环境地球化学国家重点实验室知识库

Rainwater samples collected from Guiyang city in southwestern China between May 2009 and December 2010 were analyzed to measure their calcium and strontium isotope compositions. The results show that Ca²⁺ is the most abundant cation in these rainwater samples, while SO₄²⁻ is the most abundant anion. Most rainwater samples are alkaline, with a volume-weighted mean (VWM) pH value of ~6.2 (a 1.2-unit increase since 2008). This can be explained by the neutralization of rainwater acidity by the dissolution of atmospheric aerosols in the form of calcium-rich soil and other anthropogenic dust, which are increasing because of rapid urbanization. Both stable (δ^44/40Ca and δ^88/86Sr) and radiogenic (⁸⁷Sr/⁸⁶Sr) isotope compositions of rainwater show variations correlating with each other and with the inverse of Sr or Ca concentrations. They also correlated with the concentration ratio of (NH₄⁺+K⁺)/(Ca²⁺+Mg²⁺) ―an index reflecting the anthropogenic input relative to soil dust. Based on our results, three major reservoirs can be inferred to have contributed to rainwater chemistry in Guiyang city: (1) a carbonate aerosol source characterized by low ⁸⁷Sr/⁸⁶Sr (≤~0.708), δ^88/86Sr (≤~0.32‰) and δ^44/40Ca (~0.5‰), very low (NH₄⁺+ K⁺)/(Ca²⁺+Mg²⁺), and low 1/Ca²⁺ and 1/Sr²⁺; (2) a silicate aerosol source characterized by relatively high ⁸⁷Sr/⁸⁶Sr (≥0.717), δ^88/86Sr (≥~0.34‰) and δ^44/40Ca (~0.8‰), very low (NH₄⁺+K⁺)/(Ca²⁺+Mg²⁺), low 1/Ca²⁺ and 1/Sr²⁺; and (3) an anthropogenic component containing low ⁸⁷Sr/⁸⁶Sr (~0.708), δ^88/86Sr (~0.30‰) and high δ^44/40Ca (~0.8‰), and high (NH₄⁺+K⁺)/(Ca²⁺+Mg²⁺). The chemistry of solutes in Guiyang rainwater can be explained by (1) dissolution of carbonate and silicate aerosols, as well as the anthropogenic inputs including seasonal farming activities; (2) tightened environmental protection regulations; (3) higher precipitation volumes in 2010; and possibly (4) the increased combustion of coal for heating and electricity in winter. This study demonstrated that a combination of elemental compositions, stable Ca and Sr, and radiogenic Sr isotope compositions in rainwater can be used to trace the sources of atmospheric aerosols.