Researchers in the US have shown that the shape of microplastic fibres allows them to travel further in the air than spherical beads. In a new study, the team at Cornell University and Utah State University modelled turbulent airflow around microplastic particles and found that the range of these pollutants in the atmosphere is highly sensitive to their shape. Working backwards from atmospheric models and field observations, their results suggest that the ocean is a larger source of microplastics than previous models have shown.
Microplastic particles released by industrial processes and the degradation of objects like bottles have been found in almost every part of the ocean, including the deep sea. Recently, microplastics have also been found on land in supposedly pristine environments including the French Pyrenees mountains. However, compared to the ocean, the transport of microplastics in the air has not been studied extensively. Whilst the impacts are not fully known, there is concern that the accumulation of microplastics could disrupt soil and plant processes and act as a vector for harmful chemicals.
This project was led by Shuolin Xiao, a postdoc in Qi Li’s group at Cornell University. Xiao and his colleagues wanted to know how the shape and size of microplastic particles affect their atmospheric transport across the globe. Xiao chose this problem because microplastics are long fibres, but current approaches model them as spheres. “It imposes both theoretical and modelling challenges to track these on a large scale,” says Xiao.
Turbulence enhanced transport
As well as the breakdown of consumer products, microplastics can enter the atmosphere from roads and industrial processes. It has also been suggested that wind, waves and sea spray at the ocean surface may transfer microplastics to the atmosphere.
How quickly a particle falls out of the air depends on the balance of aerodynamic and gravitational forces. Fluid flow around slender objects like microplastic fibres has been widely studied, but the turbulence of the atmosphere poses an additional challenge. Turbulent flow exerts torques on the fibre, so its orientation, and therefore its sedimentation velocity, changes constantly. The interplay between the turbulent forces and the inertia of the plastic fibre determines how much it rotates. By implementing torque into the fluid flow model, the researchers developed a prediction for how long a given microplastic fibre would remain in the air.
The model found that microplastic fibres stayed in the air longer than spherical particles of the same volume. In addition, flat fibres fell to the ground up to four and a half times more slowly than round fibres. When a fibre is very thin, it is difficult to accurately determine the cross-sectional shape, and the researchers highlight that this could introduce significant errors to models of atmospheric transport.
Microplastics are turning up everywhere
The researchers combined their results with large-scale modelling and measurements to understand how microplastics can be transported to remote areas. Field data were taken in protected areas in the US. In each place, the size, shape, and deposition rate of microplastics were measured. Sources of microplastics were identified using data on wind, sea spray, soil moisture and land use. This information, and the shape-dependent settling, were added to an existing model of atmospheric air circulation. This was fit to the observational data, resulting in a prediction of which sources contribute the most to the large-scale transport of airborne microplastics.
The research suggests that most of the microfibres in the collected samples came from the ocean. Though there are uncertainties in the model, this contrasts with a previous study that assumed spherical particles and identified roads as the largest contributor.
This work shows that even with sophisticated climate models, theories of the atmospheric transport of microplastics require an accurate treatment of microscale processes. Li says that she hopes that the role of the atmosphere in the life cycle of plastics will be further investigated. “We think that the ocean is the ultimate sink. But maybe they are in the air, they’re everywhere.”
The research is described in Nature Geoscience.
