The dust retention capacities of urban vegetation-a case study of Guangzhou, South China

Long-term open-pit mining activities at the world's largest light rare earth mine significantly affect light rare earth elements in road dust over long distances

The long-term effects of decades of open-pit mining at the Bayan Obo deposit, the world's largest light rare earth mine, on the concentrations of several elements in road dust over long distances are unknown. Here, bulk road dust (BRD) and resuspended road dust (RRD) were collected from different distances from the mine for subsequent analysis of mining impacts. As a result of mining activities, light rare earth elements (LREEs), especially La, Ce, Pr and Nd, show different statistical and spatial variations compared to other elements. These LREEs decrease with increasing distance from the mine, and the values found in RRD are higher than those in other particle sizes. Mining emissions and soil have the most significant influence on these LREEs compared to other factors. Spatially, these four LREEs changed significantly over a large area (up to 60 km from the mine) due to mining emissions. However, long-term mining activities affect these elements mainly through mining-contaminated soil as opposed to mining emissions. This study confirms the significant impact of mining activities on LREEs in road dust via a comprehensive data-driven framework, emphasizing the significant environmental effects of long-term open pit mining.

Removal of airborne particulate matter by evergreen tree species in Dhaka, Bangladesh

Urban air quality stands as a pressing concern in cities globally, with airborne particulate matter (PM) emerging as a significant threat to human health. An investigation was carried out to examine the potential of four prevalent evergreen roadside tree species grown at different locations in Dhaka to capture PM using their leaves. The distribution of PM by mass and quantity in Dhaka are presented for the first time for Bangladesh and these results will also be applicable to countries with similar climates and tree species. Separate gravimetric analyses were carried out to quantify PM in three different size ranges (0.2-2.5 μm, 2.5-10 μm, and 10-100 μm) accumulated on surfaces and trapped within waxes by using the rinse and weigh method. The method is validated for the first time through SEM-EDX analysis, which confirmed that the increase in weight from chloroform-rinsed leaves was exclusively attributable to particle deposition on the filter. The chemical composition of the deposited PM2.5 was analyzed quantitatively by determining the concentration of twenty-five trace elements employing ICP-MS. SEM-EDX analysis revealed the significance of leaf microstructural traits in effectively capturing PM. Significant variations in the deposition of PM were found among different species for two PM categories (surface PM and wax-embedded PM) and three size fractions (large, coarse, and fine) (one-way ANOVA; p < 0.05). The quantity of wax retained on the foliage of trees documented in these locations also varied (p < 0.05). Among the species studied, Ficus benghalensis demonstrated a greater ability to retain PM. Mangifera indica was identified to be the most efficient collector of wax-related PM and appears to be the ideal species for traffic-heavy areas distinguished by high concentrations of organic compounds from vehicle emissions.

Transcriptome Analysis Reveals That FvPAP1 Genes Are Related to the Prolongation of Red-Leaf Period in Ficus virens

Ficus virens is a deciduous tree that is highly valuable both economically and medicinally. Like other plants with 'red young leaves', the red-leaf period of most F. virens trees lasts only a few days, and the red leaves have little ornamental value. However, in recent years, some lines of F. virens with bright red young leaves and a prolonged red-leaf period have been utilized for urban greening. To explore the mechanism of the different lengths of the duration of F. virens leaves, we analyzed the physiology and changes in gene expression during the development of two varieties of leaves. The detection of anthocyanin in different developmental stages of the F. virens leaves showed that the changes in color of the red leaves of F. virens were primarily caused by the change in anthocyanin content. A transcriptome analysis showed that the expression of genes related to the biosynthesis of anthocyanin changed significantly during the development of leaves. A MYB gene FvPAP1, which was consistent with the change in anthocyanin content, was identified. A real-time quantitative reverse transcription PCR analysis and heterologous expression transgenic studies showed that FvPAP1 promoted the biosynthesis of anthocyanins. The difference in the expression of FvPAP1 in time and intensity in the young leaves may be the reason for the difference in the duration of the red-leaf period in different lines of F. virens. A sequence analysis showed that the cDNA sequence of FvPAP1 was polymorphic, and possible reasons were discussed. These results can provide insight for similar studies on the mechanism of the formation of red coloring in other woody plant leaves and provide molecular targets to breed new materials with more prolonged red-leaf periods in F. virens.

Influence of leaf morphological characteristics on the dynamic changes of particulate matter retention and grain size distributions

ABSTRACTPlant leaves can retain atmospheric particulate matter (PM) and thus alleviate air pollution. Herein, four plant species (Cerasus. serrulata, H. syriacus, H. tuberosus, and E. japonicus) from the central part of Shanxi Province, China, were investigated to analyse the dynamic changes in their PM retention capacity and grain size over five periods. The relationship between leaf morphological characteristics (leaf size and leaf surface microstructure) and dynamic changes were discussed in PM retention, revealing the influence of leaf morphological characteristics on the amount of PM retention and its composition. The results showed that amount and grain sizes of the retained PM differed significantly among the various studied species; however, the trends in PM retention of different species in the time series were the same. The grain size distributions of PM from the four species displayed a bimodal distribution, in which the main peak distribution range was 5-60 μm and the secondary peak distribution range was 0.4-1 μm. Leaves of smaller sizes and those with rough surfaces had a high PM retention capacity. Leaves with deep grooves are conductive to retaining PM2.5 and PM2.5-10, while leaves with hair are conductive to retaining PM>10. Therefore, the morphological characteristics of leaves should be considered when selecting the use of plant species to alleviate air pollution.Highlights Dynamics of PM retention capacity and grain size distributions of four plant species were analysed.Grain size distributions of PM retained on leaves had a bimodal distribution.Small leaves with grooves or hair are conductive to PM retention.Grooves are conductive to fine PM retention while hairs are conductive to coarse PM retention.

Particle and heavy metal accumulation by two plant species in a coal mining area of Odisha, India

Mining activities lead to severe particulate matter (PM) pollution that consequently has a detrimental effect on ecosystem. A study was therefore conducted in a coal mining area of Odisha, India with an objective to assess the particulate matter pollution on the basis of differential aerodynamic size (PM10 and PM2.5) of the particles, the metallic (Zn and Fe) composition, and also to evaluate their nature of deposition on two identified plant species. The results suggest a significant variation in particle and heavy metal fractions in the ambient air of different sites (p < 0.05). Fe dominated the finer particle (PM2.5) fraction while Zn dominated the coarser counterpart (PM10) in the ambient air. When evaluated for the particle and heavy metal deposition on leaf surface, Shorea robusta performed better in trapping the coarser particles (PM10) while Holarrhena floribunda was found to be an efficient scrubber of the finer particles (PM2.5). Fe deposition on surface of leaves was comparatively higher than Zn irrespective of plant species or size fractions. Therefore, it is concluded that both S. robusta and H. floribunda should be planted in a schematic manner to tackle the particulate pollution in coal mining areas.

