Additional focus on particulate matter wash-off events from leaves is required: A review of studies of urban plants used to reduce airborne particulate matter pollution

Urban Forestry & Urban Greening 48 (2020) 126559

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Additional focus on particulate matter wash-off events from leaves is required: A review of studies of urban plants used to reduce airborne particulate matter pollution

T

Xiaodan Xua, Jingjing Xiaa, Ya Gaob, Wei Zhengb,* a b

Department of Environmental Art, Faculty of Art and Communication, Kunming University of Science and Technology, Kunming 650500, China Department of Landscape Architecture, Faculty of Architecture and Urban Planning, Kunming University of Science and Technology, Kunming 650500, China

ARTICLE INFO

ABSTRACT

Handling Editor: A. Alessio Fini

Airborne particulate matter (PM) emissions are mainly comprised of dust and biomass produced by ground-level combustion and fossil fuel emissions. PM retention by plant leaves can reduce PM pollution from atmosphere, but in urban areas winds can cause PM resuspension, thereby preventing retention and worsening airborne pollution. Unlike winds, rainfall events can cause PM to be washed off leaves and onto the ground, which represents a net removal of PM from the atmosphere. This systematic review examines previous studies of leafPM interaction events involved PM retention, PM resuspension, or PM wash-off from leaves. Publication frequency of studies on using plants for airborne PM reduction in urban areas had grown over the past decade and we focused on 65 published papers in this review. Most of these studies were performed in Europe and East Asia, and involved PM retention on the leaves of varied urban trees in different time and space. In general, these studies indicated that rough leaves showed higher degree of PM retention than glabrous leaves. However, only six out of the 65 papers considered PM wash-off from leaves; as shown in these studies, smooth leaves may have a higher PM wash-off level than rough leaves. We conclude by recommending that future researches should be focused on studying leaf-retained PM wash-off in greater detail. We also suggest that urban plant species associated with a higher PM wash-off efficiency should be identified. Moreover, we may be able to increase PM net removal mass and thereby reduce winter haze by adding more evergreen plants with a higher leaf-retained PM wash-off efficacy. Finally, establishing a standard evaluation system for airborne PM reduction based on leafretained PM wash-off mass may be highly useful for landscape planning and design in the future.

Keywords: Airborne particulate matter Leaves PM2.5 PM retention PM net removal PM resuspension

1. Introduction Urban pollution caused by airborne particulate matter (PM, i.e. particles 0.001–100 μm in size) (Pope and Dockery, 2006) is one of the most urgent environmental problems in the world, especially in developing countries such as China and India (Huang et al., 2014). PM where 50 % of all PM are less than 10 μm in aerodynamic diameter (PM10) can enter airways and areas in the upper lung (Schwarze et al., 2006). Exposure to PM10 is related to the development of cardiovascular and respiratory diseases (Maher et al., 2013). It has been reported that air pollution leads to more than one million premature deaths and one million prenatal deaths each year (Curtis et al., 2006). Particulate pollution threats human health and urban plants could reduce the airborne PM in the ecosystem (Cavanagh et al., 2009). One study reported that the PM10 concentration outside urbanized forest areas is significantly higher than that of urbanized forest areas ⁎

(Cavanagh et al., 2009). Plants can also affect small airborne PM. In one study, a significantly lower concentration of PM2.5 was found in residential areas containing planting bands as green buffers (Suyeon et al., 2017). Recent studies have suggested that increasing tree cover of the West Midlands (UK) to a theoretical maximum of 54 % by planting all available green space would reduce the average PM10 concentration by 26 % (Mcdonald et al., 2007). Further studies have attempted to quantify the degree of PM removal from the atmosphere. In the greenbelt areas along highways, the foliar dust retention of Acacia confusa reached 564.90 g per plant (Wang, 2011). A study in Beijing, China found that the number of PM on plant leaves was up to 50,961.50 mm−2 (Shi et al., 2017). Another study in Beijing showed that the roadside leaves could remove PM at a mean rate of 7.50 g m−2 in autumn (Liu et al., 2017). Furthermore, the total amount of airborne PM removed by four tree species in Guangzhou, China was estimated to be 8012.89 t per year (Liu et al., 2013).

Corresponding author. E-mail address: [email protected] (W. Zheng).

https://doi.org/10.1016/j.ufug.2019.126559 Received 28 June 2019; Received in revised form 4 December 2019; Accepted 5 December 2019 Available online 06 December 2019 1618-8667/ © 2019 Published by Elsevier GmbH.

