Inertial angular dynamics of non-spherical atmospheric particles

Cloud-ice crystals, volcanic ash, and microplastics are ubiquitous in the atmosphere. The orientation of these non-spherical particles influences their residence times, and the radiative properties of the atmosphere. These non-spherical particles are small, but their mass density is much greater than that of air. Studying the angular dynamics of such settling non-spherical particles is a major challenge. Therefore, previous studies have focused on particles settling in liquids. We demonstrate experimentally that the orientations of heavy, submillimetre-sized spheroids settling in air fluctuate considerably, in stark contrast to the very rapid orientation alignment observed in liquids. We establish theoretically that this behaviour is a consequence of large particle inertia. Our results highlight the central role of particle inertia in the angular dynamics of atmospheric particles, in an unexplored regime of parameters. This essential physical effect must be accounted for in models of atmospheric residence time, microplastic and volcanic ash dispersion, and the radiative properties of ice-laden clouds.