Long-term trends in chemical composition of precipitation at Lijiang, southeast Tibetan Plateau, southwestern China

1090 precipitation samples were collected between 1989 and 2006 from the Lijiang region, an internationally important tourist site in Yunnan province, southwestern China. All the samples were analyzed for pH, electrical conductivity, SO42-, Cl-, NO3-, Na+, Ca2+, Mg2+ and NH4+ concentrations. pH had a mean value of 6.08 and showed a slight increase as tourism developed between 1989 and 2006. SO42- and Ca2+ were the dominant anion and cation, respectively. Most of the ions showed significantly increasing trends, especially Ca2+ and Mg2+, with the exception of NH4+ that had a clearly decreasing trend. As a result, the neutralization capacity of Ca2+ increased significantly, and the precipitation NP/AP (neutralizing potential index/acidifying potential index) ratio increased. There was a significant correlation between the soil-derived cations Ca2+ and Mg2+, and significant correlation of SO42- with Ca2+ and Mg2+. The sea-salt species Cl- and Na+ was not well correlated. Using Na+ as a sea-salt tracer, non-sea-salt source fractions were calculated as SO42-: 99.1%, Mg2+: 92.6% and Ca2+: 99.8%. Furthermore, about 95.4% of NO3- and 41.7% of SO42- were contributed by anthropogenic sources, and 57.4% of SO42- was contributed by soil/dust sources that had a remarkably strong relationship (r = 0.65, p<0.01) with the number of tourists, suggesting that human activities in a tourism-oriented city increase atmospheric dust loading. (C) 2011 Elsevier B.V. All rights reserved.