Light Scattering and Absorption by Soot Aerosols with Different Morphologies and Coating Distributions
Egor V. Demidov, Ogochukwu Enekwizu, Ali Hasani, Alexei F. Khalizov
AMS, Oral presentation, 2022
Abstract
Carbon soot, released into the atmosphere upon incomplete combustion of fossil fuels, is a major contributor to climate change through light absorption and scattering. Initially, soot particles are fractal aggregates of graphitic spherules, but as they interact with condensable gas chemicals, they acquire coatings. The coatings can be distributed either uniformly or as pendular-rings, depending on volatility of the condensed material, and the coated soot aggregates restructure at a rate that depends not only on the coating amount, but also on its distribution. In order to accurately model climatic impact of soot aerosols, effects of different coating distributions and particle morphologies on soot optical properties need to be determined. We investigated such transformations by exposing size-classified soot aerosol to supersaturated vapors of low and intermediate volatility compounds. Mass, mobility diameter, and optical properties of fresh and coated particles were then measured. We find that low volatility compounds, which form uniform coatings around particles, require relatively large coating mass increase to enhance light scattering and absorption due to combined effects of compaction of aggregates, increase in overall size and volume, and lensing. Intermediate volatility compounds, which tend to form pendular-ring coatings in junctions between monomers, induce restructuring even when present in small amounts, nearly doubling light scattering with little change in light absorption. An important finding of our study is that for lightly coated soot aggregates, not only the enhancement in absorption is weak, but also when the coating is removed, a notable decrease in absorption is observed relative to the fresh uncoated soot. This decrease in absorption is not caused by shielding and has not been predicted yet by computational methods. Based on Discrete Dipole Approximation calculations, we speculate that the decrease in absorption may be caused by breaking of necks that connect individual monomers during restructuring, decreasing their coupling.