Synoptic patterns in relation to ozone concentrations in west-central Taiwan

Recent studies have shown that regional orography and synoptic patterns greatly affect ozone concentrations. The orography of certain locations is unfavorable for pollutant dispersion, and specific synoptic patterns in these locations lead to serious air pollution episodes. Frequent ozone episodes (defined as hourly ozone greater than or equal to 120 ppbv) in west-central Taiwan during recent years have generated much concern. High ozone days (defined as hourly ozone greater than or equal to 80 ppbv) are also occurring more frequently. High ozone days occur mainly during autumn, but there has been no clear linear relationship demonstrated between any single meteorological variable and pollutant concentration. In this study, statistical data from 1997-1999 has shown that high ozone levels occur with two types of synoptic patterns. The first type consists of a continental anticyclone emanating from mainland China which is swept towards Taiwan. The second is a tropical low pressure system moving northwards closer to Taiwan. West-central Taiwan is located in the lee of the Central Ranges (altitude of 2500 similar to 3500 m). These synoptic patterns are unfavorable for pollutant dispersion and cause high ozone days.