Epidemiologic studies have demonstrated positive relationships between increases in air pollution and adverse health effecty.As major pollutants in the Shanghai atmosphere,physicochamical characterization of ambient particles,PM10 and PM2.5 (PM2.5 or PM70,particles with gerodynamics diameters less than 2.5 m or 10 m,respectively) and ultrafine particles (UFP,aerodynamic diameter less than 0.1 m) were investigated in this study.Our results desonstrated that mass concentrations of different size ambient particles collected in urban air are higher than that in suburban air,and that mass level of different size particles are different.The highest mass concentration of ambient particles was contributed by the PM 0.56-0.32,while the lowest was found in the particles with diameter 1-0.56 m and <0.0056 m.SEM/EDX results revealed that coarse particles consisted of minerals,while the fine particles were mainly compesed of sulfates,and there were few particles in the ultrafine size.ICP-MS results showed the highest mass level of heavy metals and the lowest mass level of crustal elements was found in PM1.0-0.56,while the lowest value of these elements was discovered in the ultrafine particles.Free radical-generating ability of the different size particles was tested by EPR.The free radical signals of ambient fine particles were higher than those of coarse,ultrafine particles.MTT assay was employed to investigate cellular toxicity of the ambient particles.The results of cell proliferation assay demonstrated that potency to A549 cell death ratio induced by ultrafine particles was slightly higher compared with fine and coarse particles.The mechanism of toxic phenomena caused by the different size ambient particles needs furthe exploration.