Methane, tropospheric sinks

Bainbridge and Heidt (8) estimated from the rate of decrease of the CH mixing ratio above the tropopause that at most 10% of the tropospheric CH was lost by transport into the stratosphere and suggested that a large tropospheric sink was necessary. Ehhalt and Heidt (53) found that the calculated sink for methane due to transport into the stratosphere was much too small. 

Taylor JA, Brasseur GP, Zimmermann PR, Cicerone RJ. 1991. A study of the sources and sinks of methane and methyl chloroform using a global three-dimensional Lagrangian tropospheric tracer transport model. Journal of Geophysical Research 96D 3013-3044. 

Reaction of CH4 with OH accounts for the destruction of 76% of the methane flux to the atmosphere. Other sinks of tropospheric methane are shown in Table 9.2. 

Only a small percentage of the chlorine released by photolysis of CFCs is present in the active forms as Cl or CIO, however. Most of it is bound up in reservoir compounds such as hydrogen chloride and chlorine nitrate, formed respectively by hydrogen abstraction (equation 10) from methane and addition (equation 11) to nitrogen dioxide. Slow transport of these reservoir species across the tropopause, followed by dissolution in tropospheric water and subsequent rain-out, provide sink processes for stratospheric chlorine. 

Your point is certainly well taken there are many aspects of tropospheric chemistry that are uncertain, and that involving methyl peroxide is without doubt a prime example. From my own estimation of how the methane system works in the atmosphere, I believe that a significant fraction of the methyl peroxide is removed by heterogeneous reactions before it has a chance to react. This terminates the chain making methane oxidation a net sink for OH regardless of the details of the chemistry of methyl peroxide and its daughter molecules. Nevertheless we certainly need to be aware of the many uncertainties in this chemistry. 

Quantitative understanding of the sources, sinks and atmospheric lifetime for CHa is an important future goal for several reasons. The direct increase in tropospheric CHa concentrations adds another important infrared absorbing contributor to the greenhouse effect. The calculated contribution from a CHa increase of 0.18 ppmv in a decade is a tropospheric temperature increase of 0.04 C [N.A.S., 1983], about 1/3 as large as that calculated for the observed 12 ppmv increase for CO2 over the decade from 1970-1980. As described earlier, increasing concentrations of CHa in the stratosphere have an influence on ozone-depletion by ClOx through diversion of Cl into HCl, and should in addition after oxidation increase the upper stratospheric concentrations of H2O. Methane is also a participant in tropospheric chemical reaction sequences which lead under some conditions to the formation of ozone. 

The largest sink for alkanes in the atmosphere is reaction with OH and NO3 radicals. The formation of photochemical smog is described in detail in (Chapter 9.11, Sillman). Mono-aromatic hydrocarbons react only slowly with O3 and NO3 radicals in the troposphere. The only important atmospheric processes for mono-aromatic hydrocarbons, and naphthalene and dinaphthalenes are reactions with OH radicals (Atkinson, 1990). The products of these reactions include aldehydes, cresols, and, in the presence of NO, benzylnitrates. Methane can be an important contributor to ozone formation, especially in the remote troposphere, as described in (Chapter 9.11, Sillman). 

Thus the net effect of dissociating nitrogen dioxide is neutral. Net production of tropospheric ozone occurs as a result of other reactions that convert NO into NO2 without destroying ozone. There are many such reactions, most of which involve the photooxidation of chemicals like carbon monoxide, methane and other hydrocarbons. Since these are produced by traffic and industrial processes, ozone production is a feature of polluted regions, and ozone itself is considered a pollutant at low levels of the atmosphere where it is detrimental to human and other life forms. Sinks of ozone include photodissociation and reactions with OH and HO2 (as in the stratosphere) and deposition. 

The simplest alkane is methane (CH4). Methane oxidation is the essential chemistry of the background troposphere (Logan et al., 1981 Thompson and Cicerone, 1986). Ice-core records show that methane concentrations in the atmosphere have more than doubled since preindustrial times (Khalil and Rasmussen, 1987), reaching a rate of increase of 1% yr-1 in the last decade (Khalil et al., 1989). Methane is emitted to the atmosphere by ruminants, wetlands, tundra, open waters, termites, rice paddies, biomass burning, natural gas production, and coal mining [see Jacob (1991) for a review of the literature on methane sources] the principal sink of CH4 is reaction with OH. 

A fairly general treatment of trace gases in the troposphere is based on the concept of the tropospheric reservoir introduced in Section 1.6. The abundance of most trace gases in the troposphere is determined by a balance between the supply of material to the atmosphere (sources) and its removal via chemical and biochemical transformation processes (sinks). The concept of a tropospheric reservoir with well-delineated boundaries then defines the mass content of any specific substance in, its mass flux through, and its residence time in the reservoir. For quantitative considerations it is necessary to identify the most important production and removal processes, to determine the associated yields, and to set up a detailed account of sources versus sinks. In the present chapter, these concepts are applied to the trace gases methane, carbon monoxide, and hydrogen. Initially, it will be useful to discuss a steady-state reservoir model and the importance of tropospheric OH radicals in the oxidation of methane and many other trace gases. 

Two sinks for methane in the troposphere are fairly well known. The major one is reaction with OH radicals as discussed in Section 4.2, and the other sink is a loss of methane to the stratosphere. According to Eq. (4-7),... 

The total flux of methane into the stratosphere, 60 Tg/yr, combines with the rate of CH4 oxidation by OH to give a total sink strength of 400 Tg/yr. This agrees approximately with the global emission estimates of Table 4-7. Although methane has been observed to increase in the troposphere, the budget must be about balanced since the increase is slow. If we take a value of 400 Tg/yr as representative for ( Ch4 ch4 the tropospheric residence time for methane is... 

The stratosphere as a sink was first identified by Seiler and Junge (1969). The flux of CO into the stratosphere is caused by a decline of CO mixing ratios above the tropopause toward a steady-state level lower than that normally found in the upper troposphere (see Fig. 1-14). In the lower stratosphere, CO is produced from methane and other long-lived hydrocarbons, and it is consumed by reaction with OH as in the troposphere, but the rate of vertical mixing is much slower (Seiler and Warneck, 1972 Warneck et al., 1973). The flux of CO from the troposphere into the stratosphere can be derived from the observed gradient of the CO mixing ratio above the tropopause in a manner described in Section 4.3 for methane. The loss rate obtained, llOTg/yr, is small compared with that for the reaction of CO with OH radicals. 

The troposphere has an estimated 155 Tg of hydrogen gas (H2), with approximately a two-year lifetime (Chapter 2.8.2.10). Many sources of hydrogen gas and a few major sinks account for this relatively short lifetime. The main pathway in the production of hydrogen atoms in the air is the methane (CH4) conversion by the OH radical and subsequent photolysis of formaldehyde (HCHO) see reactions (5.42) to (5.48). This process accounts for about 26 Tg H yr (Novelli et al. 1999). 

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