Mitigating airborne microplastics pollution from perspectives of precipitation and underlying surface types

The issue of airborne microplastics (AMPs) pollution is receiving increasing attention, but effective solutions are still limited. In this study, AMPs were collected in pairs from an open space and under a tree (Ficus virens) in the suburb of Chengdu, southwest China, to investigate the current pollution status. The meteorological factors and underlying surface types were analyzed to investigate whether these factors could influence and mitigate the pollution of AMPs. The results showed that the fibrous AMPs accounted for the vast majority and the dominant polymers were polypropylene-polyethylene (PP-PE) and polypropylene (PP), with an average deposition flux of AMPs of 192 n/m2/d (22.41 µg/m2/d). Rainfall was found to have the prolonged scavenging efficiency for AMPs, which could extend to 8 to 48 hours after the end of rainfall, and this is a new insight into the relationship with meteorological factors. Interactions between the key underlying surface types and AMPs were also studied. The representative tree species (Ficus virens) had a low interception rate of 6.25% for AMPs and retained mainly small-sized AMPs and more fibers. The masses of AMPs loaded into Chengdu rivers could reach 1149 kg annually, with the unit mass load of 13.6 kg/km2 in urban rivers and 8.2 kg/km2 in suburban rivers. The masses intercepted by trees and bushes throughout the city only offset the masses loading in rivers, and open or sparse buildings were found to be sensitive areas for AMPs, which indicated the urgent need to control and mitigate the pollution of AMPs, especially in these sensitive areas. This work comprehensively analyzed the AMPs pollution from various perspectives and attempted to find ways to mitigate this pollution.

Can national forest city construction mitigate air pollution in China? Evidence from a quasi-natural experiment

As air pollution in Chinese cities becomes a growing concern, measures to alleviate air pollution have attracted the attention of all sectors of society. By using the data for 283 prefecture-level cities from 2003 to 2016, we utilized the quasi-natural experiment of the national forest city construction (NFCC) and employed the difference-in-differences approach to examine the effects of NFCC on air pollution. The results show that the NFCC led to a 12.14% and 4.29% reduction in PM_2.5 concentrations and SO₂ emissions, respectively. A series of robustness tests such as instrumental variable estimates, placebo tests, and eliminating disturbing policies all supported these findings. In addition, we provided evidence that the environment benefits of the NFCC could be explained by increasing green spaces, strengthening environmental regulations, and forming green development models. Furthermore, the results from heterogeneity analysis indicate that the NFCC was more effective in smaller cities, southern cities, and western cities. Our findings are of significance to Chinese cities on the road to sustainable development and provide some insights for other developing countries and emerging markets to control air pollution.

Phytoremediation as a potential technique for vehicle hazardous pollutants around highways

With the synchronous development of highway construction and the urban economy, automobiles have entered thousands of households as essential means of transportation. This paper reviews the latest research progress in using phytoremediation technology to remediate the environmental pollution caused by automobile exhaust in recent years, including the prospects for stereoscopic forestry. Currently, most automobiles on the global market are internal combustion vehicles using fossil energy sources as the primary fuel, such as gasoline, diesel, and liquid or compressed natural gas. The composition of vehicle exhaust is relatively complex. When it enters the atmosphere, it is prone to a series of chemical reactions to generate various secondary pollutants, which are very harmful to human beings, plants, animals, and the eco-environment. Despite improving the automobile fuel quality and installing exhaust gas purification devices, helping to reduce air pollution, the treatment costs of these approaches are expensive and cannot achieve zero emissions of automobile exhaust pollutants. The purification of vehicle exhaust by plants is a crucial way to remediate the environmental pollution caused by automobile exhaust and improve the environment along the highway by utilizing the ecosystem's self-regulating ability. Therefore, it has become a global trend to use phytoremediation technology to restore the automobile exhaust pollution. Now, there is no scientific report or systematic review about how plants absorb vehicle pollutants. The screening and configuration of suitable plant species is the most crucial aspect of successful phytoremediation. The mechanisms of plant adsorption, metabolism, and detoxification are reviewed in this paper to address the problem of automobile exhaust pollution.

Trait-mediated leaf retention of atmospheric particulate matter in fourteen tree species in southern China

Particulate air pollution is a serious threat to human health, especially in urban areas, and trees can act as biological filters and improve air quality. However, studies on greening tree species selection are rare. We measured three particular matter adsorption metrics (PM_2.5, PM_2.5-10, and PM_>10 captured per leaf area) and six functional traits for each of fourteen species and estimated their minimum light requirements based on field surveys. We found that shade-tolerant species captured more coarse particles (PM_2.5-10) than light-demanding species. For traits, a strong negative correlation was found between photosynthetic capacity and adsorption capacity for all three PM size fractions, indicating that in comparison to acquisitive species, conservative species captured larger amounts of particles. Moreover, denser wood species and smaller leaves were more efficient in capturing large particles (PM_>10), while species with "expensive" leaves (high leaf N or P) were more efficient in capturing fine particles (PM_2.5), indicating that capturing large and fine particles was related to mechanical stability traits and leaf surface traits, respectively. Our results demonstrated that the metabolism (e.g., photosynthetic capacity) and chemistry (e.g., leaf N and leaf P) of leaves help explain species capacity to capture PM. We encourage future studies to investigate the ecosystem functions and stress tolerance of tree species with the same framework and trait-based methods.

Urban dust pollution tolerance indices of selected plant species for development of urban greenery in Delhi

The rise in urbanization has led to an increase in dust pollution which is hazardous to the health of living beings. The role of roadside plant species in intercepting particulate matter and improving air quality is well reported. Hence, this study was carried out to determine the ability of various plant species to intercept atmospheric dust and withstand the abiotic stress triggered by dust deposition. In the present investigation, three sites (viz., control, commercial, and industrial) differing in anthropogenic activities and vegetation were selected. Sixteen plant species entailing both trees and shrubs that are commonly occurring at all three sites were selected to estimate their dust interception capacity (DIC). The impact of dust pollution on foliage biochemistry and their tolerance in winter and summer seasons were analyzed. Based on biochemical, biological, and socio-economic parameters, air pollution tolerance index (APTI) and anticipated performance index (API) were evaluated. Both dust load and DIC were found to be two times higher in winter than in the summer season. Terminalia arjuna, Ficus benghalensis, and Plumeria alba were the best dust accumulators, while Prosopis juliflora accumulated least. The highest DIC was observed at the industrial site, for Terminalia arjuna (0.025 mg/cm²/d) in winter and Plumeria alba (0.023 mg/cm²/d) in the summer season. Photosynthetic pigments showed a negative correlation with dust load, while pH, ascorbic acid, electrolytic leakage (E.L.), and proline content showed a positive correlation. In the present study, APTI and API values were highest for Ficus religiosa, Ficus benghalensis, Alstonia scholaris, Dalbergia sissoo, and Terminalia arjuna. Such plant species with wide canopy, large and rough leaf surface area with perforated veins are found to be more suitable and, hence, recommended for the development of greenery to improve air quality in urban areas like Delhi.

Are we ignoring the role of urban forests in intercepting atmospheric microplastics?

Occurrences and characteristics of atmospheric microplastics(MPs) have been widely studied by previous studies, while the mitigation of airborne MPs pollution was not well understood. In this study, atmospheric samples of MPs were collected in pairs on the rooftop and under trees composed of representative afforested species Ficus microcarpa in Chengdu, Southwest China, to explore whether trees could intercept MPs. Results showed that the daily life of human beings and textile industries of urban areas were sources of airborne MPs as revealed by chemical compositions and air trajectories. The trees with the high coverage degree (88%) and large three-dimensional spaces formed by leaves did have the ability to intercept high-density MPs with small sizes under the force of gravity. The intercepting rate was about 16.3%, 12,593 n/m² of fibers and 347.69 kg of MPs could be intercepted by urban forests for one year. However, threshold values of rainfall intensity (12 mm/d) and rainfall amounts (14 mm) were found to limit the intercepting mechanism, and intercepting effects decrease with the increase of rainfall amounts (r =-0.71). This work provides quantitative evidence that elucidated urban forests may act as receptors of airborne MPs, thus improving the air quality and human health.