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Fig. 1. The cycle of airborne PM. A, airborne PM is comprised of dust from ground activities, biomass from ground-level combustion, and the emissions originated from underground fossil-fuel; B, Urban vegetation as well as urban buildings intercept and retain the airborne PM; C, PM captured by plants as well as building surfaces resuspended by winds, or washed off by rainfall; D, PM back to the ground and deposited into the soil.

1.1. The cycle of airborne PM

which jointly involve airborne PM retention on leaves, PM resuspension, or PM wash-off from leaves; (2) examine the methods used to analyze how urban plants reduce airborne PM accumulation; and (3) discuss the urban greening strategies for airborne PM reduction by plants, as well as future study priorities.

The main potential sources of PM pollution in urban areas are motor vehicles and industry (Barima et al., 2014). Roadside PM is derived from vehicle exhaust, tire wear particles, and re-suspended roadside soil (Leonard et al., 2016). Minor constituents, including Pb, Zn, Ni, etc., have been found in airborne PM from soot and dust (Tomašević et al., 2005). A study revealed that SiO2, CaCO3, and CaMg(CO3)2 in the airborne PM in Beijing mainly came from soil resuspension (Wang et al., 2006). In general, airborne PM is comprised of dust and biomass produced by ground-level combustion while secondary aerosol precursor emissions originated from underground fossil fuel sources (Fig. 1A). Urban vegetation as well as urban buildings improve air quality by intercepting and retaining airborne PM (Fig. 1B). A few retained PM on leaves can encapsulate into the cuticle (Ould-Dada and Baghini, 2001). However, most airborne PM captured by plants is only stored temporarily on leaf surface and resuspended in the air by winds, where it becomes airborne pollutant again (Schaubroeck et al., 2014). Unlike winds, rainfall events can remove retained PM on leaf surface through a process called wash-off (Fig. 1C); wash-off carries PM to the ground and deposits it into the soil (Ould-Dada and Baghini, 2001; Fig. 1D), which represents the net removal of PM from the atmosphere by urban plants (Schaubroeck et al., 2014). However, we still have no systematic understanding of how to use urban plants to reduce airborne PM more effectively in the above mentioned cycle. Therefore, we reviewed the literature on urban plants for airborne PM reduction airborne PM to: (1) analyze recently studies

2. Methods 2.1. Literature screening Our systematic review followed the procedures recommended in the PRISMA statement (Moher et al., 2009). Literature searching was performed by using the database of Web of Science. The literature was screened using the following phrases: “PM retention” or “PM deposition” or “PM accumulation” or “PM detention” or “PM capture” or “PM retain” or “PM wash-off” or “PM removal” or “PM resuspension” and “plants” or “trees” or “shrubs” or “herbs” or “leaves”. Papers meeting the following criteria were chosen: (1) original research article published in an English-language scientific journal with peer-reviewed; and (2) including assessments of the efficacy of one or more urban plants used to reduce airborne PM. 2.2. Data analysis Information extracted from the selected papers included: (1) study location; (2) citation details; (3) urban plant species used; (4) statistical methods used to quantify PM; (5) leaf-PM interaction events involved

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Table 1 Information extracted from the 65 selected papers on urban plants for airborne PM reduction. Type of plants

Places

Climatic zones

Leaf-PM interaction events

References

Shrub Tree Tree, shrub Tree Tree Tree Tree Tree, shrub Shrub Tree Tree, shrub Living wall plants Tree, shrub Tree Tree, shrub, herb Tree Living wall plants Wetland plants Tree, shrub, herb Tree Tree Tree Tree, shrub Tree, shrub, herb, living wall plants Tree Tree Tree Trees, shrub Living wall plants Synthetic leaves Tree Green roof plants Tree Tree Tree Living wall plants Tree, herb Tree Tree Tree, shrub, herb Tree, shrub, herb Tree, shrub, herb Tree Tree Tree Tree Tree Tree, Tree Tree, shrub Tree Trees, shrub Trees Tree Tree Tree, shrub Tree, shrub Tree Tree Tree Tree Tree Tree Tree Tree, shrub, living wall plants

