Impact of long-range transport on the levels of trace gases and fine aerosols at two high-altitude sites in western and eastern China

We present measurement results of ozone (O3), carbon monoxide (CO), non-methane hydrocarbons (NMHCs), halocarbon, and fine aerosols obtained in 2003 at the summit of Mount Tai (1534 m above sea level, 36.25 ºN, 117.10 ºE) and Mount Waliguan (3816m a.s.l., 36.28ºN 100.90ºE). The study was carried out to characterize the compositions of chemically active trace gases, aerosols, and inert pollution tracers over the highly populated North China plains and the pristine Qinghai-Tibetan Plateau and to examine the extent of intercontinental transport of air pollution to western China and the regional transport in eastern China.

The overall composition, daily, and monthly variations in the concentrations of the measured species are examined. As expected, Mount Waliguan, which is situated in a remote area in sparsely populated western China, showed very low levels of gases and aerosols of anthropogenic origins. Their levels are lower than those reported in remote mountainous sites in Europe such as Jungfraujoch and Cimone. This and the result from an examination of the variability versus lifetimes of trace gases imply the site is a good location to monitor continental background air. However, unusually high levels of reactive nitrogen (NO and NOy) were observed, which is believed to be due to enhanced emissions from soils relating to animal grazing in the region. By contrast, the concentrations at Mount Tai were much higher, reflecting the close proximity of site to major pollution sources in eastern China. An examination of diurnal variations in the trace gases levels revealed different transport processes at the two mountain sites contributing to the higher nighttime concentrations of combustion-related pollutants such as CO and NMHCs at Mount Waliguan and higher daytime levels at Mount Tai. Ozone and CO showed a moderately good (r=0.60) positive correlation in July in the eastern site with a slope of 0.08 ppbv/ppbv, which is much smaller than the slope (0.3-0.4 ppbv/ppbv) observed in North America, while there was little correlation between O3 and CO in the western site. A back trajectory analysis was carried out to examine the chemical characteristics of air masses from different geographic origins and the possible influence of biomass burning in Siberia. The results are compared with previous work in eastern Asia and the western Pacific, particularly those from TRACE-P and ACE-Asia, to elucidate the relationship between inflow to eastern Asia and outflow to downwind western Pacific.