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Particles in the atmosphere are known to have negative health effects and important but highly uncertain impacts on global and regional climate. A majority of this particulate matter is formed through atmospheric oxidation of naturally and anthropogenically emitted gases to yield highly oxygenated secondary organic aerosol (SOA), an amalgamation of thousands of individual chemical compounds. However, comprehensive analysis of SOA composition has been stymied by its complexity and lack of available measurement techniques. In this work, novel instrumentation, analysis methods, and conceptual frameworks are introduced for chemically characterizing atmospherically relevant mixtures and ambient aerosols, providing a fundamentally new level of detailed knowledge on their structures, chemical properties, and identification of their components. This chemical information is used to gain insights into the formation, transformation and oxidation of organic aerosols. Biogenic and anthropogenic mixtures are observed in this work to yield incredible complexity upon oxidation, producing over 100 separable compounds from a single precursor. As a first step toward unraveling this complexity, a method was developed for measuring the polarity and volatility of individual compounds in a complex mixture using two-dimensional gas chromatography, which is demonstrated in Chapter 2 for describing the oxidation of SOA formed from a biogenic compound (longifolene: C15H24). Several major products and tens of substantial minor products were produced, but none could be identified by traditional methods or have ever been isolated and studied in the laboratory. A major realization of this work was that soft ionization mass spectrometry could be used to identify the molecular mass and formula of these unidentified compounds, a major step toward a comprehensive description of complex mixtures. This was achieved by coupling gas chromatography to high resolution time-of-flight mass spectrometry with vacuum ultraviolet (VUV) photo-ionization. Chapters 3 and 4 describe this new analytical technique and its initial application to determine the structures of unknown compounds and formerly unresolvable mixtures, including a complete description of the chemical composition of two common petroleum products related to anthropogenic emissions: diesel fuel and motor oil. The distribution of hydrocarbon isomers in these mixtures - found to be mostly of branched, cyclic, and saturated - is described with unprecedented detail. Instead of measuring average bulk aerosol properties, the methods developed and applied in this work directly measure the polarity, volatility, and structure of individual components to allow a mechanistic understanding of oxidation processes. Novel characterizations of these complex mixtures are used to elucidate the role of structure and functionality in particle-phase oxidation, including in Chapter 4 the first measurements of relative reaction rates in a complex hydrocarbon particle. Molecular structure is observed to influence particle-phase oxidation in unexpected and important ways, with cyclization decreasing reaction rates by ~30% and branching increasing reaction rates by ~20-50%. The observed structural dependence is proposed to result in compositional changes in anthropogenic organic aerosol downwind of urban areas, which has been confirmed in subsequent work by applying the techniques described here. Measurement of organic aerosol components is extended to ambient environments through the development of instrumentation with the unprecedented capability to measure hourly concentrations and gas/particle partitioning of individual highly oxygenated organic compounds in the atmosphere. Chapters 5 and 6 describe development of new procedures and hardware for the calibration and analysis of oxygenates using the Semi-Volatile Thermal desorption Aerosol Gas chromatograph (SV-TAG), a custom instrument for in situ quantification of gas- and particle-phase organic compounds in the atmosphere. High time resolution measurement of oxygenated compounds is achieved through a reproducible and quantitative methodology for in situ "derivatization"--Replacing highly polar functional groups that cannot be analyzed by traditional gas chromatography with less polar groups. Implementation of a two-channel sampling system for the simultaneous collection of particle-phase and total gas-plus-particle phase samples allows for the first direct measurements of gas/particle partitioning in the atmosphere, significantly advancing the study of atmospheric composition and variability, as well as the processes governing condensation and re-volatilization. This work presents the first in situ measurements of a large suite of highly oxygenated biogenic oxidation products in both the gas- and particle-phase. Isoprene, the most ubiquitous biogenic emission, oxidizes to form 2-methyltetrols and C5 alkene triols, while [alpha]-pinene, the most common monoterpene, forms pinic, pinonic, hydroxyglutaric, and other acids. These compounds are reported in Chapter 7 with unprecedented time resolution and are shown for the first time to have a large gas-phase component, contrary to typical assumptions. Hourly comparisons of these products with anthropogenic aerosol components elucidate the interaction of human and natural emissions at two rural sites: the southeastern, U.S. and Amazonia, Brazil. Anthropogenic influence on SOA formation is proposed to occur through the increase in liquid water caused by anthropogenic sulfate. Furthermore, these unparalleled observations of gas/particle partitioning of biogenic oxidation products demonstrate that partitioning of oxygenates is unexpectedly independent of volatility: many volatile, highly oxygenated compounds have a large particle-phase component that is poorly described by traditional models. These novel conclusions are reached in part by applying the new frameworks developed in previous chapters to understand the properties of unidentified compounds, demonstrating the importance of detailed characterization of atmospheric organic mixtures. Comprehensive analysis of anthropogenic and biogenic emissions and oxidation product mixtures is coupled in this work with high time-resolution measurement of individual organic components to yield significant insights into the transformations of organic aerosols. Oxidation chemistry is observed in both laboratory and field settings to depend on molecular properties, volatility, and atmospheric composition. However, this work demonstrates that these complex processes can be understood through the quantification of individual known and unidentified compounds, combined with their classification into descriptive frameworks.
