Gravito-photophoresis and aerosol stratification in the atmosphere

The formation of aerosol layers in the earth's stratosphere and mesosphere under the influence of gravito-photophoretic forces is considered. The minimum set of effective particle parameters for describing gravito-photophoretic forces, and their critical values for vertical transport and levitation of particles are discussed. The behaviour of two types of particles are investigated specifically: (1) soot-like particles strongly absorbing both solar (visible) and IR radiation, and (2) sulfate-like particles which are transparent in the visible, but strongly absorb the terrestrial IR radiation. Only particles larger than one micrometer can levitate under the influence of gravito-photophoresis. Particles larger than 1 mu m can be suspended in the atmosphere. They need to have a variation in accommodation coefficient over the surface. As a rule transparent particles should have a density smaller than the one of water. Both black particles and white particles can be levitated. Gravito-photophoresis explains the existence of narrow aerosol layers at altitudes of approximately 20 and 50 km, and also at approx. 70 and 80-83 km. The thickness of the layers can be less than I km if the accomodation coefficient exhibits only small differences over the surface. Transparent particles may form thick layers above 50 km rather easily. (C) 2005 Published by Elsevier Ltd.