Sinks of atmospheric methane

Ehhalt D. H. and Schmidt U. (1978) Sources and sinks of atmospheric methane. Pure Appl. Geophys. 116, 452-464. 

Our knowledge of the sinks of atmospheric methane can be summarized as follows. Model experiments made under laboratory conditions show that the CH4 uptake of soil is not important (Ehhalt, 1974). This means that soil microorganisms do not oxidize CH4. This also means that we have to look for methane sinks in the atmosphere. 

An estimate of the annual methane flux into the atmosphere can be calculated by adding the sinks and the annual increase. These data (Table II) indicate that a flux of 375-475 trillion tons(Tg) per year would be required to account for an annual increase of50-60 trillion tons (7). Estimates of sources of atmospheric methane indicate that up to 83% is biogenic in origin (5). The other abiogenic... 

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. 

Raynaud D. and Chappellaz J. (1993) The record of atmospheric methane. In Atmospheric Methane Sources, Sinks, and Global Change (ed. M. A. K. Khalil). Springer, Berlin, pp. 38-61. 

The major sink for atmospheric methane is oxidation by highly reactive hydroxyl radicals ( OH formed by photolytic dissociation of water molecules in the atmosphere Crutzen 1988 Eisele et al. 1997). The life-time of methane in the atmosphere is, consequently, relatively short and unlikely to have exceeded 100 years... 

The total annual input of methane from all sources to the atmosphere shown in Table 6.4 is 540 Mt, while the estimated output from atmosphere to sinks is 500 Mt. The potential inaccuracies in flux data can be seen by comparing the observed carbon isotopic signature of atmospheric methane of —47%o with that calculated from the data in Table 6.4 of c— 54%o (the latter is actually equivalent to —58%o upon correcting for the kinetic isotope effect (see Box 1.3) that operates during the hydroxyl abstraction reaction). There are clearly major gaps in our understanding of the pathways of methane into and out of the atmosphere and the fluxes involved, as there are for many anthropogenic substances (see Chapter 7). 

The dominant sinks for atmospheric methane, accounting for about 90% of its loss from air, are the reaction with hydroxyl radical, OH, (reaction (1))... 

Wetlands, in addition to being an important carbon sink, are a major source of atmospheric methane. Although soil carbon in wetlands is recognized as being an important component of global... 

This means that the observed change in M mainly reflects a change in the source flux Q or the sink function. As an example we may take the methane concentration in the atmosphere, which in recent years has been increasing by about 0.5% per year. The turnover time is estimated to be about 10 years, i.e., much less than Tobs (200 years). Consequently, the observed rate of increase in atmospheric methane is a direct consequence of a similar rate of increase of emissions into the atmosphere. (In fact, this is not quite true. A fraction of the observed increase is probably due to a decrease in sink strength caused by a decrease in the concentration of hydroxyl radicals responsible for the decomposition of methane in the atmosphere.)... 

What are the likely future changes in concentrations of carbon dioxide, methane, and other carbon-containing GHGs in the atmosphere as well as changes in the sources and sinks of carbon on land and in the ocean ... 

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. 

Staffelbach T. A., Neftel A., Stauffer B., and Jacob D. J. (1991) Formaldehyde in polar ice cores a possibility to characterize the atmospheric sink of methane in the past Nature 349, 603-605. 

Table 5 Estimated sources and sinks of methane in the atmosphere in units of lO g CH4 yr . ...

Galchenko V. F., Lein A., and Ivanov M. (1989) Biological sinks of methane. In Exchange of Trace Gases between Terrestrial Ecosystems and the Atmosphere (eds. M. O. Andreae and D. S. Schimel). Wiley, New York, pp. 59-71. 

Carbon monoxide (CO), a toxic gas, is produced during combustion, both in wildfires and in fuel-burning devices CO also can be produced and consumed by bacterial activity. The presence of CO may indirectly increase the atmospheric mixing ratios of other gases by competing for oxidant species (such as the hydroxyl radical, OH-), thereby decreasing the oxidation rates of the other gases. This competition for oxidant species is believed to be one reason for the current increase in atmospheric methane, whose major atmospheric sink is reaction with the hydroxyl radical. 

The atmospheric methane budget appears in Table 4-17. As is the case with the carbon dioxide budget, many of the sink and source strengths are... 

Staffelbach T, Neftel A, Stauffer B, et al. 1991. A record of the atmospheric methane sink from formaldehyde in polar ice cores. Nature 349 603-605. 

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. 

---