Restructuring of Lightly Coated Fractal Soot Particles Exposed to Subsaturated Water Vapor

Soot particles released by combustion can rapidly develop light coatings through capillary condensation of trace chemicals in the atmosphere. If the chemicals are hygroscopic, uptake of water vapor by the coating layer can promote the restructuring of the initially fractal soot particles. We subjected size-classified soot aerosol to capillary condensation by exposing it to sub-saturated vapors of several hygroscopic chemicals and then studied the response of the lightly coated airborne soot particles to subsequent exposure to water vapor (relative humidity 10-85%). We found that the particles restructured significantly, and using a capillary condensation model [1,2] we show an agreement between our current results for aqueous solutions of condensates and previous measurements performed with neat condensates at low humidity. Achieving this agreement required invoking the presence of carbon necks between the monomer spheres comprising fractal soot particles. The neck diameter obtained from condensation-restructuring data is in agreement with the average neck diameter derived from the electron microscopy images for the same soot. Our results imply that in the presence of a few monolayers of hygroscopic chemicals, atmospheric soot particles may undergo nearly complete compaction even in the absence of cloud processing.

[1] Chen, C., O. Y. Enekwizu, X. Fan, C. D. Dobrzanski, E. V. Ivanova, Y. Ma, G. Y. Gor, and A. F. Khalizov, Single Parameter for Predicting the Morphology of Atmospheric Black Carbon, Environ. Sci. Technol., 52(24), 14169-14179, 2018, 10.1021/acs.est.8b04201[2] Ivanova, E. V., A. F. Khalizov, and G. Y. Gor, Kinetic model for competitive condensation of vapor between concave and convex surfaces in a soot aggregate, Aerosol Science and Technology, 1-19, 2020, 10.1080/02786826.2020.1846677.