A Systematic Study of the Morphological Changes Induced in Soot Aggregates by Oxidized Organics

Significant amounts of soot are released worldwide from incomplete combustion of carbonaceous materials. In the atmosphere, soot particles undergo variety of aging processes, including coagulation with other aerosols, vapor condensation, and surface oxidation. As a result of aging, the chemical composition, mixing state, and morphology of soot particles become altered significantly. Accordingly, environmental impacts of soot are modified as well. To understand the variation in soot structure and mixing state with aging, we investigated soot aggregates coated with thin layers of oxidized organics. The organic compounds spanned a broad range of physical and chemical properties, including functional groups, O/C ratios, surface tension, and viscosity. In the experiments, soot aerosol was produced in an inverted diffusion burner and then exposed to condensable organic vapors. The mixing state and morphology were characterized by scanning electron microscopy imaging and mass-mobility measurements, using parameters such as particle effective density, dynamic shape factor, mass-mobility scaling exponent, convexity, and roundness. The obtained experimental results are used to identify key parameters that control the morphology and mixing state of soot aggregates during aging transformations and to develop a physical mechanism of soot restructuring.