Abstract: As an effective means to study the change of atmospheric chemical composition,atmospheric precipitation chemistry could accurately reflect the local atmospheric environmental quality and air pollution.The precipitation chemical properties far away from human colonies can reflect the background value of atmospheric chemistry,and can also help us to study the atmosphere chemical conversion,transmission and the formation process and mechanism of acid rain.Restricted by the seasonal precipitation distribution and the observation conditions in drainage basin,the pH value,EC and ion composition in precipitation in the Yushugou River Basin in the east Tianshan Mountains from May to August 2013 were analyzed.The ionic sources were simultaneously investigated.The results showed that precipitation was neutral with mean pH value of 6.8.It suggested that there was no acid rain during the sampling period in the study area.The EC was in a range of 3.79-239 μs·cm^-1,and it was obviously affected by precipitation.With the increase of rainfall,the under cloud wash action diluted the conductive ions in precipitation,and the precipitation conductivity decreased correspondingly.The concentrations of cations and anions were in the orders of Ca²⁺ > NH⁺₄ > Na⁺ > K⁺ > Mg²⁺ and SO^2- ₄ > NO^-₃> Cl^- >F^- respectively.The cationic order was exactly the same as that of the elements in crust and different from that of cation concentration in seawater (Na⁺ > Mg²⁺ > Ca²⁺ > K⁺).It may be due to the geographic location.The Yushugou River Basin is located in an arid area where large deserts are distributed.The SO^2-₄/NO^-₃ ratio (1.76) indicated that SO^2-₄ was the main acid-causing substance in precipitation.The sources of the different ions in summer precipitation were different.SO^2-₄,NO^-₃,F^- and NH⁺₄ came mainly from man-made sources,such as coal burning,vehicle emission and agricultural production activities.Cl^- almost came from the sea salt source and crust source.The sodium concentration was mainly affected by sea salt,and a quite part of which was from terrigenous sources.Ca²⁺ and K⁺ came from the non-sea-salt and affected mainly by the earth surface and human activities.In the study area, there were four water vapor paths including the northwest,southwest,southeast and north paths.Air mass from the northwest path originated from west Russia and Kazakhstan and passed through west Xinjiang.Water vapor originated from Mongolia passed across west Inner Mongolia with low population density and relatively underdeveloped economy.Human activities were frequent along the northwest path.There were the large deserts with alkaline soil along the northwest and north paths.Large amounts of alkaline substances were carried up to the atmosphere where alkaline substances could neutralize acid rain.Water vapor from the southwest path was from southwest inland of China and passed over the east Qinghai-Tibet Plateau.Vegetation in southwest inland grew well and less affected by human activities.Air mass along the southeast path was from the midwest region of China,and the vegetation coverage was very low,so precipitation was greatly influenced by terrigenous sources.

Key words: precipitation, chemical property, ion source, air trajectories, Yushugou River Basin, East Tianshan