Date of Award
Doctor of Philosophy in Chemical Engineering - (Ph.D.)
Chemical and Materials Engineering
Edward L. Dreyzin
Ecevit Atalay Bilgili
V. Faye McNeill
The main goal of this dissertation is to generate data and parameterizations to accurately represent soot aerosols in atmospheric models. Soot from incomplete combustion of fossil fuels and biomass burning is a major air pollutant and a significant contributor to climate warming. The environmental impacts of soot are strongly dependent on the particle morphology and mixing state, which evolve continuously during atmospheric transport via a process known as aging. To make predictions of soot impacts on the environment, most atmospheric models adopt simplifications of particle structure and mixing state, which lead to substantial uncertainties. Using an experimentally constrained modeling approach, this dissertation aims to improve the predictive capabilities of atmospheric models regarding the impacts of soot. Accordingly, the study objectives are to: (1) conduct experiments and simulations to investigate how soot properties evolve during aging; (2) develop physical parameterizations between soot particle properties and aging environment using established relationships; (3) incorporate the parameterizations in a particle-resolved aerosol model.
Experiments to investigate morphological changes are conducted by exposing airborne aggregates of well-defined mass, size, and composition to vapors of chemicals condensable at atmospheric conditions. The underlying mechanisms that lead to structural change are then identified and applied in theoretical calculations for soot aging. Optical experiments are conducted to measure light absorption and scattering by soot and compared against literature reported values to resolve differences. Additionally, rigorous optical calculations are performed with morphological data from aging experiments to investigate the contributions of morphology and mixing state to parameters of interest in atmospheric models.
This work has developed a novel analytical framework for predicting the morphological mixing state and extent of restructuring of soot aggregates during atmospheric aging. The framework is validated by experimental measurements for a wide range of condensing vapors in realistic multicomponent systems, and is based on a single dimensionless parameter χ. The χ-parameter is controlled by coating material properties of vapor supersaturation and wettability for a specified soot monomer diameter. In the course of this study, the roles of vapor condensation and coating evaporation on aggregate restructuring are also found to be influenced by coating wettability. Based on rigorous optical calculations, the differences in measured and modeled optical properties of soot are resolved by varying monomer size and introducing necking material, between monomers. Additionally, the effect of the morphological mixing state on soot optical properties is found to depend strongly on the compactness of the aggregate. A simplified representation of χ-framework is incorporated into the particle resolved aerosol model, PartMC-MOSAIC and successfully tested on soot particles in an idealized urban air parcel. This demonstrates the suitability of this approach in facilitating accurate predictions of morphology-dependent soot properties in PartMC-MOSAIC.
Overall, the findings of this dissertation represent a significant advancement in understanding the processes governing the transformations and environmental impacts of soot that will benefit the atmospheric experimental and modeling research communities.
Enekwizu, Ogochukwu Yvonne, "The aging and impacts of atmospheric soot: closing the gap between experiments and models" (2020). Dissertations. 1592.