Characteristics of ozone and its precursors in Northern China: A comparative study of three sites

Regional composite pollution and variation of atmospheric oxidizing ability are significant problems in Beijing and its surrounding areas. In particular, a southwest-northeast prevailing wind makes nonlinear variation of ozone and its precursors complex. In this study, simultaneous measurements of surface ozone (O3), nitric oxide (NO), nitrogen dioxide (NO2), nitrogen oxide (NOx) and carbon monoxide (CO) were carried out from August 2009 to June 2010 to investigate ozone level and precursor concentrations in Northern China. Gucheng (GC), Longtanhu (LTH) and Xinglong (XL) were selected to represent rural area, urban area and background area in North China Plain, respectively. The results showed that there were two peaks for primary pollutants (NOx and CO) at urban and rural areas, and the highest values appeared nearly on rush hours. There was a single peak for O3 and Ox with peak values appearing at afternoon. The primary pollutants at urban and rural areas were higher than those in background region. However, the concentration of secondary pollutant O3 at downwind background areas was much higher than other sites due to transportation. Diurnal variations of pollutants indicated that O3 and Ox had peak values at afternoon, while other primary pollutants showed two peak values nearly on rush hours. Furthermore, high concentrations of O3 and Ox at background site XL also implied a robust atmospheric oxidizing ability. A positive correlation was observed between CO and O3 at XL site, which implied that air masses to the site had a large loading of anthropogenic emission experiencing strong photochemical processing. The correlation between CO and O3 was negative at GC and LTH, which indicated that local emission of precursors had significant influence on the formation of ozone. There was a positive correlation between NOx and Ox at GC and XL, which implied that NOx contributed to the formation of ozone, while a negative correlation meant titration in urban site LTH neutralized the formation of ozone due to high level of NO.