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. SO2−4 , NO− 3 , NH+ 4 , and Ca2+ 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 SO2− 4 concentration decreased by 13% compared to that during 1995– 1998, much less than the 58% reduction in the annual average SO2 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 NO2 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 and Ca2+ and increased input of NO− 3 . Furthermore, the equivalent mass ratio of NO− 3 to non-sea-salt SO2− 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 SO2 while the SO2 contribution was progressively substituted by NOx.