From 12 to 15 December 2002, the area-averaged surface ozone concentrations in south Taiwan increased to 100ppb from a background value of 40. The maximum hourly ozone concentration reached 150ppb on 15 December. GPS-radiosondes, tethered ozonesondes and an instrumented ultralight aircraft were used to measure vertical ozone concentrations and meteorological conditions during this period. An ozone concentration of 60–90ppb was observed in the nocturnal residual layer, 20–50ppb of which was estimated to be produced from local photochemical reactions. Downward mixing of ozone from the residual layer to the ground level was observed in the morning of the episode day. Back air trajectory analysis was performed to locate the source region of the accumulated ozone. The results revealed that the source of high concentration of accumulated ozone was in an ozone-polluted area with an ozone concentration in the range 90–100ppb on the previous day. About 60% of the ozone in the polluted area on the previous day persisted to the morning of the episode day.