Determination of sampling time is determined based on a preliminary study to determine the daily variation of the concentration of ozone gas. Preliminary sampling is done with gas sampling measurements of ozone for 1 hour for 12 consecutive hours from 10:00 to 22:00 as well as variations in measurements 14 hours in a row at 6:00 to 20:00. Preliminary sampling conducted at three sampling points in the city of Padang. The results of preliminary studies showed that the pattern of variation in the concentration of ozone gas is almost always the same day in which the concentration of ozone gas is quite high and stable at 10:00 to 18:00 (see Fig. 1 and Fig. 2). The results of this preliminary study can be used to determine the concentration of ozone gas sampling time for the city of Padang, where the time to be used for sampling is from 10:00 to 18:00.