Tuscany, Italy Beijing, China Stavanger, Norway Xian, China Beijing, China Beijing, China Sydney, Australia Beijing, China Beijing, China Beijing, China Santiago, Chile England, UK Beijing, China Stavanger, Norway Beijing, China Beijing, China Stoke–on–Trent, UK Beijing, China Beijing, China Beijing, China Beijing, China Siena, Italy Stavanger, Norway and Ciechanow,Poland Wuhan, China Xinjiang, China Guangzhou, China Xian, China Taiwan, China England, UK Staffordshire, UK Chongqing and Beijing, China England, UK Gujarat, India Brighton, UK Ghent, Belgium Bergen op Zoom, Netherlands Abidjan, Cote d'lvoire São Paulo, Brazilian Belgrade, Serbia Beijing, China Lucknow, India Wuhan, China Como, Italy Miskolc, Hungary West Midlands, UK Mizoram, India Antwerp, Belgium Beijing, China Terni, Italy Beijing, China Shanghai, China Sambalpur, India Kuopio, Finland Nanjing, China Beijing, China Beijing, China Beijing, China Guangzhou, China Flanders, Belgium Southampton, UK Pechcin, Poland The West Midlands and Glasgow, UK Berkshire, UK Seoul, Korea Warsaw, Poland

Cfb Dwa Dfc Dwa Dwa Dwa Cfb Dwa Dwa Dwa Dwb Cfb Dwa Dfc Dwa Dwa Cfb Dwa Dwa Dwa Dwa Csa Dfc、Cfb Cfa Bwk Cwa Dwa Cfa Cfb Cfb Cwa、Dwa Cfb Bsh Cfb Cfb Cfb Aw Cfa Cfb Dwa Cwa Cfa Cfb Cfb Cfb Cwa Cfb Dwa Csc Dwa Cfa Aw Dfc Cfa Dwa Dwa Dwa Cwa Cfb Cfb Cfb Cfb Cfb Dwa Cfb

Retention Retention Retention, wash-off Retention, wash-off, resuspension Retention Retention Retention Wash-off Retention Retention, wash-off, resuspension Retention Retention Retention Retention Retention Retention Retention, wash-off Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention Retention, resuspension Retention Retention Retention Retention Retention Retention Retention Retention, wash-off, resuspension Retention Retention Retention Resuspension Retention Retention

(Mori et al., 2018) (Lei et al., 2015) (Przybysz et al., 2014) (Wang et al., 2015) (Xu et al., 2018) (Liu et al., 2018) (Leonard et al., 2016) (Xu et al., 2017) (Zhang et al., 2017a) (Chen et al., 2017) (Guerrero-Leiva et al., 2016) (Weerakkody et al., 2017) (Shi et al., 2017) (Mori et al., 2015) (Chen et al., 2016) (Zhang et al., 2018) (Weerakkody et al., 2018c) (Yan et al., 2018) (Cai et al., 2017) (Yan et al., 2016) (Lin et al., 2017) (Blanusa et al., 2015) (Sæbø et al., 2012) (Chen et al., 2015b) (Aliya et al., 2015) (Liu et al., 2013) (Wang et al., 2013) (Wang, 2011) (Weerakkody et al., 2018b) (Weerakkody et al., 2018a) (Liang et al., 2016) (Speak et al., 2012) (Chaudhary and Rathore, 2018) (Beckett et al., 2000b) (Hofman et al., 2013) (Ottelé et al., 2010) (Barima et al., 2014) (Zampieri et al., 2013) (Tomašević et al., 2005) (Yu, 2014) (Pal et al., 2002) (Chen et al., 2015a) (Terzaghi et al., 2013) (Simon et al., 2016) (Power et al., 2009) (Rai et al., 2014) (Hofman et al., 2014) (Song et al., 2015) (Sgrigna et al., 2015) (Wang et al., 2006) (Xie et al., 2018) (Prusty et al., 2005) (Räsänen et al., 2013) (Zha et al., 2018) (Zhang et al., 2017b) (Nguyen et al., 2015) (Liu et al., 2017) (Liu et al., 2012) (Schaubroeck et al., 2014) (Beckett et al., 2000a) (Popek et al., 2012) (McDonald et al., 2007) (Ould-Dada and Baghini, 2001) (Hwang et al., 2011) (Dzierzanowski et al., 2011)

retention/resuspension/wash-off; (6) research results and conclusions. Annual variation in the publication numbers were calculated by the number of papers produced in each year according to citation details. Study locations were assigned to a Köppen-Geiger climate zone based on the closest city (Peel et al., 2007). Urban plants were divided into regular

greening plants (i.e. trees, shrubs, and herbs), living wall plants, wetland plants, and green roof plants. The percentage of papers involving the urban plant leaf PM retention, resuspension, and wash-off were calculated respectively.

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Fig. 2. The number of research papers describing urban plants used for airborne PM reduction published from 2000 to 2018.

