Combustion soot nanoparticles: Mechanism of restructuring and mechanical properties

Soot, a product of incomplete combustion of fossil fuels, is a global warming agent. The effect of soot particles on climate depends on their morphology. Freshly released soot particles are fractal lacey aggregates, but they often appear collapsed in atmospheric samples collected away from emission sources. A body of work has concluded that the collapse is caused by liquid shells when they form by vapor condensation around soot aggregates. However, some recent studies argue that soot remains fractal even when engulfed by the shells, collapsing only when the shells evaporate. To reconcile this disagreement, the effects of the condensation and evaporation on restructuring are separated in this study, by anchoring coated and coated-denuded soot, after condensation and condensation-evaporation, respectively. The morphology of the particles collected in both ways is characterized by using scanning electron microscopy images. It is shown that wetting and non-wetting liquids act differently in soot restructuring. Liquids capable of wetting the surface of soot aggregates can induce a significant restructuring by condensation. With non-wetting liquids, such as water, it is the evaporation that drives most of restructuring and there is almost no restructuring during condensation. Fractal soot particles released by combustion are typically hydrophobic, but can become hydrophilic after acquiring a coating layer made of hygroscopic atmospheric chemicals. To determine if absorption of water vapor by a thin hygroscopic coating can result in soot particle compaction at moderate relative humidities, the morphological response of soot thinly coated with hygroscopic chemicals is investigated upon humidification. Mass-mobility analysis, scanning electron microscopy, and condensation models confirm that even under subsaturated conditions capillary condensation of hygroscopic chemicals can occur into the junctions between carbon spherules, driven by the saturation pressure depression caused by the concave menisci. Furthermore, the concave menisci promote absorption of a significant amount of water vapor by the condensate at moderate relative humidities, exceeding the amounts achievable for flat and convex surfaces. Results imply that exposure of fractal soot particles to subsaturated vapors of hygroscopic chemicals in the atmosphere may be an important route towards soot compaction even at moderate relative humidities. The mechanistic details of soot aggregate restructuring remain poorly understood. In this study, atomic force microscopy is used for force-displacement measurements on bare, coated, and coated-denuded soot aggregates to determine their mechanical properties. The force curve is determined by measuring the deflection of the cantilever as it approaches and retracts from the sample. Peaks observed in the distributions of forces for bare soot may be related to the processes that occur during aggregate stretching, such as detachments between the monomers, unfolding, and breaking of the connection between monomers. These forces are significantly affected in the presence of a condensate. These results are expected to contribute to the development of physical models for soot restructuring. Overall, the findings of this dissertation advance understanding of the processes governing the transformations and environmental impacts of soot that will benefit the experimental and modeling atmospheric research communities