The absorption of light in the atmosphere and its relation to atmospheric ozone

Abstract

In the high atmosphere the absorption of solar radiation leads to fluorescence and to persistent ionization and molecular dissociation. The absorption is almost entirely in the far ultra‐violet. The degree of ionization and dissociation decreases and the degree of molecule formation increases in passing to lower altitudes, since these in general depend oppositely on collisions. At lower altitudes collisions permit frequent recombination and also the formation of new molecules which introduce new absorption. This absorption by the new molecule may in general lead to its decomposition thus setting up a photochemical steady state—the outstanding example in the atmosphere is that of ozone. Ionization will be confined chiefly to high altitudes and new molecule formation to low altitudes, not so much because of the altitudes at which the respective absorptions occur but rather because of the difference in atmospheric density. Intense absorption by oxygen in the intermediate altitudes leads to inconsiderable amounts of ozone. Moreover, the amount of new molecule formed in a photochemical steady state depends not so much upon the intensity of the absorption of the producing wave‐lengths but upon the ratio of the intensities of the producing and decomposing wave‐lengths. In a gas distributed exponentially with height, as in a gravitational field, the form of the curve, absorption versus height, is the same for all values of the absorption‐coefficient; the curve is simply transposed in height. Thus it comes about that in the production of ozone, not the strongly absorbed radiation which is absorbed at great altitudes, but the very weakly absorbed radiation, which penetrates deeply, is the important radiation in its production. It has been somewhat lost sight of that active wavelengths, extremely weakly absorbed by oxygen but carrying as much or indeed greater total energy, are absorbed just as completely but at low altitudes as those possessing high absorption‐coefficients which are absorbed at high altitudes. In this connection the pressure‐dependent absorption in oxygen is also important.