Effects of Shape and Size on Microplastic Atmospheric Settling Velocity

Fall column experiments usinglaser Doppler anemometry showthat the terminal atmospheric settling velocity of microplastic particlesis strongly influenced by their size and shape. Microplastics (MPs) have been foundin all terrestrial,marine,and riparian environments, including remote regions. This impliesthat atmospheric transport is an important pathway when consideringMP sources and global budgets. However, limited empirical data existto aid in effective development and parameterization of MP atmospherictransport models. This study measured the atmospheric settling andhorizontal drift velocities of various sizes and shapes of MPs intwo specially designed settling columns using a laser Doppler anemometer.The settling velocities were generally lower than modeled values,while shape exerted the most significant influence on the rate ofsettling. Rather than conforming to well-established, power-law models,each class of MP exhibited a linear but different relationship betweenMP size and settling velocity, with markedly higher slopes for thespheres and cylinders as compared to the films and fibers. Shape alsohad a substantial influence on particle drift, with the fibers andfilms exhibiting the greatest horizontal motion, as suggestive oftheir changing orientation in response to particle interactions andfluid drag. As a consequence, microplastic particles identified withinatmospheric deposition samples collected at a single point may derivefrom entirely different sources representing a wide range in transportdistance.