Seasonal variations and source apportionment of water-soluble inorganic ions in PM2.5 in Nanjing, a megacity in southeastern China

Daily PM_2.5 samples were collected in Nanjing, a megacity in southeastern China, for a period of one-half of a month during every season from 2014~2015. Mass concentrations of nine water soluble inorganic ions (F^?, Cl^?, SO₄^2?, NO₃^?, Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺) were determined using ion chromatography to identify the chemical characteristics and potential sources of PM_2.5. The mass concentrations of daily PM_2.5 ranged from 31.0 to 242.9?μg?m^?3, with an annual average and standard deviation of 94.4?±?31.1?μg?m^?3. The highest seasonal average of PM_2.5 concentrations was observed during winter (108.5?±?31.8?μg?m^?3), and the lowest average was observed during summer (85.0?±?22.6?μg?m^?3). The annual average concentration of total water soluble inorganic ions was 39.82?μg?m^?3, accounting for 44.4% of the PM_2.5. The seasonal variation in water soluble inorganic ions in PM_2.5 reached its maximum during autumn and reached its minimum during spring. Sulfate, nitrate and ammonium were the dominant water soluble inorganic species, with their combined proportion of 82.0% of the total water soluble inorganic ions and 36.8% of the fine particles. Seasonal variations in aerosol acidity and chemical forms of secondary inorganic ions were discussed. The average ratio of NO₃^?/SO₄^2? was 0.95. According to the results of principal component analysis, secondary sources, burning processes, and airborne dust were the dominant potential sources of PM_2.5 in Nanjing.