Soot Restructuring in Condensation-Evaporation Cycles

ALEXEI KHALIZOV, Ali Hasani, Egor Demidov, New Jersey Institute of Technology

     Abstract Number: 90
     Working Group: Carbonaceous Aerosols

Abstract
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 compositional and morphological changes, climate impacts of soot become significantly altered. Which leg of the condensation-evaporation cycle contributes the most to soot compaction is still not fully understood. Thus, we investigated soot restructuring at every stage of the cycle, starting with capillary condensation, proceeding toward full encapsulation, and ending with the condensate evaporation off a fully encapsulated aggregate. 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, these processed soot aggregates were collected on silicon wafer chips before and after condensate loss, and their morphology was examined by scanning electron microscopy. Depositing encapsulated soot particles on the 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 or sulfuric acid), compaction begins already during vapor condensation, producing partially restructured aggregates, but coating evaporation is needed achieve a fully restructured aggregate. With non-wetting liquids (water), there is almost no restructuring during vapor condensation, and full compaction occurs during shell evaporation. Interestingly, priming the hydrophobic soot aggregates with 1-2 mass percent of a water-soluble chemical, results in nearly full restructuring of the aggregates 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 at very high supersaturations, 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 scatter light.