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Article

The Role of Spontaneous Flora in the Mitigation of Particulate Matter from Traffic Roads in an Urbanised Area

by
Robert Popek
1,*,
Beata Fornal-Pieniak
2,
Piotr Dąbrowski
1 and
Filip Chyliński
3
1
Section of Basic Research in Horticulture, Department of Plant Protection, Institute of Horticultural Sciences, Warsaw University of Life Sciences—SGGW, Nowoursynowska Street 159, 02-776 Warsaw, Poland
2
Department of Environmental Protection and Dendrology, Institute of Horticultural Sciences, Warsaw University of Life Sciences—SGGW, Nowoursynowska Street 159, 02-776 Warsaw, Poland
3
Instytut Techniki Budowlanej, Filtrowa Street 1, 00-611 Warsaw, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(9), 7568; https://doi.org/10.3390/su15097568
Submission received: 25 March 2023 / Revised: 24 April 2023 / Accepted: 2 May 2023 / Published: 5 May 2023
(This article belongs to the Special Issue Biofiltration of Urban Air)
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Abstract

:
Particulate matter (PM) is a serious air pollutant that poses significant health risks. One solution to reduce PM concentrations in these areas is through phytoremediation, a process that involves using plants to remove contaminants from the environment. In this study, we investigated the capacity of spontaneous flora—herbaceous plants, shrubs, and trees growing in five zones from the road—to absorb PM on their foliage. The study found significant differences in the accumulation of PM, with the highest PM accumulation recorded in Zone V, which boasted a blend of the three mentioned vegetation types together. In contrast, Zones I and II, which were located close to the road and comprised solely herbaceous plants, exhibited 14.3% and 43.4% less PM accumulation, respectively. Similarly, Zone IV, with a mix of herbaceous plants and shrubs, showed 64.5% less PM accumulation, while Zone III, with only herbaceous plants, had a staggering PM accumulation reduction of 76.8%. The sum of Si+Al+Ca displayed a similar pattern. Furthermore, the findings highlighted the valuable role of plants in decreasing PM concentrations in the air, resulting in reductions of 76%, 39%, and 47% for PM10, PM2.5, and PM1.0, respectively. The results indicate that various spontaneous flora can work in unison to reduce PM, providing a multifaceted approach to combating air pollution.

1. Introduction

Many countries have recently suffered from high concentrations of air pollutants in urban areas [1]. This pollution mainly comes from road traffic, industry, and greenhouse gas emissions from heating systems [2,3]. Particulate matter (PM), a mixture of solid and liquid particles suspended in the air, is one of the most prominent air pollutants in cities [4,5]. PM varies in size, composition, and origin and can consist of more than 50 chemical components, such as trace elements (TE), polycyclic aromatic hydrocarbons (PAHs), sulphates, nitrates, plant pollen, and fungal spores [6,7]. PM can be produced by multiple anthropogenic sources, including industrial emissions, transport-related emissions from roads, fossil fuel combustion, construction and demolition activities, wood combustion, and mining [8,9]. In urban areas, PM transport in the air is influenced by many factors, including wind direction, thermal stratification, solar insulation, thermal capacitance of streets, and traffic conditions such as intensity and vehicle speed [10,11].
Suspended in the air, PM in urban areas consists of small inhalable particles with a diameter of 10 µm or less (PM10) and 2.5 µm or less (PM2.5) that can deeply penetrate the thoracic part of the respiratory system and lungs, causing a range of health problems. Exposure to PM, whether short-term or long-term, can result in increased respiratory and cardiovascular morbidity, worsened asthma, cardiovascular and respiratory diseases, principal cell damage, and lung cancer [1,12]. A study in Nanjing (China) found that the toxicity of PM2.5 was greater than PM10, but both had the potential to induce proinflammatory cytokines [13]. Moreover, significant and pressing are the threats posed by PM pollution in the Indo-Gangetic Plain, where major cities such as Delhi, Kolkata, and Lahore, home to millions of people, are facing severe air pollution problems [14]. Reports from the World Health Organization (WHO) [1] indicate that long-term exposure to air pollutants can lead to a shorter life expectancy. In Europe, PM pollution shortens the average citizen’s life expectancy by eight months and is responsible for 428,000 deaths annually [15]. Moreover, scientific evidence highlights the substantial influence of chronic PM pollution exposure on COVID-19 spread and mortality [16].
Urban vegetation, such as roadside and park trees, ruderals, flowering meadows, and living walls, has been shown to reduce the concentration of air pollutants through the phytofiltration mechanism [17,18]. Furthermore, several studies have demonstrated the high potential of certain species of trees, shrubs, grasses, herbaceous plants, climbers, and mosses to mitigate PM pollution [19,20,21,22]. The process of PM deposition by vegetation is complex and dynamic, involving the atmosphere, vegetation, and soil. This process includes wet deposition, chemical reactions (such as vapor-phase reactions), and the dry deposition process [23]. Some fractions of PM may be moved or washed away by wind and rain, but smaller fractions can be permanently retained once penetrating the stomata [24,25].
Unfortunately, a city is an area where green spaces are decreasing. Global strategic actions of countries take into account priority goals, i.e., sustainable development, adaptation to climate change, and mitigating the impact of pollution, including also green spaces in cities. In this case, a good idea in urbanised areas may be to use plants naturally occurring in a given area—spontaneous flora—without human intervention. Nevertheless, they can sometimes arise also due to human activities, such as the dropping of seeds or plant spores that germinate in the urban environment [26]. Spontaneous urban flora can be very diverse, depending on the location in which it grows [27]. Among such plants, one can find weeds, shrubs, trees, mosses, and lichens [20]. Often, these plants have the ability to survive in difficult conditions, such as air pollution, lack of water, or urban lighting [28]. Spontaneous flora in the city plays an important role in providing health to urban ecosystems and providing habitats for wildlife [29]. They can also introduce colours and textures to urban space, adding an aesthetic element to the urban landscape [26].
There are plenty of articles that show that planned vegetation in urban areas helps to limit the amount of PM in the air [9,10,19,20,21,22,23,24,25]. However, there is a lack of studies on the impact of spontaneous roadside vegetation. This study presents the results from a recent field study that quantified the impact of different types of plant layers on near-road air quality. The specific aim of this study was to investigate the accumulation of (1) PM (total, surface, and in wax for two size fractions) and (2) the amount of TE on the leaves of different vegetation layers along roadsides with a high level of traffic. In order to assess the efficacy of plants in mitigating PM pollution, (3) the concentration of PM particles of three distinct size fractions in the air was measured. The hypotheses of this study were that there are differences in the amount of PM and inorganic particles on the foliage (1) between species layers and (2) at different distances from the road, and (3) spontaneous flora can effectively reduce the amount of PM in the air.

