Source of surface ozone and reactive nitrogen speciation at Mount Waliguan in western China : new insights from the 2006 summer study

measurement period of 2006 (55%) than that of 2003 (25%). The abnormally high values of NOy observed in 2003 were suspected to be due to the positive interference from ammonia (NH3) to the particular catalytic converter used in that study. Varied diurnal patterns were observed for the various NOy components. The ozone production efficiencies (DO3/DNOz), which were estimated from the slope of the O3‐NOz scatterplot, were 7.7–11.3 for the polluted plumes from central and eastern China. The speciation of reactive nitrogen was investigated for the first time in the remote free troposphere in western China. PAN and particulate NO3 − were the most abundant reactive nitrogen species at WLG, with average proportions of 32% and 31%, followed by NOx (24%) and HNO3 (20%). The relatively large contribution of particulate NO3 − to NOy was due to the presence of high concentrations of NH3 and crustal particles, which favor the formation of particulate nitrate. An analysis of backward trajectories for the recent 10 years revealed that air masses from central and eastern China dominated the airflow at WLG in summer, suggesting strong impact of anthropogenic forcing on the surface ozone and other trace constituents on the Plateau.