The Interplay between Capillary Condensation and Full Encapsulation in the Collapse of Fractal Soot Particles

Abstract:

Combustion soot is made of fractal aggregates of small, nearly spherical monomers. Changes in the mixing state and morphology of soot aggregates strongly affect their optical properties and hence their climate impacts. Condensation of vapors on soot aggregates produces coatings, which can transform the aggregate morphology from fractal to compact. Here we studied the restructuring of soot aggregates subjected to vapors of intermediated volatility organic compounds (IVOC), using a range of supersaturations, from zero and above. The measurements were performed using an in-house-built miniature electrostatic classifier (EC) that could be placed immediately or at any distance after the aerosol coating chamber to accurately probe the coated particle mobility diameter. The dependence of the particle mobility diameter on vapor supersaturation showed three different regions. At a zero supersaturation, capillary condensation took place, resulting in a minor shrinkage of soot aggregates. At supersaturations slightly above zero, the shrinkage progressed due to an increased restructuring driven by additional condensation, with particle diameter decreasing to 80% of its initial value. At higher supersaturations, the particle diameter increased several times, eventually corresponding to full encapsulation of soot by the coating droplet. The removal of the IVOC coating by evaporation produced highly compacted soot particles. Our results suggest that for intermediate volatility vapors, minor restructuring of soot aggregates can occur at slightly negative or zero supersaturations because of the capillary condensation. The restructuring is promoted further by condensation at slightly-above-zero supersaturations. For higher supersaturations, the condensate encapsulates the soot aggregates and then evaporates when supersaturation is reduced to zero, leading to compact soot.