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Hourly real-time measurements of water-soluble ions including sulfate,nitrate,ammonium,sodium,calcium et al.,in PM2.5 were employed at an urban station from June to December in 2009.The average concentrations of total water soluble ions were 44 μg m-3,accounting for 38% of PM2.5 mass concentration.Monthly characterizations of these water-soluble ions in aerosol and gas phase were summarized.The highest concentration also with highest sulfate is in July,lowest in September and October.SO2 and NOX concentration increase while decrease for O3 in November related with heating emission,low temperature,weak solar radiation.It is concluded that the average SO2 oxidation ratio (SOR) for the whole campaign is about 63%,with maximum (82%) in August and minimum in November.Both nitrate oxidation ratio (NOR) and the mean mass ratio (in μg/m3) of NH3/NHX (NHR) were much lower than that of SOR,which was 15% in average,maximum in July and minimum in November.The same trend of NOR and NHR suggested that NHX (NH3+NH4 +) was also influenced by local sources and underwent the same transformation process at the atmosphere.Dramatic diurnal pattern of SO4 2-,NO3 - and NH4 + were monitored,but no obvious diurnal cycles for sodium,calcium and potassium were observed.The analysis of the equivalents of cations to anions revealed the existence of the acidic aerosols in PM2.5 at the urban site.Most of time,sulfate aerosol exists as NH4HSO4,large amount of anthropogenic emission and photo chemically aged result in the acidic aerosol.