Soot Restructuring in Condensation-Evaporation Cycles with Wetting and Non-Wetting Liquids
Atmospheric soot is made of fractal aggregates of loosely connected graphitic spherules. These aggregates can undergo condensation-evaporation cycles with various atmospheric vapors, modifying the composition of aggregates and inducing their compaction. Because of these changes, climate impacts of soot become significantly altered. There is no agreement on which of the legs of the condensation-evaporation cycle contributes the most to soot compaction. Thus, we investigated soot restructuring at every stage of the cycle, ranging from capillary condensation to full aggregate encapsulation and followed by condensate evaporation. In our experiments, airborne fractal aggregates were exposed to vapors of wetting and non-wetting liquids, and particle mobility diameter was measured before and after coating evaporation. Additionally, processed soot particles were collected on silicon wafers before and after condensate loss, and their morphology was examined by electron microscopy. Depositing encapsulated soot particles on the silicon wafer surface allowed to ‘freeze’ their morphology and obtain images of structures corresponding to their encapsulated state. The experiments show that wetting and non-wetting liquids behave differently towards restructuring of bare hydrophobic soot. With wetting liquids (e.g., oxidized organics), most restructuring occurs during vapor condensation, producing nearly fully compact aggregates. With non-wetting liquids (water), there is almost no restructuring during condensation, but instead restructuring occurs during shell evaporation. However, after priming the hydrophobic soot aggregates with a monolayer of a hydrophilic chemical, the aggregates restructured fully already during water condensation. We conclude that for wetting liquids restructuring takes place during condensation, but when non-wetting water condenses on hydrophobic soot aggregates, the aggregates remain fractal until the water shells evaporate. This finding has an important implication for evaluating the direct contribution to climate forcing by aged soot because the fractal and compact encapsulated soot aggregates differ significantly in their ability to absorb and scatter light.