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Open Access Publications from the University of California

Scholarly Works (7 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2024)

This dissertation investigates the formation of secondary brown carbon (BrC) from nighttime oxidation of heterocyclic volatile organic compounds (VOCs) such as furans and pyrroles under diverse environmental conditions. This dissertation focuses on clarifying the effects of different environmental factors such as nitrate radical (NO3) levels, pre-existing particles, and relative humidity (RH) on the optical properties of secondary BrC and the chemical composition of secondary organic aerosols (SOAs) derived from the heterocyclic VOCs. NO3 levels exhibit insignificantly influence on the overall composition of furan-derived SOA, while pre-existing particles show considerable effects on its chemical composition. The mass absorption coefficients (MAC) revealed the divergent impacts of NO3 levels and pre-existing particles on BrC light absorption, suggesting NO3 facilitates the generation of light-absorbing compounds whereas pre-existing particles may facilitate gas-to-particle partitioning of non-absorbing products. The effects of RH on secondary BrC formation were investigated through furan and pyrrole oxidation, revealing substantial effects on particulate size distribution dynamics. The pyrrole oxidation products exhibit higher potential in producing ultrafine particles via nucleation. The increase of RH leads to increased mass fractions of oxygenated compounds in both SOAs, indicating the enhanced gas-phase and/or multiphase oxidation processes in humid environments. Interestingly, higher RH leads to decreased MAC, contrasting to the trend observed in the secondary BrC derived from homocyclic precursors. This divergence is owing to the formation of non-absorbing high-molecular-weight oxygenated compounds and the reduced mass fractions of molecular chromophores. Furthermore, this dissertation highlights the crucial role of carbonyl chromophores in the light absorption of secondary BrC derived from a variety of heterocyclic VOCs. In particular, N-containing carbonyl chromophores (e.g., imides and amides) derived from pyrroles are potentially important chromophores in secondary BrC. The contribution of chromophores on the overall BrC light absorption may vary significantly across wavelengths, underscoring their distinct significance at different wavelength ranges. Overall, this dissertation contributes to the fundamental insights of secondary BrC formation in the complicated atmospheric environments, especially those within the wildfire smoke. All of these findings will be helpful for improving the evaluation of BrC's radiative forcing and its impact on climate change.

