PM2.5 and PM1 mass concentrations, and elemental carbon (EC) , organic carbon (OC) , and trace metal concentrations in PM2.5 and PM1 were measured in the Tokyo metropolitan area during winter and summer in 2004. PM2.5 and PM1 were collected by using multi-nozzle cascade impactor (MCI) . Sum of concentrations of elemental carbon and organic compounds accounted for 75 % of PM1 mass concentrations in winter, while it did only 30 % in summer. It was possibly due to the enhancement of photochemical formation of secondary inorganic compounds (mainly sulfate) in summer. Concentration of organic matter (OM) in winter was higher than that in summer; therefore, ambient OM in the sampling site was originated mainly from primary sources. Most of trace metal concentrations were high in winter and low in summer, however, vanadium and nickel concentrations in summer was higher than those in winter. We suggested that one of the reasons of the fact was due to increase of the use of oil fuel for power generation in summer season.
