Accumulation of airborne microplastics on leaves of different tree species in the urban environment

Microplastics (MPs) are omnipresent in the environment and they are linked to ecosystem and human health problems. The atmospheric transport of MPs and the role of tree leaves in MP atmospheric deposition has not been adequately studied. MP concentrations on the leaves of different tree species in urban regions of the Netherlands and Portugal, along with related MP deposition, were investigated in this study. We collected leaves from cedar, eucalyptus, oak, pine and willow trees, together with monthly deposition of particles under the trees and in the open space in Coimbra (Portugal). In Wageningen (the Netherlands), we collected leaves from a fir and a holly tree at different heights above the ground and with dry and wet weather conditions. MPs were extracted through density separation and quantified under a microscope. Polymer types were identified using μ-FTIR. The results showed a higher number of MP particles on the needle-shaped leaves from fir (2.52 ± 2.14 particles·cm^-2) and pine (0.5 ± 0.13 particles·cm^-2) and significantly lower numbers of MPs per cm² of leaf area on the bigger leaves from eucalyptus (0.038 ± 0.003 particles·cm^-2) and cedar (0.037 ± 0.002 particles·cm^-2). All tree leaves seemed to filter airborne MPs, especially the smallest particles. A non-significantly higher number of particles on leaves was detected on lower tree branches and after dry periods. The deposition of MPs under trees was generally higher than in the open space. Our results indicated that part of the MPs retained by the tree leaves floats down to lower branches and to the soil surface. We also saw that different tree species had different capacities to retain particles on their leaves over time. To control the transport of MPs through the atmosphere, it is essential to consider the role of different vegetation types in filtering small particles, especially in cities.