Browsing by Subject "Isoprene"
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Item Experimental Studies of Hydroxyl Radical Initiated Tropospheric Oxidation of Unsaturated Hydrocarbons(2011-10-21) Ghosh, BuddhadebThe tropospheric oxidation of unsaturated hydrocarbons is a central issue in atmospheric chemistry. These hydrocarbons are emitted into the atmosphere from both natural and anthropogenic sources, and their atmospheric oxidation leads to different atmospheric pollutants, including ground level ozone, photochemical smog and secondary organic aerosols. Isoprene and 1,3-butadiene represent a biogenic and an anthropogenic hydrocarbon, respectively, which primarily undergo electrophilic addition of OH radical, followed by chain propagating radical reactions. Their oxidation is the major source for ground level ozone formation in both rural and urban area and understanding their chemistry is essential for regional air quality modeling. Until recently, most of the studies of isoprene chemistry have been non-isomer specific, reflecting the reactivity of combined pathways and therefore were insensitive to specific details of the isomeric pathways. An isomeric selective approach to studying unsaturated hydrocarbon oxidation is described in this dissertation. A synthesized precursor, whose photolysis can provide a route to the formation of energy selected single isomer in the isoprene oxidation pathway, enables the study of important channels that are difficult to unravel in non isomer specific experiments. The major addition channel in OH isoprene oxidation has been studied following the isomeric selective approach and using Laser Photolysis-Laser Induced Fluorescence (LP-LIF) as the primary experimental technique. The study reveals important information about the oxidative chemistry of the ?-peroxy radicals, accounting for about 20 percent of missing carbon balance in isoprene oxidation, and isomeric specific rate constants. A similar approach was applied to study the oxidation of 1,3-butadiene, and the photolytic precursor for the dominant hydroxy alkyl isomer in the OH initiated oxidation of 1,3-butadiene was synthesized. The subsequent experiments and analysis revealed detailed information about the oxidative chemistry accounting for approximately 26 percent of the missing chemistry. Finally, non isomeric selective OH cycling experiments were carried out on the1,3-butadiene system. By analyzing the OH cycling data with the combined information obtained from the isomeric specific studies of the two isomeric channels of 1,3-butadiene oxidation, the relative branching between the two isomeric channels of OH-1,3-butadiene oxidation was determined.Item Modeling Regional Air Quality Using the Near-Explicit Master Chemical Mechanism(2014-08-01) Li, JingyiIn this study, the Master Chemical Mechanism (MCM), a near-explicit photochemical mechanism, is, for the first time, implemented into the Community Air Quality Model (CMAQ) model (referred to as CMAQ-MCM hereafter), a 3D chemical transport model, to study the spatial and temporal distribution of gaseous and particulate pollutants, and their chemical/physical properties in the eastern U.S. First, CMAQ-MCM with only gas phase chemistry (MCMv3.1) was applied to study primary VOCs in Southeast Texas during a three-week high ozone episode in 2000. The model could reproduce temporal profiles of observed ozone concentrations at major observation stations. However, predicted concentrations of alkanes and alkenes in Southeast Texas are lower than the observation by a factor of 2-5. Missing fugitive and evaporative industrial emissions were determined to be the major cause of the under-prediction. Second, the CMAQ-MCM was updated (to use MCMv3.2) and further developed to link with the inorganic aerosol module in CMAQ to study impacts of stabilized criegee intermediates (SCIs) on sulfate formation in the eastern U.S. Faster reaction rate of SCI with SO2, updated based on a recent experimental measurement (increased from 7?10^(-14) cm^(3) s^(-1) to 3.9?10^(-11) cm^(3) s^(-1)), leads to ~18% increase in surface sulfate concentration. However, the importance of this reaction greatly depends on the competition reaction rate of SCI with water vapor. Moreover, an organic aerosol module that predicts secondary organic aerosol (SOA) formation from equilibrium partitioning of semi-volatile organic compounds (SVOCs) into the organic phase and from reactive uptake of glyoxal, methylglyoxal, and isoprene epoxydiols, was incorporated into the model (termed CMAQ-MCM-SOA) to study SOA formation in this region. The episode averaged total SOA concentration is 2-12 ?g m^(-3), with the highest concentration occurring in the southeastern U.S. Isoprene epoxydiols are the major SOA component if an acidity dependent reactive uptake coefficient is used. Finally, the isoprene mechanism in the MCM was updated to successfully reproduce experimental observed isoprene SOA yield and mass concentrations, and then applied to the regional CMAQ-MCM-SOA model. While total isoprene SOA concentrations were not changed significantly from the base case, more contributions from semi-volatile components and less from surface uptake were predicted.Item The hydroxyl radical initiated oxidation of unsaturated hydrocarbons in the troposphere: a theoretical and experimental approach(2009-05-15) Tullos, Erin ElizabethIsoprene is the dominant non-methane organic compound emitted by vegetation into the atmosphere, with a global emission rate of ~ 500 Tg yr-1. Its oxidation serves as a major source of ground level ozone in North America during the summer months. Despite the significant impact on tropospheric chemistry, questions remain concerning the detailed oxidation mechanism. The initial step in the mechanism is the addition of OH to form four distinct isomers. The relative branching between these isomers influences the distribution of the final products. I present a comprehensive investigation into the mechanistic details of early steps in the oxidation mechanism of unsaturated hydrocarbons in the troposphere and employ theoretical and experimental techniques. To understand the detailed kinetics of the initial OH addition to unsaturated hydrocarbons, I first present a model developed for the ethylene-OH system. I present the details of a robust two-transition state model. I extend the developed two-transition state model to the case of OH addition to isoprene. Excellent agreement with observed temperature and pressure dependent rate constants affords a high confidence level in understanding of the kinetics and in the calculated branching ratio of the initial OH addition step. I then focus attention on the subsequent reactivity of the OH-isoprene adducts. Until recently, all four of the OH-isoprene adducts were supposed to have reacted with O2 via addition to form alkylperoxy radicals. Previous computational results suggest that two of the OH-isoprene adducts undergo an intramolecular cyclic isomerization followed by hydrogen abstraction by O2 to form stable carbonyl compounds. I have synthesized photolytic precursors, presenting a novel approach to probe the subsequent reactivity of individual hydroxyalkyl radicals. Initial verification of the cyclic isomerization pathway involved synthesis of the photolytic precursor corresponding to the 1,3-butadiene-OH adduct. A culmination of theoretical and experimental techniques allowed verification of the cyclic isomerization pathway. I synthesized the photolytic precursor, which provided a single isoprene-OH adduct. Employing laser photolysis/laser induced fluorescence, time-dependent multiplexed mass spectrometry, velocity map ion imaging, and theoretical techniques, we present the full characterization of the reactivity of the single isoprene-OH adduct in the presence of O2.