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Christian George, Barbara D’Anna, Hartmut Herrmann, Christian Weller, Veronica Vaida, D. J. Donaldson, Thorsten Bartels-Rausch, Markus Ammann - Emerging Areas in Atmospheric Photochemistry. Lisa Whalley, Daniel Stone, Dwayne Heard - New Insights into the Tropospheric Oxidation of Isoprene: Combining Field Measurements, Laboratory Studies, Chemical Modelling and Quantum Theory. Neil M. Donahue, Allen L. Robinson, Erica R. Trump, Ilona Riipinen, Jesse H. Kroll - Volatility and Aging of Atmospheric Organic Aerosol. P. A. Ariya, G. Kos, R. Mortazavi, E. D. Hudson, V. Kanthasamy, N. Eltouny, J. Sun, C. Wilde - Bio-Organic Materials in the Atmosphere and Snow: Measurement and Characterization. V. Faye McNeill, Neha Sareen, Allison N. Schwier - Surface-Active Organics in Atmospheric Aerosols.
The immense chemical complexity of atmospheric organic particulate matter ("aerosol") has left the general field of condensed-phase atmospheric organic chemistry relatively under-developed when compared with either gas-phase chemistry or the formation of inorganic compounds. In this work, we endeavor to improve the general understanding of the narrow class of oxidation reactions that occur at the interface between the particle surface and the gas-phase. The heterogeneous oxidation of pure erythritol (C4H1 00 4 ) and levoglucosan (C6H1 00 5) particles by hydroxyl radical (OH) was studied first in order to evaluate the effects of atmospheric aging on the mass and chemical composition of atmospheric organic aerosol, particularly that resembling fresh secondary organic aerosol (SOA) and biomass-burning organic aerosol (BBOA). In contrast to what is generally observed for the heterogeneous oxidation of reduced organics, substantial volatilization is observed in both systems. As a continuation of the heterogeneous oxidation experiments, we also measure the kinetics and products of the aging of highly oxidized organic aerosol, in which submicron particles composed of model oxidized organics -- 1,2,3,4-butanetetracarboxylic acid (C8H100 8), citric acid (C6 H8 0 7), tartaric acid (C4H6 0 6 ), and Suwannee River fulvic acid -- were oxidized by gas-phase OH in the same flow reactor, and the masses and elemental composition of the particles were monitored as a function of OH exposure. In contrast to studies of the less-oxidized model systems, particle mass did not decrease significantly with heterogeneous oxidation, although substantial chemical transformations were observed and characterized. Lastly, the immense complexity inherent in the formation of SOA -- due primarily to the large number of oxidation steps and reaction pathways involved -- has limited the detailed understanding of its underlying chemistry. In order to simplify this inherent complexity, we give over the last portion of this thesis to a novel technique for the formation of SOA through the photolysis of gas-phase alkyl iodides, which generates organic peroxy radicals of known structure. In contrast to standard OH-initiated oxidation experiments, photolytically initiated oxidation forms a limited number of products via a single reactive step. The system in which the photolytic SOA is formed is also repurposed as a generator of organic aerosol for input into a secondary reaction chamber, where the organic particles undergo additional aging by the heterogeneous oxidation mechanism already discussed. Particles exiting this reactor are observed to have become more dramatically oxidized than comparable systems containing SOA formed by gas-phase alkanes undergoing "normal" photo-oxidation by OH, suggesting simultaneously the utility of gas-phase precursor photolysis as an effective experimental platform for studying directly the chemistry involved in atmospheric aerosol formation and also the possibility that heterogeneous processes may play a more significant role in the atmosphere than what is predicted from chamber experiments. Consideration is given for the application of these results to larger-scale experiments, models, and conceptual frameworks.
