Insights into measurements of water-soluble ions in PM and their gaseous precursors in Beijing.

To better understand the characteristics and transformation mechanisms of secondary inorganic aerosols, hourly mass concentrations of water-soluble inorganic ions (WSIIs) in PM and their gaseous precursors were measured online from 2016 to 2018 at an urban site in Beijing. Seasonal and diurnal variations in water-soluble ions and gaseous precursors were discussed and their gas-particle conversion and partitioning were also examined, some related parameters were characterized. The (TNH) was also defined to describe the variations of the excess NH in different seasons. In addition, a sensitivity test was carried out by using ISORROPIA II to outline the driving factors of gas-particle partitioning. In Beijing, the relative contribution of nitrate to PM has increased markedly in recent years, especially under polluted conditions. In the four seasons, only a small portion of NO in the atmosphere was converted into total nitrate (TNO) and more than 80% of TNO occurred in the form of nitrate due to the abundant ammonia. The concentration of total ammonia (TNH) was much higher than that required to neutralize acid gases, and most of the TNH occurred as gaseous NH. The nitrous acid (HONO) concentration was highly correlated with NH concentration and had increased significantly in Beijing compared with previous studies. The total chloride (TCl) was the highest in winter, and ε(Cl) was more sensitive to variations in the ambient temperature (T) and relative humidity (RH) than ε(NO).