Significant amounts of ammonia, nitric acid and hydrochloric acid as ammonium particle precursors have been found in urban polluted air. In order to advance understanding the formation and dissociation of ammonium nitrate and ammonium chloride, an on-line analyzer for monitoring aerosols and gases in the ambient air was employed to measure the concentrations of sulfate, nitrate and chloride in aerosols as well as related gaseous precursors at one hour resolution. The highest levels of summed hourly-averaged sulfate, nitrate, chloride and ammonium reached up to 177.7 μg/m 3 in PM 10 from 12 to 14 June, and 101.2 μg/m 3 in PM 2.5 from 1 to 3 July, 2009 in Shanghai. Ammonium nitrate and ammonium chloride were popular in aerosols with a strong linear correlation between non-sulfate ammonium (ns-NH 4 + ) and nitrate (or nitrate plus chloride). Ammonium equivalent concentration was always less than the sum of sulfate, nitrate and chloride due to ammonium-poor aerosols, whereas the atmosphere was ammonia-rich in gas phase. The ratio of gaseous nitric acid to total nitrate less than that of hydrochloric acid to total chloride both in PM 10 and PM 2.5 indicated that ammonium nitrate was more favored to be formed than ammonium chloride.