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Daily PM2.5 and hourly water-soluble inorganic ions in PM2.5 and gaseous precursors were measured during June-November 2009 at an urban site in Beijing.The average mass concentration of the total water-soluble ions was 44 μg m-3,accounting for 38% of PM2.5.Sulfate (SO42-),nitrate (NO3-) and ammonium (NH4+) were dominant ions.The summer-fall difference in seasonal average mass concentrations was smaller than 30% for SO42-,but was up to a factor of 2.0 for NO3- and NH4+.A pronounced diurnal cycle was found for most ions and gaseous precursors and could be explained by their respective sources,formation mechanisms and meteorological conditions.The average oxidation/conversion ratio for SO42- (SOR),NO3- (NOR) and NH4+ (NHR) were estimated to be 63%,15% and 15%,respectively.The low NHR value suggests that NH3 was mainly from local sources and excessive NH3 existed and thus was not a limiting factor in the formation of NH4+.As a result,the diurnal pattern of NH4+ was similar to that of SO42- to some extent,but differed significantly from that of NH3.Based on the estimated H+ concentration and acidity purity (f),75% of data samples were strong acidic and a few percent might be alkaline.Seasonal variations in aerosol acidity and chemical forms of major ions were also briefly discussed.