Analysis of meteorological criteria leading to tropical cyclone related ozone episodes in Hong Kong

Air quality in Hong Kong has been deteriorating in the past decade with an increasing number of ozone episodes occurring over the territory. Ozone events mostly occur when a tropical cyclone or a developing low pressure area is located over the South China Sea or the western North Pacific. Owing to the non-linear characteristics of ozone formation and the complex terrain of Hong Kong, it has been a great challenge in predicting local ozone episodes. Therefore, this study attempts to unveil the mystery of the problem from the meteorological point of view. In this dissertation, meteorological data during the tropical cyclone seasons between 1999 and 2002 are analyzed. It is revealed that local ozone episodes occur when tropical cyclones are located east or southeast of Hong Kong over the seas near Luzon and Taiwan. However, ozone event tends to happen when a stronger tropical cyclone is located farther away from the territory with a broader subsidence zone. The subsidence enhances cloud dissipation; thus a more efficient photochemical reaction. Besides, a detailed analysis of synoptic patterns, both at surface and the atmosphere aloft suggests that winds from appropriate direction that bring in ozone precursors from upstream region are crucial for the occurrence of episodes. To further investigate the wind information, a diagnostic wind field model is employed to simulate the 3-dimensional wind fields over Hong Kong. It is revealed that ozone episodes occur at Tung Chung when northwesterly winds prevail over northern coast of Lantau. Southwesterlies, however, blow over Tap Mun when ozone level rises in the afternoon. The simulated downward motion suppresses the growth of mixing layer and ozone precursors are capped which favour efficient photochemical reactions. Last but not least, based on the findings of the analysis and the model simulations, a forecast scheme is developed to predict the occurrence of ozone episodes. A set of meteorological and air quality parameters are selected to be criteria. Verifications using data in 2003 show promising results and the forecast scheme is capable to capture almost all episodic events with acceptable false alarm rate and false alarm ratio