UCLA

When air pollution particles encounter water, whether in clouds or in the lungs, they partly dissolve, which starts a cascade of reactions. In clouds, this chemistry can change the mass concentration, chemical composition, and size distribution of particulate matter (PM) left behind when the cloud re-evaporates, making it an important process impacting the earth’s climate. In the lungs, particles induce health impacts via mechanisms that are areas of active research. While these two environments are distinct, both are mediated by the reactions of reactive oxygen species (ROS). Excess amounts of ROS formation triggered by the inhalation of PM is believed to be one of the causes of PM-induced diseases. I utilized different oxidative potential (OP) metrics in my research to quantify aerosol particles’ ability to generate ROS. I further combined lab measurements with statistical analysis and kinetics models to determine the most toxic components in PM and dominant emission sources contributing to OP. In this dissertation, I first delved into the chemistry of ascorbate, one of the most abundant antioxidants in the lung. Ascorbate plays a vital role in the mechanism by which particulate air pollution initiates biological responses. I synthesized the literature on ascorbate, trace metals, and ROS interactions. Recognizing the large disagreements on the ascorbate reactions with iron or copper, I developed a chemical kinetics model to constrain the mechanisms and derive key rate constants. The modeling results suggest trace metals iron and copper are efficient sinks for ascorbate. Secondly, I investigated the interactions of PM and ROS with two other OP metrics – the hydroxyl radical (OH) and dithiothreitol (DTT) assays. The source apportionment analysis on OP indicates that vehicular exhaust and brake and tire wear are primary contributors to OP. I further linked the measured PM mass and OP with the CalEnviroScreen database. This is the first study in the US to uncover a disproportionate burden of OP for people in lower socioeconomic position. The correlational analysis also suggests that the OH assay may be promising in predicting particle-induced adverse health outcomes. My study on OH chemistry extends to the cloud water context. The OH burst, a recently discovered fast OH formation phenomenon in nascent cloud droplets, contributes to OH radicals exceeding all conventional aqueous phase processes. This discovery suggests a pivotal role of OH burst in aerosol oxidation and aging, yet our understanding of this chemistry is limited. I conducted field campaigns to measure the OH burst for different types of aerosols using the newly developed Direct-to-Reagent method and investigated the variations in the OH burst. I further assessed the degree to which ambient particles we collected encountered clouds along their backward trajectories with the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. The results highlight that biomass burning organic aerosol is the largest predictor of the OH burst. Strong negative associations between OH formation and cloud processing history indicate the role of cloud water as a critical medium of OH chemistry. These findings on the dominant factors of the OH burst chemistry shed light on a parametrization of the chemistry within global models for evaluating its broader impacts on the global OH budget, aerosol oxidation, and climate.