Capillary Condensation as an Unaccounted Pathway for Rapid Aging of Atmospheric Soot

ELLA IVANOVA, Egor Demidov, Nicole Riemer, Gennady Gor, Alexei Khalizov, New Jersey Institute of Technology

     Abstract Number: 154
     Working Group: Carbonaceous Aerosols

Abstract
Combustion soot is not only an air pollutant but also a powerful climate warming agent because its high light absorptivity across a broad range of solar spectrum results in significant direct climate impact. Soot is hydrophobic initially, and hence, its indirect climate impact is insignificant at the time of emission. Experiments show that the fractal morphology of soot particles promotes rapid condensation of subsaturated vapors, such as water-soluble trace chemicals, changing the hydrophobic nature of soot, and allowing water absorption and particle compaction at moderate humidity levels. This transformation has the potential to transform initially hydrophobic soot particles into cloud condensation nuclei (CCN). However, current atmospheric models do not take into account the fractal morphology of soot particles, treating them as spheres and thereby omitting the process of soot aging by capillary condensation of subsaturated chemicals.

Recently, we developed a capillary condensation model for fractal soot aggregates and tested it against laboratory experiments. In this study, we integrate this capillary condensation model with the atmospheric aerosol model PartMC-MOSAIC to evaluate the role of subsaturated trace chemicals in atmospheric soot processing. To see the effect of capillary condensation clearly, we selected an atmospheric scenario corresponding to subsaturated conditions of condensable vapors, typical of atmospheric conditions dominated by biogenic emissions, and with low anthropogenic emissions of such chemicals as sulfur dioxide. A biogenic scenario representing the Duke Forest area (North Carolina) was configured and the compositions of the gas and aerosol species were predicted using PartMC-MOSAIC. In the next step, the capillary condensation model was applied to evaluate the supersaturations of condensable chemicals and predict their condensation on fractal soot. The hygroscopic kappa theory was used to predict the evolution of the critical supersaturation of water with soot aging. The results show that fractal soot can undergo a rapid transformation (2-3 hours) from hydrophobic to hydrophilic state, significantly faster than if the soot particles were represented as spheres. Thus, the previously overlooked mechanism of capillary condensation on fractal particles can lead to the rapid aging of atmospheric soot, which needs to be incorporated into aerosol models.