Assessment of urban tree effectiveness for submicron soot aerosols reduction through combined deposition and resuspension experiments

Abstract

Urban forests can enhance air quality by capturing aerosol particles on leaf surfaces, but the actual reduction effectiveness depends on both deposition and subsequent resuspension. In the current study, experiments were carried out in a chamber to quantitatively compare deposition and wind-driven resuspension of soot aerosols among nine tree species commonly planted as street trees in the Republic of Korea. Spatial uniformity of both air velocity and particle number concentration was verified within the test section, enabling reliable inter-species comparisons under identical flow conditions. For all species, the deposition velocity showed a V-shaped dependence on particle diameter with a minimum near 100 nm. Deposition magnitudes, including the deposited particle number per unit leaf area, differed among species, indicating that leaf surface characteristics in addition to leaf area influence deposition performance. Resuspension increased with wind speed for all species and was most pronounced during the initial stage of exposure, followed by a rapid decline over time. Smaller particles were resuspended more readily than larger particles. To assess net particle retention, the remaining particle number per unit leaf area was defined as the deposited minus resuspended particle number normalized by the measured leaf area. This metric revealed that overall particulate reduction cannot be inferred from deposition alone. Ligustrum obtusifolium and Chionanthus retusus maintained relatively high remaining particle levels, whereas Liriodendron tulipifera and Camellia japonica showed low net retention. These findings provide an experimental basis for selecting tree species considering both deposition efficiency and resuspension sensitivity under varying wind conditions.