Effects of dust particle sphericity and orientation on their gravitational settling in the earth’s atmosphere

Abstract

The effects of non sphericity and particle orientation on mineral dust gravitational settling are studied for prolate spheroids in the range of Reynolds number up to 100. New drag coefficients for prolate spheroids are defined that take into account the orientation of the particle. The drag coefficients along with the slip correction factors are applied to the momentum equation, which is numerically solved for the calculation of the steady state velocity of the particle with respect to the air. As the aspect ratio increases, the settling velocity of ellipsoids decreases compared to the settling velocity of spherical particles, and the non spherical particles can remain in the atmosphere for longer periods. Moreover, the settling velocity of horizontally oriented particles is smaller than the settling velocity of vertically oriented particles. Assuming that at time t = 0 s the particles are at altitude equal to 6 km, non spherical particles that are horizontally oriented, with aspect ratio equal to 2.4, and polar diameter up to ∼ 24 μm can remain in the atmosphere for 5 days, while particles with polar diameter up to ∼ 17 μm can remain in the atmosphere for 10 days. In the case that the particles are vertically oriented, particles with polar diameter up to ∼ 22 μm can remain in the atmosphere for 5 days, and particles with polar diameter up to ∼ 15 μm can remain in the atmosphere for 10 days.