Foliar dust particle retention and metal accumulation of five garden tree species in Hangzhou: Seasonal changes

As particulate matter and heavy metals in the atmosphere affect the atmospheric quality, they pose a threat to human health through the respiratory system. Vegetation can remove airborne particles and purify the atmosphere. Plant leaves are capable of effectively absorbing heavy metals contained by particulates. To evaluate the effects of different garden plants on the particulate matter retention and heavy metal accumulation, the seasonal changes of dust retention of five typical garden plants were compared in the industrial and non-industrial zones in Hangzhou. Results revealed that these species differed in dust retention with the descending order of Loropetalum chinense > Osmanthus fragrans > Pittosporum tobira > Photinia × fraseri > Cinnamomum camphora, which were related to the microstructure feature of the leaf. These species also showed seasonal variation in dust retention, with the highest in summer, followed by winter, autumn, and spring, respectively. The total suspended particle per unit leaf area was higher in the industrial site (80.54 g m^-2) than in the non-industrial site (19.77 g m^-2). Leaf particles in different size fractions differed among species, while coarse particles (d > ten μm) predominated in most cases. The L. chinense and C. camphora plants accumulated the greatest Pb and Ni compared to other plants. Overall, L. chinense was the best suitable plant species to improve the air quality.

Characteristics of airborne particles retained on conifer needles across China in winter and preliminary evaluation of the capacity of trees in haze mitigation

To fully understand the characteristics of particulate matter (PM) retained on plant leaves (PMR) and the effect of vegetation on haze on a large spatial scale, we investigated needle samples collected from 78 parks and campuses in 31 cities (30 provincial cities) of China and developed a comprehensive method to characterise PMR. Both the PMR load (including water-insoluble particulate matter (WIPM), water-soluble inorganic ions (WSIS) and water-soluble organic matter (WSOM)), with a mean value of 554 ± 345 mg m^-2 leaf area, and component profiles of PMR showed obvious spatial variation across the cities. Though haze pollution levels vary greatly among the 31 cities, the PM retention capacity of needles does not depend on haze level because PMR generally reaches saturation before precipitation in winter. The water-soluble component (WSC, the sum of WSIS and WSOM) accounted for 52.3% of PMR on average, among which WSIS and WSOM contributed 21.4% and 30.9% to PMR, respectively. The dominant ions of WSIS in PMR in the cities were Ca²⁺, K⁺ and NO₃^-, indicating that raised dust, biomass combustion and traffic exhaust are significant sources of PM in China. Compared with previous reports, the particle size distributions of PMR and PM across China were consistent, with fine PM (PM_2.5) constituting a substantial proportion (43.8 ± 17.0%) of PMR. These results prove that trees can effectively remove fine particles from the air, thereby reducing human exposure to inhalable PM. We proposed a method to estimate the annual amount of PMR on Cedrus deodara, with an average value of 11.9 ± 9.6 t km^-2 canopy yr^-1 in China. Compared with the load of dust fall (atmospheric particles naturally falling on the ground, average of 138 ± 164 t km^-2 land area yr^-1 in China), we conclude that trees play a significant role in mitigating haze pollution.

Season impacts on estimating plant's particulate retention: Field experiments and meta-analysis

Plants can effectively remove atmospheric particles, which contribute to air pollution. However, few studies have focused on seasonal variability of plant dust retention, an essential factor to estimate annual dust removal from the atmosphere. This study conducted a field experiment to explore the seasonal variability of particulate retention on evergreen leaved urban greening shrub plants. We performed a meta-analysis to synthesize the available literature on the subject to discuss our findings further. Results showed that particulate matter deposited on leaf surfaces (sPM) in autumn and winter was significantly higher than in spring and summer. In comparison, the particulate matter trapped in epicuticular waxes (wPM) in summer was significantly higher than in the other three seasons. The seasonal differences also existed in both sPM and wPM among particle sizes. The total dust retention of Rhododendron × pulchrum Sweet, Osmanthus fragrans Lour, and Photinia × fraseri Dress were estimated as 360.89 t, 586.66 t, and 448.84 t per year, respectively. They were significantly different from model estimates if only one season was chosen as an estimator. Furthermore, the meta-analysis revealed significant differences among seasons, particle sizes, and different leaf habits (evergreen or deciduous). In contrast, no significant differences were observed between life forms or between growth forms. Our findings both from field experiment and met-analysis highlights that seasonal variation can significantly affect the dust retention capacity of plants, which should be taken into account into particle matter retention capacity evaluations.

Relationship between leaf dust retention capacity and leaf microstructure of six common tree species for campus greening

Green plants can effectively retain atmospheric particle pollution and purify the air. As an important part of the campus ecosystem, they have a significant effect on absorbing atmospheric pollutants and improving the quality of the campus air environment. Six common greening plants were selected as the object in Xiangnan University. The dust retention capacity of plant leaves, leaf micro-structure, and leaf contact angle were measured. The results show that the highest amounts of dust removed by Osmanthus fragrans Lour was 3.451 g/m², and the least amounts of dust removed by Magnolia denudata Desr. was 1.005 g/m², and the maximum is 4.148 times of the minimum. The micro-structure of the plant leaf surface affects the amount of dust retention. The plant dust retention capacity with gully of the leaf surface and fluff, stomata, and sawtooth structure is relatively strong; The plant dust retention capacity with smooth, wax layer, leathery, and gully is relatively weak. The leaf contact area of a plant is related to its dust retention ability, that is, when the measured leaf contact angle is <90°, the leaf dust retention ability of the plant is strong. When the blade contact angle is >90°, the dust retention capacity is weak.

Evaluation of air pollution tolerance index and anticipated performance index of six plant species, in an urban tropical valley: Medellin, Colombia

High atmospheric pollution levels in urban areas have become a global problem that threatens both human health and urban ecosystems. Trees that grow near areas with vehicular and industrial emissions can be highly affected, since they constitute the main barrier for emitted pollutants, with trees being either tolerant or sensitive to them. Different methodologies worldwide have been implemented to evaluate the tolerance and sensitivity of tree species to atmospheric pollutants. In this research, the air pollution tolerance index (APTI) and the anticipated performance index (API) are evaluated in order to determine both the degree of tolerance or sensitivity of trees to pollutants in the air and their performance in urban areas. To this end, six tree species found in four biomonitoring zones in the city of Medellín, Colombia, were selected: Mangifera indica, Tabebuia chrysantha-rosea, Erythrina fusca, Jacaranda mimosifolia, Fraxinus uhdei, and Spathodea campanulata. A total of 54 individual trees were evaluated by means of the APTI and API, and it was determined that the species with the highest tolerance (APTI≥16) and the best performance (81<API<90) was Mangifera indica, which highlights the importance of this species in urban areas with air quality problems. On the other hand, it was determined that the most sensitive species (APTI≤11) are Tabebuia chrysantha-rosea, Erythrina fusca, and Spathodea campanulata, while the species with poor performance (41<API<50) are Tabebuia chrysantha-rosea, Erythrina fusca, and Jacaranda mimosifolia. These values, therefore, can be used to classify which species can be planted as pollutant sinks and which as air quality bioindicators and thus highlight the importance of urban forests and trees for environmental management and planning in big cities with air quality problems.

Dust deposition on vegetation leaves in Shanghai, China

Urban vegetation can deposit dust to reduce pollution, and dust retention capacity of vegetation has become an important indicator for urban ecological construction. We selected five representative vegetation in Shanghai to explore the regularity of dust deposition on vegetation leaves. Due to the influence of leaf area and surface characteristics, the amount of dust deposition was significantly different to each vegetation; Vegetation shows different dust retention capacity under different pollution intensity, before this capacity reaches its limit, and it will increase with the increase of dust content in the environment; Furthermore, water content of leaves was an important factor affecting dust retention capacity by vegetation. There was a linear positive correlation between the two variables. Our work suggests that the dust retention capacity of vegetation leaves was affected by various factors, but it showed certain regularity, which can provide a scientific basis for the configuration of urban green plant species.

