Vertical transport of buoyant particles under a free-surface boundary layer

Predicting the transport of buoyant particles in free-surface boundary layers is important to many environmental systems, including microplastics in the upper ocean. We study the vertical mixing of these particles with laboratory experiments of buoyant, finite-sized spheres, rods, and disks in a wind-driven free surface flow. We estimate the particle vertical diffusivity using four different methods and compare the results. From this comparison, we conclude that assuming a still-water rise velocity when interpreting the particle concentration profiles leads to an overestimation of the diffusivity. We then estimate an effective particle rise velocity and find that the flow tends to reduce the particle rise velocity relative to its still water velocity. The flow has both turbulence and waves, and we observe that both contribute to the dispersion of the particles. We also observe that the Schmidt number increases with increasing particle rise velocity in a way consistent with "crossing trajectories" effect. Overall, we find that the flow and particle characteristics can both affect the particle diffusivity and rise velocity in non-trivial ways. These effects are often overlooked in microplastic transport models which can introduce substantial errors.