CHEMICAL CHARACTERIZATION TO ELUCIDATE AMBIENT FATE AND TRANSPORT OF ORGANIC ACIDS IN WILDFIRE PLUMES

Abstract

Wildfire emissions profoundly influence air quality on both local and regional scales. Wildfire plumes can travel long distances, alter atmospheric chemical composition, and can substantially increase the burden of airborne water-soluble organic carbon (WSOC). Among these constituents, organic acids play critical roles in secondary organic aerosol (SOA) formation, cloud condensation nuclei activity, and air quality; however, the atmospheric burden of organic acids remains highly uncertain. Current chemical transport models typically only represent formic and acetic acids, leading to an underestimation of total organic acid concentrations by up to 50%. This dissertation addresses these scientific gaps by developing improved quantification methods and providing detailed molecular-level composition profiles of organic acids and related species in long-range transported wildfire and biomass burning emissions.In the first part of this dissertation, we developed and validated rapid, robust HPLC-PDA methods for quantifying six monocarboxylic acids (formic, acetic, propionic, butyric, isovaleric, and valeric) and six dicarboxylic acids (oxalic, malonic, succinic, malic, glutaric, and adipic), to improve analyte identification and quantification. By systematically optimizing chromatographic conditions, including column temperature, mobile phase composition and pH, and elution modes, the method achieved shorter analysis times, excellent linearity, low detection limits, and was successfully applied to real-world samples. Field measurements were conducted during the 2023 Canadian wildfires, which burned over 17 million hectares and severely degraded air quality across North America. Particulate matter was collected and analyzed using LC-QTOF-MS to characterize the water-soluble organic carbon (WSOC) fraction and quantify organic acids, while atmospheric transport and aging processes were assessed with HYSPLIT back-trajectory modeling. Results revealed elevated organic acid levels in urban air masses impacted by long-range smoke transport, along with a diverse chemical mixture that included oxygenated and nitrated hydrocarbons, biologically derived compounds, and species containing multiple heteroatoms. These findings highlight the significant contribution of biomass burning to regional air pollution episodes. To further elucidate formation mechanisms, controlled laboratory combustion experiments examined organic acid production from lignin, a key structural component of biomass. The experiments demonstrated that both combustion conditions and oxidative aging strongly influence organic acid yields, providing mechanistic context for field observations. In summary, this dissertation advances understanding of organic acid sources, detection, and atmospheric processing. The results bridge laboratory and ambient perspectives, offering molecular-level insights that refine emission inventories and improve the predictive capability of air quality and climate models.