Molecular-Level Investigations of Biomass Burning Organic Aerosols

<p dir="ltr">The profound and multifaceted influences ambient organic aerosols have on atmospheric, environmental, and anthropogenic processes underscore the need for their comprehensive understanding. More specifically, these particles perturb Earth’s radiative balance through light-scattering or light-absorbing interactions, thereby promoting radiative cooling or forcing trends, respectively. This dissertation seeks to discover key molecular compositional trends in the light-absorbing subclass organic aerosols – collectively termed “Brown Carbon” (BrC) – that presage their optical properties. Hybrid mass spectrometry techniques are employed in this research to comprehensively characterize atmospheric BrC species generated from biomass and biofuel pyrolytic combustion. The controlled laboratory evolution of BrC species is placed in atmospheric contexts to reconcile differences between laboratory and field studies of organic aerosols. Chapter 2 provides detailed descriptions of the instrumentation and data analysis protocols, providing a strong framework for the later chapters. Chapter 3 examines fresh and evaporatively aged biomass burning organic aerosols (BBOA) and establishes a link between photoenhancement and the increased nonvolatility and viscosity of BrC components. Additionally, this chapter demonstrates that small oxygenated monoaromatic compounds degas from fresh BBOA first, while lignin fragments and substituted polycyclic aromatic hydrocarbon species remain in the condensed phase after prolonged aging. In-lab photochemical aging of the fresh BBOA mixture is conducted in Chapter 4, and photolabile soluble and photorecalcitrant insoluble products are directly measured with an ambient ionization technique. Moreover, the measured half-lifetime of the mixture is scaled to ambient UV irradiation conditions to predict BrC longevity across various geographically distributed regions. Finally, Chapter 5 delves into the molecular and optical characterization of dung-emitted BrC species and provides a comparative analysis between dung-derived and wood-derived BBOA components. The physicochemical properties of atmospheric BrC unraveled in these studies provide a predictive understanding of their dynamic light extinction properties, essential in better constraining their effective radiative forcing effects in global models. Applications of these hybrid mass spectrometry techniques enable the identification of key BrC chromophores that determine the optical properties of BBOA plumes, thereby advancing chromophoric fingerprinting of global BBOA emissions. In sum, this research advances our molecular-level understanding of BrC compounds, providing critical insights that enhance the predictive accuracy of atmospheric and global radiation models.</p>