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It follows from the above discussion that the strength of soil sources is probably smaller than that of sinks in the soil. We may also conclude that an important part of the atmospheric hydrogen (around 50 %) is provided by anthropogenic activity. It seems to be meaningful in this way to consider H2 as a future global pollutant. This concept is further supported if the results of the first H2 measurements (Schuftan) are compared to Schmidt's concentrations. This comparison suggests that the atmospheric hydrogen level has gradually increased in past decades. This fact is particularly important since H2 takes part, as we have seen, in the formation and destruction of free radicals which participate in the control of the atmospheric cycle of many trace constituents (see later).

It is well known that protons from the Sun are continuously reaching the Earth's atmosphhere. However, this external source does not increase the atmospheric hydrogen level since hydrogen atoms are also continuously leaving the upper atmosphere. It is to be noted that Lovelock and Lodge (1972) speculate that a significant part of the escaping H2 comes from CH4 since according to the results of recent measurements the methane quantity reaching the stratosphere from the troposphere is about three times greater than that of H20. Thus, the photolysis of CH4 produces six times more hydrogen than the photodissociation of water vapour. In this way methane plays an important role in the control of the removal of H2 from the lower levels of the atmosphere.

Finally, it can be mentioned (Junge, 1963) that the deuterium content of the atmospheric hydrogen molecules is approximately 5 % of that found in ocean waters, where the D/H ratio is 3.1 x 10~4 On the other hand the abundance of tritium in the atmosphere is about 103 times greater than the value in the hydrosphere (~1018).

3.12 Helium

The results of helium analyses carried out in the atmosphere were reviewed by Glueckauf (1951) who stated that the concentration of helium is a constant value: 5.239 + 0.002 ppm. These measurements also demonstrated that the 3He/4He isotope ratio in the air is 1.2 x 10~6, which means that only the formation of 4He is of interest from the point of view of air chemistry.

Atmospheric 4He is a product of radioactive processes taking place in the Earth's crust. About 98 % of the helium is produced by the decay of two radioactive isotopes: 238U and 232Th. The rate of 4He formation can be estimated from the lithospheric quantity of these isotopes and the escape rate of the helium formed. We can assume that helium formed in the Earth's mantle cannot escape into the air. Then only 4 He produced in the crust reaches the atmosphere. This quantity is estimated to be 7 x 106 m3 STP per year (Junge, 1963).

Since there appears to be no He sinks in the soil or in the lower levels of the atmosphere, it is more than probable that this noble gas escapes from the upper atmosphere into outer space. It follows from the constant atmospheric level that this sink balances the effect of the sources. When the total atmospheric helium mass is taken into account, as well as the above formation mechanism, the residence time of He is estimated to be approximately 107 years.

3.3 Carbon compounds

The presence of methane in the atmosphere was discovered by Migeotte (1948), who identified this trace gas on the basis of absorption bands found in the infrared spectrum of the solar radiation. These first optical measurements gave an atmospheric concentration of 2 ppm. According to more recent analyses carried out by gas chromatography, the methane concentration in the lower troposphere varies between 1.3-1.6 ppm (Ehhalt, 1974). The average mixing ratios of methane in the troposphere for Northern and Southern Hemispheres were found in 1972 to be 1.41 and 1.30 ppm, respectively (Ehhalt and Schmidt, 1978). It thus follows from these data that methane concentrations are somewhat smaller over the Southern than over the Northern Hemisphere. Furthermore, the average results also show that zonal and vertical concentration changes are negligible in spite of the fact that CH4

level may fluctuate in individual profiles. In the troposphere the amplitude of annual variations is also very small. (There is some indication of a concentration maximum during the autumn.) The lack of annual variations can be explained either by the interaction of several sources, or by the similar variation of source and sink strengths during the year.

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