Seasonal variations and source apportionment of water-soluble inorganic ions in PM 2.5 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 − , SO 4 2− , NO 3 − , Na + , NH 4 + , K + , Mg 2+ and Ca 2+ ) 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 NO 3 − /SO 4 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.