The Role of Coating-Surface Interactions in the Restructuring of Soot Nanoparticles

Atmospheric soot nanoparticles are fractal aggregates of graphitic spheres that are mixed with various organic and inorganic materials. The morphological and chemical complexity of soot nanoparticles is caused by diversity in emission sources and further amplified by aging transformations in the environment. The mechanistic details of these transformations, including the formation of specific mixing states and the extent and rate of morphological changes from the condensation of different materials remain poorly understood. Also, whereas it is commonly assumed that aging transformations produce compact and uniformly coated soot aggregates, there is increasing evidence that a significant fraction of atmospheric soot is neither compact nor uniformly coated. We have generated soot particles with different initial mixing states and investigated morphological changes occurring during aging for different coating materials and aging environments. The mixing state and morphology of soot aggregates were determined from a combination of mass-mobility measurements and environmental electron microscopy imaging. The results obtained in this study will be discussed in connection with current pressing needs to identify and evaluate key parameters that control the mixing state and structure of soot nanoparticles during aging, determine the mechanism and timescale of morphology restructuring, and evaluate associated changes in cloud-forming and optical properties of particles. A better understanding of soot aging will help to reduce uncertainties in climate prediction because soot from incomplete combustion of fossil fuels is a significant contributor to direct climate forcing.