The dual role of roadside trees in shaping children’s PM exposure near schools

Children’s school commutes are repeated microenvironments where exposure to traffic-related particulate matter (PM) occurs at breathing height. Roadside trees are widely promoted to mitigate PM via deposition on foliage, yet dense tree crowns may also reduce near-road ventilation and increase pedestrian-level concentrations. We quantified these opposing effects along the most frequently used access routes to primary schools in Łódź (Poland). During June 2022, we measured PM2.5 and PM10 at 1 s resolution at 1.4 m height during morning and afternoon commuting periods (n = 242). Tree crown structure was characterised using two complementary indicators: horizontal crown density, quantified as leaf area index (LAI), representing potential particulate absorption surface, and vertical crown density, quantified as the proportion of the pedestrian-level field of view occupied by tree crowns, representing canopy obstruction and reduced ventilation. Foliar PM loads were determined by sequential leaf wash-off (n = 124). Mixed-effects models showed that higher vertical crown density was associated with higher airborne PM concentrations, with each one percentage-point increase corresponding to approximately 1.9% higher PM levels along routes. In contrast, higher horizontal crown density enhanced foliar retention: a 1% increase in LAI was associated with 0.52% and 0.56% higher foliar PM₁₀ and PM₂.₅, respectively, and species identity explained additional variation. Roadside trees, therefore, provide particulate deposition benefits but may simultaneously increase exposure at pedestrian height, highlighting the need for site-specific greening designs that explicitly account for both horizontal and vertical crown density.