Fig. 2.

Photophoretic force versus altitude. The upward photophoretic force, F_P, is normalized by the downward gravitational force, F_G, so that a particle is stably levitated at F_P/F_G = 1 (dashed line). Downward forces (F_P/F_G < 0) can occur near the stratopause when the particle’s radiative equilibrium temperature is less than ambient; forces decline to zero above the mesopause as particle temperatures become radiatively controlled. Particles may be trapped below the stratopause at “A” or below the mesopause at “B.” Forces are computed for three combinations of solar and thermal band emissivity by solving the energy balance equation shown in SI Text, assuming a disk with h = 50 nm, ρ = 3,000 kg m^-3, , and δα = 0.15. Calculations assume the 1976 Standard Atmosphere (30) and T_E = 255 K.