2. Materials and Methods

2.1. Study Area and Plant Material

The studies were conducted during the growing seasons of 2020 in Poland. According to the World Health Organization, Polish cities have some of the highest annual mean PM concentrations in the European Union [1]. For this research, the city of Tarnów (21°00′ E 50°02′ N, located in the south part of Poland), with an average annual PM10 concentration of 37 µg m−3 (a highly polluted city) was chosen as the study area. Tarnów, with a population of 106,000 inhabitants, is situated in the Lesser Poland Voivodeship at the Carpathian foothills on the Dunajec and Biała rivers. Its climate is classified as marine west coast (Cfb) by Köppen, and Tarnów is one of the warmest cities in Poland, with the longest vegetation season (118 days) and an average yearly precipitation of 713.2 mm.
The study site chosen for this research was located in the north-eastern part of Tarnów and comprised green spaces running along a paved road, a bicycle path, and an unpaved road (Figure 1). The paved road (No. 73) is used frequently by city residents, with approximately 5160 cars per 24 h, while the unpaved road is used only occasionally. These green spaces were covered by spontaneous vegetation, which was mowed in areas adjacent to the road. Five green zones (I–V) were chosen as study areas, with each zone further subdivided into four distinct study plots based on the identified plant species. Zones I, II, and III had only a herbaceous layer, while Zones IV and V had herbaceous shrubs and tree layers (Figure 1).

2.2. Flora Analysis

The methodology involved the selection of 20 study plots using the phytosociological records according to the Braun–Blanquet method [30]. These plots were located in the above-mentioned five zones. Phytosociological records of 1 m2 were located at a distance of 2 m from each other within every zone. The study encompassed a comprehensive examination of plant species within the vertical dimension, partitioned into three strata: the herbaceous layer (plants under 0.3 m in height), the shrub layer (species reaching a maximum of 1.5 m in height), and the tree layer (tree species with a maximum height of 16 m). The vegetation groups, including natural, semi-natural, and synanthropic, were assigned based on the Matuszkiewicz classification [31], while the Mirek et al. [32] nomenclature was utilised to name the plant species.

2.3. Sample Collection

Four biological replicates of the plants and leaves were harvested from each experimental zone and vegetation level. Due to the lack of shrubs and trees, only herbaceous plants were collected in the first three zones. In the fourth zone, herbaceous plants and shrubs were collected, and in the fifth and final zone, samples were also harvested from trees. Collecting herbaceous plants involved the utilisation of square plots measuring 100 × 100 cm. For shrubs and trees, leaves were taken randomly from heights of 1.0–1.5 m and 1.5–2.0 m, respectively. The sampling was conducted towards the end of the growing season, specifically in mid-September. To ensure deposition of PM on the plants, the collection was carried out only after a minimum of five consecutive dry days, as precipitation has the potential to wash away PM particles from the foliage. The specimens were then deposited in designated paper bags, labelled appropriately, and kept at room temperature until they were ready for analysis.

