Organic Aerosol Composition in the Southeastern United States and the Role of Low-Molecular-Weight Organic Acids
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Particulate matter (PM) is an important component of the atmosphere which affects the planetary energy budget, visibility, and public health. Although atmospheric PM is a complex mixture of inorganic and organic components from a variety of sources, organic aerosols (OA) represent a significant fraction (20-90%) of tropospheric submicron PM. A better understanding of atmospheric organic aerosols is essential to evaluate their impact and develop effective regulations. This thesis presents two months of ambient measurements conducted at a rural site in the southeastern United States with two different mass spectrometers: a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and a chemical ionization mass spectrometer equipped with a Filter Inlet for Gases and AEROsols (FIGAERO-CIMS). We show that molecular-based real-time measurements supplement bulk-level AMS measurements, providing a new perspective to the AMS OA factors that have been identified for over a decade and a new interpretation to the AMS-based global OA distribution map. Organic acids are ubiquitous atmospheric components that contribute to atmospheric acidity and directly participate in many reactions. Low-molecular-weight organic acids (LMWOA) are normally end products of photochemical oxidation and thus proxies of aerosol aging. Recent studies showed secondary formation from biogenic VOC oxidation is a largely underestimated source of LMWOA. In this study, the sources and partitioning of the five most abundant LMWOA in the southeastern United States were investigated. We show that the gas and particle phases of LMW monocarboxylic acids are largely decoupled: the majority of gas-phase formic and acetic acids were formed rapidly and in situ through biogenic VOC photooxidation, while their particle-phase counterparts were likely from transport. The formation of LMW dicarboxylic acids was related to aqueous processing of local biogenic secondary organic aerosols (SOA), and their gas-particle partitioning was in thermodynamic equilibrium. Transport and local OA aging were identified as sources of aged SOA, while the diurnal pattern of aged SOA was dominantly driven by boundary layer dynamics. A series of chamber experiments were conducted to investigate formic and acetic acid formation by photooxidation of isoprene and α-pinene, two of the most abundant biogenic VOC in the southeastern United States. Chamber experiments were performed under different relative humidity (RH), using both effloresced and deliquesced ammonium sulfate seed particles to study the role of water vapor and aerosol water. The introduction of both gaseous and particulate water enhanced formic and acetic acids formation. Given the relatively high RH in the southeastern United States, tropospheric water would be a non-negligible factor affecting LMWOA formation, and including it in current modeling frameworks may improve simulations and resolve the large measurement-model discrepancies observed in previous studies.
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