THE MOLECULAR DIFFUSIVE RATE OF CHANGE OF COMPOSITION IN THE ATMOSPHERE

Abstract

The atmosphere appears to be (within narrow limits) uniform in composition, as regards its permanent constituents, up to a height of at least 60 kilometers. The vertical variations of pressure and temperature produce relative motion of the different constituents by molecular diffusion ; these diffusive fluxes are calculated for an atmosphere uniformly mixed up to 80 km, and with the temperature distribution given by the Rocket Panel (1953). The downward pressure-diffusion flux of the heavier particles far exceeds the thermal-diffusion flux. But the rate at which molecular diffusion tends relatively to enrich or deplete any constituent at any height is proportional to the gradient of these fluxes, and in this respect the influence of thermal diffusion is comparable with that of pressure diffusion. It is inferred that if convection suddenly ceased throughout the atmosphere, at the outset the O2 concentration would increase, and the He concentration would decrease at about 80 km. The rates of enrichment or depletion even at this level, however, are so small, compared to the total density there, that a period of many months of rest would be needed to bring about appreciable changes of composition. Some other aspects of atmospheric diffusion are also discussed briefly.