Deposition characteristics of soot and tire-wear particles on urban tree leaves

Abstract

This study aims to analyze the deposition characteristics of particles of major tree species in urban areas of Korea by conducting experiments. To simulate black carbon (BC) particles emitted by automobiles among particulate matter floating in the roadside air, soot particles are generated using a diffusion flame burner, whereas tire wear particles are generated by the mechanical friction between tires and asphalt specimen. The deposition characteristics of the particles of each tree species are compared using a deposition chamber, which was designed and manufactured for exposing the trees to the BC particles based on air flow analysis. The five types of tree species that were used for the experiments were Zelkova serrata, Metasequoia glyptostroboides, Pinus densiflora, Euonymus japonicus, and Spiraea prunifolia f. simpliciflora. The particle deposition velocity was used to determine and compare the particle deposition per unit area for the different species and was found to be highest in Spiraea prunifolia f. simpliciflora, followed by, in descending order, Zelkova serrata, Euonymus japonicus, Pinus densiflora, and Metasequoia glyptostroboides for both soot and tire wear particles. Furthermore, the particle deposition rate was used to determine and compare the degree of particle deposition in a space of equal volume, which was found to be highest in Pinus densiflora, followed by, in descending order, Metasequoia glyptostroboides, Zelkova serrata, Spiraea prunifolia f. simpliciflora, and Euonymus japonicus; the highest particle deposition rate of the tire wear particles was in Spiraea prunifolia f. simpliciflora, followed by, in descending order, Pinus densiflora, Euonymus japonicus, Zelkova serrata, and Metasequoia glyptostroboides. For broad-leaved trees, the degree of soot particle deposition on the front side differed from that on the back side. The particle deposition velocity and particle deposition rate varied according to the tree species due to surface roughness and geometrical features of the leaves and branches. The concentration of BC particles in the roadside air, which is produced by passing motor vehicles, can be reduced if the tree species that have high particle deposition rates are planted along the roadside.