An aerosol represents a dispersion of solid or liquid particles in a gas. The propagation of radiation through Earth’s atmosphere is directly affected by the absorption and scattering of incoming solar radiation by atmospheric aerosols of both primary and secondary origin, prompting research on the relation between the complex molecular composition of aerosol particles and their optical properties. While all organic aerosols (OA) scatter incoming solar radiation, only a subset of organic aerosols known as “brown carbon” (BrC) appreciably absorb visible radiation. BrC is largely emitted from combustion processes such as through the burning of biomass and human-engineered (or urban) materials. Fires occurring at the wildland-urban interface (WUI) are increasing in both frequency and magnitude, suggesting that the composition and properties of wildfire-related OA are dictated not only by burning biomass but also by burning urban materials. Optical properties of BrC in biomass burning organic aerosol (BBOA) or urban OA are highly variable and are coupled to their fuel source and combustion conditions. Moreover, the molecular composition of BBOA and OA can evolve through atmospheric transport, such as through photochemical aging and condensed phase chemistry, to impact its optical properties.Human health is directly impacted by exposure to ambient aerosol particulate matter and volatile organic compounds (VOCs), including those coming from burning. Respiratory and cardiovascular illnesses are known outcomes of exposure to ambient air pollution. While unintended exposure to smoke from burning is a serious health risk factor, it should be kept in mind that deliberate exposure to smoking (including vaping) is a significantly greater threat to human health in terms of cancer, lung disease, cardiovascular disease, and mortality rates. Traditional cigarettes emit over 7000 known compounds, many of which are classified as toxic and carcinogenic. Secondhand exposure to cigarette smoke, even if brief, can increase risk of cancer, heart disease, and stroke. Vaping, on the other hand, exposes users to fewer toxic chemicals but is known to induce lung injuries, cardiovascular diseases, and exacerbate asthma conditions. Neither the long-term effects of vaping nor risks to secondhand vape exposure are well known.
Despite advances reported in the literature, there are still many unanswered questions about the molecular identity of BrC compounds in BBOA, connection between the optical properties and chemical composition of BBOA, and condensed phase chemical transformations leading to BBOA aging, particularly those with BBOA species and Fe(III). There is a lack of knowledge on the emissions that come from fires at the WUI and how these aerosols can change during photolytic aging. As for health-related aerosols, much is known about the smoking of traditional cigarettes, but questions remain for vaping.
Chapters 2 and 3 examine the interaction of BBOA with atmospheric iron (Fe(III)), a common component of aerosol liquid water from dissolved mineral dust. Chapter 2 presents evidence that Fe(III) can catalyze oligomerization reactions with a broad range of phenolic compounds, abundantly present in biomass burning smoke, to produce strongly light-absorbing and water-insoluble particles. Chapter 3 expands upon this Fe(III)-catalyzed chemistry by testing how it proceeds with laboratory generated BBOA instead of specific phenolic compounds. Again, we observed the formation of light absorbing, water-insoluble secondary aerosol particles that were reminiscent of primary soot particles. These chapters conclude that biomass burning smoke can become more light absorbing as it mixes with mineral dust particles during atmospheric transport, and may, therefore, have a stronger effect on climate after aging.
Chapters 4 and 5 examine the chemical composition, optical properties, and effect of sunlight on particulate matter (PM) emitted during urban fires. In Chapter 4, we pyrolyzed ten urban materials and exposed resultant PM to simulated solar radiation. We observed increased absorption and darkening of PM, suggesting that urban fires emit BrC that continues to increasingly absorb light as it travels through the troposphere. Chapter 5 utilized high resolution mass spectrometry techniques to characterize compounds acting as BrC chromophores in urban smoke and determine their fate after photolytic aging. Overall, our results demonstrate that fires at the wildland-urban interface efficiently produce chemically complex BrC PM, which evolves into species with higher light absorptive properties after UV aging.
Chapter 6 examines the composition of VOCs exhaled by participants in a human trial vaping study. We employed a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) and gas chromatography techniques to examine the composition and concentration of VOCs bystanders would be exposed to during a vaping event. Overall results indicate analytical discrepancies between the two techniques, with PTR-ToF-MS results being overwhelmed by e-liquid thermal decomposition and ionization source fragments. Online PTR-ToF-MS results should be interpreted with caution, however, it still yielded useful results in this study. Harmful VOCs present in inhaled by users are largely absorbed by the body, and less is emitted for secondhand exposures.
This PhD work furthers our understanding of how the chemical composition and evolution of smoke from both forest and urban fires impact visibility and climate. The most important result of this work is the observation that smoke can become darker in color (i.e., more light-absorbing) as a result of two very different mechanisms – transition metal catalyzed oligomerization occurring in the dark and photolysis-induced aging occurring in sunlight. Additionally, in our human trial vaping study, the most important result of this work was that ionization within the PTR-ToF-MS leads to an overestimation of VOCs, some of which have potentially significant health implications. Relying on this method alone could lead to a misrepresentation of the potential for vape emissions on adverse health effects.
