A physics-based and orientation-aware method for the direct calculation of  the settling speed of prolate spheroidal particles in the atmosphere : theoretical basis and comparison to laboratory and CFL data

We have developed a new method to calculate the settling speed of non-spherical aerosols in the atmosphere, beginning with prolate spheroidal aerosol even though the method could be generalized to other shapes such as oblate spheroids or fibers. Most existing formulations of the settling speed are empirical numerical fits designed to match the results of either laboratory measurements or CFD simulations. On the contrary, the method we expose is based essentially on theoretical results on the drag and orientation of settling particles, with a minimal use of empirical numerical fits. As a result, the present method is more simple than existing methods and (with less empirical coefficients), and permits to calculate the settling speed of a prolate particle settling in the atmosphere as a function of the characteristics of the particle and of the atmospheric conditions, with no additional information. The varying distribution of particle orientation is accounted for using the results of Mallios et al. (2021), and the force-to speed relationships are based on Mailler et al. (2024), which we have extended to intermediate orientations and systematized to reach the present results.The method presented here has been implemented in Fortran in the AerSett module, and the corresponding implementation is distributed under the free GPL-3.0 license . We hope that this novelty will permit to take into account more frequently particle elongation in chemistry-transport models, which may prove important in the case of, e.g., giant dusts, or microplastic particles with elongated shapes.