Variability, formation and acidity of water-soluble ions in PM2.5 in Beijing based on the semi-continuous observations

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 that 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 strongly acidic and a few percent might be alkaline. Seasonal variations in aerosol acidity and chemical forms of major ions were also briefly discussed.