Abstract:  As an effective means to study the atmospheric chemical composition change, atmospheric precipitation chemistry could accurately reflect the local atmospheric environmental quality and pollution condition. The precipitation chemistry features far away from human colonies can reflect the background value of atmospheric chemistry, and help to study the atmosphere chemical conversion, transmission and the formation process and mechanism of acid rain. Restricted by precipitation seasonal distribution and the conditions of observation in the river basin, the pH, EC and ion composition of precipitation were analyzed in the Yushugou River basin in EasternTianshan from May to August 2013. The ionic sources were simultaneously investigated. The results showed that the precipitation was neutral with mean pH of 6.8. It suggested that there was no acid rain during the sampling period in study area. The EC ranged from 3.79-239 μs/cm and it was obviously affected by amount of precipitation. With the increase of rainfall, the under cloud wash action diluted the conductive ions in precipitation, at the same time, precipitation conductivity decrease correspondingly. The concentration orders of cations and anions are Ca²⁺> NH4⁺>Na⁺>K⁺>Mg²⁺ and SO₄^2->NO3->Cl->F-, respectively. The order of the cationic was exactly the same as that of the elements order in the crust and different from the order of cation concentration in the seawater (Na⁺ > Mg²⁺> Ca²⁺ > K⁺). It may be due to its geographic location. The Yushugou River basin is located in arid inland areas distributing a large area of desert sand, which makes the ion concentration sequence in the atmospheric precipitation be controlled by terrigenous material. The SO₄^2-/NO₃^- ratio (1.76) indicated that SO42- was the main acid-causing substance in precipitation. The source of the different ion in summer precipitation is different. SO42^-、NO3^-、F^- and NH4⁺ mainly comes from man-made sources, such as burning coal, vehicle emissions, and agricultural production activities. Cl^- almost came from the sea salt source and crust source has little impact on it. The source of sodium affected by sea salt meanwhile quite part was from terrigenous. Ca2⁺ and K⁺ come from the non-sea-salt, which were mainly affected by the earthcrust and human activities. In the study area， there were four precipitation conveying paths which were northwest, southwest, southeast, and north four paths. Northwest path air masses originated from western Russia and Kazakhstan by the way of western Xinjiang. Path moisture originated from Mongolia passed across western Inner Mongolia with low population density, relatively underdeveloped economy, meanwhile industrial agricultural activity was frequent. The human activities exist in northwest pass. Through Northwestern and northern path there was a large area of desert with alkaline soil. It brought a large amount of alkaline substances to the atmosphere where alkaline substances could neutralize the acid precipitation. Southwest path water vapor was from southwest inland areas in China, passing along the eastern Qinghai-Tibet Plateau. The vegetation in southwest inland was in good condition and less affected by human activities. Along the southeast path air mass from the Midwest region in China, the vegetation coverage is very low, so the precipitation was greatly influenced by terrigenous. By studying the chemical characteristics of atmospheric precipitation in the river basin, this paper discussed material source of the precipitation. This research will provide data for further studies of water cycle mechanism in this mountain basin.

Key words: precipitation, chemical characteristic, ion source, Yushugou River basin