Herbaceous and Grass Roadside Vegetation as Natural Bio-Filters: Quantitative Assessment of Particulate Matter Mitigation in Urban Atmospheres

Particulate matter (PM) is a major urban air pollutant with serious impacts on human health and ecosystem integrity. Vegetation functions as a natural bio-filter, yet the PM mitigation potential of low-growing herbaceous and grass species remains largely underexplored. This study provides a novel quantitative assessment of PM accumulation on the foliage of five roadside plant species, four herbaceous and one grass, at Kyushu University, Japan, during the summer season. PM deposition was measured gravimetrically on both adaxial and abaxial leaf surfaces and categorized into coarse (>10 µm), fine (2.5–10 µm), and ultrafine (0.2–2.5 µm) fractions. Herbaceous species captured substantially higher PM loads than the grass species (232.1 vs. 29.6 µg·cm⁻²; 88.6% vs. 11.3%), with Lysimachia nummularia achieving the highest individual accumulation (87.9 µg·cm⁻²; 33.5% of total PM). Fine particles dominated deposition (40.8%), while ultrafine particles were more efficiently captured by the grass species. Evergreen species accumulated nearly twice the PM of deciduous species (201.8 vs. 102.9 µg·cm⁻²), emphasizing their year-round mitigation potential. Leaf traits, including size, margin, shape, and surface roughness, showed limited influence compared with species identity, challenging common assumptions about leaf morphology in PM capture. These findings demonstrate the untapped potential of herbaceous roadside vegetation as effective bio-filters, providing a low-cost, nature-based solution to mitigate urban air pollution. The study offers quantitative evidence and actionable guidance for urban planners, environmental managers, and policymakers to optimize plant selection for sustainable air quality management, and expands the focus of green infrastructure strategies beyond trees and shrubs to include understudied herbaceous species.