How can vegetation protect us from air pollution? A critical review on green spaces' mitigation abilities for air-borne particles from a public health perspective - with implications for urban planning

Air pollution causes the largest death toll among environmental risks globally, but interventions to purify ambient air remain inadequate. Vegetation and green spaces have shown reductive effects on air-borne pollutants concentrations, especially of particulate matter (PM). Guidance on green space utilisation for air quality control remains scarce, however, as does its application in practise. To strengthen the foundation for research and interventions, we undertook a critical review of the state of science from a public health perspective. We used inter-disciplinary search strategies for published reviews on green spaces and air pollution in key scientific databases. Using the PRISMA checklist, we systematically identified reviews with quantitative analyses. For each of the presented PM mitigation mechanisms, we conducted additional searches focused on the most recent articles published between 2016 and early 2021. The included reviews differentiate three mitigation mechanisms of green spaces for PM: deposition, dispersion and modification. The most studied mechanism is deposition, particularly measures of mass and settling velocity of PM on plant leaves. We consolidate how green space setups differ by scale and context in their potentials to reduce peak exposures, stationary (point) or mobile (line) pollution sources, and the potentially most harmful PM components. The assessed findings suggest diverse optimisation options for green space interventions, particularly concerning plant selection, spatial setup, ventilation and maintenance - all alongside the consideration of supplementary vegetation effects like on temperature or water. Green spaces' reductive effects on air-borne PM concentrations are considerable, multi-mechanistic and varied by scale, context and vegetation characteristics. Such effect-modifying factors must be considered when rethinking public space design, as accelerated by the COVID-19 pandemic. Weak linkages amid involved disciplines motivate the development of a research framework to strengthen health-oriented guidance. We conclude on an urgent need for an integrated and risk-based approach to PM mitigation through green space interventions.

Magnetism and Grain-Size Distribution of Particles Deposited on the Surface of Urban Trees in Lanzhou City, Northwestern China

Studies on the variation in the particulate matter (PM) content, Saturation Isothermal Remanent Magnetization (SIRM), and particle grain-size distribution at a high spatial resolution are helpful in evaluating the important role of urban forests in PM removal. In this study, the trees located in dense urban forests (T0) retained more PM than trees located in open spaces (T1–T4); the SIRM and PM weight of T0 were 1.54–2.53 and 1.04–1.47 times more than those of T1–T4, respectively. In addition, the SIRM and PM weight decreased with increasing distance to the road, suggesting that distance from pollution sources plays a key role in reducing the air concentration of PM. The different grain-size components were determined from frequency curve plots using a laser particle-size analyzer. A unimodal spectrum with a major peak of approximately 20 μm and a minor peak between 0.1 and 1 μm was observed, indicating that a large proportion of fine air PM was retained by the needles of the study trees. Additionally, more <2.5 μm size fraction particles were observed at the sampling site near the traffic source but, compared to a tree in a row of trees, the percentage of the >10 μm size fraction for the tree in the dense urban forest was higher, indicating that the particles deposited on the needle surface originating from traffic sources were finer than those from natural atmospheric dust. The exploration of the variation in the PM weight, SIRM, and grain size of the particles deposited on the needle surface facilitates monitoring the removal of PM by urban forests under different environmental conditions (e.g., in closed dense urban forests and in open roadside spaces), different distances to roads, and different sampling heights above the ground.

Variations in airborne pollen and spores in urban Guangzhou and their relationships with meteorological variables

Airborne pollen causes various types of allergies in humans, and the extent of allergic infection is related to the presence of different types of sporo-pollen and existing meteorological conditions in a certain area. Therefore, an aeropalynological study of 72 airborne samples with a hydrofluoric acid (HF) treatment was conducted in the Haizhu district of Guangzhou, China, in 2016, to identify the temporal variations in airborne sporo-pollen and the relationship between airborne sporo-pollen concentrations and different meteorological variables in Guangzhou, China. Forty-five types of airborne pollen, seven types of airborne spores, and some undetermined sporo-pollen taxa were identified with two separate plant habitats occurring during this period (from January to December 2016): arboreal pollen (tree-based) and non-arboreal pollen (herb, shrub, aquatic, liane, etc.). Furthermore, the daily records of four key meteorological variables (temperature, precipitation, relative humidity, and wind speed) were acquired to distinguish the pollen seasons and correlated with Spearman's rho test to establish a pollen-weather data book with the seasonal variations. The two leading seasons were identified based on pollen abundance: spring and autumn. Among them, the primary dominant sporo-pollen families during the spring season were Poaceae, Pinaceae, Euphorbiaceae, Moraceae, Microlepia sp., and Polypodiaceae. Conversely, Artemisia sp., Asteraceae, Cyperaceae, Poaceae, Alnus sp., Corylus sp., Myrtaceae, and Rosaceae were the dominant pollen species during autumn. However, few pollen grains were identified in January, May-July, and December. The statistical analysis revealed that temperature had both positive and negative correlations with sporo-pollen concentrations. However, precipitation and relative humidity had a strong impact on the sporo-pollen dispersion and exhibited a negative correlation with the sporo-pollen concentrations. The wind speed had a positive but strong correlation with the sporo-pollen concentration during the study period. Some inconsistent results were found due to environmental variations, vegetation type, and climate change around the study area. This study will facilitate the identification of pollen seasons to prevent the occurrence of pollen-related allergies in the Guangzhou city area.

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.

Micro-morphological response of some native dicotyledonous species to particulate pollutants emitted from stone crushing activities

Stone crushing processes release particulates and associated noxious substances in our surroundings that are continuously destructing environmental conditions and ecosystem health. Morpho-anatomical changes in some medicinally important native species (Aerva javanica, Calotropis procera, Digera muricata, Euphorbia prostrata, Euploca strigosa, and Peganum harmala) exposed to heavy dust pollution were evaluated. These species selected on the basis of their ubiquitous distribution in the area. Two sites were selected in the Kirana Hills, Sargodha, one near stone crushers within 500-m radius (polluted) and the other 4 km away from the crushers (control) varying significantly in amount of dust particles received. A decrease in plant height of all species from dust-polluted sites was observed. Reduction in height was more prominent in species like C. procera and D. muricata. Stem sclerification increased in C. procera and E. prostrata from the polluted site that is an indication of better tolerance to dust pollution. C. procera showed increased stem and leaf epidermis, stem sclerenchyma, and stem vascular bundles, which can increase resistance to dust pollution. E. strigosa was the most sensitive species in which all morpho-anatomical factors decreased. Survival of plant species depended on specific structural modifications in dermal, mechanical, parenchymatous, and vascular tissue. Overall, dust pollution severely affected plant morphological and micro-morphological traits, but the response of selected species to dust pollution was variable. It is concluded that stem and leaf anatomical traits like size of dermal and storage tissue thickness and stomatal density are good indicators for biomonitoring of dust pollution.

