Particulate Matter Capture Research Articles

Assessing Particulate Matter Deposition and Resuspension by Living Wall Systems in a Wind Tunnel Setup

This study examines the particulate matter (PM) capture capacity of living wall systems (LWSs), focusing on leaf traits that facilitate PM deposition. Six LWS designs, differing in structure and substrate, were tested under constant airflow conditions with and without additional PM. Results showed that planter-based LWSs reduced PM0.1 by 2% and PM2.5 by 4%, while a textile LWS reduced PM0.1 by 23% and PM2.5 by 5%, though geotextile textile increased PM by 11% for both fractions. A moss substrate LWS worsened air quality, raising PM0.1 by 2% and PM2.5 by 5%. Magnetic analysis of leaf-deposited PM (SIRM) revealed species-specific differences (p < 0.001), with SIRM values ranging from 5 ± 1 µA to 260 ± 1 µA and higher PM accumulation in plants with lower specific leaf areas. No differences were observed in SIRM between deposition and resuspension phases, indicating the PM source lacked sufficient magnetisable particles. The findings highlight the potential of LWSs in urban environments for air quality improvement but underscore the importance of selecting suitable LWS structures and plant species.

Influence of a Separation Grid on the Efficiency of Particulate Matter Capture in a Device with Arc-Shaped Elements

Influence of a Separation Grid on the Efficiency of Particulate Matter Capture in a Device with Arc-Shaped Elements

Life cycle assessment of common urban trees - The environmental performance of three Mediterranean cities

Given the increasing pressure from extreme events due to climate change, the planting of new trees has become a priority in the political agendas of cities. However, the rush to plant trees often fails to account for the reduced performance and lifespan of trees in heavily urbanized areas and the environmental impact of their production, maintenance, and eventual disposal. By means of the Life Cycle Analysis, this study aims to investigate the potential environmental benefit and impact of trees planted in three European cities located in Mediterranean areas (Perugia, Thessaloniki, and Cascais), that have adhered to the management guidelines of the LIFE Clivut project. The environmental performance of each tree is mainly affected by the tree management operations performed, by the climatic conditions, and by the tree performance in carbon dioxide (CO2) uptake and Particulate matter (PM) capture. The impact assessment obtained by ReCiPe 2016 Endpoint (H) methodology evidenced that the trees' beneficial effects widely overcome the impact of the operations carried out during the tree management. Broadleaved species showed an average environmental performance higher than conifers. The best results have been obtained by Tilia cordata Mill., Quercus ilex L., Populus spp. and Celtis australis L. in the case of broadleaved trees, and by Calocedrus decurrens (Torr.) Florin and Cedrus spp. for Conifers. The results of the Environmental Footprint (EF 3.1) method highlighted the urban trees' potentiality to mitigate Human Health problems and to improve Ecosystem Quality. Future studies may explore other management scenarios to optimize energy and materials use, reduce emissions, hence obtaining an increase of the environmental benefit for the urban areas.

Amino functionalized zeolitic imidazolate framework-8 coated cellulose aerogel for enhanced air purification

Human activities generate large quantities of air pollutants, and particulate matter (PM) in the air poses a serious threat to public health. Nanofiber filters with surface-loaded metal–organic frameworks (MOFs) are promising materials for the capture of PM. Herein, this work creatively fabricated an air filter by anchoring amino-functionalized ZIF-8 on the surface of cellulose aerogel (NZCA) by in situ growth approach. The excellent compatibility between ZIF and cellulose was demonstrated by XRD, FT-IR, SEM and XPS characterization techniques. The functionalized ligand introduces –NH2, and the polar –NH2 group enhances the electrostatic interaction between ZIF-8-NH2 and PM, resulting in NZCA with superior PM filtration performance (filtration efficiency and quality factor: PM2.5: 92%, 0.054; PM10: 96%, 0.068) compared to ZCA without sacrificing the pressure drop. Furthermore, ZIF-8-NH2 modified cellulose filters were easy to regenerate and exhibited excellent reusability. This work paves the way for the rational design of ligand-modified MOF/cellulose filters for the efficient removal of PM from air, which has important implications for solving environmental problems and energy crises.

Slot-structured catalytic membrane for sustained oily particulate matter removal with a low pressure drop

Oily particulate matter (PM) filtration faces the problems of high pressure drop and difficult regeneration. Herein, vertical arrays of MnO2 nanosheets was assembled on Zr doped TiO2 (Zr-TiO2) nanofibers to construct a core–shell structure MnO2@Zr-TiO2 (CSMT-x) catalytic membranes for highly efficient oily PM filtration and decomposition. The confining effect of the ultrathin MnO2 nanosheets enhances PM capture efficiency, and the increase of effective contact area between MnO2 and oily PM accelerate the decomposition of oily PM. The optimized CSMT-3 catalytic membrane exhibited high gas permeability and redox capacity, outperforming the Zr-TiO2 (Bare T) membrane with over 99.97 % PM filtration efficiency and a low pressure drop at the temperatures of 25–425 °C. The catalytic decomposition of PM prevented the accumulation of particles on the membrane surface, which inhibited the formation of cake layer. Therefore, the CSMT-3 membrane exhibited a slight increase in pressure drop (less than 120 Pa) even after 120 h long-term filtration. This work may contribute to the development of advanced multifunctional membranes for purification of oily PM.

