Methane atmospheric chemistry

Aselmann 1, Crutzen PJ. 1989. Global distribution of natural fresh-water wetlands and rice paddies their net primary productivity, seasonality and possible methane emissions. Journal of Atmospheric Chemistry 8 307-358. 

Seiler W, Holzapfel-Pschom A, Comad R, Scharffe D. 1984. Methane emission from rice paddies. Journal of Atmospheric Chemistry 1 241-268. 

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. 

Johnson C.E. Stevenson D.S. Collins W.J. and Derwent R.G. (2002). Interannual variability in methane growth rate simulated with a coupled Ocean-Atmosphere-Chemistry model. Geophysical Research Letters, 29(19), 1-4. 

Thus the lifetime of a constituent with a first order removal process is equal to the inverse of the first order rate constant for its removal. Taking an example from atmospheric chemistry, the major removal mechanism for many trace gases is reaction with hydroxyl radical, OH. Considering two substances with very different rate constants for this reaction, methane and nitrogen dioxide... 

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. 

The chemistry of the ozone layer in the tropics has not been extensively studied. We do not know the level of the ozone concentration with exactitude. We have demonstrated that termites do emit large quantities of methane in the tropics. The emitted methane is transformed into CO which can affect the ozone layer. Therefore, studies of ozone layer profile in the tropics would be an essential component towards our understanding of the atmospheric chemistry. 

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). 

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). 

The effects of hydrogen compounds on the behavior of other chemical species, particularly that of ozone in the mesosphere, were first examined by Bates and Nicolet (1950), and have been the subject of numerous subsequent studies (Hampson, 1966 Hunt, 1966 Hesstvedt, 1968 Crutzen, 1969 Nicolet, 1971). The high reactivity of the hydrogen free radicals, especially OH, makes these species of particular importance in atmospheric chemistry. The compounds that initiate the hydrogen radical chemistry are methane (for which the budget was discussed in Section 5.3), water vapor, and molecular hydrogen. 

Atmospheric chemistry is dominated by trace species, ranging in mixing ratios (mole fractions) from a few parts per million, for methane in the troposphere and ozone in the stratosphere, to hundredths of parts per trillion, or less, for highly reactive species such as the hydroxyl radical. It is also surprising that atmospheric condensed-phase material plays very important roles in atmospheric chemistry, since there is relatively so little of it. Atmospheric condensed-phase volume to gas-phase volume ratios range from about 3 x KT7 for tropospheric clouds to 3 x ICE14 for background stratospheric sulfate aerosol. 

The atmosphere contains many trace gases that are photochemically active. Few, however, attain a significance similar to oxygen or ozone in driving atmospheric chemistry. Table 2-6 gives an overview on the photochemical behavior of atmospheric constituents. The last column indicates whether the photodecomposition of the substance is important to atmospheric chemistry or not. The molecules listed may be subdivided into three groups. (1) For a number of gases such as methane or ammonia that enter... 

Once the importance of DMS to the global sulfur cycle was established, numerous measurements of DMS concentrations in the marine atmosphere have been conducted. The average DMS mixing ratio in the marine boundary layer (MBL) is in the range of 80-1 lOppt but can reach values as high as 1 ppb over entrophic (e.g., coastal, upwelling) waters. DMS mixing ratios fall rapidly with altitude to a few parts per trillion in the free troposphere. After transfer across the air-sea interface into the atmosphere, DMS reacts predominantly with the hydroxyl radical and also with the nitrate (N03) radical. Oxidation of DMS is the exclusive source of methane sulfonic acid (MSA) in the atmosphere, and the dominant source of S02 in the marine atmosphere. We will return to the atmospheric chemistry of DMS in Chapter 6. 

Penkett, S. A. 1982. Non-methane organics in the remote troposphere. In E. D. Goldberg, ed.. Atmospheric chemistry. Springer, Berlin, pp. 329-355. 

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