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Major ion concentrations and Sr isotope ratios (Sr-87/Sr-86) were measured in rainwater samples collected at an urban site in Beijing over a period of one year. The pH value and major ion concentrations of samples varied considerably, and about 50% of the rainwater studied here were acidic rain with pH values less than 5.0. Ca2+ and NH4+ were the dominant cations in rainwaters and their volume weighted mean (VWM) values were 608 mu eq l(-1) (14-1781 mu eq l(-1)) and 186 mu eq l(-1) (48-672 mu eq l(-1)), respectively. SO42- was the predominant anion with VWM value of 316 mu eq l(-1) (65-987 mu eq l(-1)), next was NO3- with VWM value of 109 mu eq l(-1) (30-382 mu eq l(-1)).

Using Na as an indicator of marine origin, and Al for the terrestrial inputs, the proportions of sea salt and terrestrial elements were estimated from elemental ratios. More than 99% of Ca2+ and 98% of SO42- in rainwater samples are non-sea-salt origin. The Sr-87/Sr-86 ratios were used to characterize the different sources based on the data sets of this study and those from literatures. Such sources include sea salts (Sr-87/Sr-86 similar to 0.90917), soluble soil dust minerals originating from either local or the desert and loess areas (similar to 0.7111), and anthropogenic sources (fertilizers, coal combustion and automobile exhausts). The high concentrations of alkaline ions (mainly Ca2+) in Beijing atmosphere have played an important role to neutralize the acidity of rainwater. However, it is worth noting that there is a remarkable acidification trend of rainwater in Beijing recent years. (c) 2009 Published by Elsevier Ltd.