Chemical and strontium isotope characterization of rainwater in Beijing, China: Fid-Bau Portal

Chemical and strontium isotope characterization of rainwater in Beijing, China

Xu, Zhifang / Han, Guilin

AbstractMajor 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 608μeql⁻¹ (14–1781μeql⁻¹) and 186μeql⁻¹ (48–672μeql⁻¹), respectively. SO4²⁻ was the predominant anion with VWM value of 316μeql⁻¹ (65–987μeql⁻¹), next was NO3⁻ with VWM value of 109μeql⁻¹ (30–382μeql⁻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 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.