The objective of this study was to determine comprehensive chemical components in PM2.5 from March 2011 to February 2012 in Seoul, South Korea and their contributions to light extinctions. Major chemical components in 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 extinctions were mostly correlated in their diurnal and monthly variations, which indicated that aerosol mass was the key variable for light extinctions in Seoul. However, aerosol size and compositions (of PM2.5) also played significant roles in light extinctions. We applied IMPROVE algorithm to quantify the contribution of observed chemical components to light extinctions. It was found that IMPROVE formula tended to underestimate light extinctions up to 30% in urban conditions where large sources of organic matter (OM) and element carbon (EC) existed, unless any revision were made before the light extinction calculations. IMPROVE algorithm was further optimized for observed light extinctions for OM and EC. Revised light extinction efficiencies of OM and EC in Seoul were increased about 1.5–3 times higher than those in original IMPROVE algorithm. Optimized IMPROVE scheme in this study reproduced the observed light extinctions more accurately in Seoul. Overall 41% contributions of OM and EC to light extinctions in Seoul were close to 50% of nitrate and sulfate contribution although the former mass contributions were only half of latter.