The effects of leaf size and microroughness on the branch‐scale collection efficiency of ultrafine particles

Abstract

Key Points:• UFP collection efficiency ( ) for four broadleaf species was measured• Rougher surfaces enhance ϵ even for hydraulically smooth flow • Longer leaf dimension has a thicker quasi-laminar boundary layer and smaller ϵ Abstract Wind tunnel experiments were performed to explore how leaf size and leaf microroughness impact the collection efficiency of ultrafine particles (UFP) at the branch scale. A porous media model previously used to characterize UFP deposition onto conifers (Pinus taeda and Juniperus chinensis) was employed to interpret these wind tunnel measurements for four different broadleaf species (Ilex cornuta, Quercus alba, Magnolia grandiflora, and Lonicera fragrantissima) and three wind speed (0.3-0.9 m s −1 ) conditions. Among the four broadleaf species considered, Ilex cornuta with its partially folded shape and sharp edges was the most efficient at collecting UFP followed by the other three flat-shaped broadleaf species. The findings here suggest that a connection must exist between UFP collection and leaf dimension and roughness. This connection is shown to be primarily due to the thickness of a quasi-laminar boundary layer pinned to the leaf surface assuming the flow over a leaf resembles that of a flat plate. A scaling analysis that utilizes a three-sublayer depositional model for a flat plate of finite size and roughness embedded within the quasi-laminar boundary layer illustrates these connections. The analysis shows that a longer leaf dimension allows for thicker quasi-laminar boundary layers to develop. A thicker quasi-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. It is suggested that the effects of leaf microroughness are less relevant to the UFP collection efficiency than are the leaf dimensions for the four broadleaf species explored here.

Dry deposition of particles to canopies—A look back and the road forward

“…Wedding et al [1975] reported increases by a factor of 10 in deposition rates for particles to pubescent leaves compared to smooth, waxy leaves. This was confirmed by further wind tunnel studies reported by Wedding et al [1977] and in recent work by Huang et al [2015].…”

Plants and Atmospheric Aerosols

“…Approaches to describe aerosol deposition fluxes include big-leaf schemes in climate models, rough boundary layers used in air quality models, and multilayered models used in ecosystem studies (Feng 2008;Huang et al 2014Huang et al , 2015Katul et al 2010;Peters and Eiden 1992;Petroff et al 2008;Wesely and Hicks 2000). Deposition models require a general understanding of the processes involved, and credible results can be achieved when relevant environmental factors are sufficiently known.…”

“…Deposition may be enhanced by residual turbulent fluctuations very near the deposition surface ("near-wall" effects, Botto et al 2005;Guha 2008;Marchioli et al 2006;Parker et al 2008). These effects are poorly characterized and require further consideration to reconcile theory with the results of wind tunnel experiments and environmental loading of fine aerosol to leaves (Burkhardt et al 1995;Freer-Smith et al 2004;Huang et al 2015;Wiman 1986). …”

Designing vegetation barriers for urban air pollution abatement: a practical review for appropriate plant species selection

“…This finding is supported by results from similar studies, which also found that coniferous (needleleaf) species generally offer higher deposition velocities than broadleaf species 36,37,102,105,106 . Saebø et al 36 suggest that the long and narrow shape of needle leaves offer a thinner quasi-laminar boundary layer than that of broadleaves, which offers comparatively less resistance to deposition via a shortened diffusional path length 107 . Chen et al 105 concur that many conifers are generally more effective for PM accumulation and post-rainfall re-capture due to their acicular, needlelike shape.…”

“…However, Leonard et al 78 found that leaf shape does influence PM deposition, albeit to a lesser extent than leaf surface characteristics. Indeed, a number of studies have found that complex leaf shapes are generally more effective than simple leaf shapes 78,[89][90][91]107 . In an evaluation of leaf traits for PM deposition, Weerakkody et al 91 erected experimental rigs containing both synthetic and natural leaves alongside a busy road, including synthetic leaves of different shapes but with identical surface areas and surface characteristics.…”