Monte Carlo Simulation of Photophoresis of Submicron Aerosol Particles

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Abstract
New calculations of the photophoretic force are presented for a spherical particle that may be coated with a volatile concentric spherical shell. The distribution of heat sources within the particle was obtained from an extension of the electromagnetic model of a concentric sphere of Aden and Kerker. The calculation for heat conduction within the sphere utilized a Monte Carlo technique. The nun momentum and energy transport at the particle surface was obtained in the “free molecule” regime. Computations were carried out mainly for particles at the nucleation mode (0.022 μm radius) and the accumulation mode (0.16 μm radius) of Eastern United States haze for conditions corresponding to the upper atmosphere. The photophoretic force was frequently much greater than the gravitational force and, in some cases, it was negative. Indeed, negative photophoresis for smoke, desert dust and soot coated with dirty ice might account for levitation of such particles with subsequent transport in the atmosphere over greater distances than might otherwise be anticipated. Metal particles, such as silver, also exhibited very strong negative photophoresis at wavelengths for which there was resonant excitation of surface plasmons.

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