    Cover page: Secondary Brown Carbon Formation From Nighttime Oxidation of Heterocyclic Volatile Organic Compounds
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2024)
    The production of e-cigarette aerosols through vaping processes is known to cause the formation of various free radicals and reactive oxygen species (ROS). Despite the well-known oxidative potential and cytotoxicity of fresh vaping emissions, the effects of chemical aging on exhaled vaping aerosols by indoor atmospheric oxidants are yet to be elucidated. Terpenes are commonly found in e-liquids as flavor additives. In the presence of indoor ozone (O3), e-cigarette aerosols that contain terpene flavorings can undergo chemical transformations, further producing ROS and reactive carbonyl species. Here, we simulated the aging process of the e-cigarette emissions in a 2 m3 FEP film chamber with 100 ppbv of O3 exposure for an hour. The aged vaping aerosols, along with fresh aerosols, were collected to detect the presence of ROS. The aged particles exhibited 2- to 11-fold greater oxidative potential, and further analysis showed that these particles formed a greater number of radicals in aqueous conditions. The aging process induced the formation of various alkyl hydroperoxides (ROOH), and through iodometric quantification, we saw that our aged vaping particles contained significantly greater amounts of these hydroperoxides than their fresh counterparts. Bronchial epithelial cells exposed to aged vaping aerosols exhibited an upregulation of the oxidative stress genes, HMOX-1 and GSTP1, indicating the potential for inhalation toxicity. This work highlights the indirect danger of vaping in environments with high ground-level O3, which can chemically transform e-cigarette aerosols into new particles that can induce greater oxidative damage than fresh e-cigarette aerosols. Given that the toxicological characteristics of e-cigarettes are mainly associated with the inhalation of fresh aerosols in current studies, our work may provide a perspective that characterizes vaping exposure under secondhand or thirdhand conditions as a significant health risk.
      Cover page: Ozonolysis of Terpene Flavor Additives in Vaping Emissions: Elevated Production of Reactive Oxygen Species and Oxidative Stress.
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2024)
      The chemical and optical properties of secondary organic aerosols (SOA) have been widely studied through environmental chamber experiments, and some of the results have been parametrized in atmospheric models to help understand their radiative effects and climate influence. While most chamber studies investigate the aerosol formed from a single volatile organic compound (VOC), the potential interactions between reactive intermediates derived from VOC mixtures are not well understood. In this study, we investigated the SOA formed from pure and mixtures of anthropogenic (phenol and 1-methylnaphthalene) and/or biogenic (longifolene) VOCs using continuous-flow, high-NOx photooxidation chamber experiments to better mimic ambient conditions. SOA optical properties, including single scattering albedo (SSA), mass absorption coefficient (MAC), and refractive index (RI) at 375 nm, and chemical composition, including the formation of oxygenated organic compounds, organic-nitrogen compounds (including organonitrates and nitro-organics), and the molecular structure of the major chromophores, were explored. Additionally, the imaginary refractive index values of SOA in the multi-VOC system were predicted using a linear-combination assumption and compared with the measured values. When two VOCs were oxidized simultaneously, we found evidence for changes in SOA chemical composition compared to SOA formed from single-VOC systems, and this change led to nonlinear effects on SOA optical properties. The nonlinear effects were found to vary between different systems.
        Cover page: Chemical Composition and Optical Properties of Secondary Organic Aerosol from Photooxidation of Volatile Organic Compound Mixtures.
        • Article
        • Peer Reviewed
        UC Riverside Previously Published Works (2022)
        In late 2019, the outbreak of e-cigarette or vaping-associated lung injuries (EVALIs) in the United States demonstrated to the public the potential health risks of vaping. While studies since the outbreak have identified vitamin E acetate (VEA), a diluent of tetrahydrocannabinol (THC) in vape cartridges, as a potential contributor to lung injuries, the molecular mechanisms through which VEA may cause damage are still unclear. Recent studies have found that the thermal degradation of e-liquids during vaping can result in the formation of products that are more toxic than the parent compounds. In this study, we assessed the role of duroquinone (DQ) in VEA vaping emissions that may act as a mechanism through which VEA vaping causes lung damage. VEA vaping emissions were collected and analyzed for their potential to generate reactive oxygen species (ROS) and induce oxidative stress-associated gene expression in human bronchial epithelial cells (BEAS-2B). Significant ROS generation by VEA vaping emissions was observed in both acellular and cellular systems. Furthermore, exposure to vaping emissions resulted in significant upregulation of NQO1 and HMOX-1 genes in BEAS-2B cells, indicating a strong potential for vaped VEA to cause oxidative damage and acute lung injury; the effects are more profound than exposure to equivalent concentrations of DQ alone. Our findings suggest that there may be synergistic interactions between thermal decomposition products of VEA, highlighting the multifaceted nature of vaping toxicity.
          Cover page: Formation of Redox-Active Duroquinone from Vaping of Vitamin E Acetate Contributes to Oxidative Lung Injury
          • Article
          • Peer Reviewed
          UC Riverside Previously Published Works (2023)
          The light absorption properties of brown carbon (BrC), which are linked to molecular chromophores, may play a significant role in the Earths energy budget. While nitroaromatic compounds have been identified as strong chromophores in wildfire-driven BrC, other types of chromophores remain to be investigated. Given the electron-withdrawing nature of carbonyls ubiquitous in the atmosphere, we characterized carbonyl chromophores in BrC samples from the nighttime oxidation of furan and pyrrole derivatives, which are important but understudied precursors of secondary organic aerosols primarily found in wildfire emissions. Various carbonyl chromophores were characterized and quantified in BrC samples, and their ultraviolet-visible spectra were simulated by using time-dependent density functional theory. Our findings suggest that chromophores with carbonyls bonded to nitrogen (i.e., imides and amides) derived from N-containing heterocyclic precursors substantially contribute to BrC light absorption. The quantified N-containing carbonyl chromophores contributed to over 40% of the total light absorption at wavelengths below 350 nm and above 430 nm in pyrrole BrC. The contributions of chromophores to total light absorption differed significantly by wavelength, highlighting their divergent importance in different wavelength ranges. Overall, our findings highlight the significance of carbonyl chromophores in secondary BrC and underscore the need for further investigation.
            Cover page: Contribution of Carbonyl Chromophores in Secondary Brown Carbon from Nighttime Oxidation of Unsaturated Heterocyclic Volatile Organic Compounds.
            • Article
            • Peer Reviewed
            UC Riverside Previously Published Works (2024)
            Light-absorbing secondary organic aerosols (SOAs), also known as secondary brown carbon (BrC), are major components of wildfire smoke that can have a significant impact on the climate system; however, how environmental factors such as relative humidity (RH) influence their formation is not fully understood, especially for heterocyclic precursors. We conducted chamber experiments to investigate secondary BrC formation from the nighttime oxidation of furan and pyrrole, two primary heterocyclic precursors in wildfires, in the presence of pre-existing particles at RH < 20% and ∼ 50%. Our findings revealed that increasing RH significantly affected the size distribution dynamics of both SOAs, with pyrrole SOA showing a stronger potential to generate ultrafine particles via intensive nucleation processes. Higher RH led to increased mass fractions of oxygenated compounds in both SOAs, suggesting enhanced gas-phase and/or multiphase oxidation under humid conditions. Moreover, higher RH reduced the mass absorption coefficients of both BrC, contrasting with those from homocyclic precursors, due to the formation of non-absorbing high-molecular-weight oxygenated compounds and the decreasing mass fractions of molecular chromophores. Overall, our findings demonstrate the unique RH dependence of secondary BrC formation from heterocyclic precursors, which may critically modulate the radiative effects of wildfire smoke on climate change.
              Cover page: Relative Humidity Modulates the Physicochemical Processing of Secondary Brown Carbon Formation from Nighttime Oxidation of Furan and Pyrrole.
              • Article
              • Peer Reviewed
              UC Riverside Previously Published Works (2023)
              Fluoroalkylether substances (ether PFAS) constitute a large group of emerging PFAS with uncertain environmental fate. Among them, GenX is the well-known alternative to perfluorooctanoic acid and one of the six proposed PFAS to be regulated by the U.S. Environmental Protection Agency. This study investigated the structure-biodegradability relationship for 12 different ether PFAS with a carboxylic acid headgroup in activated sludge communities. Only polyfluorinated ethers with at least one -CH2- moiety adjacent to or a C=C bond in the proximity of the ether bond underwent active biotransformation via oxidative and hydrolytic O-dealkylation. The bioreactions at ether bonds led to the formation of unstable fluoroalcohol intermediates subject to spontaneous defluorination. We further demonstrated that this aerobic biotransformation/defluorination could complement the advanced reduction process in a treatment train system to achieve more cost-effective treatment for GenX and other recalcitrant perfluorinated ether PFAS. These findings provide essential insights into the environmental fate of ether PFAS, the design of biodegradable alternative PFAS, and the development of cost-effective ether PFAS treatment strategies.
                Cover page: Aerobic Biotransformation and Defluorination of Fluoroalkylether Substances (ether PFAS): Substrate Specificity, Pathways, and Applications
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