Urban residents are exposed to high levels of ozone that are produced by photochemical reactions of ambient air in strong sunlight. The potential for ozone formation of urban air needs to be characterized in order to avoid exposure to unexpectedly high ozone concentrations during the daytime. In this study, ten sets of twin smog chamber experiments were carried out to determine the correlation between light intensity and ozone formation during photochemical reactions of urban air in Seoul. Single chamber analysis revealed a higher rate of ozone production under higher irradiation than under less intense irradiation under similar initial ambient air conditions. However, the rate of ozone production showed significant variations at the same light intensity. Here, the ratio of two light intensities for both chambers was introduced as a key parameter. The ratio of ozone production was dependent on the ratio of the two light intensities according to a power function with a factor of approximately 1.8. The proposed correlation was used to predict the daytime ozone concentration for calm, clear and dry weather conditions.