The major ions concentration and δ15N-NO3- of rain samples inin Eastern Tibetan Plateau
The deposition of atmospheric nitrate (NO3−) reflects a balance of anthropogenic and natural nitrogen oxide (NOX) sources. The Tibetan Plateau, one of the most pristine global areas, is highly sensitive to anthropogenic effects, thus is of great importance for evaluating human activity disturbance on natural system. However, the current sources of nitrate deposition on the Tibetan Plateau are still poorly constrained. In this study rainwater from the Gongga Mountain, which is located in the eastern Tibetan Plateau, was collected from Aug. 2014 to Aug. in 2015, and the chemical and nitrogen isotopic compositions were determined. The δ15N composition of nitrate in rainwater was found to be high during the early cool season (November to January: −2.8‰), and low during the late cool season (February to March: −15.3‰), with intermediate values during the warm season (April to October: −5.8‰). In the warm season, trajectory paths and similar intermediate δ15N values imply that sea aerosol from the Indian Ocean as the main source of the precipitation nitrate. In the cool season, variable δ15N values suggest a more complicated and dynamic control on precipitation nitrate in the eastern Tibetan Plateau. In the early cool season, anthropogenic NOX derived from fuel combustion in the Indian sub-continent are inferred to dominate the precipitation NOX in the study site, based on high K+ concentration and nitrogen isotopic value and on the trajectory analyses. The increased intensity of agricultural activity in the Indian sub-continent during the late cool season shifts the δ15N values lower and drives a peak of NH4+ and NO3− concentration of the precipitation. These findings represent the first constraints on the origin of precipitation nitrate in the Tibetan Plateau and demonstrates that the Tibetan Plateau responds quickly to anthropogenic nitrate derived by long−range transport from the Indian sub-continent.