0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 18 20 2 2 CH^ mixing ratio [ppm]
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 18 20 2 2 CH^ mixing ratio [ppm]
The vertical profile of methane according to Ehhalt (1974). (By courtesy of Tellus)
An interesting feature of the vertical profile of CH4 is the concentration decrease above the tropopause, which was first demonstrated by the measurements of Bainbridge and Heidt (1966) who made aircraft and balloon flights. This finding was later confirmed by the balloon obsbrvations of Ehhalt (1974) and the rocket measurements of Ehhalt et al. (1972). The result of these authors are plotted in Fig. 6. It can be seen that the methane level in the troposphere does not change with height. However, above the tropopause the volume mixing ratio decreases with increasing altitude. The initial decrease is rather rapid then, above a certain level, it becomes slower. On the basis of available data, the total atmospheric methane mass is calculated to be 4000 x 106 t.
The origin of atmospheric methane can be estimated by determining its 14C isotope abundance. This abundance should be the same as the l4C content of living plants, if methane is of biological origin or is provided by recently dead organisms. CH4 from fossil fuels or volcanic activity is practically free of radiocarbon, since fuel deposits are very old and their 14C content has already decayed. Measurement of the radiocarbon in atmospheric samples demonstrated that at least 80 % of CH4 comes from organic materials of recent origin (Ehhalt, 1974). The remaining 20 % is termed "dead" methane, and is partly due to human activity.
Global strength of biological methane sources according to Ehhalt (1974)
„ Annual production
Enteric fermentation of animals 101-220
Paddy fields, swamps, marshes 410-540
Fresh water, lakes 1.25-25
Upland fields 10
Tundra 1.3-13 Oceans
The strength of different biological methane sources was estimated by Ehhalt (1974) and by Ehhalt and Schmidt (1978). Data in Table 6 are taken from one of Ehhalt's compilations. It can be seen from these estimates that the majority of CH4 comes from swamps and marshes as well as from paddy fields; that is from an anoxic environment. In this environment, containing much organic matter, anaerobic bacteria produce a huge quantity of CH4. Furthermore, the enteric fermentation of animals is also a very important methane source. Finally, one can see from the table that from the biosphere (500-800) x 1061CH4 is liberated into the air. The strength of known "dead" methane sources (e.g. mining, industrial losses) is estimated to be between 15 x 106 and 50 x 1061 yr"1 which is less than the 20 % of the contribution of biogenic sources indicated by radiocarbon analyses. Although the exact explanation of this discrepancy is not clear it appears that the impact of mankind on the atmospheric methane cycle can be neglected at present. This is further evidenced by the fact that no concentration increases were recorded during the last twenty years.
The rate of atmospheric CH4 formation was also recently estimated by Lukshin et al. (1978) who carried out aircraft samplings over the European part of Soviet Union to measure the vertical profiles of CH4 and radon during the spring of 1975. From the results of these measurements, they calculated the vertical diffusion flux of methane liberated at the surface. The chemical destruction of CH4 (see later) in the troposphere was also taken into consideration. These authors found that the strength of global methane sources generalized on the basis of the CH4 flux calculated is about one order of magnitude greater than the total value given by
Ehhalt (1974). It follows from this difference that, while the residence time from Ehhalt's results is 4-7 years, it is only 0.4 year from the estimate of Lukshin et al. (1978), by using the same figure for the amount of atmospheric CH4 burden (4000 x 106 t). It is to be noted, however, that the number of flights in Lukshin's investigation was very limited. Clearly, more research is needed in this field.
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.
The only atmospheric chemical process that provides an adequate sink is the reaction between CH4 and OH (Ehhalt and Schmidt, 1978):
The reaction rate (see Subsection 3.4.1) of this process is 5.5 x 10 ~12 exp( —1900/7) [cm3/molecules], where T is the absolute temperature.
According to Levy (1971) the average concentration of OH radicals is 2.5 x 106 cm "3 in the troposphere. This calculated value is essentially confirmed by recent atmospheric measurements (e.g. Perner et al., 1976). On the basis of the above figures a global tropospheric CH4 destruction rate of 900 x 106 t yr_1 can be estimated, which is comparable to the total source strength given above. If this figure is correct the majority of CH4 is destroyed in the troposphere. Thus, Ehhalt and Schmidt (1978) speculate that about 85-90 % of the methane is removed from the air in the troposphere; the remainder is destroyed in the stratosphere by the same process.
As mentioned in Section 3.2, reaction [3.4] is the first step of a reaction chain which ends in the formation of carbon monoxide and hydrogen. This means that the methane cycle in the atmosphere is connected with those of the substances mentioned. Furthermore, since CH4 plays an important part in the control of the stratospheric budget of hydrogen compounds including water (formed by the oxidation of the hydrogen of CH4) and free radicals, the methane cycle is also of importance for chemical processes in the stratosphere.
It was also mentioned in Section 3.2 that tropospheric H2 formed by the above process reacts with OH to form water. The CO formed from the carbon atoms of methane is transformed into C02 by appropriate chemical processes (see Subsection 3.3.2). Both water and carbon dioxide are used by the plants during their photosynthetic activity. Thus, a major part of hydrogen and carbon of CH4 returns to the plants, thus closing the atmospheric methane cycle.
Like CH4, carbon monoxide was first optically identified in the atmosphere by Migeotte (1949). It was believed at that time that atmospheric CO was wholly due to anthropogenic sources. However, more recent studies show that carbon monoxide is emitted into the air by many other sources. Furthermore, it became clear that the atmospheric cycle of this trace gas is much more complicated than was thought at the time of its discovery.
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