Expanded and updated with new findings and new features New chapter on Global Climate providing a self-contained treatment of climate forcing, feedbacks, and climate sensitivity New chapter on Atmospheric Organic Aerosols and new treatment of the statistical method of Positive Matrix Factorization Updated treatments of physical meteorology, atmospheric nucleation, aerosol-cloud relationships, chemistry of biogenic hydrocarbons Each topic developed from the fundamental science to the point of application to real-world problems New problems at an introductory level to aid in classroom teaching
Structure determination of molecules contained within unresolved complex mixtures represents an unsolved question that continues to challenge physical and analytical chemistry. Most naturally occurring systems can be characterised as complex mixtures. These can be broadly divided according to the molecular sizes of their constituents, into mixtures of small or large molecules; the focus of this volume is on the former. While large molecules such as biomacromolecules, industrial polymers, or solid matrices are outside of the scope of this volume, the processes that are used in analysing the data originating from these studies may be of interest. Small molecule mixtures include environmental matrices (such as soil, dissolved organic matter, organic molecules contained in atmospheric aerosol particles, or crude oil), biofluids, and man-made mixtures of small molecules such as food, beverages or plant extracts. These systems are generally classed as "complex mixtures" or "unresolved complex mixtures (UCM)", emphasising our current inability to separate their individual components. The techniques best positioned to tackle such mixtures experimentally include mass spectrometry, chromatography, NMR spectroscopy, or new alternative techniques, including combinations of the above methods. For the most part, people who work on the analysis of complex mixtures are driving the progress in exploiting new methodologies and their creative combinations. In this volume, the topics covered include: Dealing with complexity: latest advances in mass spectrometry and chromatography; High-resolution techniques, from high-resolution mass spectrometry to NMR spectroscopy; Data mining and visualisation; Future challenges and new approaches.
Structure determination of molecules contained within unresolved complex mixtures represents an unsolved question that continues to challenge physical and analytical chemistry. Most naturally occurring systems can be characterised as complex mixtures. These can be broadly divided according to the molecular sizes of their constituents, into mixtures of small or large molecules; the focus of this volume is on the former. While large molecules such as biomacromolecules, industrial polymers, or solid matrices are outside of the scope of this volume, the processes that are used in analysing the data originating from these studies may be of interest. Small molecule mixtures include environmental matrices (such as soil, dissolved organic matter, organic molecules contained in atmospheric aerosol particles, or crude oil), biofluids, and man-made mixtures of small molecules such as food, beverages or plant extracts. These systems are generally classed as "complex mixtures" or "unresolved complex mixtures (UCM)", emphasising our current inability to separate their individual components. The techniques best positioned to tackle such mixtures experimentally include mass spectrometry, chromatography, NMR spectroscopy, or new alternative techniques, including combinations of the above methods. For the most part, people who work on the analysis of complex mixtures are driving the progress in exploiting new methodologies and their creative combinations. In this volume, the topics covered include: Dealing with complexity: latest advances in mass spectrometry and chromatography High-resolution techniques, from high-resolution mass spectrometry to NMR spectroscopy Data mining and visualisation Future challenges and new approaches
Atmospheric organic aerosols are composed of thousands of individual compounds, interacting with climate through changes in aerosol optical properties and cloud interactions, and can be detrimental to human health. Aerosol mass spectrometry (MS) and gas chromatography (GC)-separated MS measurements have been utilized to better characterize the chemical composition of this material that comes from a variety of sources and experiences continuous oxidation while in the atmosphere. This dissertation describes the development of a novel rapid data analysis method for grouping of major components within chromatography-separated measurements and first application using thermal desorption aerosol gas chromatograph (TAG) -- MS data. Chromatograms are binned and inserted directly into a positive matrix factorization (PMF) analysis to determine major contributing components, eliminating the need for manual compound integrations of hundreds of resolved molecules, and incorporating the entirety of the eluting MS signal, including Unresolved Complex Mixtures (UCM) and decomposition products that are often ignored in traditional GC-MS analysis. Binned GC-MS data has three dimensions: (1) mass spectra index m/z, (2) bin number, and (3) sample number. PMF output is composed of two dimensions; factor profiles and factor time series. The specific arrangement of the input data (three dimensions of variation structured as a two dimensional matrix) in a two dimensional PMF analysis affects the structure of the PMF profiles and time series output. If mass spectra index is in the profile dimension, and bin number and sample number are in the time series dimension, PMF groups components into factors with similar mass spectra, such as major contributing individual compounds, UCM with similar functional composition, and homologous compound series. This type of PMF analysis is described as the binning method for chromatogram deconvolution, and is presented in Chapter 2. If the sample number is in the time series dimension, and the bin number and mass spectra index, arranged as mass spectra resolved retention time/chromatogram (bin number), are in the profile dimension, PMF groups components with similar time series trends. This type of PMF analysis is described as binning method for source apportionment, and is described in Chapter 3. The binning methods are compared to traditional compound integration methods using previously-collected hourly ambient samples from Riverside, CA during the 2005 Study of Organic Aerosols at Riverside (SOAR) field campaign, as discussed in Chapters 2-3. Further application of the binning method for source apportionment is performed on newly acquired hourly TAG data from East St. Louis, IL, operated as part of the 2013 St. Louis Air Quality Regional Study (SLAQRS). Major sources of biogenic secondary organic aerosol (SOA), anthropogenic primary organic aerosol (POA) were identified, as described in detail in Chapter 4. Finally, our PMF separation method was tested for reliability using primary and secondary sources in a controlled laboratory system. As shown in Chapter 5, we find that for application of PMF on receptor measurements, high signal intensity and unique measurement profiles, like those found in TAG chromatograms, are keys to successful source apportionment. The binning method with component separation by PMF may be a valuable analysis technique for other complex data sets that incorporate measurements (e.g., mass spectrometry, spectroscopy, etc.) with additional separations (e.g., volatility, hygroscopicity, electrical mobility, etc.).