[Characteristics of Water-soluble Inorganic Ions in PM_2.5 in Beijing During 2017-2018]

To explore the characteristics of water-soluble inorganic ions (WSIIs) in PM_2.5 during the process of continuous improvement of air quality in Beijing in recent years, a continuous collection of PM_2.5 sample campaign was conducted in Beijing from 2017 to 2018. The PM_2.5 mass concentration and WSIIs were then determined. The results showed that the average concentration of PM_2.5 in Beijing was (77.1±52.1) μg ·m^-3, with the highest and lowest values during spring [(102.9±69.1) μg ·m^-3]and summer [(54.7±19.9) μg ·m^-3], respectively. The average concentration of WSIIs was (31.7±30.1) μg ·m^-3, accounting for 41.1% of the PM_2.5 mass, and the seasonal contributions were: autumn (45.9%) > summer (41.9%) > spring (39.9%) ≥ winter (39.2%). SNA was an important component of the WSIIs that accounted for 86.0%, 89.5%, 74.6%, and 73.0% of the total WSIIs during spring, summer, autumn, and winter, respectively. With an increase in temperature, the concentration of NO₃^- increased initially and then decreased, while the concentration of SO₄^2- increased. When the relative humidity was less than 90%, the concentrations of both NO₃^- and SO₄^2- increased with an increase in relative humidity. With the aggravation of pollution, the overall contribution of WSIIs in PM_2.5 increased significantly, and the evolution characteristics of different ions were different. Among them, the concentration and contribution of NO₃^- continued to increase, while the contributions of SO₄^2- and the ions from dust (Mg²⁺, Ca²⁺, and Na⁺) decreased. During the observation period, the primary sources of WSIIs were secondary conversion, combustion source, and dust. The control of coal combustion and motor vehicles is critical to reduce the emission of WSIIs. The backward trajectory analysis showed that the air masses from the south and west of Beijing corresponded to the high PM_2.5 concentration and proportion of WSIIs, and the contribution of secondary ions was significant. However, the concentrations and proportions of the air masses from the northwest and north were relatively low, but the contribution of Ca²⁺ was high.