Atmospheric particulate matter with diameter < 2.5 μm now makes up much of the air pollution in China, but it is the ultrafine particles (UFPs) with diameter < 90 nm that are of particular interest. This is because UFPs are strongly linked with human health for two reasons: they contain a variety of hazardous substances and they can deeply penetrate human respiratory systems. Therefore, scanning electron microscopy combined with X-ray dispersive energy spectrometry was used to characterize the morphology and surface texture, as well as the elemental composition of 60 UFPs. The UFPs was generated in a sewage sludge-incineration power plant in Zhejiang Province. This was done to determine the microstructure of the ultrafine particles and to follow the evolution of particle surface elemental composition with increasing particle size. Then, a comparison of the characteristic time for nucleation, condensation and coagulation was done to estimate the dominant mechanism. The results show that the UFPs have generally irregular shapes (cotton-like, irregular balls, sheets, etc.) and that they usually aggregate to form a mass. With increase in the size of a UFP, the mass fraction of the elements presents clearly changed: Na, K and Fe gradually decreased; while Ca, Si and Al, as well as the heavy metals Cu, Zn and Ni increased. Characteristic time estimation is a convenient and effective tool for identifying the predominant mechanisms during combustion. In this study, calculations of characteristic time were used to reveal a mechanism of vaporization, nucleation, condensation and coagulation, which drives the formation and growth of ultrafine particles.

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