Atmospheric transport of microplastic particles as a function of their size and shape

Investigation of the transport and distribution of atmospheric concentrations of microplastic (MP) particles is an important challenge, since MP may have a negative impact on human health and ecosystems. When considering particle shape, most of the atmospheric transport models assume only spherical particles, whereas MP particles cover a wide range of observed shapes. Non-spherical particles experience a larger drag in the atmosphere, which leads to a reduction of their settling velocity, hence longer atmospheric residence times. Here we study gravitational settling of one of the dominant microplastic shapes – fibers. To reduce the difference between model output and ground-based measurements, we have implemented a parameterization of the shape correction in the gravitational settling scheme of the Lagrangian transport model FLEXPART.

We have determined model sensitivity to the shape correction to explore its impact on particles transport for a range of scenarios.  This was done with a statistical comparison of 3D fields of mass concentration and deposition patterns of shape-corrected and non-corrected parameterization schemes. Using the model output, we quantified average horizontal transport distances and atmospheric residence times for spheres and fibers of different sizes and aspect ratios in different climatic regions and for different release heights of the MP particles.