2.4. Quantitative Analysis of PM on Plants/Leaves

The research investigated two types of PM, namely water-washable PM obtained from leaf surfaces (SPM) and PM captured in the leaf wax (WPM), and focused on two size ranges (2.5–10 and 10–100 µm). The approach employed followed the methodology established by Dzierżanowski et al. [33]. A representative fraction of each sample, covering roughly 300 cm2, was initially washed with water to obtain SPM, and subsequently washed with chloroform to obtain the PM present in the epicuticular waxes (WPM). The liquids were passed through a 100 µm mesh sieve and then filtered using two types of filters—Type 91 and Type 42 paper filters (Whatman, Maidstone, UK) with pore sizes of 10 µm and 2.5 µm, respectively. The filters were weighed before and after filtration, dried, and then stabilised for humidity. The PM was categorised into two size fractions: large PM (10–100 µm) and coarse PM (2.5–10 µm). Following analysis, the PM quantity was recalculated as µg/cm2 by measuring the leaf area of the samples using an Image Analysis System (Skye Instruments Ltd., Llandrindod Wells, UK).

2.5. Quantitative Assessment of PM in the Air

The Dust Air Personal Controller (Central Mining Institute and EMAG-SERWIS, Katowice, Poland), in conjunction with the Dust Air Sampler application, was employed to conduct air PM concentration measurements. For each area, measurements of PM10, PM2.5, and PM1 were performed simultaneously for a duration of 30 min each. The device was placed at a height of 1.5 m above the ground during the measurements. All measurements were conducted on seven sunny days, at two-day intervals, during the morning rush hour.

2.6. Statistical Analysis

Analysis of variance was used to determine the statistical significance of the differences between zones and vegetation layers for the amount of PM (total and categorised by size) on leaves and in the air. In order to assess the significance of differences, a Tukey’s Honest Significant Difference (HSD) test (p = 0.05) was employed. The data are presented as means with standard errors of the mean (±SE). All calculations were performed using JMP Pro 12.1.0 software (SAS Institute Inc., Cary, NC, USA).

2.7. SEM Examinations

The microscopic examinations concentrated on identifying the varieties of particles present in the PM extracted from plants, as well as establishing their source. Inorganic particles in the PM may have come from the asphalt road, pavement, or a local road. Semi-quantitative analysis was also performed to estimate the extent to which inorganic PM infiltrates the green space zones and to check the concentration gradient.
To conduct the SEM analysis, dried filter papers used for water extraction were chosen, with a pore size of 10 µm and the largest quantity of PM residue. Samples with dimensions of 5 × 5 mm were taken from the centre of each filter paper for each type and area. Prior to SEM inspection, the samples were coated with gold.
SEM observations were conducted using a Zeiss Sigma 500VP scanning electron microscope (Carl Zeiss Microscopy GmbH, Köln, Germany). Secondary electron (SE) and backscattered electron (BSE) images were collected. Phase compositions were analysed using an energy dispersive X-ray spectroscopy detector (EDX) (Oxford Ultim Max 40) with AztecLive software (Oxford Instruments NanoAnalysis & Asylum Research, High Wycombe, UK).
To assess the quantity of inorganic particles in the material under investigation, a semi-quantitative analysis was utilised. An EDX detector and 20 kV EHT (Electron High Tension) were utilised to examine five random regions at 300× magnification for each sample. The total number of silicon, aluminium, and calcium was counted, which served as markers for the inorganic PM content based on a previously developed concept [30]. Figure 2 presents an example of the analysed area using a BSE detector and EDX mapping, which shows the concentration of PM particles and indicates their origin based on the composition of elements.

3. Results

3.1. Flora Analysis

Tree, shrub, and herbaceous species were distinguished in the study area, belonging to natural, semi-natural, and synanthropic vegetation according to phytosociological classification (Table 1). Tree species were represented by Alnus incana (L.) Moench, Acer negundo L., and Juglans regia L.; shrub species included Salix sp. and Sambucus nigra L.; and herbaceous plants were represented by grasses, such as Poa annua L., and perennials such as Plantago lanceolata L., Taraxacum officinale Weber ex F.H.Wigg., Trifolium pratense L., Trifolium arvense L., and Tanacetum vulgare L. Plant species represented by semi-natural and synanthropic vegetation dominated in Zones I and II. P. annua had the highest cover (80–95%) in two zones. Semi-natural plants (e.g., T. arvense, T. pratense, and P. lanceolata) and synanthropic plants (e.g., T. vulgare and Solidago virgaurea L.) dominated in Zones I, II, and III. Shrub and tree species occurred in Zones IV and V, e.g., Salix sp. and A. incana. One rush species, represented by Phalaris arundinacea L., was also distinguished in Zones IV and V (Table 1).

