Hydrocarbons oxidation, ozone generation

The principal components of atmospheric chemical processes are hydrocarbons, oxides of nitrogen, oxides of sulfur, oxygenated hydrocarbons, ozone, and free radical intermediates. Solar radiation plays a crucial role in the generation of free radicals, whereas water vapor and temperature can influence particular chemical pathways. Table 12-4 lists a few of the components of each of these classes. Although more extensive tabulations may be found in "Atmospheric Chemical Compounds" (8), those listed in... 

The primary photochemical loss process for O3 arises from the sequence of reactions which produces OH, i.e. (R3) and (R5). Tropospheric ozone generation by photochemistry can arise from the oxidation CO and hydrocarbons via reaction sequences such as... 

Formaldehyde is one of the most important products of hydrocarbon oxidation in the atmosphere. It provides a major source of radicals formed in the atmosphere and is very important in determining the rate of ozone generation. The atmospheric photochemistry of formaldehyde has been reviewed by several groups in recent years... 

The photodecomposition of the various oxidation products of the alkanes, alkenes, and the aromatic hydrocarbons play important roles in the chemistry of the urban, mral, and remote atmospheres. These processes provide radical and other reachve products that help drive the chemistry that leads to ozone generation and other important chemistty in the troposphere. In this chapter, we have reviewed the evidence for the nature of the primary processes that occur in the aldehydes, ketones, alkyl nitrites, nittoalkanes, alkyl nitrates, peroxyacyl nitrates, alkyl peroxides, and some representative, ttopospheric, sunlight-absorbing aromatic compounds. Where sufficient data exist, estimates have been made of the rate of the photolytic processes that occur in these molecules by calculation of the photolysis frequencies ory-values. These rate coefficients allow estimation of the photochemical lifetimes of the various compounds in the atmosphere as well as the rates at which various reactive products are formed through photolysis. 

In remote tropospheric air, where NO concentrations can be quite low (17), the HO + CO oxidation mechanism can follow other pathways, leading to net ozone destruction rather than formation (18, 19). Reactions 1 through 5 typify the more complex catalytic reactivity of HO with hydrocarbons, which produce a complex array of oxidation products while generating ozone pho-tochemically (11-13). 

The addition of a gas to a reaction mixture (commonly the hydrogen halides, fluorine, chlorine, phosgene, boron trifluoride, carbon dioxide, ammonia, gaseous unsaturated hydrocarbons, ethylene oxide) requires the provision of safety precautions which may not be immediately apparent. Some of these gases may be generated in situ (e.g. diborane in hydroboration reactions), some may be commercially available in cylinders, and some may be generated by chemical or other means (e.g. carbon dioxide, ozone). An individual description of the convenient sources of these gases will be found under Section 4.2. 

This leads to an equilibrium between generation and decomposition of ozone in the stratosphere, preventing the harsh solar radiation from reaching the earth s surface. Nitrogen oxides and halogenated hydrocarbons catalyze the decomposition of ozone, thus, shifting this equilibrium [4] ... 

Consider now the interaction of nitrogen oxides with hydrocarbons in sunlit urban air and the generation of ozone from this mixture. Figure 5-1 shows the variation with time of the nitrogen oxides, hydrocarbons, and... 

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