3. Results

Table 2 The Köppen-Geiger climate zone of study locations within the 65 papers on airborne PM reduction using urban plants. 63 studies included locations in one climate zone and two studies included locations in two climate zones.

3.1. Geographical scope and classification A total of 65 original research papers published between 2000 and 2018 were screened (Table 1). In the past ten years, the number of publications on reducing airborne PM using urban plants increased each year, with one paper being published in 2009 and 11 in 2018 (Fig. 2), suggesting the continuous grow of this field. Most of the 65 studies on urban plants used to reduce airborne PM were performed in Europe and East Asia (Fig. 3), mainly in China (30 papers). 63 studies included locations in one climate zone and two studies included locations in two climate zones (Tables 1 and 2). More than ten studies were carried out in humid subtropical zones, maritime temperate (oceanic) climate zones, and humid continental climate zones (Table 2), only one study was carried out in the desert, and semi-arid climate zone. The plant species assessed were mostly urban trees (in 56 of 65 papers), followed by shrubs, herbs and living wall plants (Table 3).

Köppen-Geiger climate zones

Letter code of climatic zones

No. of papers

Tropical rainforest Tropical monsoonal Tropical savanna Desert Semi-arid Mediterranean Humid subtropical Maritime temperate (oceanic) Humid continental Subarctic Tundra Ice Cap

Af Am Aw、As Awh、Bwk Bsh、Bsk Csa、Csb、Csc Cwa、Cfa Cwb、Cwc、Cfb、Cfc

0 0 2 1 1 2 11 22

Dsa、Dsb、Dwa、Dwb、Dfa、Dfb Dsc、Dsd、Dwc、Dwd、Dfc、Dfd Et Ef

24 4 0 0

Fig. 3. The geographical distribution of study locations from the 65 research papers describing urban plants used for airborne PM reduction. Points indicate study locations. Shading represents the number of research papers per country. 4

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Wang et al., 2006; Dzierzanowski et al., 2011; Popek et al., 2012; Liu et al., 2013; Chen et al., 2015b; Guerrero-Leiva et al., 2016; Weerakkody et al., 2018b), even when different species were studied in the same location (Liu et al., 2012). In general, broad-leaved species with rough leaf surface (such as hairs, trichome, wax, furrowed areas, and surface ridges) accumulate more PM than leaves with smooth surface (Beckett et al., 2000a; Wang et al., 2006; Hwang et al., 2011; Sæbø et al., 2012; Wang et al., 2013; Lei et al., 2015; Song et al., 2015; Leonard et al., 2016; Weerakkody et al., 2017, 2018a; Chen et al., 2017; Zhang et al., 2017b; Xu et al., 2018; Zhang et al., 2018). Among all the characters associated with increased PM accumulation, leaf hairiness/ trichome presence was found to be the most important (Weerakkody et al., 2018a). One study found that fine PM is deposited more frequently in wax (40 % of total PM) than large PM (4 % of the total PM) (Sgrigna et al., 2015). Furthermore, ridges of 1–2 μm in height on the leaf surface have been shown to efficiently capture PM, especially PM2.5 (Lei et al., 2015). The leaf surface of some species—such as Firmiana simplex and Mangifera indica—are covered with tomentum and have grooves surrounding the stomata, these features facilitate the retention of airborne PM (Liu et al., 2012; Liang et al., 2016; Zha et al., 2018). Compared to species with larger leaves, smaller-leaved species, such as Buxus sempervirens L., were found with significantly higher potential to capture and retain PM (Weerakkody et al., 2017, 2018a). However, the foliar shape of broad-leaved species was found not to influence fine PM accumulation (Chen et al., 2017). The conifer species was found to more efficiently capture PM2.5 and contribute to post-rainfall recapture than broad-leaved species (Hwang et al., 2011; Yu, 2014; Chen et al., 2017). Among all urban plants in these studies that met our criteria for review, some conifer species were found most effective in capturing PM, including Juniperus chinensis (Weerakkody et al., 2018b), Platycladus orientalis (Xu et al., 2018), and Pinus species such as P. mugo (Sæbø et al., 2012), P. sylvestris (Sæbø et al., 2012; Räsänen et al., 2013; Przybysz et al., 2014), P. bungeana (Song et al., 2015; Zhang et al., 2017b), P. tabuliformis (Zhang et al., 2017b), and P. armandi (Xu et al., 2018). Conifers had a larger fine PM accumulation capacity per tree due to a larger total leaf area (Liang et al., 2016). Moreover, Pinus tabuliformis was found to retain large PM (up to 34.2 μm in diameter); the same study found that the average diameter of retained PM of Ginkgo biloba and Sabina chinensis were approximately 20.5 μm and 16.4 μm, respectively (Liu et al., 2018). The fine and complex structure of conifer foliage, including the number of stomata, the amount of epicuticular wax, and properties of the cuticle (Zhang et al., 2017b, 2018), explain their greater efficiency in capturing PM (Beckett et al., 2000b). Furthermore, in all seasons when deciduous trees are leafless, conifers are by far the most effective trees in removing airborne PM (Nguyen et al., 2015). Generally, PM densities on the adaxial leaf surface were significantly higher than that on the abaxial surface (Weerakkody et al., 2017). It has reported that only 17–24 % of the PM were found to be deposited on the abaxial side (Wang et al., 2006; Shi et al., 2017; Ottelé et al., 2010). Further studies showed that a larger contact angle (i.e. higher leaf wettability) is associated with less PM retention on the adaxial leaf surface (Räsänen et al., 2013; Wang et al., 2013; Zhang et al., 2018). The increase in wettability—when epicuticular wax is destroyed by mechanical and chemical abrasion—seemed to be the main factor leading to the seasonal variations in leaf PM accumulation (Wang et al., 2013). In addition to leaf size and morphology, PM retention on the leaf surface also varies with spatial and temporal factors, such as type of urban functional area (Speak et al., 2012; Liu et al., 2013; Yu, 2014; Chen et al., 2015b; Simon et al., 2016), deposition time, rain, and wind. Generally, the greatest retention of airborne PM occurs on plant leaves are areas exposed to traffic-related pollution (Zampieri et al., 2013; Przybysz et al., 2014; Mori et al., 2015; Wang et al., 2015). A study in Beijing revealed that PM and heavy metal retention on leaves of urban Euonymus japonicus was significantly higher in residential areas than in

