Lofting of Soot Particles Into the Middle Atmosphere by Gravito-Photophoresis

 

Rudolf F. Pueschel, Guy V. Ferry, and Anthony W. Strawa

The observed existence of soot aerosol at 20 km altitude, which arguably is generated by aircraft flying in corridors at 10-12 km, requires a transport mechanism in a thermally stable stratosphere that is different from isentropic or dynamic mixing. Such a mechanism could be provided by gravito-photophoresis, induced by the incidence of sunlight on strongly absorbing fractal soot particles. Those particles' absorptivity, in conjunction with uneven surface coating with sulfuric acid, and their fractal nature make soot particles with maximum dimensions approaching one micrometer particularly conducive to gravito-photophoresis, because the requirement of a restoring torque that orients the particle with respect to gravity that this force requires is provided by the fractal characteristics of soot, and a body-fixed photophoretic force is given by asymmetric thermal accommodation coefficients across the particles' uneven surface.

During the SONEX field campaign in 1997 we sampled soot aerosol in commercial airline flight corridors over the northeastern Atlantic and computed the gravitational and gravito-photophoretic forces acting on those soot particles. The result is that 16% by number, corresponding to 51% by mass, of a soot particle size distribution could be lofted against gravity by gravito-photophoresis. The calculated vertical velocities, exceeding settling velocities by up to a factor of 30, suggest that it takes about 30 and 20 years respectively to transport soot from 10 to 20 km and from 20 to 80 km. On the basis of current stratospheric soot loading, the resulting soot mass flux at 20 km altitude is 5e-18 g cm-2 sec-1 which is within one order of magnitude to the influx of meteoritic dust into the mesosphere from outer space.

The effect of gravito-photophoresis is strongly altitude dependent. With increasing pressure near the earth's surface, the lofting force falls off quickly. Above the mesopause, the lofting force becomes smaller because of a dominating energy loss by radiation rather than by molecular heat transfer. Thus gravito-photophoretic lofting forces are most effective within the altitude range between 10 and 85 km, making aircraft soot emitted in commercial flight corridors subject to lofting up to the mesosphere.

The current mass loading of soot in the middle atmosphere is too small to cause a direct absorption effect that would be comparable to the extinction of light by scattering on mesospheric cloud particles. However, it is conceivable that soot in the mesosphere has indirect effects by providing freezing nuclei for mesospheric ice particles to form. In addition, soot might contribute to the ionization of the mesosphere to affect the appearance of polar mesospheric summer radar echoes.

 

Collaborators: H. Rohatschek, University of Linz, Austria; Sunita Verma, SSAI; Nina Boiadjieva, San Jose State University, San Jose, CA; Steve Howard, Symtech

Point of Contact: Rudolf F. Pueschel, 650/604-5254, rpueschel@mail.arc.nasa.gov