The objective of this study was to determine comprehensive chemical components in PM2.5 from March 2011 till February 2012 in Seoul, South Korea, and their contributions to light extinction. Major chemical components in the aerosol were: ammonium sulfate, 30.3%; ammonium nitrate, 25.2%; organic matter, 21.3%; crustal mass, 16.9%; element carbon, 6.1%; and trace metals, 0.2%. PM2.5 mass concentrations and light extinction were mostly correlated in their diurnal and monthly variations, which indicates that the aerosol mass is the key variable in light extinction in Seoul. However, the aerosol size and composition (of PM2.5) also played significant roles in light extinction. We applied the IMPROVE algorithm to quantify the contributions of observed chemical components to light extinction. It was found that the IMPROVE formula tended to underestimate light extinction by up to 30% in urban conditions where large sources of organic matter (OM) and element carbon (EC) existed unless some revision was made before the light extinction calculations. The IMPROVE algorithm was further optimized for the observed light extinction for OM and EC. The revised light extinction efficiencies of OM and EC in Seoul increased by about 1.5–3 times of those in the original IMPROVE algorithm. The optimized IMPROVE scheme in this study reproduced the observed light extinction more accurately in Seoul. Overall, 41% of the contribution from OM and EC to light extinction in Seoul was close to 50% of the contribution from nitrate and sulfate, although the mass of the former contribution was only half of the latter.