1.5.2. EFFECTS OF THE TRAITS ON PM CAPTURE
Each studied trait can either help to enhance the impact of the leaf on air quality (pollutant capture), either have no impact or decrease the effect (Fig. 2).
EPIDERMIS TRAITS
Twenty-five publications concluded that the presence of hairs/trichomes on the leaf was beneficial to maximize the impact of the vegetation on PM capture (Fig. 2). From these studies, 18 were focused on trees and demonstrated that the presence of trichomes increased PM capture by leaves (Barima et al., 2014; Beckett et al., 2000 ; J. Chen et al., 2017; L. Chen et al., 2017; Chiam et al., 2019; Leonard et al., 2016; Little, 1977; Mitchell et al., 2010; Mo et al., 2015 ; Muhammad et al., 2019; Ram et al., 2014 ; Räsänen et al., 2013bø et al., 2012; Sgrigna et al., 2020; Sharma et al., 2020 Song et al., 2015 ; X. Zhang et al., 2021; Zhang et al., 2019). The positive relation between the presence of hairs/trichomes and the capacity to capture PM was confirmed for GW vegetation in six studies (Blanuša et al., 2020 ; Muhammad et al., 2019 ; Viecco et al., 2018 ; Weerakkody et al., 2017, 2018a, 2018b). As example, Weerakkody et al., (2018b) studied 20 GW plants and found that leaf hairs/trichomes showed a positive relationship with the density of PM10 present on the leaves.
In addition, ten studies demonstrated that, besides presence, higher density of leaf hairs/trichomes increase PM capture. That was concluded nine times about trees (L. Chen et al., 2017; Chiam et al., 2019; Mo et al., 2015; Muhammad et al., 2019; Ram et al., 2014; Räsänen et al., 2013bø et al., 2012; Sgrigna et al., 2020; X. Zhang et al., 2021) and confirmed by Muhammad et al., (2019) for GW climber plants having a trichome density more than 0.58 mm− 2. On the contrary, Perini et al., (2017) found that hairy leaves of four GW plants did not allow to collect more fine or ultrafine dust particles than non-hairy leaves. Few studies (two) quantified precisely the magnitude of leaf hairs/trichomes effect on PM capture. Leonard et al., (2016) showed that hair presence increased three times the PM capture by leaves, while Little, (1977) reported that hairy leaves are seven times more efficient.
Leaf roughness is related to more PM capture in 18 studies (Fig. 2) (Beckett et al., 2000 ; J. Chen et al., 2017; L. Chen et al., 2017; Little, 1977; Mitchell et al., 2010 ; Mo et al., 2015 ; Ram et al., 2014 ; Redondo-Bermúdez et al., 2021; Sgrigna et al., 2020; Sharma et al., 2020; Song et al., 2015 ; Speak et al., 2012; Viecco et al., 2018; Wang et al., 2013; Weber et al., 2014; Weerakkody et al., 2018a, 2018b; Zhang et al., 2019). Smooth leaves of trees were less effective in capturing PM than leaves with rough surfaces in ten experiences (Beckett et al., 2000; J. Chen et al., 2017; L. Chen et al., 2017; Chiam et al., 2019; Little, 1977; Mitchell et al., 2010; Mo et al., 2015; Ram et al., 2014; Sæbø et al., 2012; Song et al., 2015). That was confirmed in two GW plants experiences (Viecco et al., 2018; Weerakkody et al., 2018a). Roughness was defined by a combination of different morphological features among studies: ridges or grooves densities (Weerakkody et al., 2018b), coarseness of the epidermis (Zhang et al., 2019), furrowed areas (Song et al., 2015) or groove density (L. Chen et al., 2017). The relationship between roughness and PM capture may varied depending on the leave face (Weerakkody et al., 2018b; Zhang et al., 2019). In contrast, Perini et al., (2017) found that less rough leaves were more efficient, when studying four GW plants. Sæbø et al., (2012) did not find any relationship between leaves roughness and PM capture.
