The backward trajectories of 239 precipitation processes from 1999 to 2007 at North China regional atmospheric background station were classified. Typical north-west, south-west and south-east backward trajectory precipitation samples were selected to research the characters of the precipitation pH, electrical conductivity, chemical composition, and the main sources affect the background area. Results showed that the south-west backward trajectory coming from the east-central inland area of China, passing through Taiyuan, Shijiazhuang, and Beijing, the north-west trajectory coming from Mongolia, passing through inner Mongolia area, and the south-east trajectory coming from oceanic area, passing through east coastal cities were the main backward trajectories affecting the precipitation at Shangdianzi station. The precipitation frequency in the south-west trajectory was the highest. The pH peaked in the south-west transportation direction and reached a minimum in the south-east. The electrical conductivities reached a maximum in the south-west transportation direction and a minimum in the north-west. SO2-4, NO-3, NH+4, and Ca2+ were the major ions of precipitation at Shangdianzi. The concentrations of the major ions of the south-west trajectory samples were much higher than that of the other directions. Soil dust, farm emission related to fertilization, pollutants from traffic and coal combustion emission were the main four sources identified affecting the background area. The natural source was the primary source affecting the background area in the north-west clean transportation direction, while the anthropogenic sources were more notable in the south-west transportation direction, such as traffic and coal combustion emission. Compare to 1999-2004,the averaged pH decreased from 2005 to 2007, which attributes to the decreasing of the concentrations of alkaline ions in precipitation. The results show that rainfall is significant for the conductivity, acid rain frequency, and the components of the precipitation. The values of conductivity and components concentration decrease and the acid rain frequency increase as the rainfall increase gradually.
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