Chemistry and source identification of wet precipitation in a rural watershed of subtropical China

South China is one of the regions severely suffering from acid rain in the world. However, few systematic studies of rural precipitation chemistry have been performed in comparison with the extensive studies on their urban counterparts of this region. In order to characterize the current acid rain status and identify its possible sources in the rural area of South China, we analyzed precipitation collected event by event from a rural forested watershed in southern Anhui Province between March 2007 and February 2010. The results showed that the concentrations of major ions within precipitation in the studied rural area were significantly lower than those reported for the urban areas of the same latitude in China. Nevertheless, the precipitation acidity (with an average pH value of 4.49) and the frequency of acid rain (95%) were considerably high. The relatively high ratio of (SO₄ ^2?+ NO₃ ^?)/(Ca²⁺+NH₄ ⁺) was the main cause of acid rain in this rural area, as SO₂ and NO_x were the main precursors of acid rain, while Ca²⁺ and NH₄ ⁺ acted as the dominant neutralizers to the acidity. Source identification indicated that Ca²⁺ and Mg²⁺ mainly were derived from alkaline dust, SO₄ ^2?, NO₃ ^? and NH₄ ⁺ originated mainly from anthropogenic sources such as industrial and agricultural activities, most Na⁺, Cl^?, K⁺ and some of Mg²⁺ were derived from the sea. The results suggested that the major ions within precipitation in the rural area of South China were related to the meso-scale and long-range transport of particles and aerosols in the air.