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Abstract

Primary particle size distributions affect soot aggregate size, mass, mobility, and optical behavior. Prior studies report greater primary particle size variability between aggregates than within them. This implies aggregation of primary particles with similar flame origins, known as the external mixing hypothesis. However, hybrid soot aggregates that contain sub-aggregates with distinct primary particle sizes occur and are not explained by external mixing. This dissertation examines the hypothesis that hybrid soot forms by post-flame agglomeration. A two-stage Langevin dynamics model first generates non-hybrid aggregates representative of fresh soot near the flame. These are then scaled and filtered to match experimental distributions. Allowing the resulting aggregates to agglomerate produces hybrid aggregates with primary particle distributions consistent with experimental observations. The simulations predict an increase in projected-area diameter per mean primary particle diameter and a decrease in effective density at a given mobility diameter. Next, laboratory experiments produce soot with controlled degrees of hybridity using a miniature inverted burner. By adjusting dilution and residence time in a post-flame sampling system, four conditions yield different levels of hybridity and structural collapse. Aggregate properties are characterized using electron microscopy and tandem aerodynamic--mobility classification. Results confirm the simulated shifts in primary particle diameter versus aggregate projected-area diameter and effective density versus mobility diameter. Effective density measurements further indicate that collapse mitigates the effects of hybridity. The Langevin dynamics model is then revisited with experimental inputs to reflect the substantial dispersion observed between primary particle size and aggregate size. Incorporating this dispersion before the second aggregation stage lowers effective density at larger mobility diameters. The revised simulations reproduce experimental trends with a bias in absolute effective densities, likely arising from uncertainties in manual primary particle sizing. Analysis of the primary particle screening indicates negligible overestimation of primary particle size in the absence of collapse. The simulations and experiments presented in this dissertation are the first to investigate hybridity as an attribute of soot formed by post-flame agglomeration, demonstrating its influence on structural relationships. More realistic modeling of primary particle variability can improve soot image processing, inform property estimates for industrial carbon black, and support climate-relevant predictions.