Experimental investigation of the fallout dynamics of microplastic fragments in wind tunnel: The BURNIA agenda

We present a methodology and the first tests to estimate the settling velocity of airborne microplastics based on wind tunnel experiments. A novel approach and original perspective are proposed, discussing in detail challenges and faced problems, both on the theoretical and experimental sides. Several experiments were performed, releasing fragments of different microplastic types, PET, PVC and LPDE. A statistical analysis was applied to the measurements and the values of the settling velocities were estimated to range between 0.1 and 0.2 ms${}^{-1}$, in agreement with most values found in the related literature. Based on the observed velocities, the applicability of the Stokes’ law, which is often used also for airborne microplastics, is then addressed, highlighting its potential limitations in the context of the microplastic dynamics in the atmosphere. We confirm that using the Stokes’ law may lead to a substantial overestimation of the settling velocity for the airborne microplastics. We also recommend to consider moving to the concept of ‘effective deposition velocity’, to account for the turbulent processes characterising the real atmosphere.