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Normalized Settling Velocity Governs Short-Range Transport of Atmospheric Microplastics
Summary
Wind tunnel experiments showed that how fast a microplastic particle settles under gravity—its normalized settling velocity—is the single best predictor of how far it travels through the air before landing. This finding helps fill a major gap in atmospheric microplastic research by enabling better models of where airborne plastic particles deposit, which affects estimates of human inhalation exposure and ecosystem contamination.
After a decade of atmospheric microplastic (MP) research, there remain many knowledge gaps. In particular, there exist limited to no direct measurements of the distances traveled by MP particles during atmospheric transport. This study builds on a previous study that measured the settling velocity of various shapes and sizes of MPs. The settling velocity was found to have a distinct linear relationship for each type and shape of MP rather than one predictable relationship based on MP size and shape. In the current study, MP particles were released into the airflow from a common point within a boundary layer wind tunnel at three different wind speeds to measure the distance to which the particles were dispersed downwind. Similar to the previous study, a common relationship was not found between distance traveled and MP size and shape. However, a strong power relationship was determined between distance traveled and settling velocity when normalized by the friction velocity, which reflects the shear within the turbulent flow. This outcome stresses the importance of developing a database of MP settling velocity values that is accurate and comprehensive. The relation is suitable for use for all shapes and sizes of MP and windspeeds, providing a convenient simplification for MP dispersion modeling.