Integrated processes analysis and systematic meteorological classification of ozone episodes in Hong Kong

[ 1] We have conducted a systematic and comprehensive survey of 54 O-3 episodes that occurred over Hong Kong during 2000 - 2004 using the Pollutants in the Atmosphere and their Transport over Hong Kong ( PATH) model system. We identify the relative contributions of regional transport and local chemical production for these O-3 episodes. Two regimes with different relative regional or local contributions are identified. Subsequently, meteorological analyses are used to help identify the synoptic patterns associated with the O-3 episodes of these two regimes. In particular, we find that the O-3 episodes are dominated by regional transport when a tropical cyclone/typhoon is located over the northwestern Pacific or the South China Sea to the east or southeast ( a "C'' pattern), when an anticyclone (i.e., a local high-pressure system) appears over mainland China to the north ("A''), or both ("AC''). On the other hand, local chemical production dominates when a trough is situated to the east over the South China Sea ("T''). Strongest regional transport is associated with the "C'' or "AC'' synoptic patterns. These results are likely related to the stronger horizontal advection of O-3 under the "C,'' "A,'' or "AC'' patterns (as compared with "T''), as well as the stronger production of O-3 in the Pearl River Delta region under clear sunny skies at the periphery of a tropical cyclone/ typhoon ("C'') and/or under a high-pressure system ("A''). These four characteristic synoptic patterns, namely, "C,'' "A,'' "AC,'' and "T,'' account for 33%, 17%, 28%, and 22% of the 54 episodes, respectively, between 2000 and 2004. Finally, back trajectory analyses also show that the lower regional contributions in the trough cases ("T'') are related to their airflow pathways not passing through the O-3-rich Pearl River Estuary. This study also shows that a systematic and comprehensive application of integrated process analysis can be used to identify and summarize the salient features ( weather patterns in this case) of specific type of air quality events.