Chemical composition of rainwater samples collected at the summit of Mount Tai (1534m asl.) in eastern-central China and the relation to air-mass transport

Acid deposition has long been recognized as a serious environmental problem in eastern Asia, especially in China. Here we present recent measurements of chemical composition of wet deposition samples collected from June to December 2004 at the summit of Mt. Tai (1534 m asl, 36.25 oN, 117.10 oE), the highest mountain site in the North China Plains.

The ionic concentrations, pH value, and conductivity of these samples have been analyzed. The results show that 65% of the samples had a pH value lower than 5.6, suggesting more frequent acid-rain pollution than expected for northern China. A trajectory analysis was carried to gain insights into the influence of trans-boundary transport from different regions in Asia on the acidity and composition of the collected rain samples. The results suggest that the samples with much lower pH values were mainly brought by air masses originated from southwestern China where emissions of SO2 are high. Rainfall carried by air masses originated from northern China and those from northwestern China showed an important feature of rainwater composition in large part of North China – low acidity, high in sulfate and calcium. The latter plays a key role in neutralizing the high concentration of acidic species. However in some cases in this air-mass group, in which the air masses had spent long traveling time in the lower troposphere of the North China Plains, a pH value of 4.6 was observed, revealing the rain samples were acidified during these events. The trajectory analysis also slowed that air masses originated from western Pacific had significant oceanic characteristics such as high concentrations of sodium and chlorine. There were few trajectories passing over Korea and Japan during the sampling periods of this study, thus more samples will be needed to evaluate the possible contributions from these regions.