Oxidation Reaction Mechanisms of VOCs in Polluted Atmosphere

3 Oxidation Reaction Mechanisms of VOCs in Polluted Atmosphere 

As the influence of human activities to the atmosphere is enhanced, the atmospheric concentrations of anthropogenic species such as nitrogen oxides (NOx = NO +NO2), volatile organic compounds (VOCs), etc. increase over a certain level. Tropospheric chemistry then exceeds the range of perturbation to the natural atmosphere, a chemical reaction system characteristic to the polluted atmosphere, sometimes called smog reactions, is brought about. Oxidation reaction mechanisms for the NOx-VOC mixtures directly related to the OH chain reaction are described in this section. 

In contrast to the NOx mixing ratios 10-100 pptv in the clean lower atmosphere, those of NOx in the urban polluted air are t3 pically one to tens of ppbv, more than 100 times higher than the former (Finlayson-Pitts and Pitts 2000). Similarly, the mixing ratios of components of non-methane volatile organic compounds (NMVOCs) or non-methane hydrocarbons (NMHCs) in the polluted atmosphere, are also 0.1-100 ppbv, typically 100 times higher than 1-1000 pptv in the clean atmosphere (Finlayson-Pitts and Pitts 2000). In such a polluted atmosphere, most of the OH radicals formed by reactions (7.1) and (7.2) react with anthropogenic NHVOCs rather than CH4 and CO. 

The oxidation reaction processes of VOC in the presence of NOx are different for alkanes (saturated hydrocarbons), aUcenes (unsaturated hydrocarbons with a double bond, also called olefins), alkynes (unsaturated hydrocarbons with a triple bond), and aromatic hydrocarbons (unsaturated hydrocarbons with a benzene ring). However, all NMVOCs react with OH, and the HOx chain cycle can formally be expressed as, 

The oxidation reactions of alkanes, alkenes and aromatic hydrocarbons treated in this section are described in detail in monographs by Calvert et al. (2000, 2002, 2008), and reaction mechanisms for air quality models are summarized by StockweU et al. (2012). Also, Calvert et al. (2011) and Mellouki et al. (2003) reviewed the oxidation reactions of oxygenated volatile organic compounds (OVOCs) which are not treated in this book. Detailed models for the photolysis 

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In a polluted or urban atmosphere, O, formation by the CH4 oxidation mechanism is overshadowed by the oxidation of other VOCs. Seed OH can be produced from reactions 4 and 5, but the photodisassociation of carbonyls and nitrous acid [7782-77-6]y HN02, (formed from the reaction of OH + NO and other reactions) are also important sources of OH in polluted environments. An imperfect, but useful, measure of the rate of O 3 formation by VOC oxidation is the rate of the initial OH-VOC reaction, shown in Table 4 relative to die OH-CH4 rate for some commonly occurring VOCs. Also given are the median VOC concentrations. Shown for comparison are the relative reaction rates for two VOC species that are emitted by vegetation isoprene and OC-pinene. In general, internally bonded olefins are the most reactive, followed in decreasing order by terminally bonded olefins, multialkyl aromatics, mono alkyl aromatics, C5 and higher paraffins, C2 C4 paraffins, benzene, acetylene, and ethane. 

The results obtained for reactions of NO3 with RO2 (R = H, CH3 and C2H5) show that rate constants at 298 K are comparable (1 - 3.6 x 10 cm molecule" s" at 298 K), and they all proceed through RO2/RO conversion. These reactions can consequently be rather fast and make important the proposed chain mechanism of VOC oxidation during nighttime in moderately polluted atmosphere. In addition, if the high rate constant found for the NO3 + CH3C(0)02 reaction was confirmed (k 2 X 10 cm molecule s" at 298 K) this reaction could affect PAN concentrations [5]. 

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