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| Book | PreJuSER-22995 |
2012
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-804-4
Please use a persistent id in citations: http://hdl.handle.net/2128/4615
Report No.: D 468
Abstract: Benzene, toluene, the xylenes, and the trimethylbenzenes are among the most abundant aromatic trace constituents of the atmosphere mainly originating from anthropogenic sources. The OH-initiated atmospheric photo-oxidation of aromatic hydrocarbons is the predominant removal process resulting in the formation of O$_{3}$ and secondary organic aerosol. Therefore, aromatics are important trace constituents regarding air pollution in urban environments. Our understanding of aromatic photo-oxidation processes is far from being complete. This work presents novel approaches for the investigation of OH-initiated atmospheric degradation mechanisms of aromatic hydrocarbons. Firstly, pulsed kinetic studies were performed to investigate the prompt HO$_{2}$ formation from OH + aromatic hydrocarbon reactions under ambient conditions. For these studies, the existing OH reactivity instrument, based on the flash photolysis/laser-induced fluorescence (FP/LIF) technique, was extended to the detection of HO2 radicals. The experimental design allows for the determination of HO$_{2}$ formation yields and kinetics. Results of the pulsed kinetic experiments complement previous product studies and help to reduce uncertainties regarding the primary oxidation steps. Secondly, experiments with aromatic hydrocarbons were performed under atmospheric conditions in the outdoor atmosphere simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction chamber) located at Forschungszentrum Jülich. The experiments were aimed at the evaluation of up-to-date aromatic degradation schemes of the Master Chemical Mechanism (MCMv3.2). The unique combination of analytical instruments operated at SAPHIR allows for a detailed investigation of HO$_{x}$ and NO$_{x}$ budgets and for the determination of primary phenolic oxidation product yields. MCMv3.2 deficiencies were identified and most likely originate from short-comings in the mechanistic representation of ring fragmentation channels. These shortcomings relate to the formation of peroxy radicals, the NO to NO$_{2}$ conversion, and the O3 production. Conceptual ideas were presented to overcome these MCMv3.2 shortcomings and an improved reaction mechanism was constructed. However, major eficiencies still remain that require further investigations. Regarding the primary oxidation steps, the results of this work confirm the current MCMv3.2 recommendations. Proposed primary oxidation products are, e.g., phenols and epoxides. Phenol yields are in line with the MCMv3.2 values. The results of the complementary pulsed kinetic studies are consistent with the proposed combined formation yields of phenols plus epoxides. So far, epoxides have only been identified tentatively and the results of this work provide more quantitative information.
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