How Does Leaf Surface Micromorphology of Different Trees Impact Their Ability to Capture Particulate Matter?

Impact of atmospheric particulate matter retention on physiological characters of five plant species under different pollution levels in Zhengzhou

Atmospheric particulate matter (PM) pollution has become a major environmental risk, and green plants can mitigate air pollution by regulating their enzymatic activity, osmoregulatory substances, photosynthetic pigments, and other biochemical characteristics. The present investigation aims to evaluate the mitigation potential of five common evergreen tree species (Photinia serrulata, Ligustrum lucidum, Eriobotrya japonica, Euonymus japonicus, Pittosporum tobira) against air pollution and to assess the effect of dust retention on plant physiological functions exposed to three different pollution levels (road, campus, and park). The results found that the amount of dust retained per unit leaf area of the plants was proportional to the mass concentration of atmospheric particulate matter in the environment, and that dust accumulation was higher on the road and campus than in the park. There were significant differences in dust retention among the five tree species, with the highest leaf dust accumulation observed for E. japonica (5.45 g·m-2), and the lowest for P. tobira (1.53 g·m-2). In addition, the increase in PM adsorption by different plants was uneven with increasing pollution levels, with significant decreases in chlorophyll content, photosynthetic and transpiration rate. From a physiological perspective, P. tobira exhibited greater potential to respond to PM pollution. Biochemical indicators suggested that PM pollution caused changes in plant protective enzyme activities, with a decrease in superoxide dismutase (SOD) and peroxidase (POD) activities, as well as promoting membrane lipid peroxidation, and appropriate stress also enables plants to counteract oxidative damage. In particular, PM exposure also induced stomatal constriction. Overall, PM retention was significantly associated with physiological and photosynthetic traits. In conclusion, our study contributes to the understanding of the effects of PM on plant physiology. Furthermore, it also provides insights into the selection of plants that are tolerant to PM pollution.

Understanding mercury accumulation in mosses of two subalpine forests in China

The role of forest ecosystems in the global mercury (Hg) biogeochemical cycle is widely recognized; however, using litterfall as a surrogate to assess the Hg sink function of forests encounters limitations. We investigated the accumulation characteristics and influencing factors of Hg in mosses from two remote subalpine forests in southwestern China. The results indicated that there was high Hg accumulation in subalpine forest mosses, with average concentrations of 82 ± 49 ng g-1 for total mercury (THg) and 1.3 ± 0.8 ng g-1 for methylmercury (MeHg). We demonstrated that the accumulation capacity of Hg in mosses was significantly dependent on species and substrates (micro-habitats), the mosses on tree trunks exhibited significantly elevated Hg accumulation levels (THg 132 ± 56 ng g-1, MeHg 1.6 ± 0.2 ng g-1) compared to mosses in other substrates. The surface morphologies and biochemical components of leaf (phyllidia), such as cation exchange capacity (CEC), pectin, uronic acid, and metallothionein, play a crucial role in the accumulation of Hg by mosses. These findings provide valuable insights into Hg accumulation in forest mosses. Suggesting that the contribution of mosses Hg accumulation should be considered when assessing atmospheric Hg sinks of forests.

Exploring the interplay between particulate matter capture, wash-off, and leaf traits in green wall species

