Differential foliar particulate matter retention in Urumqi park trees: influence of canopy structure and spatial pattern

Urban trees intercept atmospheric particulate matter (PM), but the extent to which canopy structure and within-canopy position shape retention across particle-size fractions remains poorly resolved. We sampled 27 trees of Quercus robur , Ulmus densa , and Rhus typhina in Urumqi People’s Park. For each species, three trees were selected in each of three canopy structure classes (intact, moderate, and poor), and leaves were collected from 12 positions spanning three heights and four orientations on a single date. Retained particles were separated into PM 0.2–3 , PM 3–10 , PM >10 , and PM total fractions. Foliar PM retention increased with canopy structure, following the order intact > moderate > poor. Canopy height exerted a highly significant effect on PM retention ( P < 0.01), with the vertical gradient ranked as lower canopy > middle canopy > upper canopy; maximum-to-minimum differences across canopy positions ranged from 1.4- to 2.9-fold. Orientation effects were generally weak and exhibited no consistent directional pattern. Canopy structure had a highly significant effect on coarse-particle retention (PM 3–10 ; P < 0.01), whereas canopy height significantly affected all particle-size fractions ( P < 0.05). Orientation effects were particle-size-specific: a significant effect was detected only for the PM 3–10 fraction ( P < 0.05), while no significant directional pattern emerged for other size fractions. The interaction among canopy structure, canopy height, and orientation was not significant ( P > 0.05). Total leaf area and leaf area index were the structural factors most strongly correlated with foliar PM load, and intact-crowned trees were associated with the greatest foliar PM load. These findings suggest that urban green-space planning should prioritize tree species with intact, multi-layered canopies and adopt orientation-specific planting arrangements to maximize PM 3–10 retention, thereby strengthening the air-purification function of park forests in arid cities.