Variation of the oxygen level in the Earth's atmosphere during geological time (Berkner and Marshall, 1967). (By courtesy of Academic Press)

Walker (1972) it is not excluded that the biologically protective ozone shield was formed at this altitude at an earlier time.

After the late Silurian era the rate of oxygen production further increased. It is probable that in the Carboniferous period the rapid increase of the oxygen level resulted in a higher concentration than the present one. After this age the oxygen level varied (see Fig. 3) as a function of the photosynthetic oxygen production and of the oxidation of decaying organic materials.3 Finally, as a result of different processes a constant atmospheric oxygen level came into being. In the present quasi-equilibrium state, the 02 production is estimated to be 7 x 1013 molecules cm"2 s"1 (Berkner and Marshall, 1967) and the atmospheric residence time of this species is around one thousand years.

3 While photosynthesis makes free 02, decay consumes it. Net 02 is produced by the mechanism proposed by Berkner and Marshall (1967) only if fixed carbon is buried. If not, for the explanation of the 02 content of the atmosphere by the presence of the biosphere some other biological process is necessary as discussed by Lovelock and Lodge (1972).

An important task in air chemistry research is to study the effect of the activity of mankind on this quasi-permanent oxygen concentration. Industrial and other combustion processes, together with damage to photosynthetic plants, can modify this quasi-stable level. According to Davitaya (1971) the relative decrease of 02 during the past fifty years, calculated on the basis of the quantity of fossil fuels used, should have been 0.02 %. Very precise measurements of the 02 content of the atmosphere, however, did not reveal any change in the time interval mentioned (Machta and Hughes, 1970). This difference between calculations and measurements can only be explained if we suppose that the atmosphere has some self-regulation capacity which eliminates even such small changes. It also goes without saying that the lack of anthropogenic effects at present does not mean that in the future we may neglect this problem. The survey of the oxygen concentration in the atmosphere must be involved in global monitoring programs aimed at determining anthropogenic changes in the composition of our atmospheric environment.

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