Ca and Sr isotope compositions of rainwater from Guiyang city, Southwest China: Implication for the sources of atmospheric aerosols and their seasonal variations

Abstract

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 (δ44/40Ca and δ88/86Sr) 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), δ88/86Sr (≤~0.32‰) and δ44/40Ca (~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), δ88/86Sr (≥~0.34‰) and δ44/40Ca (~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), δ88/86Sr (~0.30‰) and high δ44/40Ca (~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.