In an effort to estimate secondary $PM_{10}$ concentrations and their diurnal variations at different photochemical activities, $PM_{10}$, CO, and $O_3$ concentrations obtained from the ambient air quality network located in Seoul are analyzed for the period from 2000 to 2005. In order to classify the photochemical activities on a daily basis, measured ${Delta}O_{3,;max-min}$ (maximum $O_3$-minimum $O_3$) and ${int}(hv)dt$ which represents accumulated daily insolation, were used to classify each day into three regimes: 1) low photochemical reactivity; ${Delta}O_{3,;max-min};{leq};40;ppb$, and ${int}(hv)dt;{leq};4000;W/m^2$, 2) moderate photochemical reactivity; $40;ppb;<;{Delta}O_{3,;max-min};{leq};60;ppb$, and $4000;{leq};{int}(hv)de;{leq};6000;W/m^2$, and 3) high photochemical reactivity; ${Delta}O_{3,;max-min};>;60;ppb$, and ${int}(hv)dt;{geq};6000;W/m^2$. The ratio of ($PM_{10}$/CO) obtained at low photochemical activity regime was used as an index of tracer for the estimation of secondary $PM_{10}$ at higher photochemical activity regimes. The results show that the estimated secondary $PM_{10}$ concentrations for moderate and high photochemical regimes are found to be 18.8% ($10.9;{mu}g/m^3$), and 35.0% ($26.2;{mu}g/m^3$), respectively. Diurnal variation of secondary $PM_{10}$ for the moderate photochemical regime shows weak but noticeable patterns. However, the highly activated photochemical regime shows strong diurnal variations of secondary $PM_{10}$ concentrations with the maximum value of $35.1;{mu}g/m^3$ at 1300LST.