Table 3 The types of urban plants within the 65 research papers on airborne PM reduction using urban plants. Some papers involved more than one type of urban plants. Type of urban plants

No. of papers

Regular greening plants Tree Shrub Herb Living wall plants Green roof plants Wetland plants Synthetic leaves

56 19 7 6 1 1 1

Fig. 4. Proportions of leaf-PM interaction events discussing the retention, washoff, or resuspension for airborne PM using urban plants.

Only one paper reported green roof or wetland plants. 85.14 % of leafPM interaction events described PM retention by urban plant leaves, but only 8.10 % and 6.76 % of leaf-PM interaction events examined leaf-retained PM wash-off and PM resuspension, respectively (Fig. 4). 3.2. PM retention on plant leaves Retained PM is generally classified into three size fractions: fine PM (0.2–2.5 μm), coarse PM (2.5–10 μm), and large PM (> 10 μm) (Xu et al., 2018). In urban areas, fine PM generally accounts for the smallest proportion of the total PM in mass but its particle number is the highest, accounting for more than 90 % of the total number of PM (Ottelé et al., 2010; Song et al., 2015; Cai et al., 2017; Xu et al., 2018). However, this pattern is not the same everywhere; in the arid oasis city of Aksu, northwestern China, large PM accounts for more than 90 % of total PM (Aliya et al., 2015). Retained PM is primarily composed of C, O, Si, Al, Ca, K, Mg, Nb, Fe, Na, and Ti (Song et al., 2015; Liu et al., 2017). Metals associated with vehicle use—including Cu, Cr, and Mn—have been found in PM collected from leaves (Leonard et al., 2016). Minor constituents associated with soot and dust, including Pb, Zn, Ni, V, Cd, As, and Cu, have also been identified in urban areas (Tomašević et al., 2005). Furthermore, PM collected from leaves near a roadway contains carbon and oxygen (Mori et al., 2018), such as SiO2, CaCO3, CaMg(CO3)2, NaCl, and 2CaSO4·H2O (Wang et al., 2006). Another study of the chemicals present in leaf-retained PM revealed that water soluble ions account for 28 % of the total PM mass (Xu et al., 2018). Other studies have shown that PM retained by foliage varied significantly among species (Beckett et al., 2000a; Prusty et al., 2005; 5

