Behaviors of Surface ozone along with the other two precursor gases No_x and CO during the year 1996-1997 in the vincinity of Chiang Mai , a city in northern Thailand , have been analyzed.

Monthly means of surface ozone measured in downtown Chiang Mai for the two-year study are between 10-30 ppb. Diurnal variation of the surface ozone is explained by the photochemical activities.

The increase of the surface ozone is noticeable during the dry season from November through April of the following year and attains the maximum concentration during March and April.

The time of maximum ozone and No_x concentrations supports the enhancement of the photochemical surface ozone .