Entrainment and horizontal atmospheric transport of microplastics from soil

Soils are an important source of microplastics (MPs) to the atmosphere but the fluxes and mechanisms involved in MPs entrainment are not well understood. In the present study, a series of horizontally aligned sediment traps have been deployed at different heights within 1 m above the ground for a two-month period at various locations in an arid region (Sarakhs, Iran). MPs were isolated from sediments and were quantified and characterised ac-cording to size, colour, shape and polymer composition by established techniques. Most MPs were <250 mu m in length, fibres were the most important shape, black and blue-green were the dominant colours, and polymer abundance decreased in the order polyethylene > nylon > polypropylene > polystyrene > polyethylene tere-phthalate. The distributions of sediment mass (range <0.01-9 g) and number of MPs (range = 0 to 21) were heterogeneous, both between sites and at the different heights sampled, and yielded median, vertically-averaged horizontal fluxes for the region of about 450 g m- 2 d-1 and 2600 MP m- 2 d-1, respectively. However, when data were pooled, the number of MPs normalised to sediment mass exhibited a significant inverse relationship with sediment mass, an effect attributed to the presence of ambient suspended MPs and sediment that are diluted by the suspension of soil and deposited MPs at higher wind speeds. The mechanisms of MP saltation and entrainment were not ascertained but a theoretical framework for threshold shear velocity based on regularly -shaped particles and density considerations is presented. Further experimental work is required to verify this framework, and in particular for fibrous MPs with different aerodynamic properties to soil particles.

Automated summary

Soils are a significant contributor of microplastics to the atmosphere, but the processes and mechanisms involved in the entrainment of microplastics are not fully understood. In this study, sediment traps were deployed at different heights above the ground in an arid region in Iran, and the isolated microplastics were characterized based on size, color, shape, and polymer composition. The majority of microplastics were less than 250 μm in length, with fibers being the most common shape and black and blue-green as the dominant colors. The distribution of sediment mass and the number of microplastics varied spatially and vertically, with median fluxes of approximately 450 g m-2 d-1 and 2600 MP m-2 d-1 for the region, respectively. The number of microplastics normalized to sediment mass showed an inverse relationship, possibly due to the dilution of ambient suspended microplastics and sediment by higher wind speeds. Although the mechanisms of microplastic saltation and entrainment were not determined in this study, a theoretical framework based on particle shape and density is presented. More experiments are needed to validate this framework, particularly for fibrous microplastics with different aerodynamic properties compared to soil particles.