Particulate pollution capture by urban trees: effect of species and windspeed

Particulate pollution is a serious health problem throughout the world, exacerbating a wide range of respiratory and vascular illnesses in urban areas. The use of trees to reduce the effects of these pollutants has been addressed in the literature, but has rarely been quantified. The aim of the present study was to quantify the effectiveness of five tree species-pine (Pinus nigra var. maritima), cypress (x Cupressocyparis leylandii), maple (Acer campestre), whitebeam (Sorbus intermedia), poplar (Populus deltoides x trichocarpa 'Beaupre')-in capturing pollutant particles. This was achieved by exposing them to NaCl droplets of approximately 1 mum diameter at a range of windspeeds in two windtunnels. The deposition velocity (V-g) and particle trapping efficiency (C-p) were calculated from these exposures. In addition, a variable dependent on foliage structure [Stokes number (St)] was correlated with C-p to gauge the effect of tree morphology on particle capture. Maximum C-p values ranged from 2.8% for P. nigra, to 0.12% and 0.06% for P. trichocarpa x deltoides and A. campertre, respectively. The finer, more complex structure of the foliage of the two conifers (P. nigra and C. leylandii) explained their much greater effectiveness at capturing particles. The data presented here will be used to model the effectiveness of tree planting schemes in improving urban air quality by capturing pollutant particles.