Sulfuric Acid and New Particle Formation in the Nocturnal Boundary Layer

<p dir="ltr">Atmospheric aerosols play a critical role in Earth’s climate through their direct interaction with solar radiation and their indirect effects as cloud condensation nuclei (CCN). A major contributor to CCN is new particle formation (NPF), a process during which gas-phase species such as sulfuric acid (H2SO4) react with other stabilizing molecules and form clusters. While NPF and H2SO4 formation are typically associated with daytime photochemistry, recent evidence suggests that oxidation pathways active at night may also contribute to H2SO4 and particle formation during nocturnal and early morning hours. However, nocturnal H2SO4 chemistry remains poorly understood, in part due to limited instrumentation capable of measuring atmospheric H2SO4.</p><p dir="ltr">This thesis investigates the formation and vertical transport of H2SO4 and newly formed aerosol particles in the nocturnal boundary layer through a combination of field observations and instrument development. Chapter 2 presents measurements from Pittsburgh, PA, where increases in nighttime H2SO4 concentrations were observed in the absence of photochemical hydroxyl radical production. The resulting concentrations of H2SO4 were comparable to daytime values but did not display NPF and growth events associated with daytime H2SO4. These results point to possible heterogeneous catalytic oxidation of sulfur-containing species on aerosol surfaces and suggest persistent, nonphotochemical mechanisms of H2SO4 formation. </p><p dir="ltr">Chapter 3 analyzes the relation between nocturnal boundary layer mixing and the ground-level particle size distribution at the Department of Energy Atmospheric Radiation Measurement Southern Great Plains site in Oklahoma. These events occurred during nocturnal low-level jets (NLLJ) and were characterized by sudden increases in sub-10 nm particle concentrations at the surface. During the dispersion of an NLLJ, increases in H2SO4 and sub-5 nm concentrations suggest that nocturnal formation of H2SO4 and small particles was happening aloft, which impacted particle concentrations both at night and in the early morning. </p><p dir="ltr">To address the limitations of existing H2SO4 detection methods, Chapter 4 introduces a new instrument, the Sulfuric Acid Dimethylamine-Reactive Condensation Particle Counter (SAD-RCPC), designed for portable, low-power, and vertically resolved H2SO4 measurements. The SAD-RCPC was deployed on a tethered balloon system and demonstrated its ability to detect H2SO4 in locations where chemical ionization mass spectrometers are impractical. The development of this instrument opens new opportunities for investigating nighttime and vertically distributed H2SO4. </p><p dir="ltr">Together, the findings presented in this thesis highlight the potential significance of nighttime H2SO4 formation and its contribution to NPF, challenging the assumption that NPF is restricted to photochemical regimes. These results underscore the importance of investigating H2SO4 formation in new environments and expanding models to include nonphotochemical oxidation pathways that contribute to NPF and particle growth.</p>