Nadine Borduas-Dedekind

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.

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The development and deployment of advanced aerosol instrumentation is essential to address gaps in understanding atmospheric processes. In this thesis, three primary projects were undertaken: the first involved constructing an LED-based environmental chamber (ATMOX) to simulate atmospheric conditions for studying photochemical and oxidative reactions under controlled light wavelengths. This chamber was characterized to ensure its capability in reproducing realistic atmospheric environments, with potential applications in wavelength-specific photochemistry. The second project focused on characterizing locomotive exhaust emissions during a field campaign at an undisclosed railyard, examining the effects of biodiesel fuel blends on particulate matter emissions. The third project investigated cooking aerosol emissions by employing measurement techniques to evaluate particle size distributions and their potential to generate harmful reactive oxygen species. Collectively, these projects highlighted the versatility and importance of aerosol measurement tools in bridging laboratory and real-world applications, advancing knowledge in air quality and atmospheric chemistry.

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Mixed-phase clouds are important climate regulators, providing significant annual precipitation and global cloud coverage. An increase in wildfires in the past decade is changing cloud cover and cloud composition which are currently difficult to predict due to a lack of mechanistic understanding. Although there is evidence of the ability of organic aerosols to nucleate ice under mixed-phase cloud conditions, the underlying physicochemical mechanism through which organic materials promote ice nucleation is an active area of research. The goal of this thesis is to study the mechanism through which lignin, and, by extension, biomass burning organic aerosols, nucleate ice in the atmosphere. To determine ice activity of lignin, we employ a bottom-up approach by synthesizing lignin nanoparticles (LNPs). LNPs have controllable, measurable, characteristics; valuable for elucidating the mechanism of ice nucleation of lignin. To determine the role of size and surface properties, we synthesized lignin nanoparticles (LNPs) from commercially available Kraft lignin and tested their freezing ability in our home-built Freezing Ice Nuclei Counter (FINC). LNPs were characterized for their size and dispersity with dynamic light scattering (DLS). Transmission Electron Microscopy (TEM) provided additional confirmation of shape and dispersity. We then prepared LNP solutions of different sizes at equal concentration via thermogravimetric analysis (TGA) to isolate the role of size on ice nucleation. Our FINC and DFA results confirmed that lignin nanoparticles, ranging in size from 80 – 500 d.nm, at the same concentration (0.2 mg/mL) are ice active with T\textsubscript{50} values ranging from -14.96 to -15.91 ºC. We found that few large LNPs nucleate ice with the same ability as many small LNPs. Normalizing the freezing data to mass and number suggests that aggregation facilitates ice nucleation of LNPs in the 10s and 100s of nanometers in size. From this research, we further contribute to the growing literature suggesting the key role of aggregation, and therefore likely of chemical composition on organic aerosols' ability to nucleate ice. These findings help understand how lignin within wildfire organic aerosols are able to nucleate ice and hence impact the ice crystal concentration in mixed-phase clouds.

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