The study investigated inter-species variation in particulate matter (PM) accumulation, wash-off, and retention on green wall plants, with a focus on leaf characteristics. Ten broadleaf plant species were studied in an experimental green wall. Ambient PM concentrations remained relatively stable throughout the measurement period: PM1: 16.60 ± 9.97 μgm-3, PM2.5: 23.27 ± 11.88 μgm-3, and PM10: 39.59 ± 25.72 μgm-3. Leaf samples were taken before and after three rainfall events, and PM deposition was measured using Scanning Electron Microscopy (SEM). Leaf micromorphological traits, including surface roughness, hair density, and stomatal density, exhibited variability among species and leaf surfaces. Notably, I.sempervirens and H.helix had relatively high PM densities across all size fractions. The study underscored the substantial potential of green wall plants for atmospheric PM removal, with higher Wall Leaf Area Index (WLAI) species like A.maritima and T.serpyllum exhibiting increased PM accumulation at plant level. Rainfall led to significant wash-off for smaller particles, whereas larger particles exhibited lower wash-off rates. Leaf micromorphology impacted PM accumulation, although effects varied among species, and parameters such as surface roughness, stomatal density, and leaf size did not consistently affect PM deposition. The composition of deposited particles encompassed natural, vehicular, salt, and unclassified agglomerates, with minimal changes after rainfall. Air Pollution Tolerance Index (APTI) assessments revealed that I.sempervirens displayed the highest air pollution tolerance, while O.vulgare had the lowest. APTI showed a moderate positive correlation with PM deposition across all fractions. The study concluded that the interplay of macro and micromorphology in green wall plant species determines their PM removal potential. Further research is needed to identify the key leaf characteristics for optimal green wall species selection for effective PM removal.

The cellular consequences of particulate matter pollutants in plants: Safeguarding the harmonious integration of structure and function

Particulate matter (PM) pollution is one of the pressing environmental concerns confronting human civilization in the face of the Anthropocene era. Plants are continuously exposed to an accelerating PM, threatening their growth and productivity. Although plants and plant-based infrastructures can potentially reduce ambient air pollutants, PM still affects them morphologically, anatomically, and physiologically. This review comprehensively summarizes an up-to-date review of plant-PM interaction among different functional plant groups, PM deposition and penetration through aboveground and belowground plant parts, and plants' cellular strategies. Upon exposure, PM represses lipid desaturases, eventually leading to modification of cell wall and membrane and altering cell fluidity; consequently, plants can sense the pollutants and, thus, adapt different cellular strategies. The PM also causes a reduction in the photosynthetically active radiation. The study demonstrated that plants reduce stomatal density to avoid PM uptake and increase stomatal index to compensate for decreased gaseous exchange efficiency and transpiration rates. Furthermore, genes and gene sets associated with photosynthesis, glycolysis, gluconeogenesis, and the TCA cycle were dramatically lowered by PM stress. Several transcription factors, including MYB, C2H2, C3H, G2-like, and WRKY were induced, and metabolites such as proline and soluble sugar were accumulated to increase resistance against stressors. In addition, enzymatic and non-enzymatic antioxidants were also accumulated to scavenge the PM-induced reactive oxygen species (ROS). Taken together, this review provides an insight into plants' underlying cellular mechanisms and gene regulatory networks in response to the PM to determine strategies to preserve their structural and functional blend in the face of particulate pollution. The study concludes by recommending that future research should precisely focus on plants' response to short- and long-term PM exposure.

Nature-Inspired Regenerative Fine-Dust-Catching Coatings to Improve Air Quality

Increased particulate matter (PM) concentrations in our ambient air are the cause of various life-threatening diseases and consequently need to be reduced to nonhazardous levels. The natural PM removal capabilities of leaves inspired the development of a low-cost coating technology that exploits natural weather phenomena for its PM catching and removal processes. The herein presented coating is based on microparticle-filled silicone with optimized chemical and physical surface properties. Its surface roughness was tuned using differently sized spray-dried particles, and its surface contact angle was adjusted through silicone tensides, polar ether groups incorporated in the silicon backbone, and the used amount of spray-dried particles. In such a way, optimized silicone coatings showed in laboratory experiments improved catching abilities (>300% relative to glass surfaces), a full retention of adsorbed PM during wind events, and the formation of large PM aggregates. Upon (simulated) rain events, these coatings were regenerated, and the content of harmful PM of various sizes dispersed in water was reduced between ∼73 and 100%. Furthermore, an outdoor test over 100 days showed the functioning of the coating under real-world conditions. These regenerative coatings are readily applicable on diverse surfaces and do not require any further technical infrastructure. Thus, they present an extension of the toolbox for PM reduction technologies.

