in Typical Urban Areas of 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 PM2.5 samples as precursors to analyze the characteristics of water-soluble inorganic ions. The concentrations of water-soluble inorganic ions in PM2.5 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 PM2.5 in Beijing during the sampling period was 94.28 +/- 52.49 mu g/m3. As compared to the winter of 2016, the average daily PM2.5 concentration in Beijing decreased by 29 mu g/m3 in 2017 (28.2% decrease), with a remarkable decline in the number of days with pollution. During the pollution period, the concentrations of NO3-, SO42-, and NH4+ were significantly higher in PM2.5 as compared to the cleaning period. The ratio of the concentrations of [NO3-]/ [SO42-] 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 SO42- emissions from fixed sources, such as coal. Furthermore, TOF-SIMS analysis results showed that NH4+ tended to exist in the form of molecular ammonium sulfate or ammonium hydrogen sulfate during the period of pollution.

Automated summary

This study analyzed the characteristics of water-soluble inorganic ions in PM2.5 samples before and after the implementation of coal-to-gas conversion policy in Beijing. The results showed a significant decrease in the concentrations of water-soluble inorganic ions in PM2.5 after the policy implementation, indicating the effectiveness of the policy in reducing pollution.