Influence of leaf wax on PM capture is controversial. In total, seven experiences found that leaf wax allowed to catch more PM, against four that found no relation or a negative one (Fig. 2). Three stated that the quantity of wax is related to PM accumulation on leaf trees (Barima et al., 2014; Mo et al., 2015bø et al., 2012). Perini et al., (2017) confirmed this positive relation between waxy leaves and particle deposition for four GW plant species. The two species comporting waxy-leaves in the study of Weerakkody et al., (2017) also captured high levels of PM. Two more studies draw the same conclusion about GW plant species (Redondo-Bermúdez et al., 2021; Weerakkody et al., 2018a). In contrast, Dzierżanowski et al., (2011) did not find any effect of wax presence, and the lowest rate of deposition occurred for leaves with waxy surfaces in Leonard et al., (2016), Mitchell et al., (2010) and Wang et al., (2013).
The role of stomata in PM deposition is unclear, as noticed by Räsänen et al., (2013). An increasing stomatal density was related to increase PM capture in five papers, but four papers stated the opposite. As far as trees are concerned, four articles found a positive relationship between an increasing stomatal density and PM capture (J. Chen et al., 2017; Ram et al., 2014; Sgrigna et al., 2020; Song et al., 2015), and three articles did not (L. Chen et al., 2017; Muhammad et al., 2019sänen et al., 2013). For GW, adaxial faces caught more PM2.5 and PM10 when the stomatal density was higher (Weerakkody et al., 2018b). The same conclusion was drawn about the PM10 on the abaxial surface. On the contrary, Muhammad et al., (2019) and Redondo-Bermúdez et al., (2021) did not find any relation between the density of stomata on GW plants and the capacity of leaves to capture PM.
LEAF SURFACES AND SHAPE TRAITS
Leave size was reported to influence PM capture with small leaves reported to increase PM capture in the majority of studies (8/12) and large leaves unanimously reported to be less efficient in PM capture (6 studies) (Fig. 2), while comparing small, medium and large leaves. Small leaves were defined by a surface inferior to 200 mm² (Viecco et al., 2018; Weerakkody et al., 2018a, 2018b) and large leaves by a surface between 1280 et 6990 mm2 (Weerakkody et al., 2017, 2018a, 2018b).
Four studies on trees found that smallest leaved plants were particularly good at immobilizing PM (Beckett et al., 2000; Chiam et al., 2019; Leonard et al., 2016sänen et al., 2013). In contrast, three studies did not find any correlation between trees leaf size and PM accumulation on the leaves surfaces (J. Chen et al., 2017; Muhammad et al., 2019bø et al., 2012). However, six studies about trees concluded that broad leaves were not efficient in the PM accumulation (Beckett et al., 2000; J. Chen et al., 2017; Freer-Smith et al., 2005; Leonard et al., 2016; Muhammad et al., 2019bø et al., 2012).
Studies on GW indicated that the smaller the leaf, the more particles it captured (Viecco et al., 2018; Weerakkody et al., 2017, 2018a, 2018b), except for the creepers in Muhammad et al., (2019). Four studies stated a lower PM capture for large leaves (Muhammad et al., 2019; Weerakkody et al., 2017, 2018a, 2018b). According to Leonard et al., (2016), large leaf could both increase and decrease PM deposition because it provides a greater surface to catch PM, but it also increases leaf movement that can lead to PM dislodgement.
Not enough publications were found about leaves shapes to make solid claims, except for the case of needles. Four studies stated that trees with needles were more efficient in capturing PM (Beckett et al., 2000; L. Chen et al., 2017; Freer-Smith et al., 2005sänen et al., 2013). As examples, coniferous scots pine collected the largest number of particles in the study of Räsänen et al. (2013) on four trees. That was confirmed for Juniperus chinensis L. present on a GW (Weerakkody et al., 2018b).
In addition, three studies on trees stated that lanceolate leaves (broadest below the middle) were more efficient to capture PM (L. Chen et al., 2017; Leonard et al., 2016; Muhammad et al., 2019). That was also to case for two climbers in Muhammad et al., (2019).