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Abstract

Singlet molecular oxygen (¹O₂*) is the first excited state of molecular oxygen (O₂) and is formed through indirect photochemistry during irradiation of atmospheric brown carbon. ¹O₂* acts as a competitive oxidant to OH radicals, ozone, hydrogen peroxide, and superoxide in the photochemical processing of atmospheric aerosols and droplets. ¹O₂* has been quantified in atmospherically relevant environments such as fog water, cloud water, rain, and particulate matter extracts. As more researchers investigate the atmospheric photochemistry of ¹O₂*, standardized protocols are needed to minimize errors and ensure results are comparable across laboratories. In this thesis, I present an intercomparison of ¹O₂* measurements produced from four photosensitizing molecules for four photoreactor setups at three research institutions. The production of ¹O₂* from perinaphthenone, Rose Bengal, lignin, and juglone served as a representative suite of atmospherically relevant and previously characterized photosensitizers. Furfuryl alcohol was used as a chemical probe for ¹O₂* quantification. Two chemical actinometers, 2-nitrobenzaldehyde and p-nitroanisole/pyridine, were used to quantify photon flux and calculate rates of light absorbance for photosensitizer molecules in unique photoreactor setups. Rate of light absorbance and ¹O₂* steady-state concentrations ranged by up to three orders of magnitude, depending on the light source (xenon vs UV) and the intensity. Normalizing to ¹O₂* quantum yield showed agreement within the standard deviation of triplicate measurements for perinaphthenone and Rose Bengal, while discrepancies for lignin and juglone were attributed to wavelength dependencies. Time-dependent density functional theory calculations were used to explore molecular properties that govern efficient ¹O₂* production. Triplet state energies, redox potentials, and S₀→T₁ orbital transition types were calculated for a functionally diverse set of BrC components to assess their potential as predictive metrics for quantum yield. These metrics proved to be useful predictors of efficient ¹O₂* production for single molecules, and represent a first step toward developing predictive tools for identifying highly efficient ¹O₂* photosensitizers in complex atmospheric samples. Based on this intercomparison study and theoretical investigation, we make recommendations on how to increase the accuracy, reproducibility, and predictive capacity of ¹O₂* studies to advance understanding of the role of excited state oxidants in photochemical processing of atmospheric aerosols and droplets.