Anthropogenic influence on rainwater in the Xi'an City, Northwest China: Constraints from sulfur isotope and trace elements analyses

Abstract

Sulfur isotopes of sulfate and trace elements in the rain and snow water samples from the Xi'an urban and rural sites were measured from 2007 to 2008 to investigate atmospheric pollution and identify anthropogenic influence on precipitation. The results show that the δ34S values of SO42− ranging from +8.71‰ to +19.05‰ (average +13.41‰, n=30) in the Xi'an urban site and from +4.67‰ to +20.59‰ (average +11.23‰, n=31) in the suburb site are significantly higher with respect to those generally reported for precipitation samples around the world. This indicates a dominant source of coal-burning emissions of SO2 in the studied precipitation samples due to sulfur in the coal from Northern China having relative high δ34S value. Trace elements such as Ti, Cr, Mn, Co, Ni, Cu, Zn, As, Mo, Cd, Pb, Al and Fe were also analyzed by or Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) in the present study. Enrichment factors related to Si in both crust and loess suggest that most of the studied trace elements have an anthropogenic origin. Only Al, Ti and Fe show a significant loess source. While Cr, Co, Cu and Pb in the precipitation samples from both of the two sampling sites have similar distribution patterns compared to those of local fly ashes mostly originated from coal combustion. Zinc in the precipitation samples appears to be controlled mainly by industrial and/or traffic sources. Both of the trace elements and sulfur isotopic composition indicate that coal combustion is the main source for air pollutants in the Xi'an City and its surrounding regions. The study results also reveal that serious air pollution in the Xi'an City is widely diffused significantly affecting also the adjacent rural area up to 100km from the city.