Modelling the Potential Long-Range Dispersion of Atmospheric Microplastics Reaching a Remote Site

The study of the microplastic dispersion in the atmosphere is a relatively new field of research. In the studies available in literature, it was suggested that long-range transport may play a main role in the motion of microplastics towards remote sites, where no significant local sources are present. In this framework, here the Lagrangian particle dispersion model MILORD was used in the backward mode to investigate the potential long-range dispersion of microplastics in the atmosphere and to assess possible source areas that may determine microplastic pollution in a remote site. The chosen case study refers to the work of Allen et al. (2019), who analysed the observations of atmospheric microplastics deposition in a remote and pristine site in the French Pyrenees, also through numerical backward trajectories. Based on their dataset, they showed that microplastics could arrive at the site from distances up to about 100 km. Yet, they considered that it was unlikely to have only microplastic emissions from local sources, because of the low population density in the area. The results of MILORD simulations for different periods showed that for this case study the dominant scale of the microplastics transport process is regional up to mesoscale, while long-range transport may contribute when the tracer is mostly travelling in the free atmosphere before being deposited at the site. The relative contributions of dry and wet depositions and corresponding rates were also assessed. It was confirmed that plastic particles characterised by the highest settling velocities, reasonably associated to large particles, are unlikely to be subjected to long-range transport.