Methane radiation chemistry

The reactions of free radicals in ethylene are well known (5, 24), and the applicability of this information to radiation chemistry has been demonstrated (32). Kinetic analysis of radical reactions revealed that the formation of methyl radicals and methane cannot be ascribed to excited precursors alone (32, 37). 

Schuchmann H-P, von Sonntag C (1981) Photolysis at 185 nm of dimethyl ether in aqueous solution Involvement of the hydroxymethyl radical. J Photochem 16 289-295 Schuchmann H-P, von Sonntag C (1984) Methylperoxyl radicals a study ofthey-radiolysis of methane in oxygenated aqueous solutions. Z Naturforsch 39b 217—221 Schuchmann MN, von Sonntag C (1977) Radiation chemistry of carbohydrates. Part 14. Hydroxyl radical-induced oxidation of D-glucose in oxygenated aqueous solution. J Chem Soc Perkin Trans 2 1958-1963... 

A very interesting technique has been used by Walker and Back124 in which the photolyses of methane, ethylene, and ethane have been carried out in a windowless system at 584 A., the helium resonance line. Since this is a study which has crossed the line into the realm of radiation chemistry, no discussion of the work will be presented. It does, however, represent a radiation chemical study using monochromatic ionizing radiation and it deserves, therefore, the attention of interested researchers. 

Both the high methane yield and the pressure dependence of the yields present an interesting facet to the radiation chemistry of ethylene, but without a much more complete product analysis no conclusions can be drawn. This is not within the scope of the present paper. 

In the following we shall discuss the radiation chemistry of methane in the gas phase, which may serve as an illustration of the various aspects of the radiation chemical effects in the gas phase. We shall also consider the effect of pressure and dose rate on the competition of the various processes. Finally we shall discuss briefly the effect of pressure on the fragmentation in cyclohexane. For a more detailed study the reader is referred to the review article of Gyorgy and the references cited therein. 

The radiation chemistry of gaseous methane was often studied. Radiolysis decomposition results in the formation of hydrogen, ethane, and ethylene as main products longer chains and even polymer-like products also form in low yields (O Table 23.4) (Yang and Manno 1959 Hummel 1970 Lias and Ausloos 1975 Gyorgy 1981). 

The necessary starting point for any study of the chemistry of a planetary atmosphere is the dissociation of molecules, which results from the absorption of solar ultraviolet radiation. This atmospheric chemistry must take into account not only the general characteristics of the atmosphere (constitution), but also its particular chemical constituents (composition). The absorption of solar radiation can be attributed to carbon dioxide (C02) for Mars and Venus, to molecular oxygen (02) for the Earth, and to methane (CH4) and ammonia (NH3) for Jupiter and the outer planets. 

The central role of hydroxyl radicals in atmospheric chemistry is well illustrated by examining the atmospheric cycles of methane and carbon monoxide. A quantitative assessment of both of these species was carried out in the 1920s in Belgium by Marcell Migeotte, who detected their absorption lines in the spectrum of infrared solar radiation reaching Earth s surface. 

Methane is the most abundant hydrocarbon in the atmosphere. It plays important roles in atmospheric chemistry and the radiative balance of the Earth. Stratospheric oxidation of CH4 provides a means of introducing water vapor above the tropopause. Methane reacts with atomic chlorine in the stratosphere, forming HCl, a reservoir species for chlorine. Some 90% of the CH4 entering the atmosphere is oxidized through reactions initiated by the OH radical. These reactions are discussed in more detail by Wofsy (1976) and Cicerone and Oremland (1988), and are important in controlling the oxidation state of the atmosphere. Methane absorbs infrared radiation in the troposphere, as do CO2 and H2O, and is an important greenhouse gas (Lacis et al., 1981 Ramanathan et al., 1985). 

The role of carbon dioxide in the Earth s radiation budget merits this interest in atmospheric CO2. There are, however, other changes of importance. The atmospheric methane concentration is increasing, probably as a result of increasing cattle populations, rice production, and biomass biuning (Crutzen, 1983). Increasing methane concentrations are important because of the role it plays in stratospheric and tropospheric chemistry. Methane is also important to the radiation budget of the world. 

As well as raising global temperatures by the trapping of thermal radiation, increasing methane concentration in the atmosphere influences atmospheric chemistry in two further ways first, by the increase in amounts of oxidation products (H20, CO and C02) second, by the rapid abstraction of hydroxyl radicals, which are important in a variety of oxidation processes and in ozone chemistry (see Section 7.2.1). 

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