Modification of soot by volatile species in an urban atmosphere

Aerosol samples in the urban atmosphere of Kumamoto (32°48′N, 134°45′E) in southwestern Japan were collected onto aluminum foil strips. Parts of the samples were heated to 550 °C in pure helium gas, and oxygen (2%)–helium (98%) mixture gas. Particles in unheated and heated parts were characterized individually by their morphology and elemental composition using a scanning electron microscope equipped with an energy dispersive X-ray spectrometer. There were mainly two types of soot-containing particles according to the morphology: chain-like and sub-round. Chain-like particles were likely young soot particles because such particles in heated specimens showed similar morphology to those in unheated specimen. In contrast, the sub-round particles were composed of volatile species encapsulated with soot. The heating caused partial evaporation of such particles, and the soot inclusions could be identified only after the heating. The volatile species frequently contained sulfur compounds, but sulfur was not detected in the residues, suggesting that the volatile species were mainly produced on soot particles in the atmosphere. The sub-round soot-containing particles were approximately 3 times larger in diameter than the inclusions. These results suggest that soot particles could be substantially modified in size and composition by volatile species in the urban atmosphere.