Long-term visibility observations, including that of aerosol chemistry, is necessary to improve the visibility of the mega-city of Seoul. In this study, the contributions of size-resolved aerosols to light extinction were estimated during an extensive visibility monitoring period, from 2007 to 2009. Optical measurements of a light extinction coefficient, a light scattering coefficient, and a light absorption coefficient were made using a transmissometer, nephelometer, and aethalometer. Size-resolved aerosol measurements, including the collection of submicron (Dp < 1.0 µm), fine (Dp < 2.5 µm), and coarse (2.5 < Dp < 10 µm) particles were conducted for the determination of their mass extinction coefficients and contributions of chemical components to light extinction. A total of 386 measurement data sets were used to construct the predictive mass extinction coefficients for the size-resolved particles using regression analysis. The mass extinction coefficients of the sized-resolved aerosols of PM1.0, PM2.5, and PM10 were found to be 8.7 ± 0.8, 4.7 ± 0.2, and 2.7 ± 0.2 m2/g, respectively. The aerosol light extinction varied with aerosol size distribution, the chemical composition of the aerosol, and ambient relative humidity. It was found that the ammonium sulfates were the largest contributor to light extinction and visibility impairment due to aged aerosols in the urban atmosphere of Seoul.