Date of Award
2-26-2025
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Aerosol-radiation interactions remain a critical yet uncertain component of Earth’s climate system. Carbonaceous aerosols, particularly black carbon (BC) and brown carbon (BrC), are significant contributors to radiative forcing due to their strong light-absorbing characteristics. However, systematic measurement biases and incomplete representations of aerosol properties across multiple scales can hinder our understanding of their interaction with solar radiation and climatic impacts. This dissertation addresses these gaps through a multipronged investigation that integrates measurement corrections, atomic-scale simulations, particle-scale optical modeling, and large-scale radiative forcing evaluations. Firstly, filter-based instruments commonly used for aerosol light absorption measurements, such as the Particle Soot Absorption Photometer, suffer from artifacts which leads to overestimation of light absorption. To address these biases, the first major component of this dissertation systematically evaluates both analytical (e.g., Virkkula and Bond–Ogren corrections) and machine learning-based approaches (Random Forest Regression) at the Southern Great Plains site and Tracking Aerosol Convection Interactions Experiment campaign. The results show that machine learning based corrections can reduce filter-based measurement errors by as much as 50%, highlighting the importance of site-specific, nonlinear correction frameworks. These data-driven corrections were also used to determine the impact of the key input parameters on biases in the Particle Soot Absorption Photometer measurements, such as absorption by dark-BrC particles. Beyond aerosol light absorption measurement corrections, a deeper understanding of the black-brown continuum in carbonaceous aerosols requires exploring the atomic-scale processes that drive their optical diversity. Building on this need, the second major component of this dissertation uses density functional theory calculations to reveal how hydrogen-tocarbon (H/C) ratios and carbon hybridization states (sp2 vs. sp3) modulate light absorption. By correlating reduced sp2 hybridization with lower imaginary refractive indices, this study elucidates the continuum from strongly absorbing “black” carbon to more wavelengthdependent absorption of “brown” carbon. Subsequently, this dissertation advances to the next scale through particle-scale modeling of light absorption and scattering by BrC aggregates. The third major component of this dissertation employs polydisperse diffusion-limited cluster aggregation and the discrete dipole approximation to quantify how BrC subclasses (dark and weakly absorbing) and their aggregation morphologies (monomer numbers and sizes) alter absorption and scattering. The findings underscore that both absorption and scattering are enhanced due to near-field interactions among aggregated monomers, underscoring the necessity of incorporating realistic non-spherical aerosol morphologies into climate models. Finally, the fourth major component of this dissertation evaluates the broader radiative impacts of particle-scale diversity using BC injected into pyrocumulonimbus plumes during extreme wildfire events. Particle-resolved in situ measurements from the FIREX-AQ campaign reveal that simplifying BC morphology and mixing state can lead to a substantial underestimation of absorption and direct radiative forcing in the upper troposphere and lower stratosphere region. Refined particle-resolved BC morphology model that capture the heterogeneity of BC coatings predict up to a 17% enhancement in amount of solar energy absorbed by pyrocumulonimbus BC aerosol layer. In sum, this dissertation bridges fundamental and applied aerosol research by: • Demonstrating advanced correction techniques for filter-based absorption biases. • Elucidating the atomic-scale structure-property relationships shaping the black-brown carbon continuum. • Quantifying aggregation-driven enhancements in BrC’s optical properties. • Assessing the implications of particle-scale aerosol heterogeneity on radiative forcing. Together, these contributions offer a robust “first principles” framework for improving the accuracy of aerosol light absorption measurements, refining our atomic- and particle-scale understanding of carbonaceous aerosols, and reducing uncertainties in aerosol-radiation interactions within climate models. This dissertation lays the groundwork for future efforts to more accurately represent aerosols in climate models.
Language
English (en)
Chair
Rajan Chakrabarty
Committee Members
Brent Williams; Hiren Jethva; Randall Martin; Rohan Mishra