Dry deposition of particulate matter and its associated soluble ions on five broadleaved species in Taichung, central Taiwan

Many studies have estimated particulate matter (PM) removal by urban trees using dry deposition models; however, few studies have quantified the accuracy of their results. Thus, this study investigated the dry deposition of PM and its associated soluble ions in five broadleaved species in three districts of Taichung, central Taiwan, through field experiments. The total suspended particulate (TSP) dry deposition flux on leaf surfaces varied with sampling time, site, and tree species. By contrast, single-factor effects were observed for PM₁₀ and PM_2.5. The average dry deposition velocities of TSPs, PM₁₀, and PM_2.5 were 0.63, 0.062, and 0.028 cm s^-1, respectively. Moreover, the dry deposition velocities of sulfate and nitrate were estimated to be 0.186 and 0.194 cm s^-1, respectively. A significant relationship was observed between the ambient concentration and the dry deposition flux for all size fractions of PM. By contrast, weak and negative correlations were found between particle deposition velocity and wind speed. The measured PM_2.5 dry deposition velocity was approximately equal to the dry deposition velocity obtained with the i-Tree model (0.03 cm s^-1), which indicated the promising application potential of i-Tree in Taiwan. Compound and rough leaves, such as leaves of the Taiwan golden-rain tree, intercepted a high amount of PM_2.5, whereas the pongam tree, which has thin leaves and wax surfaces, exhibited the lowest TSP interception. Species difference mostly occurred in the dry deposition flux of nitrate rather than sulfate; however, the interception of sulfate by trees revealed the possibility of the long-range transport of air pollutants. The results of this study elucidate the dry deposition of PM and its associated soluble ions in real-world situations.

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.

Quantifying particulate matter reduction and their deposition on the leaves of green infrastructure

The green infrastructure (GI) is identified as a passive exposure control measure of air pollution. This work examines particulate matter (PM) reduction by a roadside hedge and its deposition on leaves. The objectives of this study are to (i) quantify the relative difference in PM concentration in the presence of GI and at an adjacent clear area; (ii) estimate the total mass and number density of PM deposited on leaves of a hedge; (iii) ascertain variations in PM deposition at adult (1.5m) and child (0.6 m) breathing levels on either side of a hedge; (iv) illustrate the relationship between PM deposition to leaves and ambient PM concentration reductions; and (v) quantify the elemental composition of collected particles of the leaves on different heights and sides of hedge. PM reduction of 2-9% was observed behind hedge compared to a clear area and followed a trend of ΔPM₁ >ΔPM₁₀ >ΔPM_2.5. Counting of particles was found to be an effective method to quantify deposition than weighting methods. Sub-micron particles (PM₁) dominated particle deposition on leaves at all sampling points on both sides of the hedge. PM mass deposition and number concentration to the leaves on traffic-facing side was up to 36% and 58% higher at 0.6m compared with 1.5m height, respectively. Such a difference was absent on the backside of the hedge. The SEM-EDS analysis showed up to 12% higher traffic-originated particles deposited to leaves on the traffic-facing side compared to the backside. The naturally occurring particles dominated in identified particles on leaf samples from all collection points on the hedge. These new evidence expand our understanding of PM reduction of GI in the near-road environment and its variations in particle deposition, depending on height and sides of GI, which could allow a better parameterisation of dispersion-deposition models for GI assessment at micro-scale.

Urban wastelands: On the frontline between air pollution sources and residential areas

In urban areas, particulate matter (PM) represents an increasing threat to human health. The ability of plants in parks and along roads in cities to accumulate PM has already been demonstrated, but nothing is known about the effect of wasteland vegetation on air quality, despite a significant proportion of greenery in polluted areas being on wastelands. The aim of this study was to document the accumulation of PM and trace elements (TE) by wasteland species (Robinia pseudoacacia L., Populus × canescens (Aiton) Sm., Acer negundo L., Solidago gigantea (Aiton) and Poaceae) growing on Central European urban wastelands with differing levels of air pollution. On average, the largest amounts of PM accumulated on the foliage of R. pseudoacacia and S. gigantea, and the smallest amounts accumulated on P. × canescens leaves. However, accumulation of PM depended more on the distance from the emission source than on species selection, and was higher on the polluted wasteland where the plants' gas exchange was the lowest. The results also suggest that in order to effectively accumulate PM from the air, it is critical to have the correct configuration of plants, with the wasteland vegetation having a layered structure and layers differing in PM retention, as shown in this study using the examples of R. pseudoacacia (a tall tree with low PM retention) and S. gigantea (below-tree vegetation with high PM retention). P. × canescens accumulated the highest concentrations of Cd and Zn, S. gigantea accumulated the highest concentration of Cu, and Poaceae accumulated the highest concentrations of Cr and Ni. These findings have implications for urban vegetation management in areas where there is no organised greenery, and offer proof that vegetation in wasteland areas should be maintained since it is an excellent tool for reducing concentrations of PM at its place of origin.

Abundance of human pathogen genes in the phyllosphere of four landscape plants

The surface of leaf, also known as phyllosphere, harbors diverse microbial communities which include both beneficial microorganisms promoting plants growth and harmful microorganisms, such as plant pathogens and human pathogens. Several studies have investigated the interaction between plants and human pathogens, while few works have focused on the quantitative analysis of pathogenic bacteria. On the basis of real-time polymerase chain reaction (qPCR), this study aimed to evaluate the abundance of following genes: the nuc and pvl of Staphylococcus aureus, the lytA and psaA of Streptococcus pneumoniae, and the ttr and invA of Salmonella enterica in the phyllosphere of four landscape plants (Nandina domestica, Rhododendron pulchrum, Photinia serrulata, and Cinnamomum camphora) growing in two habitats. Our results indicated that the relative abundance of pathogenic genes in the phyllosphere ranged from 10^-9 to 10^-6. The specific genes of S. aureus, S. pneumoniae and S. enterica in landscape plants were pvl, lytA and ttr, respectively. The two pathogenic genes of S. pneumoniae and the 16S rRNA gene were mainly affected by habitats, host species, and habitats-species interaction. Moreover, for the abundance of lytA and 16S rRNA, results showed that plants present in roadside with traffic pollution were relatively higher than that of campus with less pollution. The N. domestica and C. camphora were recommended for planting along the roadsides due to lower abundance of pathogenic genes. However, we have observed no significant difference in the abundance of pathogenic genes among four plants in the campus. Thereby, this study provided a valuable reference for selecting landscape plants in view of human health.

Deciphering source contributions of trace metal contamination in urban soil, road dust, and foliar dust of Guangzhou, southern China

Trace metal contamination prevails in various compartments of the urban environment. Understanding the roles of various anthropogenic sources in urban trace metal contamination is critical for pollution control and city development. In this study, the source contribution from various contamination sources to trace metal contamination (e.g., Cu, Pb, Zn, Co, Cr and Ni) in different environmental compartments in a typical megacity, Guangzhou, southern China, was investigated using the receptor model (Absolute Principal Component Scores-Multiple Linear Regression, APCS-MLR) coupled with the Kriging technique. Lead isotopic data and APCS-MLR analysis identified industrial and traffic emissions as the major sources of trace metals in surface soil, road dust, and foliar dust in Guangzhou. Lead isotopic compositions of road dust and foliar dust exhibited similar ranges, implying their similar sources and potential metal exchange between them. Re-suspended soil contributed to 0-38% and 25-58% of the trace metals in the road dust and foliar dust, respectively, indicating the transport of the different terrestrial dust. Spatial distribution patterns implied that Cu in the road dust was a good indicator of traffic contamination, particularly with traffic volume and vehicle speed. Lead and Zn in foliar dust indicated mainly industrial contamination, which decreased from the emission source (e.g., a power plant and steel factory) to the surrounding environment. The spatial influence of industry and traffic on the contamination status of road dust/foliar dust was successfully separated from that of other anthropogenic sources. This study demonstrated that anthropogenic inputs of trace metals in various environmental compartments (e.g., urban soil, road dust, and foliar dust) can be evaluated using a combined APCS-MLR receptor model and geostatistical analysis at a megacity scale. The coupled use of APCS-MLR analysis, geostatistics, and Pb isotopes successfully deciphered the spatial influence of the contamination sources in the urban environment matrix, providing some important information for further land remediation and health risk assessment.

