Gravitational settling of microplastic fibers: experimental results and implications for global transport

Gravitational settling is a crucial parameter to study the transport and distribution of atmospheric concentrations, sources, and sinks of particles. Although the settling velocity is highly dependent on the particle shape, most atmospheric transport models assume particles to be spherical, ignoring other geometries. In this study, we focus on the gravitational settling of microplastics (MP) particles, which often deviate strongly from sphericity. For instance, MP fibers can be approximated more closely by cylinders rather than spheres. Here, we present the results of conducted experiments with extremely elongated MP particles to define their settling velocity. This was done with the settling column and 3D-printed MP particles of different shapes (straight cylinders, half-circled cylinders, and quarter-circled cylinders), lengths, and aspect ratios. The experimental data shows that the parameterization scheme for shape correction proposed by Bagheri and Bonadonna, 2016 is a reliable tool to predict the gravitational settling of fibers considering different types of particle orientation (random, horizontal, and average of both).This scheme was implemented in the gravitational settling scheme of the Lagrangian transport model FLEXPART to eliminate uncertainties regarding the shape of a particle when simulating solid particle transport. As a study case, the mass concentration and deposition 3D fields of MP fibers were estimated according to the global population density to understand the contribution of the individual sites/regions to MP contamination of the atmosphere, land, and World Ocean.