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Scholarly Works (4 results)

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UC Riverside Electronic Theses and Dissertations (2024)

Wildfires have shown an increase in occurrence and severity in recent years. These biomass burning events release volatile organic compounds (VOCs) and particulate matter in the atmosphere which impact air quality, human health, and Earth’s radiative balance. The oxidation of these wildfire-emitted VOCs at nighttime by the nitrate radical (NO3) can lead to the formation of secondary organic aerosol (SOA) and light-absorbing brown carbon (BrC). Despite their significance, the understanding of NO3-initiated oxidation mechanisms of wildfire-emitted VOCs leading to SOA and BrC in previous studies have been very limited. The major SOA constituents from these reactions have remained a challenge to uncover largely due to limitations in analytical techniques used to uncover the gas- and particle-phase chemical composition. In the following projects, we use a suite of analytical methods to elucidate the major reactions leading to the formation of SOA and BrC from representative wildfire-emitted VOCs. In Chapter 2, we studied the NO3 oxidation of seven phenolic VOCs. From this study, we discovered many products that were previously undiscovered from this system and found nitrophenol products were very dominant adding to evidence that these are a major class of compounds responsible for significant light absorption in BrC. We also found evidence of diphenyl ether dimers forming from NO3 oxidation of each of the studied phenolic VOCs. In Chapter 3, we investigated the NO3 oxidation of limonene. From this project, we discovered that the primary nitrooxy peroxy radical formed from limonene can rapidly undergo autoxidation leading to highly oxidized organonitrates. We also identified the formation of several dinitrate compounds highlighting the importance of sequential oxidation for limonene. In Chapter 4, we investigated the NO3 oxidation of selected N-containing heterocyclic VOCs: pyrrole, 1-methylpyrrole (1-MP) and 2-methylpyrrole (2-MP). From the observed product distribution in the SOA from these systems, we concluded that the presence of an easily abstractable hydrogen in precursor structure regulates the mechanism of initial NO3 oxidation and has significant effect on light absorption of the SOA. Furthermore, we propose a novel gas-phase mechanism for the addition of three NO2 groups to the backbone of pyrrole.

    Cover page: Investigating Nighttime Oxidation of Wildfire-Emitted VOCs: Implications for Oxidation Mechanisms, Secondary Organic Aerosol, and Brown Carbon Formation
    • 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)
        We assessed the efficacy of ozonation as an indoor remediation strategy by evaluating how a carpet serves as a sink and long-term source of thirdhand tobacco smoke (THS) while protecting contaminants absorbed in deep reservoirs by scavenging ozone. Specimens from unused carpet that was exposed to smoke in the lab ("fresh THS") and contaminated carpets retrieved from smokers' homes ("aged THS") were treated with 1000 ppb ozone in bench-scale tests. Nicotine was partially removed from fresh THS specimens by volatilization and oxidation, but it was not significantly eliminated from aged THS samples. By contrast, most of the 24 polycyclic aromatic hydrocarbons detected in both samples were partially removed by ozone. One of the home-aged carpets was installed in an 18 m3 room-sized chamber, where its nicotine emission rate was 950 ng day-1 m-2. In a typical home, such daily emissions could amount to a non-negligible fraction of the nicotine released by smoking one cigarette. The operation of a commercial ozone generator for a total duration of 156 min, reaching concentrations up to 10,000 ppb, did not significantly reduce the carpet nicotine loading (26-122 mg m-2). Ozone reacted primarily with carpet fibers, rather than with THS, leading to short-term emissions of aldehydes and aerosol particles. Hence, by being absorbed deeply into carpet fibers, THS constituents can be partially shielded from ozonation.
          Cover page: Remediation of Thirdhand Tobacco Smoke with Ozone: Probing Deep Reservoirs in Carpets
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