Characteristics of gaseous pollutants at Gucheng, a rural site southwest of Beijing

During the Campaigns of Air Quality Research in Beijing and Surrounding Region 2006, ambient O-3, NOx, SO2, and CO are observed for the first time at Gucheng site southwest of Beijing. This paper characterizes these pollutants, analyzes their sources, and studies their pollution levels. From July 2006 to September 2007 the average concentrations of NOx, CO, SO2, O-3, and O-x(= O-3 + NO2) are 29.0 +/- 19.4 ppb, 1.52 +/- 0.73 ppm, 17.1 +/- 12.8 ppb, 28.5 +/- 13.8 ppb, and 47.1 +/- 8.5 ppb, respectively. The levels of these gases varied greatly from season to season, with the levels of NOx, CO, and SO2 being higher and the level of O-3 being lower in winter and the levels of NOx, CO, and SO2 being lower and the level of O-3 being higher in summer. Levels of O-3 and SO2 often exceed Grades I or II of the Chinese Ambient Air Quality Standards. According to estimation of source contributions, biomass burning seems to be one of the major sources for the air pollutants, particularly CO and NOx. The ground measurements of NO2 and O-3 at Gucheng correlate well with the satellite measurements of tropospheric columns NO2 and O-3, respectively, which suggests that the site has good regional representativeness. Therefore the site is well situated for monitoring regional background concentrations of air pollutants in the North China Plain and an ideal upwind site for studying air quality in Beijing. In August, over 80% of trajectories of more polluted air masses moved northward from Gucheng and passed over Beijing.