The photooxidation of volatile organic compounds (VOCs) in the atmosphere can lead to the formation of secondary organic aerosol (SOA), a major component of fine particulate matter. Improvements to air quality require insight into the many reactive intermediates that lead to SOA formation, of which only a small fraction have been measured at the molecular level. This thesis describes the chemistry of secondary organic aerosol (SOA) formation from several atmospherically relevant hydrocarbon precursors. Photooxidation experiments of methoxyphenol and phenolic compounds and C12 alkanes were conducted in the Caltech Environmental Chamber. These experiments include the first photooxidation studies of these precursors run under sufficiently low NOx levels, such that RO2 + HO2 chemistry dominates, an important chemical regime in the atmosphere. Using online Chemical Ionization Mass Spectrometery (CIMS), key gas-phase intermediates that lead to SOA formation in these systems were identified. With complementary particle-phase analyses, chemical mechanisms elucidating the SOA formation from these compounds are proposed. Three methoxyphenol species (phenol, guaiacol, and syringol) were studied to model potential photooxidation schemes of biomass burning intermediates. SOA yields (ratio of mass of SOA formed to mass of primary organic reacted) exceeding 25% are observed. Aerosol growth is rapid and linear with the organic conversion, consistent with the formation of essentially non-volatile products. Gas and aerosol-phase oxidation products from the guaiacol system show that the chemical mechanism consists of highly oxidized aromatic species in the particle phase. Syringol SOA yields are lower than that of phenol and guaiacol, likely due to unique chemistry dependent on methoxy group position. The photooxidation of several C12 alkanes of varying structure n-dodecane, 2-methylundecane, cyclododecane, and hexylcyclohexane) were run under extended OH exposure to investigate the effect of molecular structure on SOA yields and photochemical aging. Peroxyhemiacetal formation from the reactions of several multifunctional hydroperoxides and aldehyde intermediates was found to be central to organic growth in all systems, and SOA yields increased with cyclic character of the starting hydrocarbon. All of these studies provide direction for future experiments and modeling in order to lessen outstanding discrepancies between predicted and measured SOA.
This book highlights new cross-disciplinary advances in aerosol chemistry that involve more than one phase, for example, unique chemical processes occurring on gas-solid and liquid-solid interfaces.
Biomass burning organic aerosol (BBOA), organic aerosol that derived from burning of biomass fuels, has been a major research focus because of its special role in the global budget of atmospheric chemistry and radiative forcing. Due to its chemical complexity, there are gaps in our knowledge about the chemical aging processes of BBOA in the atmosphere. Since many photochemical aging experiments on BBOA are usually conducted for only a few hours, less is known about the photo-aging pathways of the system over an extended timescale. This study presents the analyses of three BBOA filter samples derived from three types of fuels that were photolytically aged over a timeframe of up to ~3.5 days. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Offline-Aerosol Mass Spectrometry (Offline-AMS) were used to measure the chemical changes in the aqueous sample extracts and evaluate how those changes can relate to their specific fuel type. This study finds an overall increase in oxidation states and decrease in the nitro group (NO2) compounds in the samples. The level of levoglucosan, a tracer organic species of BBOA, is also observed to decrease in the sample mixture due to photolysis alone for the first time. Several unique chemical characteristics were observed for each sample, which possibly relate to their individual fuel type. In order to further support those observations and obtain a full picture of the chemical compositions of the samples, future studies will focus on examining the acetonitrile extracts of our samples, investigating the corresponding on-line AMS data set, and applying more analytical methods to the sample extracts.