Size distribution of particulate matter in runoff from different leaf surfaces during controlled rainfall processes

The presence of plant leaves has been shown to lower the risks of health problems by reducing atmospheric particulate matter (PM). Leaf PM accumulation capacity will saturate in the absence of runoff. Rainfall is an effective way for PM to "wash off" into the soil and renew leaf PM accumulation. However, little is known about how PM wash-off varies with PM size and health problems caused by particulate pollution vary with PM size. This study thus used artificial rainfall with six plant species to find out how size-fractioned PM are washed off during rain processes. Total wash-off masses in fine, coarse and large fractions were 0.6-10.3 μg/cm², 1.0-18.8 μg/cm² and 4.5-60.1 μg/cm² respectively. P. orientalis (cypress) and E. japonicus (evergreen broadleaved shrub) had the largest wash-off masses in each fraction during rainfall. P. cerasifera (deciduous broadleaved shrub) had the largest cumulative wash-off rates in each fraction. Rainfall intensity had more influence on wash-off masses and rates of large particles for six species and for small particles in evergreen species, but limited effect on wash-off proportions. Wash-off proportions decreased in large particles and increased in small particles along with rainfall. The results provide information for PM accumulation renewal of plants used for urban greening.

How Much Does Weather Matter? Effects of Rain and Wind on PM Accumulation by Four Species of Australian Native Trees

As interest in improving urban air quality grows, phytoremediation—amelioration through plants—is an increasingly popular method of targeting particulate matter (PM), one of the most harmful pollutants. Decades of research has proven that plants effectively capture PM from air; however, more information is needed on the dynamics of PM accumulation. Our study evaluated the effects of meteorological conditions on the dynamics of PM deposition, wash off and resuspension using four Australian tree species growing under natural conditions near a busy highway. Accumulation of PM on foliage was analyzed over the short term (daily changes) and over a longer time period (weekly changes). The results obtained were correlated with ambient concentrations of PM2.5 and PM10, rain intensity and wind strength. The highest accumulation of PM was recorded for Eucalyptus ovata (100.2 µg cm−2), which also had the thickest wax layer while the lowest was for Brachychiton acerifolius (77.9 µg cm−2). PM accumulation was highly changeable, with up to 35% different PM loads on the foliage from one day to the next. Importantly these dynamics are hidden in weekly measurements. Changes in PM deposition on the leaves was mostly affected by rain and to a lesser extent by wind, but the extent of the effect was species specific. The large PM fraction (10–100 µm) was the first to be removed from leaves, while the smallest PM fraction (0.2–2.5 µm) was retained for longer. Precipitation affects also PM retained in waxes, which until now were believed to be not affected by rain. This work demonstrates important interactions between PM load and weather, as well as adding to the small inventory of Australian native tree PM accumulation data.

Particle removal in polluted cities: Insights from the wash-off process dynamics for different wetland plants

Particulate matter (PM) in the atmosphere is a threat to human health. Wetland plants were confirmed to accumulate particles on the leaf surface; at the same time, rainfall could wash-off particles and accelerate the whole removal process, however, the dynamic processes occurring during rainfall events on wetland plants remain unclear. In order to provide sustainable strategies for authorities to take measures, we need to figure out how to reduce PM on leave surface by artificial rainfall efficiently. Four wetland species (Scirpus validus, Typha orientalis, Phragmites australis, and Iris wilsonii) were selected to examine for leave surface accumulation and simulate the experiment. We estimated the wash-off ability of rainfalls with three different intensities (15, 30, and 60 mm h^-1) and determined the proportions of different PM size-fractions washed by the rains. The results showed that particles accumulated on the surface could be washed off efficiently (78% ∼ 89%) by the simulated rainfalls. The removal rates were high in the first 30 min and large particles comprised a large proportion of the removed particles. The rainfall with the intensity of 30 mm h^-1 removed the most particles among three different rainfall intensities. When the rainfall intensity increased, fine particles (PM_2.5) could be washed off more easily. Moreover, with a thinner wax layer, fine particles on wetland plants' leaf surfaces might be more easily removed by the rains. While wash off the plants, spraying rains with the intensity of 30 mm h^-1 for about 30 min every time (high intensity with shorter time) may be particle removal efficiency.

Magnetic characteristics of Juniperus formosana needles along an urban street in Lanzhou, Northwest China: the variation of different season and orientation

The magnetic properties of particulate matter (PM) deposited on the needles of Juniperus formosana along an urban street in Lanzhou city were measured to evaluate the variations of PM concentration in different seasons by varying distance from the road. The magnetism of PM deposited in this context was significantly higher in winter than in summer, which may reflect changes of atmospheric particle concentrations. Needle samples which were collected from the road-facing side exhibiting significantly stronger magnetism compared with those which were collected from the opposite side of the road, indicating the distance from pollution source to the needles as a factor controls the amount of PM. The results of this study show that the needles of Juniperus formosana are effective traps for PM and can therefore be used to monitor pollution fluxes in different seasons in an important urban context in NW China.

Resuspension of settled atmospheric particulate matter on plant leaves determined by wind and leaf surface characteristics

Atmospheric particulate matter (APM) is temporarily settled on the leaf surface of plants and will return to the air via the resuspension process under certain meteorological conditions. How leaf surface characteristics affect the resuspension of settled APM on the leaf surface has been rarely studied. Therefore, the resuspension of APM after settling on plant leaves was analyzed using four common urban greening species, including Prunus triloba, Platanus acerifolia, Lonicera maackii, and Cercis chinensis. The results show that the leaf hair density has a significantly positive correlation with the maximum particulate matter (PM) retention and natural PM retention (p < 0.05). Under the same wind speed, the proportions of the resuspended PM that settled on the leaf surfaces of the four plant species increase with the wind blowing time. During the same wind blowing time, the resuspension rate of the settled PM on leaf surfaces of P. triloba, P. acerifolia, and L. maackii increase with the wind speed. The leaf hair and stomatal density is negatively correlated to the resuspension rate of PM under the wind speed of 1 m s^-1 (p < 0.05), and the stomatal density is also negatively correlated to the resuspension rate of PM under the wind speed of 5 m s^-1 for 10 min or 20 min (p < 0.05). However, as the wind speed further increase, the leaf characteristics are no longer correlated to the resuspension rate of PM (p > 0.05). These results indicate that when the wind force (wind speed + wind blowing time) is small, the stomatal density and leaf hair density have a significant effect on APM resuspension. When the wind force is large, the influence of leaf surface structure on APM resuspension becomes less profound. APM resuspension is comprehensively affected by the external wind and the leaf surface characteristics, and these two factors jointly determine the fate of the PM after it settles on leaves.

The Effect of Vegetation Enhancement on Particulate Pollution Reduction: CFD Simulations in an Urban Park

Vegetation in parks is regarded as a valuable way to reduce particulate pollution in urban environments but there is little quantitative information on its effectiveness. The aim of this study was to conduct on-site measurements and computational fluid dynamic (CFD) simulations to determine the aerodynamic and deposition effects of vegetation enhancement on particulate matter (PM) dispersions in an urban park in Xi’an, China. Initially, the airflow characteristics and deposition effects of vegetation were predicted and compared with measured air velocities and particulate pollution data to validate the numerical modeling. Then, associated coverage ratios and supplementary green areas (tree coverage ratio, crown volume coverage (CVC), and roof greening) were added to numerical simulations. After a series of numerical simulations and comparisons, results indicated that: (1) Numerical models with simplified vegetation method could reproduce the distribution of particulate matter concentrations in the real park environment; (2) with a tree coverage ratio >37.8% (or CVC > 1.8 m3/m2), the pedestrian-level PM2.5 could meet the World Health Organization’s air quality guidelines (IT-1) standards in the park; (3) roof greening on leeward buildings produced greater PM removal effects compared with windward buildings; and (4) the most economical and reasonable tree coverage ratio and CVC to reduce atmospheric PM in urban parks should be 30% and 1.8 m3/m2, respectively. These results are useful guidelines for urban planners towards a sustainable design of vegetation in urban parks.

