Single Parameter for Predicting the Morphology of Atmospheric Black Carbon

Black carbon (BC) from fuel combustion is an effective light absorber that contributes significantly to direct climate forcing. The forcing is altered when BC combines with other substances, which modify its mixing state and morphology, making the evaluation of its atmospheric lifetime and climate impact a challenge. To elucidate the associated mechanisms, we exposed BC aerosol to supersaturated vapors of different chemicals to form thin coatings and measured the coating mass required to induce the restructuring of BC aggregates. We found that studied chemicals fall into two distinct groups based on a single dimensionless parameter, χ, which depends on the diameter of BC monomer spheres and the coating material properties, including vapor supersaturation, molar volume, and surface tension. We show that when χ is small (low-volatility chemicals), the highly supersaturated vapor condenses uniformly over aggregates, including convex monomers and concave junctions in between monomers, but when χ is large (intermediate-volatility chemicals), junctions become preferred. The aggregates undergo prompt restructuring when condensation in the junctions dominates over condensation on monomer spheres. For a given monomer diameter, the coating distribution is mostly controlled by vapor supersaturation. The χ factor can be incorporated straightforwardly into atmospheric models to improve simulations of BC aging.