Effects of spatial heterogeneity of leaf density and crown spacing of canopy patches on dry deposition rates

Trees have a large role in improving urban air quality, among other mechanisms, through dry deposition of scalars and aerosols on leaf surfaces. We tested the role of leaf density and canopy structure in modulating the rate of dry deposition. We simulated the interactions between a virtual forest patch and deposition rate of an arbitrary scalar using the Parallelized Large Eddy Simulation Model (PALM). Two canopy structures were considered: a homogenous canopy and canopy stripes. For each canopy stripe scenario, we considered thin, intermediate, and wide stripes, while the space between stripes equals the stripes' width. Four leaf area densities were considered for each case (LAI = 0.5, 1, 1.5, and 2). The results showed that denser canopies and canopy stripes experienced more total deposition, noting that stripes had a larger per leaf area deposition than homogeneous canopies. Our results can be explained by canopy-induced turbulence structures that couple the air within and above the canopy and lead to more effective leaf area where this coupling is stronger. We aggregate our results to the whole-patch scale and suggest a canopy-structure and leaf-area dependent correction to the canopy resistance parameter so to be used in coarse models that resolve dry deposition.

Automated summary

Denser canopies and canopy stripes lead to increased total deposition, with canopy stripes showing higher per leaf area deposition compared to homogeneous canopies. The canopy-induced turbulence structures enhance the interaction between air layers, improving the effectiveness of leaf area deposition.