Chemical and Sr isotopic characteristics of rainwater on the Alxa Desert Plateau, North China: Implication for air quality and ion sources

The major ions and Sr isotopes in rainwater have been studied during 2013-2015 on the Alxa Desert Plateau in order to identify the source of rainwater chemistry and to assess air quality in the desert area of northern China. The pH and EC values of rainwater vary from 6.7 to 8.1 and from 35 to 1237 mu S cm(-1), respectively, at the two meteorological stations (AYQ and YBL) in the Alxa Desert Plateau. Ca2+, SO42- , Na+ and Cl- are the dominant ions in rainwater, possessing > 85% of total ions. The mean daily wet deposition fluxes of soluble ions are 8709 mu eq/m(2)/d at YBL and 5459 mu eq/m(2)/d at AYQ, approaching the values at Xi'an, Beijing, Guangzhou, and Chengdu. Statistical analysis shows that SO42- and NO3- in rainwater were mainly from anthropogenic sources while Ca2+ and K+ originated from terrestrial sources. Cl- was mainly from seawater sources, and Na+ was partly from mineral weathering. Major ions are well correlated with each other in rainwater, revealing that substances of various origins were synchronously carried into the atmosphere by wind. By using Sr isotope techniques, three main end-members controlling base cations of rainwater are identified: silicates, carbonates and seawater. Based on the analyses of acid-soluble fractions of desert soils, local soil dust could be the most. important source of base cations in rainwater whereas the effect of the anthropogenic sources could be neglected.