In order to investigate the causes of visibility impairment in Seoul metropolitan area, extensive aerosol chemical and optical monitoring had been conducted at two urban sites, Junnong, Seoul and Younghyun, Incheon during three seasonal intensive monitoring periods; 5 - 26 August, 20 - 28 October 2002, and 10 - 24 January 2003. Light extinction and scattering coefficient were measured simultaneously with a transmissometer and a nephelometer, respectively. During the first IMP, average light extinction coefficient and visual range were measured to be 692±283 Mm^-1 and 5.7±8.0 km at Junnong, 366±232 Mm^-1 and 10.7±9.6 km at Younghyun, respectively. They were measured to be 354±194 Mm^-1 and 11.5±7.7 km, 229±119 Mm^-1 and 17.1±7.7 km during the second IMP and 702±567 Mm^-1 and 5.6±6.9 km, 1123±831 Mm^-1 and 3.5±4.7 km during the third IMP. Mean light extinction budget for five major aerosol components; ammonium sulfate, ammonium nitrate, fine carbonaceous particles (EC+OC), fine soil, and coarse particle were calculated to be approximately 13, 15, 48, 2, 22% for Junnong and 12, 9, 53, 2, 24% for Younghyun, respectively. The contribution of EC to visibility impairment was about three times larger than that of OC. In addition, visibility impairment occurred not only under atmospheric stagnation condition with low wind speed < 1m/sec, but also under higher wind speed > 3m/sec. It was implied that regional or long-range transport of secondary air pollutants might affect local visibility impairment.