Understanding the heterogeneous oxidation of organic particulate matter ("aerosol") is an active area of current research in atmospheric and combustion chemistry. The chemical evolution of organic aerosol is complex and dynamic since it can undergo multiple oxidation reactions with gas phase oxidants to form a mixture of different generations of oxidation products that control the average aerosol mass and volatility. In many of these systems, hydrocarbon free radicals, formed by reaction with gas phase oxidants, play key roles as initiators, propagators and terminators of surface reactions. This dissertation presents a detailed study of the reaction kinetics and mechanisms of the heterogeneous oxidation of unsaturated organic aerosol, and aims to provide new molecular and mechanistic insights into the reaction pathways in heterogeneous organic aerosol oxidation. The heterogeneous oxidation of unsaturated fatty acid (oleic acid C18H34O2, linoleic acid C18H32O2 and linolenic acid C18H30O2) aerosol by hydroxyl (OH) radicals is first studied in Chapter 2 to explore how surface OH addition reactions initiate chain reactions that rapidly transform the chemical composition of unsaturated organic aerosol. Oleic acid, linoleic acid and linolenic acid have the same linear C18 carbon backbone structure with one, two and three C=C double bonds, respectively. By studying carboxylic acids with different numbers of C=C double bonds, the role that multiple reactive sites plays in controlling reaction rates can be observed. The kinetic parameter of interest in these studies is the effective uptake coefficient, defined as the number of particle phase unsaturated fatty acid molecules reacted per OH-particle collision. The effective uptake coefficients for the unsaturated fatty acids are larger than unity, providing clear evidence for particle-phase secondary chain chemistry. The effective uptake coefficients for the unsaturated fatty acids decrease with increasing O2 concentration, indicating that O2 promotes chain termination in the unsaturated fatty acid reactions. The kinetics and products of squalene (a C30 branched alkene with 6 C=C double bonds) oxidation are compared to that of the unsaturated fatty acids in Chapters 3 and 4 to understand how molecular structure and chemical functionality influence reaction rates and mechanisms. The squalene effective uptake coefficient, which is also larger than one, is smaller than that of linoleic acid and linolenic acid despite the larger number of C=C double bonds in squalene. In contrast to the unsaturated fatty acids, the squalene effective uptake coefficient increases with O2 concentration, indicating that O2 promotes chain propagation in the squalene reaction. Elemental and product analysis of squalene aerosol shows that O2 promotes particle volatilization in the squalene reaction, suggesting that fragmentation reactions are important when O2 is present in the OH oxidation of branched unsaturated organic aerosol. In contrast, elemental and product analysis of linoleic acid aerosol shows that O2 does not influence the rate of particle volatilization in the linoleic acid reaction, suggesting that O2 does not alter the relative importance of fragmentation reactions in the OH oxidation of linear unsaturated organic aerosol. Lastly, depending on the aerosol phase (e.g. solid and semi-solid) and the timescale for homogeneous mixing within the aerosol particle, the chemical composition may vary spatially within an aerosol particle. This necessitates the need for new techniques to characterize the interfacial chemical composition of aerosol particles. In the last portion of the dissertation, direct analysis in real time mass spectrometry (DART-MS) is used to analyze the surface chemical composition of nanometer-sized organic aerosol particles in real time at atmospheric pressure. By introducing a stream of aerosol particles in between the DART ionization source and the atmospheric pressure inlet of the mass spectrometer, the aerosol particles are exposed to a thermal flow of helium or nitrogen gas containing some fraction of metastable helium atoms or nitrogen molecules. In this configuration, the molecular constituents of organic aerosol particles are desorbed, ionized and detected with reduced molecular ion fragmentation, allowing for compositional identification. The reaction of ozone with sub-micron oleic acid particles is also measured to demonstrate the ability of DART-MS to identify products and quantify reaction rates in a heterogeneous reaction.
