On the local and regional influence on ground-level ozone concentrations in Hong Kong

Hong Kong is a densely populated city situated in the fast developing Pearl River Delta of southern China. In this study, the recent data on ozone (O3) and related air pollutants obtained at three sites in Hong Kong are analyzed to show the variations of O3in urban, sub-urban and rural areas and the possible regional influences. Highest monthly averaged O3was found at a northeastern rural site and lowest O3level was observed at an urban site. The levels of NOx, CO, SO2and PM10showed a different spatial pattern with the highest level in the urban site and lowest at the rural site. Analysis of chemical species ratios such as SO2/NOxand CO/NOxindicated that the sites were under the influences of local and regional emissions to varying extents reflecting the characteristics of emission sources surround the respective sites. Seasonal pattern of O3is examined. Low O3level was found in summer and elevated levels occurred in autumn and spring. The latter appears different from the previous result obtained in 1996 indicating a single maximum occurring in autumn. Principal component analysis was used to further elucidate the relationships of air pollutants at each site. As expected, the O3variation in the northeastern rural area was largely determined by regional chemical and transport processes, while the O3variability at the southwestern suburban and urban sites were more influenced by local emissions. Despite the large difference in O3levels across the sites, total potential ozone (O3+NO2) showed little variability. Cases of high O3episodes were presented and elevated O3levels were formed under the influence of tropical cyclone bringing in conditions of intense sunlight, high temperature and light winds. Elevated O3levels were also found to correlate with enhanced ratio of SO2to NOx, suggesting influence of regional emissions from the adjacent Pearl River Delta region.