Study and assessment of the dispersion of microplastics in the atmosphere through numerical modelling

The novel topic of airborne microplastics (MPs) dispersion in the atmosphere has recently gained attention due to the detection of MPs in remote regions of the planet. The low anthropogenic activity in such areas suggests that long-range atmospheric transport may be responsible for these observations. To date, airborne MPs have been mainly studied through observational campaigns, while numerical modelling has only recently begun to play a role. This Ph.D. research investigates airborne MPs dispersion using a multi-strategy approach. The first involves the use of MILORD, a global-scale Lagrangian particle dispersion model, to simulate backward and forward trajectories and explore MPs’ aerodynamic behaviour, source regions, and exchanges between the Planetary Boundary Layer (PBL) and the Free Atmosphere (FA). The second focuses on extending the SPRAY model with a new resuspension module designed to simulate the reintroduction of deposited MPs into the atmosphere. The third approach involves wind tunnel experiments to characterize the settling velocity of three types of plastic fragments, providing empirical input for numerical models. MILORD simulations revealed that vertical exchanges between the PBL and FA typically occur around 200 km from the source, mostly after the first day of transport, indicating that local sources rarely inject particles directly into the FA. Additionally, air mass back-trajectories showed that marine regions, including seas and oceans, may significantly contribute to the global airborne MPs load. The newly developed resuspension module in SPRAY demonstrated that resuspension mainly affects areas within ~1–10 km of the source. It reduces ground deposition and increases near-surface pollutant concentrations. Finally, wind tunnel experiments confirmed that such facilities are suitable for studying MPs fallout and yielded settling velocity values of approximately 0.1 m/s for the tested fragments. Overall, this research advances the understanding of atmospheric MPs transport by integrating numerical modelling and experimental approaches. The results highlight the complexity of MPs dispersion and provide tools and data to support future investigations on the environmental fate of airborne plastics. This work contributes to a growing field and addresses key knowledge gaps in the atmospheric science of microplastic pollution.