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Abstract. To investigate the chemical characteristics of precipitation in the polluted urban atmosphere in Beijing and possible mechanisms influencing their variations, a total of 131 event-based precipitation samples were collected from March 2001 to August 2005. The concentrations of major ions in the samples were analyzed by using ion chromatography. Intermediate pH (6.1-7.3) was recorded in approximately two-thirds of the precipitation samples and acidic pH (4.2-5.6) in only 16 % of the samples. However, the precipitation acidity was on the growth track and the process was likely being accelerated. SO 2− 4 , NO − 3 , NH + 4 , and Ca 2+ were the most abundant ions in the precipitations, with their single volume-weighted mean (VWM) concentration all above 100 µeq l −1 . The two major anions and two major cations accounted for more than 80 % of total anionic and cationic mass, respectively. The VWM SO 2− 4 concentration decreased by 13 % compared to that during 1995-1998, much less than the 58 % reduction in the annual average SO 2 concentration from 1998 to 2005 in Beijing. What seems more counterintuitive is that the VWM NO − 3 concentration nearly doubled over the period although the annual average NO 2 concentration decreased by 5 % from 1998 to 2005. These results imply that the conversion of gaseous precursors to acid compounds and/or the regional transport were reinforced over the decade. The average ratio of neutralizing potential to acidifying potential (i.e. NP/AP) was as high as 1.2 but experienced an evident decline trend. This was mainly ascribed to reduced input of NH 4 presented an increasing trend over the study period, suggesting that the contribution of NO − 3 to the precipitation acidity increased in recent years. However, the mean ratio was only 0.37 ± 0.11 in the study period, which is significantly lower than those reported in some metropolitan areas in developed countries. This shows that the precipitation acidity in Beijing was still dominantly from SO 2 while the SO 2 contribution was progressively substituted by NO x .

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