chlorophyl. According to recent laboratory experiments of Seiler et al. (1978) this source provides carbon monoxide in an amount similar to that of the above sources, except for human activity.

Unfortunately rather different opinions can be found in the literature concerning the rate of CO formation by the oxidation of methane in the atmosphere. Thus, according to Seiler (1974) the global strength of this source is between 1500 x 10® t yr~1 and 4000 x 1061 yr"1. However, Warneck (1974) gives a figure of 300 x 106 t yr~based in part on a calculation of the removal of OH radicals from the air by aerosol particles. On the other hand, Lukshin et al. (1978) argue that the quantity of CO formed yearly by this process in the atmosphere is about one order of magnitude greater than Seiler's estimate. Finally, Ehhalt and Schmidt (1978) propose a figure similar to that given by Seiler (1974). Because of this latter agreement the present author uses a value of about (1000-1500) x 1061 yr"1 for this term (see later Fig. 8). This discussion makes it clear that the sources of carbon monoxide are not well defined numerically. Efforts are needed in the future to obtain more realistic figures. This is particularly important if we want to estimate correctly the man-made contribution to the atmospheric budget of carbon compounds.

At present it appears that there are two major carbon monoxide sinks. The first is provided by the soil surface. Thus, laboratory experiments show that the CO absorbing capacity of different soils can be very important (see Seiler, 1974). The generalization of the results of these laboratory works gives a value of 450 x 106 t yr ~1 for the global sink intensity (see Table 7). An other possibility is the reaction of carbon monoxide with OH radicals which removes CO rapidly from the air. In the previous section we have discussed the chemical destruction of CH4 by OH. One end product of these processes is carbon monoxide (see [3.2]) which also reacts with free radicals to form carbon dioxide in the following way (e.g. Bortner et al. 1974)

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