Synergy of different leaf traits determines the particulate matter retention capacity and its susceptibility to rain wash-off

Rainfall plays a crucial role in the removal of particulate matter (PM) from plant leaves, influencing PM retention and the environmental behaviour of harmful substances that accumulate in PM. This study examined the PM retention capacity, particle size distributions, and wash-off rates of leaf surface PM from three common green tree species in northern China during two natural rainfall events (light rain: 8.3 mm; heavy rain: 54.2 mm), to investigate the relationship between the leaf traits, PM retention capacity, and PM wash-off process. Our results found that leaf morphometric characteristics, such as leaf size, length, width, and aspect ratio (length-to-width), had a negative and significant correlations with the PM retention capacity, but had no significant correlation with the leaf surface PM wash-off rate. Smaller leaves with low aspect ratios exhibited greater stability under external disturbances than large leaves with high aspect ratios, resulting in a higher PM retention capacity and lower wash-off rate. Ridges and grooves enhanced the PM retention capacity by increasing the leaf roughness. Rainfall could wash off all particle size ranges of leaf surface PM without altering their mechanical composition. Larger particles were more easily washed off. Euonymus japonicus, with its small leaf size and low aspect ratio, exhibited the highest PM retention capacity. Its curled leaf shape also hindered light rain from washing off leaf surface PM. Forsythia suspensa, with denser grooves and ridges compared with Prunus serrulata, exhibited a rougher leaf surface and higher PM retention capacity. However, this roughness may reduce wettability, making it easier for runoff to form on the leaf surface and dislodge leaf surface PM, resulting in F. suspensa having the highest wash-off rate. Our results highlight the synergy of different leaf traits on PM retention capacity and the PM stability after rainfall.

Submerged leaves of live indoor foliage plants adsorb H1N1 influenza virus from suspension

Control of hazardous indoor particles using plants has attracted interest due to the increasing worldwide air pollution and spread of pandemic-causing viruses. However, the interaction between human pathogenic viruses (HPVs) and live plants has not been examined largely due to issues in detecting tiny amounts of infectious viruses in a carrier (such as an aerosol) and the lack of suitable examination methods. In this study, as a novel evaluation method, the effect of submerged leaves of live plants on HPVs in water was examined, using the H1N1 influenza virus as a model. Selected plant foliage of a live plant was immersed in a small bag containing HPV water suspension. In an initial screening test, the activities of 20 different plant species on the virus suspension were evaluated using a rapid virus detection kit. Ten plant species had the capability to decrease virus concentrations in the water suspension within 72 h. Among the experimental plant species, Epipremnum aureum showed the highest virus decreasing characteristics when examined using both the kit and quantitative real time polymerase chain reaction. The capacity of immersed leaf of live E. aureum to decrease viral content was enhanced when the plant-containing pot was electrically grounded to the earth (approximately 70% decrease in virus concentration). The foliage sample analysis showed that virus adsorption to the plant foliage surface could be the major reason for the decrease in the suspension. These results suggest that the proposed method can be applied to select plants to further investigate plant-HPV interactions.

Differences in particulate matter retention and leaf microstructures of 10 plants in different urban environments in Lanzhou City

Particulate matter (PM) is a major primary environmental air pollutant and poses a threat to human health. Differences in the environment and leaf microstructures of plants will result in varying abilities to retain PM, but the effects of changes in these factors on PM retention are not yet well understood. This study selected 10 plant species in four urban areas (sports field, park, residential green space, and greenway) as the study objects. The amount of retained PM by the different species was measured, and the leaf microstructures were observed. It was found that the environment significantly affected both PM retention and leaf microstructure. The ranking of PM retention in the 10 species in four areas was greenway > residential green space > park > sports field. The ranking of average stomatal width and length was park > sports field > residential green space > greenway, while that of average stomatal density was greenway > residential green space > park > sports field. Different environments affected the length and density of trichomes in the leaves. These changes represented the adaptation of plant species to the growth environment. The stomata and grooves of the leaf surface significantly affected the ability of plants to retain PM. The amount of PM retained by different species varied. In all four urban areas, Prunus × cistena N. E. Hansen ex Koehne (purple leaf sand cherry), Prunus cerasifera Ehrhart f. atropurpurea (Jacq.) Rehd. (cherry plum), Buxus sinica var. parvifolia M. Cheng (common boxwood), and Ligustrum × vicaryi Rehder (golden privet) showed strong PM retention. The results of this study will provide information for planners and urban managers for the selection of plant species.

