Additional focus on particulate matter wash-off events from leaves is required: A review of studies of urban plants used to reduce airborne particulate matter pollution

Airborne particulate matter (PM) emissions are mainly comprised of dust and biomass produced by ground-level combustion and fossil fuel emissions. PM retention by plant leaves can reduce PM pollution from atmosphere, but in urban areas winds can cause PM resuspension, thereby preventing retention and worsening airborne pollution. Unlike winds, rainfall events can cause PM to be washed off leaves and onto the ground, which represents a net removal of PM from the atmosphere. This systematic review examines previous studies of leaf-PM interaction events involved PM retention, PM resuspension, or PM wash-off from leaves. Publication frequency of studies on using plants for airborne PM reduction in urban areas had grown over the past decade and we focused on 65 published papers in this review. Most of these studies were performed in Europe and East Asia, and involved PM retention on the leaves of varied urban trees in different time and space. In general, these studies indicated that rough leaves showed higher degree of PM retention than glabrous leaves. However, only six out of the 65 papers considered PM wash-off from leaves; as shown in these studies, smooth leaves may have a higher PM wash-off level than rough leaves. We conclude by recommending that future researches should be focused on studying leaf-retained PM wash-off in greater detail. We also suggest that urban plant species associated with a higher PM wash-off efficiency should be identified. Moreover, we may be able to increase PM net removal mass and thereby reduce winter haze by adding more evergreen plants with a higher leaf-retained PM wash-off efficacy. Finally, establishing a standard evaluation system for airborne PM reduction based on leaf-retained PM wash-off mass may be highly useful for landscape planning and design in the future.