Evidence shows that trees and vegetation in general have the capability of cleaning the air by filtering out air pollutants (Vos et al., 2013). Due to its relatively large surface area, plant canopy functions as a sink for particulate matter (Wei et al., 2017). Most plants have a large surface area, increasing the likelihood of deposition of airborne particles compared with the smooth, hard surfaces present in urban areas. The amount of deposited material depends among others on the vegetation surface area, the deposition rate, and the concentration of the pollutant. (Janhäll, 2015)
Vegetation serves also as an effective windbreak, as on the leeward side, due to a decrease in flow velocity, suspended particles are deposited and their overall concentration in the air is reduced (Raupach et al., 2001).
Foliage longevity was found to be one of the most important traits for the capture of air pollutants (Grote et al., 2016). Since evergreen species retain functional leaves throughout the year, in terms of deposition, coniferous species are preferable to deciduous species. In case of deciduous species those ones should be preferred that exhibit longer in-leaf seasons. (Barwise and Kumar, 2020)
The various leaf characteristics influence the efficiency of air pollutant capture. Small leaf size, complex leaf shape, and various micromorphological features, such as a hairy or waxy surface or surface ridges, were found to be beneficial traits for the capture of particulate matter (Weerakkody et al., 2018). Due to the needle-like shape of their leaves, conifers are generally more effective for the accumulation of particulates (Chen et al., 2017).
Stomata that are the small pores found on leaf surfaces that control gas exchange have a relevant role in pollutant capture, particularly in case of gaseous pollutants (Lawson et al., 2014). Air pollutant removal may be enhanced by the selection of species that have stomata with extensive opening periods, such as poplar and some oak species (Grote et al., 2016). Growing evidence indicates that plant leaves have a role in bioremediation of air pollutants, acting as biofilters. Plant leaves and leaf-associated microbes are able to capture air pollutants, and biodegrade or transform them into less or nontoxic molecules (Wei et al., 2017).
Not only the structure and other properties of the leaves are relevant for particle capture efficiency, but also the canopy shape and shoot structure. There is an indication that tree species with more complex shoot structure have higher particle capture efficiency. (Watanabe, 2015) A higher density of vegetation generally results in lower downwind concentrations of particulates and gaseous pollutants (Barwise and Kumar, 2020).