Effects of Urban Surface Roughness on Potential Sources of Microplastics in the Atmospheric Boundary Layer

Understanding the local transport of microplastics (MPs) emitted from the urban environment, such as those from vehicle tire wearing in streets and highways, is a necessary first step for quantifying their global transport cycle. By approximating microplastics as heavy particles, we conduct numerical simulations using large-eddy simulations (LESs) to understand how spatially organized sources and complex urban surface roughness affect their transport. Three sets of cases are considered, namely: (i) spatially uniform source and (ii) spatially organized source, and (iii) spatially organized source with explicitly resolved roughness elements, respectively. Results suggest that for a spatially organized source without buildings, source heterogeneity in streamwise direction only influences the vertical concentration profile up to $$z/L_z=0.11$$ . In contrast, that in spanwise direction influences the profile till $$z/L_z=0.58$$ , where z and $$L_z$$ are the vertical coordinate and the domain height, respectively. Simulations with buildings reveal that the buildings impede the transport of particles and particles accumulate on the leeward side of the buildings, which are characterized by wake turbulence and relatively quiescent flow. Within the canopy sub-layer, the gravitational settling effect of the particles becomes more significant because of the reduced wind. Because of that, the escape fraction of particles is smaller than in cases with no buildings and it decreases with increasing building height h and increasing building plan area fraction $$\lambda _p$$ . Finally, by finding suitable scalar displacement height $$d_s$$ and scalar roughness length $$z_{os}$$ , we find that similar to a passive scalar, an inertial sub-layer (ISL) still exists for heavy particles. This study highlights that for spatially organized particle sources, the momentum sinks due to urban roughness and the gravitational settling jointly affect the transport of heavy particles, which implies that the surface heterogeneity effect can be substantial in quantifying the atmospheric transport of microplastics of urban origins.