Leaf Morphology Controls Particulate Matter Deposition on Urban Roadside Trees: Evidence from Size-Fractionated Gravimetric Analysis

Atmospheric particulate matter (PM) is a critical urban air pollutant with significant implications for environmental quality and human health. Urban vegetation can mitigate PM pollution through dry deposition on leaf surfaces; however, PM removal efficiency varies widely among plant species and is strongly influenced by leaf morphological and structural traits. This study investigates the role of leaf morphology in controlling PM accumulation on roadside trees under real urban conditions. Four native tree species with contrasting leaf morphologies were examined on an urban university campus in Fukuoka, Japan, including two broad-leaved species (Camellia oleifera and Zelkova serrata) and two needle-leaved coniferous species (Pinus thunbergii and Chamaecyparis obtuse). Particulate matter deposited on leaf surfaces was quantified using a size-resolved gravimetric method following sequential washing and filtration. PM was separated into coarse (10–100 μm), fine (2.5–10 μm), and ultra-fine (0.2–2.5 μm) fractions to assess differences in deposition patterns among species. Results revealed pronounced differences in PM accumulation between leaf types. Needle-leaved species accumulated substantially higher total PM loads (1913.8 μg·cm⁻²) compared with broad-leaved species (94.7 μg·cm⁻²). Fine particles constituted the dominant fraction of PM on coniferous species, whereas ultra-fine particles accounted for the largest proportion of PM on broad-leaved species. Among the studied trees, Chamaecyparis obtuse exhibited the highest PM accumulation, while Zelkova serrata showed the lowest. These findings demonstrate that leaf morphology, particularly needle-like geometry, surface roughness, and shoot structure, plays a decisive role in regulating PM deposition processes. The results provide quantitative evidence supporting species-specific selection of urban trees to enhance the effectiveness of green infrastructure as a nature-based solution for mitigating particulate air pollution.