Evaluation of historical atmospheric pollution in an industrial area by dendrochemical approaches

We conducted a dendrochemical study in order to evaluate the exposure of territories and populations to different types of pollutants and to characterise the history of pollution in one of the most intensely industrialised areas of Europe: the industrial port zone of Fos, also heavily urbanised. To perform the study, two tree species have been selected, Pinus halepensis and Populus nigra, on a rural plot located roughly 20 km away from the industrial harbour, an urban plot located in the city of Fos-sur-Mer and an industrial plot. Our study indicated that poplar was a more relevant model for the dendrochemical studies, exhibiting a higher bioaccumulation capacity than pine except for Hg, Sb and Mn. Moreover, thanks to this work, we observed significant exposure of the trees in the urban and industrial areas to As, Cd, Co, Cu, Mo, Sb, Zn, Al, Ca, and Mg, highlighting the exposure of the territory and populations living in the vicinity of the industrial harbour. The temporal variability of the concentrations measured in the tree rings corresponds to the increasing industrialisation of the territory as well as to the evolution of the industrial processes. Thus, this project highlighted the exposure of the Gulf of Fos to atmospheric emissions (industrial, road and urban) of the industrial harbour as well as the changes over time. It also pointed out the relevance of using dendrochemistry to measure atmospheric exposure of metals and metalloids and its temporal variability.

Phytoextraction of heavy metals from contaminated soil, water and atmosphere using ornamental plants: mechanisms and efficiency improvement strategies

Accumulation of heavy metals (HMs) in soil, water and air is one of the major environmental concerns worldwide, which mainly occurs due to anthropogenic activities such as industrialization, urbanization, and mining. Conventional remediation strategies involving physical or chemical techniques are not cost-effective and/or eco-friendly, reinforcing the necessity for development of novel approaches. Phytoextraction has attracted considerable attention over the past decades and generally refers to use of plants for cleaning up environmental pollutants such as HMs. Compared to other plant types such as edible crops and medicinal plants, ornamental plants (OPs) seem to be a more viable option as they offer several advantages including cleaning up the HMs pollution, beautification of the environment, by-product generation and related economic benefits, and not generally being involved in the food/feed chain or other direct human applications. Phytoextraction ability of OPs involve diverse detoxification pathways such as enzymatic and non-enzymatic (secondary metabolites) antioxidative responses, distribution and deposition of HMs in the cell walls, vacuoles and metabolically inactive tissues, and chelation of HMs by a ligand such as phytochelatins followed by the sequestration of the metal-ligand complex into the vacuoles. The phytoextraction efficiency of OPs can be improved through chemical, microbial, soil amending, and genetic approaches, which primarily target bioavailability, uptake, and sequestration of HMs. In this review, we explore the phytoextraction potential of OPs for remediation of HMs-polluted environments, underpinning mechanisms, efficiency improvement strategies, and highlight the potential future research directions.

How Outdoor Trees Affect Indoor Particulate Matter Dispersion: CFD Simulations in a Naturally Ventilated Auditorium

This study used computational fluid dynamics (CFD) models, coupling with a standard k-ε model based on the Reynolds-averaged Navier-Stokes (RANS) approach and a revised generalized drift flux model, to investigate effects of outdoor trees on indoor PM1.0, PM2.5, and PM10 dispersion in a naturally ventilated auditorium. Crown volume coverage (CVC) was introduced to quantify outdoor trees. Simulations were performed on various CVCs, oncoming wind velocities and window opening sizes (wall porosities were 3.5 and 7.0%, respectively, for half and fully opened windows). The results were as follows: (1) A vortex formed inside the auditorium in the baseline scenario, and the airflow recirculation created a well-mixed zone with little variation in particle concentrations. There was a noticeable decrease in indoor PM10 with the increasing distance from the inlet boundary due to turbulent diffusion. (2) Assuming that pollution sources were diluted through the inlet, average indoor particle concentrations rose exponentially with increasing oncoming wind speed. PM10 changed most significantly due to turbulent diffusion and surface deposition reduction intensified by the increased wind velocity. (3) Increasing the window opening improved indoor cross-ventilation, thus reducing indoor particle concentrations. (4) When 2.87 m³/m² ≤ CVC ≤ 4.73 m³/m², indoor PM2.5 could meet requirements of the World Health Organization's air quality guidelines (IT-3) for 24-hour mean concentrations; and (5) average indoor particle concentrations had positive correlations with natural ventilation rates (R² = 0.9085, 0.961, 0.9683 for PM1.0, PM2.5, and PM10, respectively, when the wall porosity was 3.5%; R² = 0.9158, 0.9734, 0.976 for PM1.0, PM2.5, and PM10, respectively, when the wall porosity was 7.0%).

SUNSPACE, A Porous Material to Reduce Air Particulate Matter (PM)

The World Health Organization reports that every year several million people die prematurely due to air pollution. Poor air quality is a by-product of unsustainable policies in transportation, energy, industry, and waste management in the world's most crowded cities. Particulate matter (PM) is one of the major element of polluted air. PM can be composed by organic and inorganic species. In particular, heavy metals present in PM include, lead (Pb), mercury (Hg), cadmium, (Cd), zinc (Zn), nickel (Ni), arsenic (As), and molybdenum (Mo). Currently, vegetation is the only existing sustainable method to reduce anthropogenic PM concentrations in urban environments. In particular, the PM-retention ability of vegetation depends on the surface properties, related to the plant species, leaf and branch density, and leaf micromorphology. In this work, a new hybrid material called SUNSPACE (SUstaiNable materials Synthesized from by-Products and Alginates for Clean air and better Environment) is proposed for air PM entrapment. Candle burning tests are performed to compare SUNSPACE with Hedera Helix L. leafs with respect to their efficacy of reducing coarse and fine PM. The temporal variation of PM10 and PM2.5 in presence of the trapping materials, shows that Hedera Helix L. surface saturates more rapidly. In addition, the capability of SUNSPACE in ultrafine PM trapping is also demonstrated by using titanium dioxide nanoparticles with 25 nm diameter. Scanning electron microscope (SEM) and Transmission electron microscope (TEM) images of SUNSPACE after entrapment tests highlight the presence of collected nanoparticles until to about 0.04 mm in depth from the sample surface. N2 physisorption measurements allow to demonstrate the possibility to SUNSPACE regeneration by washing.

Phyllostachys edulis forest reduces atmospheric PM2.5 and PAHs on hazy days at suburban area

This study is aim to illustrate Phyllostachys edulis' role in affecting air quality under hazy day and solar day. P. edulis is a crucial plants growing well at suburban area at China Southern. In this manuscript, on 2 weather conditions (hazy day; solar day), changes in atmospheric particulate matter (PM), polycyclic aromatic hydrocarbons (PAHs), associated volatile organic compounds (VOCs), and PAHs in leaves and soils were measured, with PM-detection equipment and the GC-MC method, in a typical bamboo forest at suburban areas. The results showed that: (1) Bamboo forest decreased atmospheric PM_2.5 and PM₁₀ concentrations significantly by 20% and 15%, respectively, on the hazy day nightfall time, when they were times higher than that on any other time. Also, similar effects on atmospheric PAHs and VOCs were found. (2) Significant increases in PAHs of leaves and soil were found inside the forest on the hazy day. (3) Bamboo forest also reduced the atmospheric VOC concentrations, and changed the compounds of 10 VOCs present in the highest concentration list. Thus, bamboo forests strongly regulate atmospheric PM_2.5 through capture or retention, for the changes in atmospheric VOCs and increase in PAHs of leaves and soil.