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parks (Zhang et al., 2017a). One study in Nanjing, China indicated that the concentration of polycyclic aromatic hydrocarbons (PAHs) in leaf PM in urban areas was significantly higher than that in suburban and rural areas (Zha et al., 2018). Furthermore, compared with 3.0 m above ground, pollutants were more abundant at 1.5 m above ground (Mori et al., 2018). Shrubs and tree-grass were found to be the most effective configurations for reduction of PM2.5 (Chen et al., 2015a, 2016). Higher planting density was found to enhance the deposition of large and coarse PM on plant leaves (Mori et al., 2018). Although the increase in leaf density appears to reduce wind velocity—suppressing turbulent deposition of airborne PM through impaction—the effect of tree crown morphology on PM deposition is almost negligible compared with the influence of physical factors like height, azimuth, and tree position (Hofman et al., 2014). This was also observed in a different study, where leaves at a height of 1 m retained more PM than leaves at 2 m and 4 m in height (Aliya et al., 2015). Seasonal variation and the presence of monsoons also have a significant effect on PM deposition (Prusty et al., 2005; Liu et al., 2013; Zhang et al., 2017a, 2017b). Generally, leaf PM retention increases from spring to autumn/winter (Wang et al., 2013; Przybysz et al., 2014; Yu, 2014; Wang et al., 2015; Chaudhary and Rathore, 2018). It appears that broad-leaved trees have the highest efficiency only after the leaves are fully developed (Nguyen et al., 2015). This pattern was also evident in the conifer Pinus sitchensis, where larger deposition amounts were found on two–year–old as compared to one–year–old needles (Mori et al., 2015). Researchers have also identified important daily cycles: the PM2.5 retention is generally the highest in the morning and decreases in the afternoon and evening (Nguyen et al., 2015), because more frequent winds and human activities in the afternoon maybe increase the PM resuspension from plant leaves. Taken together, the effects of species diversity, pollution level, spatial and temporal variation, and local weather conditions (including the possibility that strong winds may cause PM resuspension) are important factors determining total PM retention on plant leaves.

Moreover, leaf-retained large PM showed relatively higher wash-off levels compared to fine and coarse PM (Terzaghi et al., 2013; Przybysz et al., 2014; Wang et al., 2015; Weerakkody et al., 2018b). It has been suggested that PM wash-off from the leaf surface is closely related to plant type (Xu et al., 2017). For example, the smoothleaved living wall species Bergenia cordifolia has relatively higher PM wash-off levels than the waxy-leaved species Hedera helix (Weerakkody et al., 2018c). Generally, the PM wash-off from abaxial leaf surface is negligible because it does not contact the raindrops (Potter and Ragsdale, 1991). As for the living wall species Hedera helix, however, a rainfall intensity of 41 mm hr−1 was found to result in significant washoff levels on both the adaxial and abaxial sides (Weerakkody et al., 2018c). In a comparison of urban plants that were continuously exposed to automobile emissions with those that were not, the leaves exposed to the emissions changed significantly compared with the control leaves (Pal et al., 2002). Their cell boundaries were irregularly fused, their epidermal cells collapsed, and their stomatal number as well as trichome length increased to two-fold (Pal et al., 2002). The epicuticular wax also lost its original shape; it subsequently eroded, and became disorganized, forming a crusty patch on the cuticle (Pal et al., 2002). Furthermore, leaf retained PM was found to negatively influence leaf dry weight, membrane permeability, contents of the photosynthetic pigments, oxidative stress response, and stomatal index in urban trees (Chaudhary and Rathore, 2018). However, most urban plants can remain healthy and continue to effectively capture PM after rainfall (Pal et al., 2002; Weerakkody et al., 2018c). After being washed off from plant leaves, airborne PM is generally deposited into soil as the net removal. However, some deposited PM on the ground is resuspended by wind back into the air (Fig. 1D), while urban grass can effectively prevent this resuspension (Nguyen et al., 2015). 3.5. Research methods for urban plant-mediated airborne PM reduction Conventional weighting, the derivative rinse-and-weigh method (Zhang et al., 2017a), and the supplemental ultrasonic cleaning (UC) weighting method (Liu et al., 2018) are techniques used to measure PM retention on plant leaves. The rinsing and weighing method can be used to quantify the accumulation of water-soluble ions and insoluble PM (Xu et al., 2018). Furthermore, the weighting method reported by Xu et al. (2017) can be used to detect PM wash-off mass under controlled rainfall. Microscopy is often used to examine the correlation between the morphological features of the leaf surface and the number of retained PM (Lin et al., 2017). New methods include combined approaches: Scanning Electron Microscopy (SEM) combined with object-based image analysis (Yan et al., 2016), and Environmental Scanning Electron Microscope (ESEM) supplemented with ImageJ image analysis software (Weerakkody et al., 2017, 2018c). Furthermore, SEM supplemented with energy dispersive X–ray (SEM-EDX) can be used to identify the size, distribution, morphology, and chemical composition of PM (Tomašević et al., 2005; Song et al., 2015; Shi et al., 2017). The metals of trace amount present in deposited PM can be determined using microwave plasma atomic emission spectrometry (MP-AES) (Simon et al., 2016). The SEM approach can also be used to compare PM densities on leaf surface before and after rainfall to calculate the PM wash-off efficiency (Weerakkody et al., 2018c). Generally, PM number per leaf sample per unit leaf area were counted. These counts (N) can be used to calculate particle volume by using the following formula: V = NπD3/6. Here, D represents the average particle diameter and all PM are assumed to be spherical. PM mass can then also be given by: M = ρV, where ρ is the particle density, assumed to be 1.30 g cm−3 (Held et al., 2006). However, these equations may not be suitable for calculating the mass of PM with a diameter larger than 10 μm, since larger particulates are usually flat and irregularly shaped. Biomagnetic leaf monitoring of magnetic concentration parameters

