Mechanisms of soot-aggregate restructuring and compaction

Soot aggregates form as open, fractal-like structures, but aged atmospheric particles are often observed to be restructured into more-compact shapes due to internal mixing ("coating"). This compaction has a major effect on the radiative properties of the aggregates, and may also influence their aerosol-cloud interactions, toxicity, and deposition in human lungs. Recent laboratory studies have presented conflicting arguments on whether this compaction occurs during condensation or during the evaporation of coatings. In this three-part study, we combine theory and experiments to explain these conflicting results. First, we review the surface-science literature and identify explicit mechanisms for condensation-compaction as well as evaporation-compaction. We also identify a mechanism for avoiding compaction during condensation, based on heterogeneous nucleation theory and the kinetic barriers to capillary formation. Second, we review the soot-restructuring literature and find clear evidence for both condensation- and evaporation-compaction, with condensation-compaction being the norm. Third, we present new experimental results where the capillary forces due to anthracene coatings were "switched on" or "switched off" by using solid or liquid phases during coating addition and removal. Consequently, we demonstrate condensation-compaction, evaporation-compaction, and no compaction, for the same soot source. Overall, our study indicates that soot particles will typically undergo compaction when internal mixing occurs by the condensation of liquid coatings, while compaction may be avoided when internal mixing occurs through coagulation or the gas-to-particle formation of solid or highly viscous coatings.