Distribution of Plastics of Various Sizes and Densities in the Global Ocean From a 3D Eulerian Model

Abstract

We study the global transport and distribution of microplastics (MPs) using a three-dimensional Eulerian model. For the first time, the effects of both particle size and density are accounted for, influencing the vertical transport of MPs, and leading to an advanced understanding of their global distribution. Our simulations reveal two key findings: Only particles with low density and sufficiently large size (e.g., density 900 kg $m^{-3}$ and size $\gtrsim$10 $\mu$m) aggregate in the five subtropical gyres that were identified in previous studies. In contrast, sufficiently small particles ($\lesssim$1 $\mu$m), regardless of their density, behave like neutrally buoyant particles and can penetrate down to 1 km deep into the ocean. In addition, we observe a seasonal variation in the surface concentration of positively buoyant MPs—higher in summer and lower in winter—which reasonably agrees with the satellite observations made by the Cyclone Global Navigation Satellite System (CYGNSS) in terms of the phase of the variation. A quantitative analysis shows that the seasonal variation in the surface particle concentration correlates well globally with the variation in mixed layer depth. We attribute this correlation to the vertical stretching/squeezing effect of the seasonally varying mixed layer, where the total amount of positively buoyant particles is conserved.