Synoptic weather patterns and their relationship to high ozone concentrations in the Taichung Basin

Frequent high ozone days (defined as daily maximum ozone concentration grea ter than or equal to 80ppb) during recent years in the Taichung Basin have caused much concern. High ozone days occur mainly during autumn and spring. Statistically, there is no clear linear relationship between a single mete orological variable and ozone concentration. In this study, data from 1996- 21000 has shown that high ozone concentrations occur during two types of sy noptic weather patterns. The first type is a continental cyclone emanating from mainland China, the southern part of it swept towards Taiwan by easter ly winds. The second pattern is a tropical depression moving northwards tow ard the region, the northern part of it affecting Taiwan via easterly winds . Both types cover Taiwan with easterly winds, which are blocked by the Cen tral Mountain Ranges (altitude of 2000-3000m). The ranges create lee cyclog enesis to the west, which is unfavorable for pollutant dispersion and leads to serious air pollution episodes. The statistical results of the synoptic weather patterns in relation to ozo ne concentrations are based on the 5 yr data (1996-2000). This was obtained from a network of air-pollution monitoring sites in the study area, while the vertical data come from two 3-day tethersonde experimental campaigns co nducted during March and October 2000, measuring air pressure, air temperat ure, relative humidity, wind speed and direction, non-methane hydrocarbons, NOx and O-3, (C) 2001 Elsevier Science Ltd. All rights reserved.