Organic aerosols comprise hundreds, if not thousands, of distinct chemical compounds. Traditional analytical techniques for analysis of chemical composition lack the ability to completely characterize complex mixtures such as organic aerosol. Until recently, the best available methods could only provide information on selected aerosol compounds, on selected groups of compounds, or on sample-averaged elemental ratios. Such experimental limitations posed significant barriers to understanding the detailed chemical composition of organic aerosols and its atmospheric evolution. The unique HR ESI-MS methods developed in this research are able to not only characterize the organic aerosols average elemental ratios, but also simultaneously obtain information about hundreds or even thousands of individual compounds in organic aerosols. One of the key achievements of this work was the development of new methods for classification of individual compounds in organic aerosols by their functional groups using reactive HR ESI-MS. This contribution made it possible to track organic aerosols throughout their atmospheric evolution via functional group composition and average elemental ratios while still retaining the chemical composition of each individual compound. Other important scientific advances described in this thesis include: complete characterization of the chemical composition of limonene SOA as a function of particle size and reaction time; adaptation of PILS (particle-into-liquid sampler) to the HR ESI-MS platform; chemical characterization of the water soluble component of several types of organic aerosols; the effects of photochemical aging on the water soluble component of limonene SOA through characterization of the optical properties coupled with chemical composition; and investigation of photochemistry of carbonyls in model SOA matrices. The research included in this dissertation reviews the development of unique aerosol characterization tools utilizing the facilities at UCI and the Environmental Molecular Sciences Laboratory at the Pacific Northwest National Laboratory. The research project answered important questions regarding organic aerosol formation, evolution, and chemical composition that impact the direct and indirect influences of aerosols on Earth's climate.
Atmospheric organic aerosols have a significant impact on climate and human health. However, our understanding of the physical and chemical properties of these aerosols is inadequate, thus their climate and health influences are poorly constrained. In this study, we investigated the secondary organic aerosol (SOA) formation from OH-initiated oxidation of -pinene. The majority of experiments were conducted in the York University smog chamber. The main objective was to identify the gas and particle phase products with an atmospheric pressure chemical ionization mass spectrometer (APCI-MS/MS). A wide variety of products were identified containing various functional groups including alcohol, aldehyde, carboxylic acid, ketone and nitrate. Following the chemical composition characterization of products, the shape, phase state and density of generated particles were determined. Images from a scanning electron microscope (SEM) revealed that SOA particles from -pinene were commonly spherical in shape, and adopted an amorphous semi-solid/liquid state. Additionally, the density was determined for SOA particles generated from -pinene/OH, nopinone/OH and nopinone/NO3 experiments for the first time using a tapered element oscillating microbalance-scanning mobility particle sizer (TEOM-SMPS) method. Our results showed a correlation between the determined particle density and the particle chemical composition of the respective system. This demonstrates that changes in particle density can be indicative of the changes in chemical composition of particles. We also investigated the chemical aging of oxidation products by exposing them to additional OH radicals or ozone. The observed changes in chemical composition of products and additional SOA mass production during OH-induced aging were attributed to further oxidation of gas phase intermediate products. The NOx dependence of SOA formation from -pinene photooxidation was investigated in the York University smog chamber and the Jlich Plant Atmosphere Chamber (JPAC). Consistent with previous NOx studies, SOA yields increased with increasing [NOx] at low-NOx conditions, whereas increasing [NOx] at high-NOx conditions suppressed the SOA yield. This increase was attributed to an increase of OH concentration. After removing the effect of [OH] on SOA yield in the JPAC, SOA yields only decreased with increasing [NOx]. Finally, the formation mechanisms of identified products were probed based on the information acquired throughout our study.
The heterogeneous reaction of hydroxyl radicals (OH) with squalane and bis(2-ethylhexyl) sebacate (BES) particles are used as model systems to examine how distributions of reactionproducts evolve during the oxidation of chemically reduced organic aerosol. A kinetic model of multigenerational chemistry, which is compared to previously measured (squalane) and new(BES) experimental data, reveals that it is the statistical mixtures of different generations of oxidation products that control the average particle mass and elemental composition during thereaction. The model suggests that more highly oxidized reaction products, although initially formed with low probability, play a large role in the production of gas phase reaction products. In general, these results highlight the importance of considering atmospheric oxidation as a statistical process, further suggesting that the underlying distribution of molecules could playimportant roles in aerosol formation as well as in the evolution of key physicochemical properties such as volatility and hygroscopicity.