Environmental impacts of air pollution and its abatement by plant species: A comprehensive review

Air pollution is one of the major global environmental issues urgently needed attention for its control through sustainable approaches. The release of air pollutants from various anthropogenic and natural processes imposes serious threats to the environment and human health. The green belt development using air pollution-tolerant plant species has become popular approach for air pollution remediation. Plants' biochemical and physiological attributes, especially relative water content, pH, ascorbic acid, and total chlorophyll content, are taken into account for assessing air pollution tolerance index (APTI). In contrast, anticipated performance index (API) is assessed based on socio-economic characteristics including "canopy structure, type, habit, laminar structure, economic value and APTI score" of plant species. Based on previous work, plants with high dust-capturing capacity are identified in Ficus benghalensis L. (0.95 to 7.58 mg/cm2), and highest overall PM accumulation capacity was observed in Ulmus pumila L. (PM10 = 72 µg/cm2 and PM2.5 = 70 µg/cm2) in the study from different regions. According to APTI, the plant species such as M. indica (11 to 29), Alstonia scholaris (L.) R. Br. (6 to 24), and F. benghalensis (17 to 26) have been widely reported as high air pollution-tolerant species and good to best performer in terms of API at different study sites. Statistically, previous studies show that ascorbic acid (R2 = 0.90) has good correlation with APTI among all the parameters. The plant species with high pollution tolerance capacity can be recommended for future plantation and green belt development.

Capture of fire smoke particles by leaves of Cunninghamia lanceolata and Schima superba, and importance of leaf characteristics

Emission of particulate matter (PM) during forest fires is a major source of air pollution and hence purification of atmospheric pollution has gained increasing importance. Trees can absorb polluting gases and fine particles by their leaves from the atmosphere and act as a sustainable air purification filter. However, the capture efficiency varies among tree species; thus exploring the ability of forest trees to capture smoke PM released during forest fires provides a basis for assessing net emissions from forest fires and the impact of smoke on forest ecosystems. In this study, the main afforestation tree species, Cunninghamia lanceolata (Lamb.) Hook, and a fire-resistant tree species, Schima superba Gardn.et Champ, in southern China were exposed to different smoke concentrations by simulating forest fire. The amount of PM per unit leaf area, absorption of nutrient element, leaf surface characteristics and antioxidant enzyme activities were determined. The main findings were: (1) The total quantity of PM captured by unit leaf area (μg·cm^-2) of C. lanceolata was 28.25 ± 1.12, 30.52 ± 3.43 and 33.14 ± 3.00 in low, intermediate and high smoke concentrations, respectively. The corresponding values for S. superba was 5.96 ± 0.56, 10.09 ± 1.13 and 12.27 ± 0.39, respectively. (2) Both species had weak absorption capacity for inorganic ions in the PM. (3) The purification of smoke PM by leaves was mainly related to leaf surface roughness, where it was higher for C. lanceolata than S. superba leaves. (4) Smoke treatment positively affected the contents of chlorophyll and soluble protein as well as increased antioxidant enzyme activities. In conclusion, the findings highlight the importance of leaf structural characteristics in capturing smoke particles and C. lanceolata is better suited for purification of atmospheric smoke particles following forest fire than S. superba.