3.2. PM Analysis

The level of total PM accumulation differed significantly across the vegetation in the five zones, as depicted in Figure 3. The first two zones, which contained mostly herbaceous plants such as grasses, mowed biweekly, exhibited the highest accumulation of total PM, particularly in the region nearest to the roadway.
Interestingly, the trend for shrub plants was quite different from that of herbaceous plants, although no shrubs were present in the first two zones. In this case, the amount of PM increased with distance from the source, with the highest accumulation observed in the fifth zone, where the amount of PM was three times higher than in Zone IV. These findings suggest that the characteristics of vegetation cover play a crucial role in determining the extent of PM accumulation in a given area.
The study also revealed significant differences in the accumulation of different sizes of PM across the different vegetation zones. The build-up of large PM (10–100 µm) on plants exhibited a pattern comparable with that of the total PM accumulation, which can be attributed to the fact that this fraction constituted the greatest portion of total PM, as shown in Figure 3. Coarse (2.5–10 µm) PM showed a similar trend to large PM (Figure 4A,B). However, the amount of coarse PM on leaves was, on average, one-third less than that of large PM.
When looking at the different plant types separately, the zone differences were less pronounced. For herbaceous plants, the difference in the accumulation of large PM between Zones I and III, IV, and V was 74%, while for coarse PM, it was 10% less. The same was observed for shrubs, with a difference of 71% for coarse PM and 64% for large PM. Trees accumulated much less of the smaller fraction of PM (58%) than the larger fraction.
The concentration of rain washable PM (SPM) that gathered on plants across various zones and distances from the road aligned with the patterns of total PM accumulation (as depicted in Figure 4C). The highest accumulation was found in the complex of plants in Zone V, followed by herbaceous plants in Zone I. The lowest accumulation was observed in Zone III, where it was 71% less than the most effective zone.
Another type of PM, immobilised in epicuticular wax layers (WPM), was accumulated on a smaller scale than SPM but showed the same pattern (Figure 4D). The amount of WPM on leaves was 23% lower than that of SPM. When comparing different species of herbaceous plants, the amount of SPM and WPM decreased from the road (with a 71% difference between Zones I and II, III, and IV on average), but for shrubs, there was a huge increase between Zones IV and V, amounting to 70% for both types of PM.

3.3. Concentrations of PM in the Air

The levels of PM in the air fluctuated according to the size of PM and the distance from the road. The average PM10 concentration in the entire measured area was the highest, reaching 27.4 µg/m3, which exceeded the amount of PM2.5 and PM1.0 by 21% and 49%, respectively (Figure 5). The highest concentrations of PM10 and PM2.5 were recorded in the zone closest to the road, while for PM1.0, it was in the second zone.
The reduction in PM10 concentrations was the most significant, with 23%, 43%, 53%, and 76% decreases observed in Zones II, III, IV, and V, respectively, compared with Zone I. The levels of PM2.5 and PM1.0 were the same in the first two zones, and they decreased successively, reaching their minimum in Zone V, with concentrations of 39% and 47% of the highest concentration recorded, respectively, for PM2.5 and PM1.0. The PM concentrations in the last zone were only slightly different and amounted to 10.7 µg/m3, 10.5 µg/m3, and 9.2 µg/m3 for PM10, PM2.5, and PM1.0, respectively.

3.4. SEM Imaging and Analysis

The SEM analysis revealed two main categories of PM particles: inorganic particles originating from roads or pavements, and organic particles from vegetation zones (Figure 6).
Under SEM examination, two types groups of particles were identified. The first group comprised inorganic particles commonly found in dust from asphalt roads, local roads, or pavement, such as quartz, plagioclase, pyroxene, calcium carbonate, dolomite, fly ash, and others. The second group consisted mainly of various types of plant pollen and plant particles, which were organic in nature.
Upon closer examination, it was found that samples collected from the herbaceous plant zones (I–III) located closer to the road contained a higher concentration of inorganic particles. However, in the shrubs and trees zones, the opposite trend was observed, and in order to confirm these observations, a semi-quantitative analysis was conducted.

