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The chemistry of the natural atmosphere and the influence by long-rangetransport of air pollution are key issues in the atmospheric sciences. Herewe present two intensive field measurements of volatile organic compounds(VOCs) in late spring and summer of 2003 at Mt. Waliguan (WLG,36.28° N, 100.90° E, 3816 m a.s.l.), a baseline stationin the northeast part of the Qinghai-Tibetan Plateau. Most VOC species exhibitedhigher concentrations in late spring than in summer. A typical diurnalvariation was observed with higher nighttime levels, in contrast to resultsfrom other mountainous sites. Five different air masses were identified frombackward trajectory analysis showing distinct VOC speciation. Air massesoriginating from the central Eurasian continent contained the lowest VOClevels compared to the others that were impacted by anthropogenic emissionsfrom China and the Indian subcontinent. A photochemical box model based onthe Master Chemical Mechanism (version 3.2) and constrained by a full suiteof measurements was developed to probe the photochemistry of atmosphere atWLG. Our results show net ozone production from in situ photochemistry duringboth late spring and summer. Oxidation of nitric oxide (NO) by thehydroperoxyl radical (HO2) dominates the ozone production relative tothe oxidation by the organic peroxy radicals (RO2), and the ozone isprimarily destroyed by photolysis and reactions with the HOx(HOx = OH + HO2) radicals. Ozone photolysis is thepredominant primary source of radicals (ROx = OH + HO2 + RO2), followed by the photolysis of secondaryoxygenated VOCs and hydrogen peroxides. The radical losses are governed bythe self and cross reactions among the radicals. Overall, the findings of thepresent study provide insights into the background chemistry and the impactsof pollution transport on the pristine atmosphere over the Eurasiancontinent

Sources and photochemistry of volatile organic compounds in the remote atmosphere of western China: results from the Mt. Waliguan Observatory