Volatile organic compounds (VOCs) are those chemical species having sufficiently high vapor pressures to exist largely or entirely in the gaseous phase, whereas reactive organic carbon (ROC) encompasses all organics except methane. ROC can be emitted biogenically and anthropogenically, usually in a pure hydrocarbon form that is susceptible to reaction with common atmospheric oxidants such as hydroxyl and ozone in the initial steps to the formation of particulate matter, the criteria pollutant most strongly implicated in human mortality. The diversity of both the emitted VOCs and their possible atmospheric reactions yields countless different compounds existing in the atmosphere with a correspondingly wide range of volatility, solubility, reactivity, etc.. Moreover, the temporal and spatial variability of a given analyte is often large. Real-time chemical characterization of gaseous and particulate organic compounds can be achieved by instrumentation utilizing chromatographic and/or mass spectrometric techniques, but these methods are expensive, often logistically challenging, and require high levels of skills for both operation and data analysis. Conversely, filter-based measurements for organic particulates are inexpensive and straightforward, but do not give real-time data and analytes may be lost or transformed before analysis. There is a niche for robust, low-maintenance, moderate-cost instrumentation that offers chemical information on atmospheric carbon. Presented here are two projects that develop and validate instrumentation for measuring ROC. The first combines flame ionization detection (FID) with a CO2 detector to estimate the O/C ratios of sampled gases and particulates. O/C ratios are a particularly valuable piece of chemical information as higher ratios give lower volatility and higher solubility, meaning increased propensity to partition into the condensed phase. The second project utilizes portable VOC samplers with sorbent tubes that trap and protect analytes for detailed analysis. The samplers' portability and programmable microcontrollers offers the investigator great flexibility, both spatially and temporally. A third project analyzed the chemical composition of commercially available fragrance mixtures and modeled their emissions' impact on oxidant reactivity. It was observed that terpenes, despite their low mole fractions in the mixtures, represent the vast majority of emitted reactivity and are quantitatively evolved from the mixtures in a matter of hours. / Doctor of Philosophy / Organic (i.e., carbon-containing) compounds are emitted into the atmosphere from a variety of natural and anthropogenic sources. Respective examples would include the agreeable aroma of a pine forest (from terpene compounds) or the pungent smell of gasoline (from additives such as toluene). These emitted compounds are often pure hydrocarbons (molecules formed of carbon and hydrogen atoms), and the category VOCs (volatile organic compound) encompasses hydrocarbons and the products of their chemical reactions with atmospheric oxidants like the hydroxyl radical and ozone. In the presence of pollutant nitrogen oxides, oxidants modify these VOCs; adding oxygen lowers the VOCs' vapor pressure and increases aqueous solubility, resulting in higher likelihood of condensation from the gaseous phase into particulates (liquid or solid phases). "Smog" is a colloquial term for the entire suite of noxious chemical compounds produced in the air from reactions of largely anthropogenic organic precursors. Particulates, a.k.a. aerosols, are the most concerning atmospheric pollutant due to deleterious effects on respiratory and cardiovascular health and has shown strong correlations with increased mortality in exposed groups such city dwellers. Determining the chemical identities of the VOCs is useful for pollution forecasting and possibly identifying and quantifying VOC sources. Current methods for chemical identification are cumbersome, expensive, complex, and wholly unsuitable for many investigators. In this work, we introduce two new approaches to gathering chemical information about organic gases and particulates. The first instrument has been demonstrated to give accurate estimates of oxygen/carbon (O/C) ratios; higher O/C ratios represent higher propensities to condense into particulate forms. The second instrument developed is a portable VOC sampler, which traps (and prevents reaction of) a broad range of organics on a sorbent (such as activated charcoal) in a small metal tube. After sampling in remote locales, the tubes can be analyzed in the lab and the VOCs identified and quantified. The third study investigated the chemical composition of fragrance mixtures (present in perfumes, cleaning agents, etc.) and modeled (i.e., estimated) VOC emissions based on the fragrance components as well as the effects on atmospheric oxidant levels. Fragrance mixtures represent a significant source of atmospheric carbon, so a more thorough understanding of the fragrances' impacts on oxidant levels gives further insight into atmospheric processes and aerosol formation.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/113347 |
| Date | 20 January 2023 |
| Creators | Hurley, James Franklin |
| Contributors | Civil and Environmental Engineering, Isaacman-VanWertz, Gabriel, Dietrich, Andrea M., Hankey, Steven C., Marr, Linsey C. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | Creative Commons Attribution-NonCommercial 4.0 International, http://creativecommons.org/licenses/by-nc/4.0/ |
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