Planting design for urban overpasses based on atmospheric particulate matter retention in Changsha.

Atmospheric particulate matter (PM) emission from overpasses is serious. To optimize the solution of planting design in the overpass, it is essential to understand the plant's ability to capture PM. In this study, leaf samples were collected from 11 plant species commonly existing in five overpass greening areas in Changsha, China. The PM retention per unit leaf area (M_leaf), PM retention on a unit greening land (M_land), and leaf surface microstructure were measured and analyzed. Results showed that the M_leaf of Ophiopogon japonicus (1.59 g/m²) exhibited the highest value, and that of Nandina domestica (0.23 g/m²) was the lowest value. The M_land of O. japonicus, Fatsia japonica, and Magnolia grandiflora was the highest based on the leaf area index. Leaves with wide gullies and cuticular wax significantly affected the PM retention ability of plants. Plant height played an important role in the PM retention ability in the overpass. Multilayered canopy structures such as arbor-shrub-herb had great potential for PM retention in overpasses. Based on the results, a PM retention design scheme of overpass greening space was proposed. This study provided an optimal solution for providing critical insights and guidance for developing effective PM reduction strategies in urban overpass environments.