Air pollutant removal by four sidewalk tree species in the largest city in Taiwan

Air pollutants pose risks to human health, especially in densely populated cities. We compared the interception of suspended particles and metal elements by four sidewalk tree species with different leaf surface wettability (based on contact angle), leaf area, and phenology in Taipei, Taiwan. Suspended particles were enriched 2.0-2.5 times in throughfall relative to rainfall due to wash-off of suspended particles deposited on leaf surfaces during rainless periods. The enrichment in throughfall was greater in tree species with larger leaf areas. Despite greater concentrations of suspended particles in rainfall during the low-leaf-area period, enrichment was greater in the high-leaf-area period, indicating that leaf area was a key factor affecting canopy interception of pollutants. Throughfall enrichment of suspended particles positively correlated with water quantity, indicating that air pollutants intercepted by tree canopies were not fully washed off by rainfall. Annually, ∼830 g of suspended particles were intercepted and washed off from one tree canopy, with a crown area of 42 m² . Scaling up, a rough estimate of 72.7 Mg of suspended particles were intercepted annually by the 90,000 sidewalk trees in Taipei City. Copper, chromium, and aluminum were enriched in throughfall compared with rainfall. However, lead was depleted in throughfall, indicating greater interception than wash-off. Based on our results, leaf area and length of foliated period are key characteristics affecting canopy interception of particulate matter and associated metal elements, whereas leaf surface wettability is of secondary importance.

Effect of airflow pattern and distance on removal of particulate matters and volatile organic compounds from cigarette smoke using Sansevieria trifasciata botanical biofilter

Botanical biofilters can effectively remove indoor air pollution. However, to apply botanical biofilters in situ, the distance of botanical biofilter to the pollutants and airflow pattern can be important factors impacting efficiency. This study examined the removal efficiency of particulate matters (PMs) and volatile organic compounds (VOCs) from cigarette smoke, such as formaldehyde and acetone, at various distances (100 cm, 175 cm, 240 cm, and 315 cm) using a Sansevieria trifasciata botanical biofilter. The botanical biofilter was placed inside a testing room (24 m³) and exposed to cigarette smoke. The pollutants removal efficiency was evaluated for six cycles (24 h/cycle) and one cycle as a recovery period where botanical biofilter was placed under normal conditions for 30 days. Results showed that the botanical biofilter could remove 140-250 μg m^-3, 147-257 μg m^-3, 212-455 μg m^-3 for PM₁, PM_2.5, and PM₁₀, respectively, at 8 h. Total VOCs, formaldehyde, and acetone removal were 40%-65%, 46%-69%, and 31%-61% at 24 h. PMs and VOCs removal efficiency can be affected by both distance and pattern of airflow in the testing room. The highest PM₁ and PM_2.5 elimination appeared at 240 cm and 315 cm, while VOCs removal was high at 100 cm. Botanical biofilter creates airflow vortices around 100 cm, indicating low removal of PMs. This is the first study that demonstrated the effect of airflow patterns on different pollutants removal efficiency.

The Nature and Size Fractions of Particulate Matter Deposited on Leaves of Four Tree Species in Beijing, China

Particulate matter (PM) in different size fractions (PM0.1–2.5, PM2.5–10 and PM>10) accumulation on four tree species (Populus tomentosa, Platanus acerifolia, Fraxinus chinensis, and Ginkgo biloba) at two sites with different pollution levels was examined in Beijing, China. Among the tested tree species, P. acerifolia was the most efficient species in capturing PM, followed by F. chinensis, G. biloba, and P. tomentosa. The heavily polluted site had higher PM accumulation on foliage and a higher percentage of PM0.1–2.5 and PM2.5–10. Encapsulation of PM within cuticles was observed on leaves of F. chinensis and G. biloba, which was further dominated by PM2.5. Leaf surface structure explains the considerable differences in PM accumulation among tree species. The amounts of accumulated PM (PM0.1–2.5, PM2.5–10, and PM>10) increased with the increase of stomatal aperture, stomatal width, leaf length, leaf width, and stomatal density, but decreases with contact angle. Considering PM accumulation ability, leaf area index, and tolerance to pollutants in urban areas, we suggest P. acerifolia should be used more frequently in urban areas, especially in “hotspots” in city centers (e.g., roads/streets with heavy traffic loads). However, G. biloba and P. tomentosa should be installed in less polluted areas.

