Sources and photochemistry of volatile organic compounds in the remote atmosphere of western China: Results from the Mt. Waliguan Observatory
The chemistry of the natural atmosphere and the influence by long-range transport of air pollution are key is- sues in the atmospheric sciences. Here we present two in- tensive field measurements of volatile organic compounds (VOCs) in late spring and summer of 2003 at Mt. Waliguan (WLG, 36.28◦ N, 100.90◦ E, 3816ma.s.l.), a baseline sta- tion in the northeast part of the Qinghai-Tibetan Plateau. Most VOC species exhibited higher concentrations in late spring than in summer. A typical diurnal variation was ob- served with higher nighttime levels, in contrast to results from other mountainous sites. Five different air masses were identified from backward trajectory analysis showing dis- tinctVOCspeciation. Air masses originating from the central Eurasian continent contained the lowest VOC levels com- pared to the others that were impacted by anthropogenic emissions from China and the Indian subcontinent. A pho- tochemical box model based on the Master Chemical Mech- anism (version 3.2) and constrained by a full suite of mea- surements was developed to probe the photochemistry of atmosphere at WLG. Our results show net ozone produc- tion from in situ photochemistry during both late spring and summer. Oxidation of nitric oxide (NO) by the hydroper- oxyl radical (HO2) dominates the ozone production rela- tive to the oxidation by the organic peroxy radicals (RO2), and the ozone is primarily destroyed by photolysis and re- actions with the HOx (HOx =OH+HO2) radicals. Ozone photolysis is the predominant primary source of radicals Instrumentation (ROx =OH+HO2+RO2), followed by the photolysis of Methods and secondary oxygenated VOCs and hydrogen peroxides. The Open Access radical losses are governed by the self and cross reactions Data Systems among the radicals. Overall, the findings of the present study provide insights into the background chemistry and the im- pacts of pollution transport on the pristine atmosphere over Geoscientific the Eurasian continent.