Reduction of urban traffic-related particulate matter-leaf trait matters.

Particulate matter in European cities, especially in urban areas, is mainly from urban road traffic and constitutes a great threat to the health of inhabitants. Therefore, understanding of the role of common urban roadside plant plays for particulate matter (PM) reduction would have important meaning for the mitigation of urban air pollution. The leaf surface PM capturing capacity of 12 representative urban plants (five shrubs and seven trees) alongside one main street in Hanover Germany was explored in this study. Significant differences for the amount of captured PM (both PM10 and PM2.5) on the leaf surface were found between plant species. Leaves of Taxus baccata, Berberis thunbergii, Pinus nigra, and Quercus robur showed the highest capturing capacity, while leaves of Prunus laurocerasus and Carpinus betulus showed the lowest value. Leaf shape is an important factor which affects the capturing capacity of the tested plant species. Needle-leaved pants were relatively more effective than broad-leaved species. Besides, species with small leaf surface area (0-10 cm2) showed a notably higher capacity than species with large leaf surface area (over 100 cm2). Additionally, by the observation of leaf surface characteristics, plant species with rough and hairy leaf surface tended to be more effective than species with smooth leaf surface. More PM was found distributed on the upper leaf surface close to the midrib rather than on the surface area away from it. At last, no significant correlation was found between the leaf surface contact angle and its PM capturing capacity for the 12 tested plant species in September. In summary, leaf traits including leaf form, leaf shape, leaf surface area, leaf surface hydrophilicity and leaf surface characteristics all have notable effects on the PM capturing capacity of the roadside plant species. This study provides a scientific basis for roadside plants as an eco-friendly solution for the reduction of urban traffic-related PM and stresses the importance of leaf traits as criteria for the plant selection.