Seasonal variations of chemical composition and optical properties of aerosols at Seoul and Gosan were investigated using the ground-based aerosol measurements and an optical model calculation. The mass fraction of elemental carbon was $8{sim}17%$, but its contribution on light absorption was high up to $29{sim}48%$ in Seoul. In Gosan, the contribution of water soluble aerosols on aerosol extinction was $83{sim}94%$ due to the high mass fraction of these particles in the range of $56{sim}88%$. Model calculation showed that the water holding capacity of aerosols was larger in Gosan than in Seoul because of higher relative humidity and temperature along with abundant water soluble aerosols. Difference between measured and calculated aerosol optical depths was the highest in summer. This was because aerosol optical depth calculated from ground-based measurements could not consider aerosol loadings at high altitude in spite of high column-integrated aerosol loadings observed by Sun photometer. Although hygroscopic growth was expected to be dominant in summer, the mass concentration of water soluble aerosols was too low to permit this growth.