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One hundred ninety-five chemically speciated samples were collected from March 2003 to February 2005 in the Seoul Metropolitan area to investigate the characteristics of the major components in PM2.5 and to characterize the chemical variations between smog and non-smog events. The annual average PM2.5 concentration was 43μgm−3 that is almost three times higher than the US NAAQS annual PM2.5 standard of 15μgm−3. During this sampling period, smog and yellow sand events were observed on 27 and 10 days, respectively. The PM2.5 concentrations and its constituents during smog events were about two–three times higher than those during non-smog and yellow sand events. In particular, the mass fractions of secondary aerosols such as sulfate, nitrate, and ammonium during the smog events were higher than those of the other constituents. The mean concentration and mass fraction of secondary organic carbon (SOC) were highest during the winter smog events. Sulfate, nitrate and SOC that can have long residence times were important species during the smog events suggesting that regional scale sources rather than local sources were important. Five-day backward air trajectory analysis showed that the air parcels during smog events passed through the major industrial areas in China more often than those during non-smog events. [Copyright &y& Elsevier]