[PDF] Kinetic And Mechanistic Studies Of The Hydroxyl Radical Initiated Photo Oxidation Of Saturated Hydrocarbons Under Simulated Atmospheric Conditions eBook

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File Size : 17,61 MB
Release : 2001
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The aim of this work was to improve the understanding of the OH-radical initiated oxidation of aromatic hydrocarbons (benzene, toluene, p-xylene (BTX) and 1,3,5-trimethylbenzene (TMB)). These mechanisms are considered a major uncertainty in state-of-the-art photochemical models as they are used to predict photooxidant formation from urban air. Differential Optical Absorption Spectroscopy (DOAS) was employed in a systematic outdoor smog-chamber study at the European Photo Reactor (EUPHORE) located at the CEAM-Institute, Valencia/Spain. The available DOAS system was improved for this purpose. The yields of ring-retaining products (phenol from benzene, phenol-type and aldehyde-type compounds from p-xylene and TMB) and glyoxal (from BTX) were investigated. The phenol yield (Y(PHEN)= 53) was found more than two times higher than presently available literature values. Further, the bicycloalkyl-radical pathway was identified as a major pathway from BTX. It was demonstrated that the results of this study are representative for the atmosphere. Deviations from the the degradation pathways of BTX and TMB were further observed under conditions of high NOx (e.g. several ppm). The results of this work indicate that the representations of aromatics in photochemical models need to be updated. The results indicate that the contribution of aromatic hydrocarbons to the formation of photooxidants (i.e. ozone) is underestimated today.

Author : Erin Elizabeth Tullos
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File Size : 25,86 MB
Release : 2010
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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.

Author : Buddhadeb Ghosh
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File Size : 19,85 MB
Release : 2011
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The 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 [delta]-peroxy radicals, accounting for about 20% 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% 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.