Enhanced light scattering and absorption by soot aerosols with different coating distributions
Egor V. Demidov, Alexei F. Khalizov
ACS MARM, Oral presentation, 2021
Abstract
Carbon soot is released into the atmosphere upon incomplete combustion of fossil fuels and is a major contributor to climate change through light absorption and scattering. In the atmosphere, soot particles, which are structurally fractal carbon aggregates, interact with other chemicals, to acquire coatings and undergo changes in their composition, size, and morphology. Particles with different sizes, morphologies, and coating distributions affect the climate differently. In order to accurately model behavior of soot aerosols in the atmosphere we need to determine the relationship between different coating distributions and optical properties of soot. Theoretical studies had been conducted in this area and experimental data are needed to verify the predictions. We studied the dependence of optical properties on mixing with intermediate and low volatility chemicals relevant to those encountered in the atmosphere. To simulate interactions that occur in the atmosphere, soot aerosol was passed through a temperature-controlled chamber, containing organic compounds. Evaporated compounds condensed on soot aggregates and formed coatings of various thickness and distributions, which depended on volatility of the compounds and chamber temperature. Mass, size, and optical properties of bare and coated soot were measured. Optical properties included absorption, scattering, extinction, and single-scattering albedo. Aerosols with different coating distributions were compared. Presence of liquid coatings on soot aggregates alters their morphology and optical properties. Our results suggest that increasing coating thickness leads to a significant (up to 1000%) increase in light scattering. Absorption is also enhanced (up to 50%). Coated particles’ mobility diameter first decreases due to prevalent restructuring of the soot core, reaches a minimum, and then starts increasing again due to increase in the volume of coating material. Removal of the coating material by thermal denuding allows quantifying the degree of compaction of soot aggregates. Scattering of light by coated soot is increased from both restructuring and coating material addition, while absorption enhancement is only due to coating material. Climate impact of coated soot is a combination of cooling due to light scattering and heating due to light absorption, with scattering contribution dominating over absorption contribution for thicker coatings.