Vapor Condensation and Coating Evaporation Are Both Responsible for Soot Aggregate Restructuring

Abstract

Fresh soot is made of fractal aggregates, which often appear collapsed in atmospheric samples. 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, we investigated soot restructuring under conditions ranging from capillary condensation to full encapsulation, also including condensate evaporation. In these experiments, airborne fractal aggregates were exposed to vapors of wetting liquids, and particle size was measured before and after coating loss, allowing us to isolate the contribution from condensation toward restructuring. We show the existence of three distinct regions along the path connecting the initial fractal and final collapsed aggregates, where minor restructuring occurs already at zero vapor supersaturation due to capillary condensation. Increasing supersaturation increases the amount of condensate, producing a more notable aggregate shrinkage. At even higher supersaturations, the aggregates become encapsulated, and subsequent condensate evaporation leaves behind fully compacted aggregates. Hence, for wetting liquids, minor restructuring begins already during capillary condensation and significant restructuring occurs as the coating volume increases. However, at this time, we cannot precisely quantify the contribution of condensate evaporation to the full aggregate compaction.

Using the Black Carbon Particle Mixing State to Characterize the Lifecycle of Biomass Burning Aerosols

“…Scatter in m R,100 values and in growth rates derived for individual BBOP flights shown in Figure are likely due to differences in prevailing combustion conditions (e.g., active flaming, smoldering, pyrolysis), source fuel, and plume dilution and cooling . Condensation and coagulation of BC particles with oxidized hydrophilic vapors and particles, and direct heterogeneous oxidation of the BC particles, will rapidly convert the fresh, hydrophobic BC particles to hydrophilic ones. , This conversion represents a critical step in a BC particle lifecycle, given the importance of wet removal processes to their atmospheric lifetimes.…”

“…29 Condensation and coagulation of BC particles with oxidized hydrophilic vapors and particles, and direct heterogeneous oxidation of the BC particles, will rapidly convert the fresh, hydrophobic BC particles to hydrophilic ones. 30,31 This conversion represents a critical step in a BC particle lifecycle, given the importance of wet removal processes to their atmospheric lifetimes.…”

Impact of Cloud Process in the Mixing State and Microphysical Properties of Soot Particles: Implications in Light Absorption Enhancement

“…Such difference indicates that organics lead to less compaction of soot aggregates, rather than sulfate. The restructure of soot aggregates depends on the uptake of water by the coating materials (Guo et al., 2016), and the following evaporation of water (Enekwizu et al., 2021; Ma et al., 2013). Complete encapsulation of organic coating produces lower surface tension than partly coated structure and sulfate coating during water evaporation (Schnitzler et al., 2017), resulting in less degree of reconstruction between monomers of soot aggregates fully embedded in organics.…”

Condensation‐Controlled Toposelective Vapor Deposition in Nano‐ and Microcavities: Theory, Methods, Applications, and Related Technologies

“…[ 115 ] On the other hand, although the forces can be weaker during advancing wetting, their variation is probably larger. [ 112 ] The difference in wetting and drying has been highlighted in studies of, for instance, restructuring of soot aggregates especially during adsorption [ 118 ] or water sorption in cellulose, where the extent of structural changes happened also during drying and depended on the maximum humidity that the samples were exposed to before drying. [ 119 ] Similar observations were done with nanoparticle agglomerates, where capillary forces caused cluster restructuring only at higher partial pressures.…”