Particulate species were measured in April–May 2003 at two sites, one in the megacity of Seoul and the other in the Deokjeok Island to the west of Seoul, to examine the effects of long-range transport under the influence of prevailing westerlies. The effects of Asian dust were observed in April, and a severe smog episode occurred in late May. During May, all air pollutants showed negative correlations with wind speed at Seoul, particularly for NO2 and CO, a large proportion of which result from vehicle emissions. SO42– continued to increase in association with an inflow of air pollutants from China with heavy use of coal, with fluctuations depending on wind speed. The smog episode in late May occurred as emissions from Siberian forest fires were superimposed on pollutant inflows from China that had persisted since early May and local emissions accumulated under stagnant conditions. During the episode, Siberian forest fires increased K+ while local emissions primarily from vehicles increased NO3–, OC, and EC. The effects of an inflow of air pollutants from the outside were significant at Deokjeok, with small local emissions, resulting in substantial increases in Ca2+ during the Asian dust event and SO42– during the smog episode, compared to those at Seoul. Because both sites were strongly influenced by the effects of long-range transport in May, PM2.5 along with SO42– and K+ exhibited a strong correlation between Seoul and Deokjeok.