Particulate Matter (PM) Adsorption and Leaf Characteristics of Ornamental Sweet Potato (Ipomoea batatas L.) Cultivars and Two Common Indoor Plants (Hedera helix L. and Epipremnum aureum Lindl. & Andre)

Particulate matter (PM) is a serious threat to human health, climate, and ecosystems. Furthermore, owing to the combined influence of indoor and outdoor particles, indoor PM can pose a greater threat than urban PM. Plants can help to reduce PM pollution by acting as biofilters. Plants with different leaf characteristics have varying capacities to capture PM. However, the PM mitigation effects of plants and their primary factors are unclear. In this study, we investigated the PM adsorption and leaf characteristics of five ornamental sweet potato (Ipomea batatas L.) cultivars and two common indoor plants (Hedera helix L. and Epipremnum aureum Lindl. & Andre) exposed to approximately 300 μg m−3 of fly ash particles to assess the factors influencing PM adsorption on leaves and to understand the effects of PM pollution on the leaf characteristics of plants. We analyzed the correlation between PM adsorption and photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (Tr), leaf area (LA), leaf width/length ratio (W/L), stomatal density (SD), and stomatal pore size (SP). A Pearson’s correlation analysis and a principal component analysis (PCA) were used to evaluate the effects of different leaf characteristics on PM adsorption. The analysis indicated that leaf gas exchange factors, such as Pn and Tr, and morphological factors, such as W/L and LA, were the primary parameters influencing PM adsorption in all cultivars and species tested. Pn, Tr, and W/L showed a positive correlation with PM accumulation, whereas LA was negatively correlated.

Particulate matter and volatile organic compound phytoremediation by perennial plants: Affecting factors and plant stress response

Air pollution by particulate matter (PM) and volatile organic compounds (VOCs) is a major global issue. Many technologies have been developed to address this problem. Phytoremediation is one possible technology to remediate these air pollutants, and a few studies have investigated the application of this technology to reduce PM and VOCs in a mixture of pollutants. This study aimed to screen plant species capable of PM and VOC phytoremediation and identify plant physiology factors to be used as criteria for plant selection for PM and VOC phytoremediation. Wrightia religiosa removed PM and VOCs. In addition, the relative water content in the plant and ethanol soluble wax showed positive relationships with PM and VOC phytoremediation, with a high correlation coefficient. For plant stress responses, several plant species maintained and/or increased the relative water content after short-term exposure to PM and VOCs. In addition, based on proteomic analysis, most of the proteins in W. religiosa leaves related to photosystems I and II were significantly reduced by PM_2.5. When a high water content was achieved in W. religiosa (80% soil humidity), W. religiosa can effectively remove PM. The results suggested that PM can reduce plant photosynthesis. In addition, plants might require a high water supply to maintain their health under PM and VOC stress.

'Green barriers' for air pollutant capture: Leaf micromorphology as a mechanism to explain plants capacity to capture particulate matter

