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

Surface ozone (O₃), carbon monoxide (CO), and total and speciated reactive nitrogen compounds (NO_y, NO, NO₂, PAN, HNO₃, and particulate NO_3-) were measured at Mount Waliguan (WLG; 36.28°N, 100.90°E, 3816 m above sea level (asl)) in the summer of 2006 to further understand the sources of ozone and reactive nitrogen and to investigate the partitioning of reactive nitrogen over the remote Qinghai-Tibetan Plateau. The mean mixing ratios of O₃, CO, NO _y, and daytime NO were 59 ppbv, 149 ppbv, 1.44 ppbv, and 71 pptv, respectively, which (except for NO_y) were higher than those measured from a previous campaign in summer 2003, which is consistent with more frequent transport of anthropogenic pollution from central and eastern China in the measurement period of 2006 (55%) than that of 2003 (25%). The abnormally high values of NO_y 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 NO_y components. The ozone production efficiencies (ΔO₃/ΔNO_z), which were estimated from the slope of the O₃-NO_z 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 NO_3- were the most abundant reactive nitrogen species at WLG, with average proportions of 32% and 31%, followed by NO_x (24%) and HNO₃ (20%). The relatively large contribution of particulate NO_3- to NO_y 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.