Chemical and strontium isotope characterization of rainwater in Beijing, China

Major ion concentrations and Sr isotope ratios (87Sr/86Sr) 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 μeq l−1 (14–1781 μeq l−1) and 186 μeq l−1 (48–672 μeq l−1), respectively. SO42− was the predominant anion with VWM value of 316 μeq l−1 (65–987 μeq l−1), next was NO3− with VWM value of 109 μeq l−1 (30–382 μeq l−1). 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 87Sr/86Sr 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 (87Sr/86Sr∼0.90917), soluble soil dust minerals originating from either local or the desert and loess areas (∼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.