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

Abstract

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, SO 4 2−, Cl −, NO 3 −, Na +, Ca 2+, Mg 2+ and NH 4 + concentrations. pH had a mean value of 6.08 and showed a slight increase as tourism developed between 1989 and 2006. SO 4 2− and Ca 2+ were the dominant anion and cation, respectively. Most of the ions showed significantly increasing trends, especially Ca 2+ and Mg 2+, with the exception of NH 4 + that had a clearly decreasing trend. As a result, the neutralization capacity of Ca 2+ 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 Ca 2+ and Mg 2+, and significant correlation of SO 4 2− with Ca 2+ and Mg 2+. 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 SO 4 2−: 99.1%, Mg 2+: 92.6% and Ca 2+: 99.8%. Furthermore, about 95.4% of NO 3 − and 41.7% of SO 4 2− were contributed by anthropogenic sources, and 57.4% of SO 4 2− 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.