This study measured vertical distributions of ozone, sulfur dioxide, nitrogen oxide, carbon monoxide, six biogenic-related volatile organic compounds, wind vector, humidity and temperature within the Planetary Boundary Layer (PBL) below 1200 m using a tethered balloon sounding system at a downwind rural site in the Taichung Basin, Taiwan, during the 2002 summer. During ozone episodes, both nocturnal inversion and valley-mountain wind circulation were stronger than those during non-episode days. The observed vertical distributions of these pollutants indicate that the concentrations of carbon monoxide, nitrogen monoxide and biogenic volatile organic compounds decreased as the height increased. Conversely, the highest ozone concentrations were measured during early afternoon at heights of 100-600 m above ground level. Notably, these high ozone concentrations were normally accompanied by relatively higher concentrations of nitrogen dioxide and sulfur dioxide, but not carbon monoxide. From emission inventory, trajectory analysis and statistical analysis, all suggest that high-stack point sources have an important role in causing the summer ozone episodes. This suggestion is supported by 11-y summer data at three stations in the Basin. In addition, it is found that part of the early-afternoon surface ozone was from ozone stored aloft in the residue layer at heights above the nocturnal boundary layer of the preceding night.