Chemical composition of rainwater at Lijiang on the Southeast Tibetan Plateau: influences from various air mass sources

Daily rainwater samples collected at Lijiang in 2009 were analyzed for pH, electrical conductivity, major ion (SO4 (2-), Cl-, NO3 (-), Na+, Ca2+, Mg2+, and NH4 (+)) concentrations, and delta O-18. The rainwater was alkaline with the volume-weighted mean pH of 6.34 (range: 5.71 to 7.11). Ion concentrations and delta O-18 during the pre-monsoon period were higher than in the monsoon. Air mass trajectories indicated that water vapor from South Asia was polluted with biomass burning emissions during the pre-monsoon. Precipitation during the monsoon was mainly transported by flow from the Bay of Bengal, and it showed high sea salt ion concentrations. Some precipitation brought by southwest monsoon originated from Burma; it was characterized by low delta O-18 and low sea salt, indicating that the water vapor from the region was mainly recycled monsoon precipitation. Water vapor from South China contained large quantities of SO4 (2-), NO3 (-), and NH4 (+). Throughout the study, Ca2+ was the main neutralizing agent. Positive matrix factorization analysis indicated that crustal dust sources contributed the following percentages of the ions Ca2+ 85 %, Mg2+ 75 %, K+ 61 %, NO3 (-) 32 % and SO4 (2-) 21 %. Anthropogenic sources accounted for 79 %, 68 %, and 76 % of the SO4 (2-), NO3 (-) and NH4 (+), respectively; and approximately 93 %, 99 %, and 37 % of the Cl-, Na+, and K+ were from a sea salt source.