Some statistical and meteorological analysis on O 3 near the ground was carried out using the data obtained at three sites in respectively, island, urban and rural areas, for 9 months from winter to summer. Night-time O 3 is usually low but sometimes increases at the three sites almost at the same time. The increase in night-time O 3 in winter is often associated with a winter monsoon pressure pattern. In spring, night-time O 3 often increases near the passage of a cold front. However, in summer there are no special synoptic weather conditions for increase in night-time O 3. Using the stability parameter defined by — Rn/ U 2 (Rn: net-radiation near the ground, U: surface wind speed), we showed that an increase in O 3 at night is due to a decrease in this parameter and NO x concentration, irrespective of the season. Variation of daytime O 3 in winter and early spring was found to be well described by the entrainment of background O 3 aloft due to the development of mixing layer and the emission of NO near the observation site. On a typical day of high level O 3 episode, O 3 at the island site sometimes exceeds 140 ppb in the sea breeze. O 3 precursors carried by the land breeze in the previous night or early morning was inferred to be a possible cause of this high level O 3.