Improved relationships between the effective density and morphology of compacted soot

Fresh soot is composed of chain-like lacey aggregate of primary particles that can compact when coated with liquids. The transition from a lacey to a compact structure is a key restructuring mechanism for these carbonaceous nanoparticles. In this study, fresh propane soot aggregates of mobility diameter (d m ) between 35 nm and 800 nm were coated with oleic acid to induce compaction. The restructuring process was observed using aerosol-particle size measurements as well as fixed-angle and, for the first time, tilt-stage TEM images. Analysis of the TEM images showed that the circularity increased from ∼0.1 to 0.4 and the sphericity increased from ∼0.55 to 0.7 as the particles transitioned from a lacey to a fully compacted structure at volume growth factors > 5. For such fully compacted aggregates, we confirmed the applicability of the universal packing factor of 0.35, and corresponding effective density of 650 ± 20 kg/m 3 for large aggregates (d m > 50) from several studies, compiled in previous work (Sipkens & Corbin, 2024) . That compilation identified a literature gap for the morphology of small aggregates. Here, we present measurements filling this gap. We measured negligible restructuring for small aggregates (d m < 50 nm for d pp,100 ∼ 19 nm) and corroborated this observation with TEM. Hence the effective density of small aggregates was similar to that of fresh soot. We also quantified and parameterized the effective-density transition from the negligible-restructuring regime to the constant-packing-density regime (650 ± 20 kg/m 3 ). This parameterization estimates the effective density of fully compacted soot aggregates based on their mobility diameter and can be easily applied to other nanoparticle aggregates of known material density. • Circularity and sphericity increase as soot evolves from lacey to compacted. • Orientation effects are strongest for partially compacted soot. • Smaller aggregates show negligible restructuring even under heavy coatings. • Improved effective density-based relation captures compacted soot morphology across sizes.