The Effects of Leaf Size and Micro-Roughness on the Collection Efficiency of Ultrafine Particles (UFP)

Abstract:

While the significance of ultra-fine particle (UFP) deposition onto vegetated surfaces is rarely questioned, how to incorporate leaf attributes into global climate models continues to draw research attention. How leaf dimension and micro-roughness impact UFP collection efficiency is explored here through wind-tunnel experiments across a wide range of broadleaf species. Ilex cornuta with its partially folded shape and sharp edges is shown to be more efficient at collecting UFP than the other flat broadleaf species, but less efficient than the needle-like coniferous species. This finding suggests that UFP collection efficiency is linked to leaf attributes (i.e., dimension and micro-roughness). Analogies to flat-plate boundary layer theory are used to explain these findings, where the laminar boundary layer is assumed to be pinned to the leaf surface. For scaling arguments with maximum simplicity, the solid boundary is flat and possesses a finite dimension and micro-roughness. These simplifications allow explicit description of the area-averaged velocity and diffusivity profiles as a function of leaf dimension and micro-roughness. Further assuming the boundary behaves as hydraulically smooth (roughness elements do not protrude outside the viscous sub-layer), the analysis here shows that longer leaf dimension allows for thicker laminar boundary layers to develop. A thicker laminar boundary layer depth in turn increases the overall resistance to UFP deposition due to an increase in the diffusional path length thereby reducing the leaf-scale UFP collection efficiency. The mean velocity and diffusivity profiles over hydraulically smooth surfaces are not altered by the presence of micro-roughness elements. However, rougher surfaces in hydraulically smooth flow lead to shorter depositing distances (i.e., shorter diffusional path length) from the boundary, which enhances the particle deposition velocity. When the mean micro-roughness height is sufficiently larger than the size of UFP but still embedded within the viscous sub-layer, the dependence of the UFP collection efficiency on the size of UFP vanishes. The proposed flat-plate boundary layer analogy explains the observed features of UFP collection efficiencies onto leaves.