New explicit formulae for the settling speed of prolate spheroids in the atmosphere: theoretical background and implementation in AerSett v2.0.2

Abstract

We propose two explicit expressions to calculate the settling speed of solid atmospheric particles with prolate spheroidal shapes under the hypothesis of horizontal and vertical orientation. The first formulation is based on theoretical arguments only. The second method, valid for particles with mass median diameter up to 1000 µm, is based on recent heuristic drag expressions based on numeric simulations. We show that these two formulations show equivalent results within 2 % for deq≤100 µm and within 10 % for particles with deq≤500 µm falling with a horizontal orientation, showing that the first, more simple, method is suitable for virtually all atmospheric aerosols, provided their shape can be adequately described as a prolate spheroid. Finally, in order to facilitate the use of our results in chemistry transport models, we provide an implementation of the first of these methods in AerSett v2.0.2, a module written in Fortran.