Ozone episodes occurred from 1999 to 2004 were investigated using the PATH (Pollutants in the Atmosphere and their Transport over Hong Kong) model system. PATH is consisted of three main modules: a meso-scale meteorological model (MM5), an emission processing module (EMS-95) and an Eulerian transport and chemistry model (SAQM). ₃...[ Read more ]

Ozone episodes occurred from 1999 to 2004 were investigated using the PATH (Pollutants in the Atmosphere and their Transport over Hong Kong) model system. PATH is consisted of three main modules: a meso-scale meteorological model (MM5), an emission processing module (EMS-95) and an Eulerian transport and chemistry model (SAQM).

The synoptic patterns conducive to the occurrence of O₃ episodes in Hong Kong were categorized by an inspection of the weather charts during the 6-year period from 1999 to 2004. They included Cyclonic, Anti-cyclonic and Trough synoptic patterns. About 56% of ozone episodes over the past 6 years were resulted from cyclonic (C) weather patterns. About 28% of ozone episodes were attributed to the anti-cyclonic (A) synoptic pattern, and the remaining high ozone episodes (about 16%) happened when the low-pressure trough (T) extended over south China sea.

As an example, a typhoon-related ozone episode occurred in 1999 was studied in detail using PATH model system. The direct observation-simulation comparisons and statistical evaluations suggested very good performance on modeling of meteorology and acceptable performance on simulations of surface ozone concentrations. The integrated process rate (IPR) analysis method was implemented in SAQM to identify the relative contribution of regional transport and local chemical reactions to O₃ formation during the O₃ episode. The integrated reaction rate (IRR) was applied to examine the O₃-NO_x-VOC relationship through quantifying NO_x-cycle and OH-cycle in both urban and rural areas in Hong Kong.

IPR analysis was employed to three different synoptic patterns of ozone episodes occurred from year 2000 to year 2004 to identify the relative contributions of regional transport and chemical reactions to the ozone formation in Hong Kong. They included twenty-four cases related to the activities of tropical cyclones or typhoons traveling over the South China Sea and Northwestern Pacific, fifteen cases associated with anti-cyclonic synoptic patterns present over the mainland of China and thirteen cases resulting from low pressure troughs. Results showed the regional transport had a significant impact on occurrence of episodic levels of ozone in the lower atmospheric boundary layer of Hong Kong.

O₃ formation has been found to be VOC-limited in both Hong Kong and PRD region through the sensitivity studies. Sensitivity tests have provided some scientific evidences to policy-makers for consideration of effective control strategies.