3.3. PM resuspension from leaves back into the air It has been reported that 27–36 % of the accumulated PM on leaves can be resuspended by strong winds (Wang et al., 2015). Furthermore, a case study was performed in Belgium, and the researchers found that 76 % of the accumulated fine PM was resuspended (Schaubroeck et al., 2014). Because of the winds, changes in the PM retained by plant leaves over time are complex dynamic processes in which maximum values could exceed minimum values by 10-fold (Xie et al., 2018). Results from a study in Guangzhou, China, showed that PM retention reached a saturated maximum after about 24 days (Liu et al., 2013). The dynamic processes associated with the effect of wind on PM retention can be simulated by using polynomial functions of continually-rising, inverse U-shaped, or U-shaped (Zhang et al., 2017b; Xie et al., 2018). 3.4. PM wash-off from plant leaves via rainfall Rainfall generally does not completely remove all PM present in the leaves. One study found that 28 % and 48 % of accumulated PM were washed off from leaves with 10.4 mm and 31.9 mm precipitation, respectively, and that more precipitation can remove more PM from the leaf surface (Wang et al., 2015). In a controlled rainfall simulation experiment (Xu et al., 2017), a wash-off treatment removed up to 70 % of the leaf-retained PM by evergreen shrub Euonymus japonicus. In this study, the maximum wash-off occurred during the first 2.5 mm of rain, whereas cumulative PM wash-off rates increased with rainfall volume. This remained true until the rainfall volume exceeded 12.5 mm, when the curve of wash-off rates became steady. However, according to a case study in Belgium, where researchers observed that only 24 % of the total amount of fine PM was washed off (Schaubroeck et al., 2014). 6

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can be used to assess the spatial distribution of PM (Power et al., 2009; Rai et al., 2014), especially the technique of Saturation Isothermal Remanent Magnetization (SIRM) (Barima et al., 2014; Rai et al., 2014). Using these techniques, crown-deposited PM were used to assess the spatial distribution of PM inside individual tree crowns and an urban street canyon (Hofman et al., 2013; Barima et al., 2014). In addition, using a ground-based Light Detection and Ranging (LiDAR) approach, the three-dimensional tree crown morphology could assist the obtaining of leaf-deposited PM weight (Hofman et al., 2014). Furthermore, the degree of airborne PM removal by urban foliage can be estimated using remote sensing images (Liu et al., 2013), and Geographic Information System (GIS) techniques (Mcdonald et al., 2007).

it is unknown which type of plant crown and planting configuration will best facilitate PM wash-off. Just like how PM is retained on leaves, i.e. Cu particles accumulate on vine leaves (Ottelé et al., 2010; Weerakkody et al., 2017, 2018b), this might be another complex question that should be studied soon. Generally, the haze caused by airborne PM during winter is more serious than in summer in north China. In addition to the increase in heating emissions during winter, there are two other major causes of this haze: (1) winter has less rainfall than summer leading less wash-off events; and (2) many deciduous plants have no leaves in winter, only evergreen plants that can capture PM effectively in winter. In order to reduce winter haze, therefore, evergreen trees are a better choice to mitigate the impact of airborne PM year-round (Blanusa et al., 2015). Furthermore, incorporating the regular spraying of water on leaves may help increase the net removal of airborne PM using evergreen plants, if possible.

