Tullos, Erin Elizabeth (2007-05). The hydroxyl radical initiated oxidation of unsaturated hydrocarbons in the troposphere: a theoretical and experimental approach. Doctoral Dissertation. Thesis uri icon

abstract

  • Isoprene 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.
  • Isoprene 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.

publication date

  • May 2007