3.5. Semi-Quantitative Analysis

The results of a semi-quantitative analysis of inorganic dust compounds collected in each zone are shown in Figure 7.
An inorganic dust marker (IDM) that measures the sum of Si+Al+Ca elements was selected to determine the gradient of inorganic PM concentration. This approach has been effectively utilised in prior studies by Popek et al. [34].
Based on the SEM-EDX analysis results, it can be observed that the IDM factor increases in shrub zones located further away from the roads and pavement. The lowest value of IDM for the trees zone supports this theory, as trees are taller and would therefore experience less dust settling. Conversely, the IDM factor in herbaceous layers has an opposite gradient than that of shrub zones. In the herbaceous layer, the IDM factor reaches the highest values in Zone I, which is located closest to the asphalt road and next to the pavement.
In Zones II and III, which are located on both sides of a local unpaved road, the IDM factor has similar low values. This suggests that the local unpaved road is just a marginal source of PM pollution in this area. The IDM factor decreases in herbaceous areas in Zones IV and V, located further from the roads and pavement. This may be due to the distance from the PM sources and also the protective role of shrubs and trees present in Zones IV and V.

4. Discussion

In highly urbanised areas, PM pollution is a major environmental concern, primarily caused by the use of roads. Green spaces in urban areas have not only environmental benefits but also significant economic advantages. For example, urban greenery and green infrastructure offer direct monetary benefits through the provision of goods and services, such as recreational areas, biodiversity, and carbon sequestration [35,36]. Additionally, urban green areas provide numerous indirect monetary benefits. Properties located in close proximity to green spaces are likely to have a higher value than those located farther away from vegetation cover due to a number of factors, including improved aesthetics, noise reduction, and air quality [1]. Research has also shown that urban green spaces have positive effects on the mental health and well-being of residents [37]. For example, studies have found that access to green spaces can reduce stress, improve mood, and increase cognitive function. Furthermore, green areas in urban settings provide opportunities for social interaction, physical activity, and relaxation, which can contribute to a higher quality of life for city dwellers [38].
One of the main challenges for urban managers is to minimise the negative impacts of anthropogenic pressures on urban ecosystems, such as air pollution [3]. The results of our research are significant, as they demonstrate that spontaneous vegetation has the potential to reduce the accumulation of PM in the air. Every layer of vegetation, including herbaceous plants, shrubs, and trees, can accumulate PM and contribute to reducing its concentration in the air. This finding, together with the conclusions of other authors [22,34], highlights the importance of promoting diverse vegetation in urban areas. This is important not only for visual values and biodiversity but also to minimise air pollution, which was confirmed by our research results.
Often, green barriers designed to reduce air pollution near roads only consider trees and shrubs while disregarding other types of urban greenery, such as herbaceous plants, that have different vegetation structures [10,39,40]. However, due to safety concerns and limited space, trees and tall shrubs cannot be planted in close proximity to roads, resulting in dispersed PM emissions that can be transferred to new locations [41]. Our research shows that herbaceous plants in Zones I and II accumulated a significant amount of PM, which had not been absorbed by any physical barriers, resulting in the permanent retention of PM in the immediate vicinity of the road. Similar results were found by Przybysz et al. [18] for flowering meadows.
The challenge is to find plants that can tolerate extreme growing conditions. Spontaneous plants are well-suited to urban environments. They are resistant to high temperatures, lack of water, and other challenges, while maintaining aesthetic value throughout the vegetation period [42]. These plants were found to thrive in areas with notably higher levels of airborne PM, positioning them as the initial physical barrier in filtering air pollution before it spreads further. Herbaceous species in this study presents a high accumulation ability of PM due to their morphology and layer structure, with many plants being covered with hairs (Plantago lanceolata L.) and a thick waxy coating (Sonchus arvensis L.) that effectively accumulates PM [22,23,29]. Therefore, more attention should be given to herbaceous plants in designing vegetation barriers to mitigate near-road air pollution. Furthermore, the results of our study demonstrated that a dense layer of grass (Poa annua L.), which was intentionally left largely unmoved, played a crucial role in effectively filtering PM from the air. Previous studies by Janhäl [43] and Popek et al. [34] have already demonstrated the high effectiveness of herbaceous plants in accumulating PM, while Przybysz et al. [44] specifically showed the PM accumulation ability of grasses. One way to maximise the phytoremediation potential of herbaceous plants is by reducing roadside vegetation mowing, particularly in areas where aesthetics are not a top priority. Mowing reduces their effectiveness in air purification from suspended particles, as it shortens their lifespan and reduces the surface area on which pollutants can be accumulated [34,44]. In particular, in the case of herbaceous plants growing along roads, limiting mowing allows for an increase in the density and porosity of the filtering vegetation, which further increases their effectiveness in removing suspended particles [43].