4. Discussion 4.1. Additional focus on PM wash-off events from plant leaves is required Up to 85.14 % of leaf-PM interaction events considered here focused on comparing PM retention on the leaves of varied urban trees from the atmosphere in different time and space. Throughout the year, however, most of the airborne PM captured by plants is stored temporarily on leaf surface before being resuspended in air by wind, or washed off into the soil by rainfall. Thus only a small portion of accumulated PM remains attached to the leaves, and is deposited into soil after the leaves fall from the canopy (Xu et al., 2017). Furthermore, the PM retained on leaf surface can either be washed off and then deposited into soil, or be resuspended in air by wind and become airborne PM (Xu et al., 2017). Although only a portion of surface PM retention can be removed during rainfall (Xu et al., 2017), in many urban areas there are many rainfall episodes each year, whereas leaves fall only once every year to deposit the leaf-retained PM into soil. Thus, from a net removal standpoint, investigating of how PM can be washed off from leaves may be more informative than investigating how PM accumulates on leaf surface. However, based on our analysis, only six of the selected papers (Table 1) examined the net removal events of urban plants relative to airborne PM reduction, revealing that the leaf retained PM wash-off rate increased and become steady after rain exceeded 12.5 mm (Xu et al., 2017). Therefore, future studies may focus on the features of urban plants that facilitate efficient wash-off to identify species with high net removal efficacy of airborne PM. One recent paper found that the net removal efficiency was affected by rainfall intensity in all tested species except for Populus tomentosa (Xu et al., 2017). This finding suggests that the trichomes present on the adaxial leaf surface may help prevent the leaf-retained PM from being washed off. However, to date no studies have focused on comparing the leaf-retained PM wash-off mass or efficiency between glabrous and hairy plants, as well as between broad-leaved and needle-leaved species. These questions may be addressed in future studies in this field.

4.3. Establish a unified evaluation system for airborne PM reduction by urban plants At present, we still lack an effective system to evaluate the relative efficiency of different plants in reducing airborne PM pollution in urban areas. Compared to SEM and magnetic concentration parameters SIRM, the weighting method is more quantitative. With respect to PM retention detection, however, this should be combined with the process of artificial rinsing to prevent systematic errors. For example, appendages on the leaf surface, such as wax, may dissolve during the rinsing process, and can result in the overestimation of PM retention on urban plant leaves. Moreover, the leaf surface can absorb water during rinsing, causing the results to be lower than the actual retained PM content (Janhäll, 2015). For natural wash-off events, the wash-off content can be calculated by weighing the leaves before and after rainfall without rinsing. Finally, with respect to the quantification of retained PM in the cited literature, experiments are rarely identical in terms of interval time, rinsing method, drying conditions, and leaf area measurement, which can result in low comparability of the experiments (Janhäll, 2015). To fix this issue, in the future, a unified method to identify and quantify the PM reduction of urban plant leaves is in need, and this method should probably be based on PM net removal. Therefore, a standard evaluation system for PM reduction of urban plants may be developed. Finally, the economic benefits can also be evaluated by using biomagnetic leaf monitoring of an urban plant or community, and by using remote sensing images to examine the whole urban greening system. These systematic and comprehensive measures may offer both economic and practical benefits related to landscape planning and design, as well as the greening of urban environments.

4.2. Urban greening based on the net removal of airborne PM

5. Conclusions

Leaf surface microstructures, including epicuticular wax, hair/trichomes, and surface ridges, facilitated the capture of PM, suggesting that it is better to select species with more trichomes or hairs on the leaf surface for airborne PM reduction (Weerakkody et al., 2018a; Yan et al., 2018). This may be especially true for conifers and species with smaller leaves (Weerakkody et al., 2018b). Some researchers have discussed that designing effective planting configurations is of more importance than tree species selection in attenuating the ambient PM concentrations in urban areas (Chen et al., 2016). By comparing PM net removal rates, however, urban plants with smooth leaves are generally less effective accumulators of PM, but positively affect the efficiency of PM wash-off events (Weerakkody et al., 2018c). Therefore, increasing the proportion of urban plants with smooth leaves in urban greening may improve PM net removal. Further,

A total of 65 published papers, showing a continuous growth over the past decade, were examined to characterize the airborne PM reduction rates of urban plants. Most of the studies that met our inclusion criteria were performed in Europe and East Asia and involved PM retention on the leaves of urban trees. However, only six and five of these studies examined leaf retained PM wash-off and PM resuspension, respectively. Therefore, future studies should focus more on wash-off events—which could reduce the amount of PM present—and developing strategies to screen urban plant species with greater wash-off potential for PM deposition into soil. Ongoing improved methods and a standard evaluation system for PM reduction of urban plant leaves could be established using the net removal of PM as the common unit of comparison. 7

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Declaration of Competing Interest

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