Numerical simulation and process analysis of typhoon-related ozone episodes in Hong Kong

In this study the synoptic patterns conducive to the occurrence of O-3 episodes in Hong Kong are categorized by an inspection of the weather charts over the period of 1999-2003. The synoptic patterns associated with tropical cyclones originating in the North Pacific Ocean and the South China Sea are found to be the most optimal weather conditions for the occurrence of ozone episodes in Hong Kong. A high-resolution version of the regional, three-dimensional, multiscale photochemical air quality model ( Pollutants in the Atmosphere and Their Transport in Hong Kong ( PATH)), developed by the Hong Kong Environmental Protection Department in 2000, has been employed to investigate the evolution of one type of ozone episode related to tropical cyclones. A nonhydrostatic meteorological model (MM5) was applied with four-dimensional data assimilation to provide necessary meteorological fields to the air quality model. The performances of both the meteorological and chemical models are evaluated by comparing the simulated results with the available observed data. An integrated process rate analysis is used to examine the relative contributions of individual physical and chemical processes in the formation of ozone episodes for obtaining a better understanding of the mechanisms of photochemical smog events in Hong Kong. Results show that about 30% of the total ozone production is due to local chemical production in the lower atmosphere boundary layer, and about 70% is contributed by interregional transport from southern China into Hong Kong. In addition, four main processes, including horizontal advection, vertical transport, photochemical reactions, and deposition are found to have a significant influence on the ground-level concentration of ozone. The sensitivity experiments indicate that the chemical regime for ozone formation in Hong Kong seems to be limited by volatile organic compounds.