Study Of Spatial And Temporal Variations Of Ozone And Its Main Causes In Hong Kong

Based on the measurements of chemical compositions on several sites of geographical difference (an upwind site, a downtown site, several downwind sites and a mountain site) in Hong Kong from 2011 to 2014, this article aims to gain insights into the spatiotemporal variations of ozone and the dominate meteorological factors and chemical processes influencing the ozone distribution in Hong Kong, by means of several statistical and analysis methods, air mass trajectory analysis and specific case study.With a detailed comparison of the seasonal profiles and diurnal patterns of ozone and total oxidants (Ox) in different measurement sites in Hong Kong, horizontal distribution of ozone is made available. The seasonal profile of ozone in Hong Kong shows a peak in October and a trough in July, and the diurnal variation of ozone displays a bimodal pattern, with a maximum in 1500-1600LT and a secondary peak at about 0400LT. Furthermore, downtown site has a shorter time gap between these two peaks when compared with suburban site and the new town site. Besides, with three years’ global analysis data, the HYSPLIT model made it possible that backward trajectories were calculated and clustered into four groups, representing four air mass transporting scenarios which influence the ozone and its precursors in Hong Kong, respectively. The dominant air mass type varies considerably with the seasons; precisely speaking, air masses from the south sea dominate in summer while those from the coastal cities in the mainland of China take over in winter and autumn.Analysis of the causes of four ozone episodes in the annual downwind location of Hong Kong, i.e. the Hong Kong international airport and its surrounding regions, sheds light upon the ozone distribution variance on a smaller spatial scale. With a maritime site as the background site of air mass from the Pearl River Delta (PRD) areas in the mainland of China transporting to Hong Kong under certain circumstance, the local emissions of the Hong Kong airport were then evaluated, characteristics of the airport emissions were analyzed and contributions of the air mass from PRD areas to Hong Kong in respect of NO, NO2 and CO were assessed. It’s found that the mixing ratio of NO2 among the airplane exhausts is much higher than that of the auto emissions on road, which strongly implies that airport sites are not suitable as representative of the downwind site of Hong Kong. In the light of the fact that Tung Chung site can be greatly influenced by air masses from PRD areas through regional transportation, it is obvious that another site is needed to characterize the oxidizing properties of the downwind area of Hong Kong.Lastly, with comparing the measurements of trace gases on the mountain site Tai Mo Shan with that of the ground site Tap Mun, vertical distribution and annual change of ozone and its main causes were made accessible. A comprehensive analysis of the reasons for two types of ozone episodes on the mountainous area in spring and summer renders it clear how the daily variation of boundary layer height, the mesoscale circulation and the typhoon in west Pacific influence the ozone distribution on the mountain site in Hong Kong. It’s found that the annual change of monsoon has a pronounced impact on the long-distance transportation of certain trace gases, which can further influence the annual variation of ozone distribution in Hong Kong. Besides, typhoon originated in the west Pacific in summer can act as a passage for air masses enveloped with high concentrations of ozone precursors in northern China transferring to Hong Kong, bringing ozone episodes to mountainous areas like Tai Mo Shan in Hong Kong.