Quantifying particulate matter accumulated on leaves by 17 species of urban trees in Beijing, China

Airborne particulate matter (PM) has become a serious environmental problem and harms human health worldwide. Trees can effectively remove particles from the atmosphere and improve the air quality. In this study, a washing and weighing method was used to quantify accumulation of water-soluble ions and insoluble PM on the leaf surfaces and within the wax of the leaves for 17 urban plant species (including 4 shrubs and 13 trees). The deposited PM was determined in three size fractions: fine (0.2-2.5 μm), coarse (2.5-10 μm), and large (> 10 μm). Significant differences in the accumulation of PM were detected among various species. The leaves of Platycladus orientalis and Pinus armandi were the most effective in capturing PM. Across the species, 65 and 35% of PM, on average, deposited on the leaf surface and in the wax, respectively. The greatest PM accumulation by mass on leaves was in the largest PM size fraction, while the accumulation of coarse and fine particle size fractions was smaller. Water-soluble ions accumulated on the leaf surfaces contributed 28% to the total PM mass, on average. This study demonstrated that leaves of woody plants accumulate PM differently, and the most effective plant species should be selected in urban areas for attenuating ambient PM.

Capacity of six shrub species to retain atmospheric particulates with different diameters

Garden plants can absorb and retain atmospheric particles and are important for remediating environmental pollution. In this study, the dust retention characteristics of six typical shrub species were measured in the greenbelt of a road in the Chengyang District of Qingdao, China, and the maximum capacity for dust retention of each species was determined. The different diameters and areas occupied by particulate matter (PM) were analyzed on the leaf surfaces of the plants. Based on the results for the six shrub species, the rank order of average content of dust retention per unit leaf area was Euonymus japonicus > Pyracantha fortuneana > Ligustrum vicaryi > Amygdalus triloba > Ligustrum sinense > Forsythia suspensa, whereas the rank order of average content per unit volume was E. japonicus > A. triloba > P. fortuneana > L. vicaryi > L. sinense > F. suspensa. The maximum content of dust retention per unit leaf area was reached in approximately 24 days. Plants retained atmospheric PM primarily on the upper leaf surfaces. The primary portion of particles on the leaves was PM₁₀ (over 80%), and PM_2.5 was the principal component of PM₁₀. Leaf surface structure significantly affected the abilities of plants to retain PM, and the plants with a thick wax layer or large and dense stomata adsorbed more PM, such as E. japonicus. This study provides a scientific basis for the capacity of landscape plants to retain different diameter particulates.

Influence of rainfall duration and intensity on particulate matter removal from plant leaves

Rainfall influences removal of airborne particulate matter (PM) from leaf surfaces through a process called wash off resulting in throughfall that carries PM to the ground. The present study examined the effects of rainfall characteristics on PM wash-off mass and rate from the foliage of four broadleaf species, to investigate retention of PM pollution. In a controlled rainfall simulation experiment, rainfall intensity was set to 15, 30, and 50mmh^-1, and sampling intervals for the three rainfall intensities were divided into 10, 5, and 3min, respectively. Of the plants examined, the evergreen shrub Euonymus japonicus had the greatest surface PM accumulation before rainfall (165μgcm^-2), maximum wash-off during the first 2.5mm of rain (30μgcm^-2), and maximum surface PM retention after rainfall (24μgcm^-2). Fitting observations with the Box Lucas regression model, cumulative PM wash-off rates increased with cumulative rainfall amount, until the curves tended to become steady after rain exceeded 12.5mm. Wash off removed 51 to 70% of surface PM accumulation. As rainfall intensity increased, the duration of PM wash-off decreased, and wash-off rates were highest during the first rainfall interval. However, there was no significant difference between PM wash-off rates for rainfall intensities of 30 and 50mmh^-1 in each rainfall interval. In addition, rain did not remove all PM completely, and PM retention following rainfall differed with rainfall intensity, except for Populus tomentosa.

Spatio-temporal variations in PM leaf deposition: A meta-analysis

Particulate matter (PM) pollution in urban cities is of great concern for public health due to its global and adverse effect of human health while ecosystems function and vegetation control is an effective and eco-friendly way to alleviate PM pollution. We reviewed 150 studies conducted in 15 countries that were published between 1960 and 2016 and used a meta-analysis to examine the time trends and regional differences in leaf deposited PM of urban greening plants. The results suggested that the weekly PM leaf deposition varied markedly with both plant species and space-time and the average value was 1.71 ± 0.05 g m^-2·wk^-1, and the variations occurred because of vegetation factors, characteristics of the PM source and meteorological factors. Moreover, fine particulate matter accounts for the minimum proportion of the total PM mass but its number ratio is maximum, more than 90% of the total number of particles. This meta-analysis illustrated the spatio-temporal trends and variations in PM leaf deposition and the influencing factors, which provides a scientific basis for the mechanism of PM deposition on leaf surface as well as plant selection and configuration in urban greening.

Foliar heavy metal uptake, toxicity and detoxification in plants: A comparison of foliar and root metal uptake

Anthropologic activities have transformed global biogeochemical cycling of heavy metals by emitting considerable quantities of these metals into the atmosphere from diverse sources. In spite of substantial and progressive developments in industrial processes and techniques to reduce environmental emissions, atmospheric contamination by toxic heavy metals and associated ecological and health risks are still newsworthy. Atmospheric heavy metals may be absorbed via foliar organs of plants after wet or dry deposition of atmospheric fallouts on plant canopy. Unlike root metal transfer, which has been largely studied, little is known about heavy metal uptake by plant leaves from the atmosphere. To the best of our understanding, significant research gaps exist regarding foliar heavy metal uptake. This is the first review regarding biogeochemical behaviour of heavy metals in atmosphere-plant system. The review summarizes the mechanisms involved in foliar heavy metal uptake, transfer, compartmentation, toxicity and in plant detoxification. We have described the biological and environmental factors that affect foliar uptake of heavy metals and compared the biogeochemical behaviour (uptake, translocation, compartmentation, toxicity and detoxification) of heavy metals for root and foliar uptake. The possible health risks associated with the consumption of heavy metal-laced food are also discussed.

Effects of weather, time, and pollution level on the amount of particulate matter deposited on leaves of Ligustrum lucidum

This paper investigated the spatial and temporal variations in the amounts of PM accumulated on leaves of Ligustrum lucidum, a common evergreen tree species in North China. The effects of rainfall and wind on the amounts of PM deposited on foliage were also determined. The amounts of PM (g·m⁻²) retained by leaves of L. lucidum differed significantly among the sites (from 0.96 to 5.56) and over time (from 2.51 to 4.48). The largest amounts of PM on foliage of L. lucidum were observed on plants growing at the most polluted site. During the year, the highest and lowest accumulation of PM occurred in November and August, respectively. A considerable proportion of the accumulated PM on leaves was removed by rainfall events (28–48% of PM) and strong winds (27–36% of PM), and more precipitation or higher maximum wind speed could remove more PM from leaves. Rainfall removed mainly large and coarse particles, while fine particles adhered more strongly to the foliage. These results suggested that the effects of local weather conditions (e.g., rainfall, strong wind), different seasons, and pollution levels should be considered in evaluating total PM accumulation on leaves.