Interestingly, the trend for shrub plants was quite different from that of herbaceous plants, although no shrubs were present in the first three zones. In this case, the amount of PM increased with distance from the source, with the highest accumulation observed in the fifth zone, where the amount of PM was three times higher than in Zone IV. When comparing different vegetation types, trees, which were only present in the fifth zone, accumulated one-fifth less PM than shrubs in the same zone. However, their PM accumulation was more than half lower than that of herbaceous plants in the first zone, indicating the significant role played by herbaceous plants in PM retention. Overall, the highest accumulation occurred at the furthest distance from the emission source by the three types of plants together, which was even 15% higher than in the zone closest to the emission source and four times higher than the retained amount of PM in the least effective zone (III).
The research revealed that herbaceous perennials accumulated significantly more PM than trees, which was unexpected. Until now, street-adjacent trees were considered the only urban greenery that could effectively mitigate PM pollution (8–10, 22–25). The higher efficiency of herbaceous plants in PM accumulation is likely due to their proximity to the road, which was the sole source of PM in the area. This suggests that herbaceous plants act as a barrier of the polluted air before it could reach the trees [34,44]. Although herbaceous plants effectively accumulate PM, the high concentration of PM between the road and the first row of trees indicates that the air was still polluted. It is likely that more PM was deposited on shrubs and tree leaves due to the re-suspension by wind and rain [10,25]. To prevent further re-suspension of PM, it is recommended that there should be no hardened surfaces areas (such as pavements, bicycle paths, and parking lots) under trees and shrubs, especially near the roadside.
Based on our results, it can be concluded that there are significant differences in accumulation also in size and types of PM across different vegetation zones. Moreover, a study carried out in Beijing revealed that various plant species exhibited different capacities to remove PM of diverse sizes, with certain species demonstrating higher efficacy than others [28]. In this research, large PM, which has the largest share in total PM, shows a similar trend to total PM accumulation, while coarse PM accumulates less on leaves than large PM. The differences between zones were less pronounced when looking at different plant species separately. Many studies have had similar results [22,39]. An example study in Italy found that urban trees could accumulate PM, particularly large PM, but the accumulation varied depending on the plant species [45]. Some studies found that herbaceous plants tend to accumulate larger PM particles, while trees can accumulate smaller PM particles [40], but our study did not confirm this. These results indicate diversity in PM accumulation in plants depending on the type of plant, size of PM particles, and distance from the emission source.
The quantity of SPM accumulated on vegetation due to rain followed the same pattern as total particulate matter, with the greatest accumulation in Zone V and the lowest in Zone III. The same pattern was observed for PM, immobilised in epicuticular wax layers (WPM), although the accumulation was on a smaller scale than SPM. PM on the surface is loosely attached and can be easily removed by external factors, while WPM embedded in wax is more difficult to remove and cannot aggregate into larger particles [3,10,18,19,20,21,22]. The retention of PM by wax decreases towards the end of the growing season when the wax deteriorates [46]. Experimental plants protected from rain by higher plant layers had higher amounts of surface and in-wax PM than those exposed to rainfall. However, the difference was greater for surface PM. The results suggest that the development of leaves is crucial for the phytoremediation of PM, and urban greenery should be organised accordingly [8,9,10,11,18,19,20,21,22,23,24,25,46].
Comparing different species of herbaceous plants, the amount of SPM and WPM decreased from the road, but for shrubs, there was a significant increase between Zones IV and V. It is important to mention that the present study did not analyse the amount and chemical makeup or structure of the waxes. It should be noted that different plant species may exhibit distinct types of waxes [22,25,33]. Additionally, the wax may vary over the growing season and respond differently to meteorological conditions, which can be crucial for PM accumulation. For example, during the summer, elevated temperatures can partially cause the wax to melt and increase its viscosity, which may contribute to higher levels of PM accumulation [25].
The aim of this study was also to examine the spatial dispersion of inorganic PM containing Si, Al, and Ca emitted from non-exhaust road sources. It was found that the percentage of inorganic PM in the total accumulated PM decreased with distance from the road in herbaceous plants. The proportion of organic PM increased in Zones IV and V together for the foliage of herbaceous plants, trees, and shrubs, corresponding to the increase in total PM deposited on leaves. Unexpectedly, an increase in the accumulation of inorganic PM was observed despite the presence of plant barriers, low concentration of air PM in shrub and tree zones, and long distance from the PM emission source. This may be due to the high PM retention on leaves [10,25,34]. In this research, it was observed that the canopy of a tall and dense tree acted as a shield against wind and precipitation, resulting in the accumulation of PM by shrubs and trees growing beneath it. This is in contrast to previous studies where PM accumulation was interrupted by rain events [9,10]. The findings indicate that urban trees, in conjunction with herbaceous plants along the roadside, effectively mitigate the scattering and spreading of PM to other regions by efficiently amassing air pollutants discharged from the road. This is true even when these plants are located at a considerable distance from the road. Additionally, properly planned and maintained urban forests and roadside vegetation can reduce PM concentrations considerably [34].

