Photophoresis and the descent of particles into the lower stratosphere

Calculations are presented for the photophoretic force acting on an opaque spherical particle moving in the free molecule regime. The theoretical results are used to estimate the time of descent of small particles from high levels of an idealized stagnant atmosphere to the lower stratosphere. The theory indicates that a difference in settling times should exist between nonmetallic particles of low thermal conductivity and metal particles of higher conductivity. Despite such differences, the estimated descent times, for example, of 0.1-micron radius particles ranging from conductivities of 8 × 10−4 to 8 × 10−2 cal sec−1 cm−1 °K−1, traveling from 100 to 20 km altitude, remains approximately the same, the order of 1000 days. The photophoretic effect on particles of low conductivity can become appreciable below 30 km altitude. Nevertheless, this force does not appear to contribute significantly to decreasing the total time required for material to settle from very high altitude to the lower stratosphere.