Finding ways to mitigate atmospheric particulate matter (PM) is one of the key steps towards fighting air pollution and protecting people's health. The use of green infrastructure is one option that could help improving urban air quality and promoting more sustainable cities. Detailed knowledge of how plants capture particulate matter can support plant selection for this purpose. Previous studies have primarily focused on 2D techniques to assess the micromorphology of plant leaves. Here, 3D optical profilometry and SEM imaging (2D) are used to quantify leaf roughness and other micromorphological leaf traits of three contrasting plant species (Hedera helix 'Woerner', Thuja occidentalis 'Smaragd', and Phyllostachys nigra) located within a mixed-species green barrier. These techniques have allowed us to identify the relative distribution of adhered atmospheric PM with respect to the surface topography of leaves, with high spatial resolution. Leaf surface roughness did not show a direct relationship with PM deposition; however, the descriptors width, depth and frequency of the grooves are important to explain PM capture by the leaves. Additionally, the presence of wax on leaves was relevant for PM adherence. All species captured PM, with their overall PM capture efficiency ranked from highest to lowest as follows: Thuja occidentalis > Hedera helix > Phyllostachys nigra. All green barrier species contributed to air quality improvement, through PM capture, regardless of their location within the barrier. Having multiple species in a green barrier is beneficial due to the diverse range of leaf micromorphologies present, thus offering different mechanisms for particulate matter capture.

Screening of Particulate Matter Reduction Ability of 21 Indigenous Korean Evergreen Species for Indoor Use

The formation and pollution of particulate matter (PM), a side effect of rapid industrialization and urbanization, is considered a global issue. However, various plant species are able to effectively capture and reduce atmospheric PM concentrations. We investigated the indoor growth and morphology of 21 indigenous Korean evergreen species at low light intensities to ascertain their ability to reduce PM of aerosol particles in a closed acrylic chamber. The decrease in PM mass concentration differed significantly across species, with a significant correlation (8 h; p < 0.001). The reduction in the mass concentration of PM differed with particle size and across species. The highest reduction of PM_2.5 occurred after 8 h with Dryopteris lacera (86.8%), Ilex × wandoensis (84.9%), Machilus thunbergii (84.3%), and Rhododendron brachycarpum (84.0%). Reduction of PM₁₀ after 8 h was highest with Cephalotaxus harringtonii (98.3%), I. × wandoensis (98.5%), M. thunbergii (98.5%), and R. brachycarpum (98.3%). Plant morphological characteristics (category, plant height, leaf shape, leaf area) and relative humidity were closely related to the decrease in PM mass concentration. In conclusion, our findings can be used to identify Korean plant species that can reduce PM concentration and are suitable for indoor use.

The relationship between particulate matter retention capacity and leaf surface micromorphology of ten tree species in Hangzhou, China

Atmospheric particulate matter (PM) is one of the main environmental air pollutants, but it can be retained and adsorbed by plants. To systematically and comprehensively conduct qualitative and quantitative research on the relationship between the leaf PM retention ability and the microstructure of leaf surfaces, this study evaluated the PM retention abilities of ten common tree species (1860 leaf pieces in total) in the greenbelts around the Lin'an toll station of the Hang-Rui Expressway in Hangzhou, China, in October 2019. The leaf surface roughness and contact angle were measured with confocal laser scanning microscopy and a contact angle measuring instrument. Scanning electron microscopy was applied to collect data on the stomata and groove morphology. The PM retention ability of the leaves was assessed by quantifying the PM mass and number density on the leaves. The results revealed that Platanus acerifolia and Sapindus mukorossi had a strong ability to retain particulates of different sizes. The mass of the retained PM_2.5 on their leaves accounted for the lowest proportion (mean: 8.12%) among the total retained particulate mass, but the number density of the retained PM_2.5 accounted for the highest proportion (mean: 97.49%) among the total number density. A significant negative correlation between the PM_2.5 mass and the groove width on the adaxial surface (R² = 0.746, P < 0.05) and a significant positive correlation between the roughness and the PM number density on the adaxial surface (R² = 0.702, P < 0.01) were observed. No obvious correlations were found among the groove width, roughness and number density of the retained PM on the abaxial surface. Leaf surfaces with dense and narrow grooves, strip-like projections, high roughness and high wettability had strong retention abilities. This study can provide a theoretical reference for selecting plants with strong PM retention ability for green urban garden design.

