The observed existence of soot aerosol at 20-kilometer (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. This particle absorptivity, in conjunction with uneven surface-coating of sulfuric acid and their fractal nature, makes soot particles (with maximum dimensions approaching 1 micrometer) particularly conducive to gravito-photophoresis. It is the requirement of a restoring torque that orients the particle with respect to gravity. This required force is provided by the fractal characteristics of soot, and a body-fixed photophoretic force is given by asymmetric thermal accommodation coefficients across the uneven surface of the particle.

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