SPM and CO concentrations are low during the day time because the atmosphere is well mixed. In summer, solar radiation is twice as high as that in winter. So the atmospheric mixing during the day is stronger in summer than winter. However, in Kyoto, SPM and CO concentrations are not low in summer compared to winter. In order to reveal the reasons of these diurnal concentration variations, we estimated SPM and CO background concentrations using the data of the Atmospheric Environmental Regional Observation System in Kyoto. We conclude that SPM and CO diurnal concentration variations are caused by the following reasons; (a) SPM or CO concentrations decrease to SPM or CO background concentrations and (b) SPM background concentration is the highest in summer and is the lowest in winter and CO background concentration is almost constant through all seasons.