Characteristics and sources of atmospheric composition based on precipitation chemistry in the Shiyang River Basin,Northwestern China

Through collected precipitation samples continuously to research the precipitation chemical characteristics and the water vapor source in Shiyang River Basin, Northwestern China, a total of 121 precipitation samples had been collected in the Shiyang River Basin from July 2013 to July 2014. The results showed that, during the period of sampling, the variation range of pH value of precipitation ranged from 6.62 to 8.53 in Shiyang River Basin and the average was 7.46. The EC values ranged from 17.28 to 787.00 μs/cm, with a mean 186.66 μs/cm. Ca²⁺ and Na⁺ dominated cations and accounted for 71.80% of total cations composition, while SO₄ ^2? and NO₃ ^? dominated anions with a contribution of 82.18%. The main ionic sources are local dust aerosols and the dust from Central Asia and Northwestern China arid regions brought by the westerly, and the back trajectories also supported it. Back trajectories suggested that the air mass arrived in Shiyang River Basin mainly from the dust sources region in Central or West Asia or Northwestern China through westerly circulation in dry season, while from the Indian Ocean or the Pacific Ocean through monsoon circulation in wet season. The precipitation can be divided into three types based on the vector of water vapor transportation: the monsoon precipitation, the westerly precipitation, and the interaction precipitation (precipitation influenced both by monsoon and westerly). The type of interaction precipitation was major precipitation patterns in study area, and the westerly precipitation came next. The results are also helpful for further understanding the air pollution situation caused by dust events in study area and providing scientific basis for the effective prevention and control of atmospheric pollution.