5. Conclusions

The results of this study suggest that in order to harness the full potential of plants in mitigating PM pollution, it is crucial to gain a comprehensive understanding of PM accumulation within entire plant communities, including spontaneous flora, rather than merely concentrating on individual species. To accurately evaluate the effectiveness of spontaneous urban flora in air purification, it is important to recognise it as a multifaceted and intricate mechanism. Herbaceous plants, shrubs, and trees work as a team and fulfil distinct roles in air purifying.
In addition to its air pollution mitigation function, spontaneous flora provides various other ecological services that benefit both humans’ and the environment’s well-being. Urban green spaces offer opportunities for recreation and exercise and have been shown to impact mental health positively. They also provide crucial habitats for urban wildlife. Unfortunately, spontaneous urban flora is often destroyed by human activities such as sidewalk cleaning or the removal of plants deemed “undesirable”. That is why educating society about the value of spontaneous urban flora and taking action to safeguard and preserve it in urban spaces is so important. A limitation of this study is that it was conducted in a specific geographical location, and the findings may not be generalizable to other areas with different environmental conditions, pollution sources, or plant species. Future studies should examine the effectiveness of spontaneous flora in reducing air pollution in a variety of urban locations with different environmental conditions, pollution sources, and plant species.

Author Contributions

Conceptualisation, R.P. and B.F.-P.; methodology, R.P., B.F.-P. and F.C.; software, R.P.; validation, R.P.; formal analysis, R.P. and B.F.-P.; investigation, R.P., P.D., B.F.-P. and F.C.; resources, R.P. and F.C.; data curation, R.P.; writing—original draft preparation, R.P.; writing—review and editing, B.F.-P. and F.C.; visualisation, R.P., B.F.-P. and F.C.; supervision, R.P.; project administration, R.P.; funding acquisition, R.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financed by the Ministry of Science and Higher Education in Poland under Agreement No. 6967/IA/2019 of 9 July 2019.

