Trait-mediated, size-dependent retention of particulate matter by urban roadside vegetation

Urban vegetation can act as an effective sink for airborne particulate matter (PM) in densely populated cities, yet how specific leaf functional traits regulate the retention of different PM size fractions remains poorly resolved. In particular, it is unclear whether fine and coarse particles are governed by similar trait mechanisms under realistic urban roadside conditions. Here, we examined 44 common roadside plant species along roads with contrasting traffic intensity and surrounding land-use characteristics in Nanjing, China. Foliar retention of multiple PM size fractions (total suspended particles, PM >10 μm, PM10-3 μm, and PM3_-1 μm) was quantified using a three-stage membrane filtration method, together with measurements of leaf functional traits and surface micromorphology. PM retention capacity differed significantly among species and life forms (p < 0.05). Retention of coarse particles was primarily associated with structural traits that enhance mechanical interception, whereas fine-particle retention was more strongly linked to micro-scale leaf traits, particularly stomatal width, indicating a clear size-dependent shift in trait importance. Although particle-size distributions varied among roadside environments, trait-PM relationships remained broadly consistent across sites. These findings demonstrate that particulate matter retention by urban vegetation is regulated by distinct, size-dependent trait mechanisms rather than by a uniform set of functional traits. By highlighting functional specialization in plant-PM interactions, the present study provides a trait-based framework for optimizing urban roadside vegetation design according to local pollution profiles and mitigation objectives.