Green infrastructure for air quality improvement in street canyons

Street canyons are generally highly polluted urban environments due to high traffic emissions and impeded dispersion. Green infrastructure (GI) is one potential passive control system for air pollution in street canyons, yet optimum GI design is currently unclear. This review consolidates findings from previous research on GI in street canyons and assesses the suitability of different GI forms in terms of local air quality improvement. Studies on the effects of various GI options (trees, hedges, green walls, green screens and green roofs) are critically evaluated, findings are synthesised, and possible recommendations are summarised. In addition, various measurement methods used for quantifying the effectiveness of street greening for air pollution reduction are analysed. Finally, we explore the findings of studies that have compared plant species for pollution mitigation. We conclude that the influences of different GI options on air quality in street canyons depend on street canyon geometry, meteorological conditions and vegetation characteristics. Green walls, green screens and green roofs are potentially viable GI options in existing street canyons, where there is typically a lack of available planting space. Particle deposition to leaves is usually quantified by leaf washing experiments or by microscopy imaging techniques, the latter of which indicates size distribution and is more accurate. The pollutant reduction capacity of a plant species largely depends on its macromorphology in relation to the physical environment. Certain micromorphological leaf traits also positively correlate with deposition, including grooves, ridges, trichomes, stomatal density and epicuticular wax amount. The complexity of street canyon environments and the limited number of previous studies on novel forms of GI in street canyons mean that offering specific recommendations is currently unfeasible. This review highlights a need for further research, particularly on green walls and green screens, to substantiate their efficacy and investigate technical considerations.

Response of the particulate matter capture ability to leaf age and pollution intensity

Differences in leaf surface microstructure characteristics can lead to differences in the ability of trees to capture suspended particulate matter (PM). The influence of changes in leaf surface microstructure caused by growth and environmental pollution on the PM capture ability is poorly understood. This study assessed the influence of growth on leaf microstructure in leaves of different ages, and the influence of pollution intensity was assessed by studying trees growing under different pollution conditions. It was found that the ability of leaves of Taxus cuspidata var., Platycladus orientalis, and Pinus tabuliformis to absorb total suspended particles (TSP), PM10, PM2.5, and PM1 increased with leaf age. The amounts of TSP and PM10 captured by P. orientalis, P. tabuliformis, Sophora japonica, Populus tomentosa, and Ginkgo biloba were higher in heavily polluted areas than in clean areas. This may be because particle capture is influenced by leaf microstructure changes. With age increasing, the root mean square roughness (Rq) of three evergreen species leaves increased. Environmental pollution will change the leaf surface microstructure and its ability to capture PM. Compared with a clean area, in a heavily polluted area, the stomatal index of the leaves decreased, stomata were occluded, the leaf wax layer was degraded, the leaf surface contained more particles, the surface texture of S. japonica and G. biloba leaves became irregular, the boundaries of the epidermal cells became more irregular, and the trichrome of S. japonica became thinner, longer, and harder. The Rq value was generally higher in the heavily polluted area, and the roughness of the abaxial surface increased more than on the adaxial surface. In the heavily polluted area, the leaf microstructure changes were the main reason for the increase in the Rq value. With the increase in leaf roughness, the amount of PM on the leaf surface increased.

Urban Vegetation in Air Quality Management: A Review and Policy Framework

Recent episodes of high air pollution concentration levels in many Polish cities indicate the urgent need for policy change and for the integration of various aspects of urban development into a common platform for local air quality management. In this article, the focus was placed on the prospects of improving urban air quality through proper design and protection of vegetation systems within local spatial planning strategies. Recent studies regarding the mitigation of air pollution by urban greenery due to deposition and aerodynamic effects were reviewed, with special attention given to the design guidelines resulting from these studies and their applicability in the process of urban planning. The conclusions drawn from the review were used to conduct three case studies: in Gdańsk, Warsaw, and Poznań, Poland. The existing local urban planning regulations for the management of urban greenery were critically evaluated in relation to the findings of the review. The results indicate that the current knowledge regarding the improvement of urban air quality by vegetation is not applied in the process of urban planning to a sufficient degree. Some recommendations for alternative provisions were discussed.