Background/Question/Methods There is keen interest in increasing the amount of urban tree cover to improve human health and well-being, as evidenced by planting programs like MillionTreeNYC in New York City. Despite the resonance of improving ecosystem services by deliberate design, engineering forest stands to achieve specific objectives like filtering fine particulates (PM2.5) is elusive. Factors limiting rational design approaches include the absence of a solid theory for dry deposition kinetics, an emphasis on deposition instead of dispersion in service valuation models, imprecise accounting of  spatiotemporal scaling factors, a lack of empirical data to test models, confounding of local and regional pollution sources, and a lack of an appropriate metrics for human exposure and health outcomes. We conducted controlled wind tunnel experiments in which species, leaf morphology, canopy density and wind speed were varied to quantify deposition velocity of fine particulates to leaf surfaces in real time. We also performed field experiments comparing ambient  PM2.5 under deciduous and evergreen canopies with an open field during spring leafout and over short distance gradients from roadways to determine the effects of canopy cover on air quality in forested landscapes. Finally, we used immunoassays to determine the proinflammatory effect of particles collected in different landscape locations relative to roadside canopies.
Results/Conclusions

Wind tunnel experiments demonstrated interspecific differences in deposition to leaves but deposition was not inversely related to leaf size as often believed. Leaf area and leaf density had a negligible effect on deposition, suggesting that not all surfaces in a canopy capture PM2.5 equally. Further, the average residence time of particles in the airstream was lowest in the empty tunnel as opposed to a tunnel filled with leafy branches, suggesting that particles were entrained in eddies formed in the wakes of  leaves and branches.

Field experiments consistently showed lower PM2.5 counts in an open field than under adjacent deciduous or evergreen canopies. PM2.5 concentration along distance gradients from roads showed that 20-50m are needed to de-couple ambient PM2.5 from ambient traffic effects regardless of canopy density and structure, suggesting that narrow buffer strips have limited effectiveness.

Cytokines responsible for inflammation of the respiratory tract were induced more strongly by samples collected in urban parks than samples collected at curbside, suggesting that endotoxins and other biological substances are more concentrated under canopies. It may be harder to improve human health by increasing canopy cover than both scientists and policy makers would hope.