Data Availability Statement

Detailed data supporting the findings of this article are available from the authors upon request.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Schematic representation of the study area locations.
Figure 1. Schematic representation of the study area locations.
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Figure 2. An example of an analysed area from a filter using BSE and EDX mapping techniques (herbaceous plants from a sample taken in Zone III).
Figure 2. An example of an analysed area from a filter using BSE and EDX mapping techniques (herbaceous plants from a sample taken in Zone III).
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Figure 3. The quantity of total PM accumulated on the leaves of plants from various vegetation zones in relation to the proximity to the pollution source. The white-coloured letters indicate significant distinctions among plant layers, while the black letters (above the columns) signify differences in distances.
Figure 3. The quantity of total PM accumulated on the leaves of plants from various vegetation zones in relation to the proximity to the pollution source. The white-coloured letters indicate significant distinctions among plant layers, while the black letters (above the columns) signify differences in distances.
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Figure 4. Amount of (A) 10–100 µm and (B) 2.5–10 µm PM fractions; two types of PM: (C) SPM and (D) WPM, on the foliage of plants in diverse vegetation zones varied depending on the proximity to the pollution source. The black letters (above the columns) indicate statistically significant differences in distances, while the white letters represent significant distinctions in plant layers.
Figure 4. Amount of (A) 10–100 µm and (B) 2.5–10 µm PM fractions; two types of PM: (C) SPM and (D) WPM, on the foliage of plants in diverse vegetation zones varied depending on the proximity to the pollution source. The black letters (above the columns) indicate statistically significant differences in distances, while the white letters represent significant distinctions in plant layers.
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Figure 5. The concentration of PM10, PM2.5, and PM1.0 in the air of different zones. Statistically significant differences within the PM fraction are indicated by different letters in each colour.
Figure 5. The concentration of PM10, PM2.5, and PM1.0 in the air of different zones. Statistically significant differences within the PM fraction are indicated by different letters in each colour.
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Figure 6. Various types of dust particles with different origin (1—quartz, 2—dolomite, 3—organic particle, 4—fly ash).
Figure 6. Various types of dust particles with different origin (1—quartz, 2—dolomite, 3—organic particle, 4—fly ash).
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Figure 7. Results of energy dispersive X-ray spectroscopy (EDX) analysis for each zone.
Figure 7. Results of energy dispersive X-ray spectroscopy (EDX) analysis for each zone.
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Table
Table 1. Occurrence of plant species in the study area (green spaces—Zones I to V).
Table 1. Occurrence of plant species in the study area (green spaces—Zones I to V).
Zone I
Plant Species% Cover of Species in Plot No. 1Plant Species% Cover of Species in Plot No. 2Plant Species% Cover of Species in Plot No. 3Plant Species% Cover of Species in Plot No. 4
Herbaceous layer
Poa annua (sm)80Poa annua (sm)90Poa annua (sm)85Poa annua (sm)85
Sonchus arvensis (s)5Plantago lanceolata (sm)5Sonchus arvensis (s)5Medicago falcata (sm)5
Medicago falcata (sm)5Sonchus arvensis (s)2Trifolium pratense (sm)5Sonchus arvensis (s)5
Taraxacum officinale (sm)2Trifolium pratense (sm)2Tanacetum vulgare (s)5Tanacetum vulgare (s)2
Plantago lanceolata (sm)2Tanacetum vulgare (s)1Trifolium arvense (sm)2Centaurea cyanus (s)1
Melilotus albus (s)1------
Trifolium arvense (sm)5------
Zone II
Plant Species% Cover of Species in Plot No. 1Plant Species% Cover of Species in Plot No. 2Plant Species% Cover of Species in Plot No. 3Plant Species% Cover of Species in Plot No. 4
Herbaceous layer
Equisetum arvense (s)5Poa annua (sm)80Poa annua (sm)85Poa annua (sm)80
Medicago falcata (sm)5Equisetum arvense (s)5Hieracium pilosella (sm)5Hieracium pilosella (sm)10
Trifolium pratense (sm)5Trifolium pratense (sm)5Medicago falcate (sm)5Medicago falcata (sm)5
Sonchus arvensis (s)3Hieracium pilosella (sm)5Tanacetum vulgare (s)3Trifolium pratense (sm)5
Tanacetum vulgare (s)2Sonchus arvensis (s)3Trifolium pratense (sm)2--
--Taraxacum officinale (sm)2----
Zone III
Plant Species% Cover of Species in Plot No. 1Plant Species% Cover of Species in Plot No. 2Plant Species% Cover of Species in Plot No. 3Plant Species% Cover of Species in Plot No. 4
Herbaceous layer
Solidago virgaurea (s)85Solidago virgaurea (s)60Solidago virgaurea (s)70Solidago virgaurea (s)70
Calamagrostis epigejos (sm)10Calamagrostis epigejos (sm)15Calamagrostis epigejos (sm)20Calamagrostis epigejos (sm)25
Tanacetum vulgare (s)5Tanacetum vulgare (s)15Tanacetum vulgare (s)5Rubus sp. (sm)4
--Achillea millefolium (sm)3Daucus carota (sm)2Phleum pratense (sm)1
- Trifolium pratense (sm)2Rubus sp. (sm)3--
Zone IV
Plant Species% Cover of Species in Plot No. 1Plant Species% Cover of Species in Plot No. 2Plant Species% Cover of Species in Plot No. 3Plant Species% Cover of Species in Plot No. 4
Herbaceous layer
Phalaris arundinacea (n)70Rubus sp. (sm)70Rubus sp. (sm)60Rubus sp. (sm)60
Rubus sp. (sm)20Juglans regia (s)30
Urtica dioica (s)10
Shrub layer
Salix sp. (n)80Salix sp. (n)60Salix sp. (n)60Salix sp. (n)60
Tree layer
------Juglans regia (s)20
Zone V
Plant Species% Cover of Species in Plot No. 1Plant Species% Cover of Species in Plot No. 2Plant Species% Cover of Species in Plot No. 3Plant Species% Cover of Species in Plot No. 4
Herbaceous layer
Phalaris arundinacea (n)70Phalaris arundinacea (n)70Phalaris arundinacea (n)70Phalaris arundinacea (n)90
--Rubus sp. (sp)30Rubus sp.30--
Shrub layer
----Sambucus nigra (s)30Salix sp. (n)60
Tree layer
Alnus incana (n)95Alnus incana (n)90Alnus incana (n)90Alnus incana (n)70
Acer negundo (s)5Acer negundo (s)5Acer negundo (s)5Salix sp. (n)60
(n)—plant belongs to natural vegetation (forest, water, and rush communities); (sm)—plant belongs to semi-natural vegetation (grasses communities); (s)—plant belongs to synanthropic vegetation (anthropogenic communities and alien plants).
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Popek, R.; Fornal-Pieniak, B.; Dąbrowski, P.; Chyliński, F. The Role of Spontaneous Flora in the Mitigation of Particulate Matter from Traffic Roads in an Urbanised Area. Sustainability 2023, 15, 7568. https://doi.org/10.3390/su15097568

AMA Style

Popek R, Fornal-Pieniak B, Dąbrowski P, Chyliński F. The Role of Spontaneous Flora in the Mitigation of Particulate Matter from Traffic Roads in an Urbanised Area. Sustainability. 2023; 15(9):7568. https://doi.org/10.3390/su15097568

Chicago/Turabian Style

Popek, Robert, Beata Fornal-Pieniak, Piotr Dąbrowski, and Filip Chyliński. 2023. "The Role of Spontaneous Flora in the Mitigation of Particulate Matter from Traffic Roads in an Urbanised Area" Sustainability 15, no. 9: 7568. https://doi.org/10.3390/su15097568

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