Analysis of particulate matter (PM) trapped by four different plant species in an urban forest: Quantification and characterization

Urban forests provide a series of ecosystem services, as recognized from the European 2030 Forestry Strategy; among them, they have the ability to capture particulate matter (PM) from the atmosphere, retaining it on leaves surface. This study analyses the efficiency in PM capturing of four broadleaved species: Celtis australis, Fraxinus ornus, Morus alba and Tilia cordata, set in an experimental urban forest in the city of Reggio Emilia (Italy). The airborne particles have been quantitatively and qualitatively analysed through SEM/x-EDS. Furthermore, the role played by leaf micromorphology in the efficiency of these four species to intercept PM has been assessed. Considering the total average number of particles trapped and collected by leaves for three years (from 2019 to 2021), F. ornus showed the highest capture-capacity (5.0 ± 3.1 × 103 particles/mm2), collecting 49 % more particles than Celtis australis, 86 % more than Tilia cordata and 135 % more than Morus alba. Qualitative analyses on PM have found that the majority (from 68 % to 74 %) of total particles show a diameter lower than 2,5 μm. Leaf micromorphology proved to be important, however it was no possible to identify a micromorphological trait more influential than others in determining the ability of the species to capture PM. The particles trapped by leaves were mainly rich in non-toxic or low-toxic elements, but some of them can be considered potentially dangerous for human health and the environment for the presence of Ni, Cr and Ba. The PCA highlights as the principal source of pollution in this study seems to be the erosion soil, followed by vehicle traffic. These results can give useful knowledge to understand what the contribution of different plants species in PMs retention ability is, also identified as a regulation ecosystem service.