Outflow patterns of air pollutants, and mechanisms associated with a monsoon condition that is peculiar to the East Asian region in winter, have been studied. To capture the continental outflow, airborne and shipboard measurements of aerosols and trace gases were conducted over the northern part of the East China Sea and the western part of the Sea of Japan on January 17--26, 1991 (airborne), January 28--29, 1992 (airborne), January 22--31, 1993 (shipboard), and January 29--30, 1993 (airborne). At least two types of highly polluted air masses were found at different times of the monsoon. Type 1 polluted air mass, which contained more than 10 μg m-3 of non-sea-salt SO42- and more than 5 μgC m-3 of black carbon aerosols, typically appeared over the East China Sea at the beginning of the monsoon period. Chemical and microphysical characteristics suggest that the type 1 polluted air mass is chemically aged. Type 2 air mass appeared in the later half of the monsoon period and had characteristics of freshly emitted pollutants. It consisted of several high concentrations of SO2 (>10 ppbv) confined in narrow horizontal width (10 to 30 km wide). Analyses using an Eulerian chemical transport model and detailed backward trajectories indicate that the type 1 polluted air mass was subcontinental in extent and its formation and the transport were dominated by a synoptic-scale weather cycle; it was formed over the Asian continent under anticyclonic conditions that persisted for 3--4 days, and transported intermittently to the east over the Pacific Ocean by the passage and the development of traveling cyclones. This study suggests that aerosols that flow out from East Asia are enriched with carbonaceous materials and consequently have low single-scattering albedo, which may affect the direct radiative forcing by aerosols over the North Pacific. ¿ 2000 American Geophysical Union