(494d) The Impact of Vapor Supersaturation on the Morphology, Mixing State and Optical Properties of Atmospheric Soot

Soot nanoparticles, produced from the incomplete combustion of fossil fuels, absorb solar radiation and contribute to climate change by direct radiative forcing. The magnitude of forcing is strongly influenced by the changes in morphology and mixing state of the soot aggregates as they interact with other aerosols during atmospheric transport. Presently, the pathways leading to these structural transformations and their associated impacts are not well understood. In this study, soot aerosols were exposed to supersaturated vapors of organic and inorganic liquids with a broad range of physical and chemical properties to generate thinly coated soot aggregates. We observed two distinct restructuring behaviors which were governed by the level of vapor supersaturation in our coating chamber. With the aid of a simple analytical model, we show that high vapor supersaturation results in uniform condensation over the aggregates. Conversely, low vapor supersaturation causes preferential condensation in the junctions between contacting primary spheres to form pendular rings which induce structural collapse. We surmise that the degree of vapor supersaturation is the driving factor for the different mixing states and morphological changes observed in lab and field studies. We also discuss the impacts of changes in morphology and mixing state on the optical properties of soot for both condensation behaviors. By incorporating vapor supersaturation in atmospheric models, improvements can be made in simulations of the radiative forcing of soot.