Orientation of non spherical prolate dust particles moving vertically in the Earth’s atmosphere

Abstract

Preferential particle orientation has been previously reported for desert dust and attributed on atmospheric electricity (Ulanowski et al., 2007). Depending on its strength, the total electric field within the dust layer can: (a) counteract the gravitational settling of large particles and (b) cause a preferential orientation of the non-spherical particles along the vertical direction (Fucks, 1958; Ulanowski et al., 2007). Both effects change the particle aerodynamics and impact radiative transfer. Here, we quantify the electrical and aerodynamic torques on the non-spherical prolate model that is frequently assumed for dust particles. The transport of these particles is assumed to only depend on the influence of electric and gravitational fields. We provide an analytical approximation for the calculation of the mean orientation angle of both charged and uncharged dust particles. We find that electric fields with strength greater than 60 kV/m are required for the alignment of particles up to 20 μm in diameter, with the electric field lines. If the particles are moving with the same speed and in the same direction with the wind, the aerodynamic torque is zero and particle orientation along the electric field requires 1–2 orders of magnitude lower electric field strengths.