Influence of settling and rising on the vertical distribution of microplastics in the marine environment

Microplastics (MPs), defined as plastic particles less than 5 mm in size, are ubiquitous in the marine environment. These microscopic particles, with a generally accepted lower limit of 20 μm, pose an environmental and human threat. MPs can absorb hazardous contaminants such as heavy metals and persistent organic pollutants (POPs) and are ingested by marine and terrestrial organisms, entering the food chain. The scientific community investigates the problem through in situ sampling, laboratory analysis and dispersion modelling. Among these, Lagrangian models are the most widely used to study MPs in the marine environment. In such models, particle displacement is determined by an average component derived from an oceanographic model in Eulerian form and a generated stochastic component that simulates turbulent diffusion. In three-dimensional models, with regard to the vertical component, it is essential to consider the terminal velocity of the MPs, which depends on the physical properties of the MPs, in particular their size, shape and density. This velocity is called settling or rising, depending on whether the polymer density is greater or less than the water density. Terminal velocity represents one of the main factors determining the vertical distribution of MPs. The initial goal of this work was to represent the full spectrum of MPs present in the marine environment to conduct Lagrangian simulations accounting for the variability in size, shape, and density of the polymers. The influence of settling/rising velocities on the vertical distribution was investigated through a three-dimensional Lagrangian model developed by the research group, with the final objective of validating the model through comparison with experimental sampling data. An original probability distribution of terminal settling and rising velocities was developed to represent different types of MPs within dispersion models. In the initial phase, a sensitivity analysis was conducted using a simplified two-dimensional Eulerian model, followed by the application of the Lagrangian model to a case study in the Tyrrhenian Sea. The Eulerian analysis showed that the vertical distribution is mainly influenced by terminal velocity and vertical turbulent diffusivity. Lagrangian simulations showed that considering only rising, settling, or neutral particles leads to markedly different vertical concentration profiles of MPs. Furthermore, the lower size limit of the particles used in the generation of settling/rising velocity distributions proved to be the most influential parameter on the vertical distribution. Increasing this limit results in higher terminal velocities and a transition from nearly uniform to increasingly exponential vertical profiles. The vertical profiles simulated with the Lagrangian model were validated through comparison with experimental data, using the same particle size range in the simulations as in the sampled data. This comparison revealed two different similarity laws, depending on the size range of the MPs considered.