Synoptic and meteorological drivers of summer ozone pollution events in the Kaoping Region, Taiwan

Most studies on ozone pollution in Taiwan have focused on surface meteorological conditions and atmospheric dispersion, whereas the influence of mid-level synoptic patterns (e.g., 500 hPa systems) and vertical convective stability remains underexplored. This study investigates ozone pollution events during July and August from 2008 to 2023, using data from Taiwan's Ministry of Environment, the Central Weather Administration, and upper-air soundings. Principal Component Analysis (PCA) was employed to identify key atmospheric drivers of ozone variability, including convective stability, dispersion capacity, humidity, precipitation, solar radiation, and convective top height. Ozone events were more frequent when the East Asian monsoon trough extended eastward, positioning Taiwan on the western flank of the 500 hPa subtropical high-pressure ridge and low-pressure trough. These conditions coincided with strong convection at 850 hPa, below-average temperatures over Taiwan and the adjacent seas (relative to the long-term climatological average), a weakened surface high-pressure ridge, and the presence of low-pressure systems-of which typhoons accounted for 45.8% of cases. The PCA identified three primary meteorological drivers of ozone pollution. Convective stability (explaining 18.50% of the variance)-represented by Lifted Index (LI), Showalter Index (SHI), and Showalter Index (SHI)-was the dominant factor. Humidity and precipitation (16.50%)-characterised by RH (Relative Humidity), RR, and RA-highlighted the influence of moisture-related processes. Dispersion capacity (10.86%)-defined by mixing height (MH) and the product of wind speed (WS) and MH (WS × MH)-reflected atmospheric dilution conditions. Severe ozone events were associated with high atmospheric stability, limited dispersion, dry conditions, strong solar radiation, and low convective tops. In conclusion, mid-level synoptic patterns play a critical role in ozone pollution, primarily through their modulation of convective stability and atmospheric dispersion. While tropical low-pressure systems influence ozone variability, convective stability emerges as the dominant mechanism driving ozone pollution events in Taiwan.