Atmospheric transport dynamics of microplastic fibres

Microplastics have been identified in most terrestrial areas of Earth including rural, remote and isolated locations where the only likely source is through atmospheric transport and deposition.  To date there has been limited attention paid to the fundamentals of microplastic transport by wind, and in particular, the similarities and differences between the motion of mineral grains and microplastic particles within boundary layer flows.  These fundamentals are key to future modelling of mineral-microplastic interaction in the atmosphere.  This research examines the dynamics of microplastic entrainment and transport by wind, focusing on fibres which are one of the most common shapes associated with aeolian systems.  A series of particle tracking velocimetry (PTV) experiments was conducted in a boundary layer wind tunnel to determine how nylon fibres (4 mm length) travel through the air and interact with the ground surface. The high-speed camera images show that the silhouette area presented to the wind has a high degree of temporal variability for fibres, as compared to sedimentary particles, affecting the fluid drag (e.g. form versus skin friction), translational versus rotational energy, and lift.  The motion of plastic particles in the flow follows a variety of different patterns, including end-over-end cartwheeling and horizontal transport with the long-axis oriented flow parallel. The progression of an airborne plastic particle through different motion types (a "lifecycle") appears to be orderly, despite a wide variability in the length of time spent in each particular motion type. Travelling across a mobile sand bed, microplastic fibres are observed to dislodge and cause the ejection of sand particles suggesting they can contribute to the development of the saltation cloud and may have the potential to reduce the threshold velocity for sand transport.