Is transport of microplastics different from that of mineral dust? Results from idealized wind tunnel studies

Abstract. Atmospheric transport disperses microplastic particulate matter to virtually every environment on the planet. Despite the well-known long-range transport, only few studies have examined the fundamental transport mechanisms for microplastics and contrasted it with the existing body of knowledge accumulated for mineral dust over the past decades. Our study addresses this research gap and presents results from wind tunnel experiments, which examine the detachment behavior of microplastics ranging from 38 to 125 µm in diameter from idealized substrates. We here define 'detachment' as microspheres detaching from a substrate and leaving the field of observation, which includes several transport modes including creeping, rolling, directly lifting off. The detachment behavior of polyethylene microspheres (PE69) and borosilicate microspheres (GL69) of nominally the same physical diameter (63–75 µm) are contrasted across hydrophilic to hydrophobic substrates. We further examine the effect of microsphere-microsphere collisions on the detachment behavior of both polyethylene and borosilicate microspheres. Differentiating between collision independent microspheres and collisions dependent microspheres revealed that collisions impact detachment from enhancing to mitigating. Further, results indicate that GL69, as a hydrophilic particle, is highly dependent on substrate hydrophobicity and PE69 is less affected by it. A more detailed comparison between GL69 and PE69 regarding surface and substrate hydrophobicity is masked by the influence of capillary forces. Moreover, the smallest polyethylene microspheres behave similar to mineral microspheres. Results demonstrate that PE69 and GL69 as proxy for plastic and mineral dust, respectively, detach at u* between 0.1 to 0.3 ms-1 fitting to the prediction of the simple wind erosion model by Shao et al. (2000). In the observed range of rH, capillary forces can increase the median detachment by about 0.2 ms-1 for PE69 and GL69. Polyethylene microspheres, smaller than 70 µm in diameter, behave like borosilicate microspheres of the same size. For bigger microspheres, the lesser density of polyethylene drives their higher erodibility. We conclude that it is no surprise, that like mineral dust, plastic dust is found all around the globe, transported via the atmosphere.