Effect of meteorology and air pollutant transport on ozone episodes at a subtropical coastal Asian city, Hong Kong

Abstract

Hong Kong is a subtropical coastal city situated at the rapid urbanizing and industrializing region of South China. High frequencies of elevated ozone are found in various remote, rural, and metropolitan districts of Hong Kong. Meteorological conditions related to synoptic-scale flow pattern, mesoscale weather phenomenon, and local micrometeorology have strong effects on the pollutant transport to South China and the occurrence of ozone episodes in Hong Kong. The transport processes and the source regions of ozone are analyzed with the combined use of surface pressure patterns, streamline charts and prevalent winds, and with representative episodes. The synoptic meteorological conditions can be classified into nine patterns. The patterns associated with winter monsoon and traveling cyclones are found to be most conducive to the occurrence of ozone episodes. Long-range transport of ozone with enhanced CO level from the aged continental air accompanying northeast monsoon contributes to substantial cases of episodes, which are especially noticeable in the upwind remote areas. These episodes feature consistently high ozone at night in the remote station and early morning peaks in the metropolitan stations. The possible source regions of this ozone may include Asian continent, the coastal areas of Mainland China, southern Japan, and Taiwan. This high inflow of background ozone together with elevated ozone enhanced by the channeling effect, and local transport causes the highest frequencies of ozone episodes in the downwind western side of the territory. The short-range transport of ozone and pollutants from the industrial and urban centers in the Pearl River Delta region and Guangdong Province in southern China is another source of ozone. When transported ozone is being superimposed on ozone formed from local sources, territory-wide and extremely high concentration episodes are found. Our meteorological and ozone episode diagnosis have also demonstrated that the ‘background’ outflow of Asian pollutants can be significantly modified on its way to the Pacific Ocean when passing through the fast growing Pearl River Delta cities such as Hong Kong.