THE PHOTO-CHEMISTRY OF SOME MINOR CONSTITUENTS OF THE EARTH'S ATMOSPHERE (CO2, CO, CH4, N2O)

Abstract

Carbon dioxide and carbon monoxide: The rate of photo-dissociation of CO2 is appreciable only in the region above about 100 km. If local equilibrium prevails the carbon would occur mainly as the monoxide in this region, and as the dioxide below. However, oxidation may proceed so slowly that the life of a CO molecule is long compared with the characteristic time associated with atmospheric mixing effects. In this case the CO resulting from photo-dissociation would not be confined to above the 100 km level, but would extend much lower; its total abundance could scarcely be great enough to produce the observed absorption lines. It is estimated that the various forms of combustion taking place on the Earth would provide the CO content of the atmosphere within perhaps four years or even less. Methane: Dissociation of CH4 in the upper atmosphere is brought about mainly by collision processes. These prevent the existence of appreciable CH4 above the 100 km level and probably keep the concentration low even down to 70 km. The yield to date from oil wells and coal mines appears to be less than the amount of the gas now in the atmosphere. Seepage from fuel beds, and the anaerobic decay of vegetable matter, are the only obvious naturally occurring sources of significance. As far as can be judged at least ten years (and probably much longer) is required for the production of the atmospheric abundance. Nitrous oxide: Information on the necessary rate of formation of N2O is obtained by calculating the photo-dissociation rate. One hypothesis is that soil micro-organisms produce the gas, but it appears that the yield would only be sufficient if N2O were a major end-product of denitrification. Many of the homogeneous gas reactions suggested by earlier workers must be rejected, since there is an inadequate supply of the requisite parent particles. The only acceptable parent particles seem to be those arising directly or indirectly from the photo-dissociation of O2 in the Herzberg continuum. Various reactions involving O, O3 and N2 are discussed. Though they are very slow they might nevertheless give rise to sufficient N2O.