Numerical investigation of microplastic dispersion in the water column

The issue, widely recognized by the scientific community, of microplastic marine contamination is addressed through a 3D Lagrangian dispersion model. Simulations are conducted in a domain comprising the Tyrrhenian Sea and over a ten-year period (2010-2019). Simulate particle displacements are due to the combined effect of mean currents, turbulence and, along the vertical direction, rising or settling velocity function of microplastic density. An innovative parameterization of vertical turbulent diffusivity is implemented in the model to consider the sub-grid turbulence occurring in the marine mixed layer which is not completely resolved by oceanographic models. By returning in output 3D maps of concentration and vertical profiles, the proposed model allows the investigation of the vertical distribution of microplastics along the water column as well as of accumulation zones. The results show that vertical turbulent diffusivity is an essential factor for the vertical dispersion of microplastics within the mixed layer. The influence of seasonality in particle dispersion is shown. Finally, it is observed that the settling or rising velocity assumes a key role in the accumulation of pollutants on the seafloor or surface, respectively.