Characteristics of water-soluble ions in dry deposition in urban Beijing

To understand the chemical composition and temporal variations of precipitation in Beijing, one-week integrated dry deposition samples were collected for more than four years. Among 222 dry deposition samples, there existed more or less anion deficits. SO₄^2- and Ca²⁺ were the most abundant anion and cation, followed by NO₃^- and NH₄⁺, respectively. SO₄^2-,NO₃^- and NH₄⁺ exhibited similar seasonal pattern, with higher and lower concentrations occurring in summer and winter, respectively. Dry deposition samples usually appeared neutral, with narrow variations in their monthly averages but an evident seasonal pattern, i.e., lower level in summer and higher level in spring.