| dc.description.abstract | Reactive organic carbon (ROC), defined as all volatile organic compounds (VOCs) and particulate organic carbon except methane, is the largest source of reactive emissions in the atmosphere and therefore plays a central role in atmospheric chemistry. After emission, ROC undergoes rapid chemical transformations driven by sunlight and atmospheric oxidants, forming peroxy radicals (RO₂) whose fate shapes the resulting oxidation products. This sequence of reactions, known as the ROC lifecycle, continues until ROC is removed via wet or dry deposition or fully oxidized to carbon dioxide (CO₂). Throughout its evolution, ROC contributes to the formation of ozone, particulate matter and CO₂, making its study necessary for understanding air quality and climate. While many aspects of this cycle have been explored through modeling, laboratory experiments, and field campaigns, our understanding of the ROC lifecycle remains incomplete due to the large number of compounds and its chemical complexity as it evolves in the atmosphere. This thesis investigates key aspects of the ROC lifecycle, focusing on its multigenerational chemical evolution over extended atmospheric aging and on how RO₂ chemistry shapes gas-phase species distributions. The first part investigates the multigenerational oxidation of ROC through a series of laboratory experiments designed to simulate atmospheric aging over extended timescales. These experiments make comprehensive measurements of both gas- and particle-phase carbon to gain a holistic understanding of ROC evolution in the atmosphere. While models have been used to simulate the full evolution of ROC, few experiments have constrained this process. These experiments provide the first direct constraints on the lifetime of ROC against heterogeneous oxidation, the formation of small and long-lived oxygenated VOCs, and the evolution of carbon properties (e.g. carbon number, carbon oxidation state) over multiweek atmospheric equivalent aging. We then compare these experimental results to long-term aging simulations using several chemical mechanisms that are implemented in models. Most chemical mechanisms achieve near-total carbon closure, however, they differ in species composition, ROC mineralization rates, and OH reactivity, especially at later stages of oxidation. These discrepancies highlight the gaps that remain in our understanding of the long-term chemical evolution of ROC. Finally, we conduct experiments that simulate a variety of RO₂ environments representative of atmospheric conditions. Past chamber experiments have primarily focused on RO₂ chemistry under extreme conditions that involve short bimolecular lifetimes and “low” or “high” NOₓ conditions. Our findings suggest that traditional metrics used to assess species distributions are insufficient to explain the full range of changes with shifts in the RO₂ environment. A more comprehensive accounting of all relevant RO₂ fates is necessary to explain gas-phase species composition, which in turn influences HOₓ and NOₓ cycling, ozone production and ultimately our understanding of secondary organic aerosol formation. Together, these results provide new constraints on the atmospheric evolution of ROC across a wide range of oxidative and photochemical conditions, improving our understanding of ROC and its role in air quality and climate. | |
