Modeling the gravitational settling of microplastic fibers in the atmosphere*

The ubiquitous presence of microplastics imposes multiple threats to the environment and ecosystem. There are extensive studies on their global cycles in the aquatic environment and increasing evidence from recent studies has highlighted the importance of the atmosphere as an equally important medium and transport pathway in the plastic cycle. The gravitational settling speed of microplastics, which is important to quantify the atmospheric limb of the plastic cycle, remains poorly understood, especially for asymmetrically-shaped microplastics such as fibers. It is estimated that microplastic fibers (MPFs) account for a significant portion of airborne and deposited microplastics found in both the natural and built environments with a more adverse effect on the health of organisms compared to nonfibrous ones. In this talk, a semi-analytical model of the gravitational settling velocity of MPF has been developed to include the effect of their morphology and ambient atmospheric turbulence. Particularly, in the framework of linearized slender body theory, the orientation variance of MPF that is the consequence of net balance among fluid convective inertia, rotational drag torque, and torque from turbulent strain has been parameterized to model the time-averaged MPF gravitational settling velocity. The results show that the settling speed of MPF is primarily determined by its cross-sectional configuration with nonnegligible dependence on ambient turbulence for certain types of MPFs, which is different from what is based on the volumetric equivalent spherical particle model. Specifically, flat fibers have an averaged approximately 80% reduction in dry deposition rate and enhancement of dry deposition lifetime above 500% compared with round fibers with their lengths and widths sampled from field measurement.