Process Analysis of Different Synoptic Patterns of O3 Episodes in Hong Kong

The Pearl River Delta (PRD) region and Hong Kong, located in the coastal region of South China, have complex terrains. The severity, duration, and frequency of ozone episodes in Hong Kong increase significantly in recent years. More than 36 ozone episode days (i.e., an ozone day referred to as a day with hourly mixing ratios of ozone higher than 120 ppb) was observed in 2004. The episodes are found to be associated with three categories of synoptic patterns: Cyclonic (C), Anti-cyclonic (A) and Trough (T). About 56%, 28% and 16% of ozone episode days are attributed to these three types of synoptic patterns, respectively, over the past five years. All ozone episodes occurring in these five years are investigated using the Pollutants in the Atmosphere and their Transport over Hong Kong (PATH) model system. PATH consists of three major modules: a meso-scale meteorological model (MM5), an emission modeling system (EMS-95) and an Eulerian transport and chemistry model (SARMAP Air Quality Model, SAQM). The up-to-date high-resolution (30 m) land use datasets are used to replace those of United States Geological Survey (USGS) over Hong Kong and PRD in MM5. Four-dimensional data assimilation (FDDA) is applied to MM5 simulations. The model evaluation is performed in terms of temporal variations, spatial distributions, and statistical calculation. The results suggest good performance for meteorological predictions and acceptable performance for surface ozone predictions. The integrated process rate (IPR) analysis method embedded in SAQM is used to identify the relative contributions of regional transport and local chemical production of O3 in the planetary boundary layer (PBL) during the O3 episodes. The IPR analysis results show that the percent contributions of regional transport vs. local chemical production are 63% vs. 37%, 59% vs. 41% and 42% vs. 58% for the episodes related to the synoptic patterns of categories C, A and T, respectively. Therefore, regional transport from the PRD region has a significant impact on the occurrence of episodic levels of ozone in the lower PBL in Hong Kong. This could be the consequences of the rapid change of land use and hence modified regional land-sea breeze circulation, as well as a rapid and substantial increase of anthropogenic emissions in the PRD region.