Efficient capture of particulate matter with composite air filters comprising bamboo-derived nanofiber and base paper

Particulate-matter (PM) air filters comprising chemically synthesized polymers and petroleum-based materials are not environmentally sustainable because they do not biodegrade, causing secondary environmental pollution. To overcome this problem, we synthesized and analyzed the PM-capturing performance of composite sandwich-structured air filters containing bamboo-derived lignocellulose nanofibers and lignocellulose base paper. Bamboo pulp was produced through an organosolv process, and lignocellulose nanofibers and lignocellulose base paper were manufactured via electrospinning and paper processes, respectively. The manufacturing process was not only optimized via a rheological analysis of the precursor solution with different mixed solvent composition (6:4, 5:5, and 4:6 of 1-ethyl-3-methylimidazolium acetate:dimethylformamide) and different lignocellulose weight percent (3.3, 4.0, and 4.6 wt%), but also via nanofiber characterization, including electrospinnability, diameter distribution, and basis weight. The base paper condition was optimized based on analyses of the air permeability, PM filtration efficiency, and strength according to the paper thickness of 0.38 ± 0.05 and 0.10 ± 0.05 mm. As the lignocellulose nanofibers and base paper effectively captured particles smaller and larger than 2.5 μm, respectively, the structure of their composite sandwich-structured air filter was optimized to improve its PM-capturing performance. The study results provide new insights into the manufacturing of highly efficient PM air filters suitable for practical applications with diverse PM size distributions.

Particulate Matter Capture and Air Pollution Tolerance of Six Roadside Plants in Cheongju, South Korea

Particulate matter (PM), a highly hazardous air pollutant with known adverse health effects, has been proven to be effectively mitigated by using plants. This study aimed to evaluate the capacity of various plants in capturing PM and their tolerance to air pollution. This will facilitate the selection of suitable species for roadside planting. Accordingly, this research quantified the accumulation of particulate matter in six different plant species. Four biochemical parameters [total chlorophyll, leaf pH extract, relative water content, and ascorbic acid] were evaluated to determine the Air Pollution Tolerance Index (APTI), PM accumulation results varied among the plant species. The tolerance level was categorized into four groups: tolerant, moderately tolerant, intermediate, and sensitive. Ligustrum obtusifolium demonstrated the highest PM accumulation and tolerance index values. The presence of leaf hair and roughness on leaf surfaces was observed to facilitate higher PM accumulation in certain plants. PM accumulation on leaves was significantly correlated with biochemical parameters and APTI of plants. Based on these outcomes, Ligustrum obtusifolium, Hibiscus syriacus, and Chamaecyparis obtusa were identified as suitable for roadside planting to mitigate atmospheric particulate matter.

Image based analytical approaches for study of particulate matter (PM) in air

Particulate matter (PM) stands as a predominant pollutant in developing countries, demanding effective source identification and remediation strategies. This review centers on the scanning electron microscopy (SEM) image-based methodology for PM analysis, particularly emphasizing the passive technique of utilizing plant leaves for PM capture. The SEM-image-based approach serves as a powerful tool for unraveling the morphological characteristics of PM, crucial for source identification. Additionally, SEM, when equipped with energy dispersive spectroscopy (EDS), enables chemical and mineralogical characterization, providing insights into the origin of PM. The first part of the review describes the plant as the best bio-sampler for PM. In this context, removal of PM from the environment through plant-based interventions is described. Subsequently, the application of SEM for size-based analysis using ImageJ and morphological analysis for source identification of PM is detailed. Following this, the PM chemical and mineralogical composition for source identification are described based on EDS analysis. Image-based techniques play a pivotal role in selecting the most effective plant species for PM removal from the air. The review comprehensively outlines the morphological, chemical, and mineralogical attributes utilized for PM source identification and their subsequent remediation by plants. Finally, the benefits of SEM-image-based techniques for PM analysis are elucidated. This review offers a holistic understanding of the SEM-EDS and plant-based approach, presenting a promising avenue for addressing PM pollution and enhancing environmental quality.

Puffy hierarchical nanonet structured multifunctional membranes for NO degradation and ultrafine dust filtration

Multifunctional nanofibrous catalytic membranes are used for comprehensive treatment of fine particulate matter (PM) and toxic gaseous contaminants. However, the need for high purification efficiency with low resistance presents a unique challenge. Herein, a method of optimizing the morphology of manganese oxide (MnOx) on Zr-TiO2 (TZ) nanofiber membranes was developed to remove ultrafine PM and nitric oxide (NO) efficiently with relatively low pressure drops. After tailoring the hydrothermal solution properties, the MnO2 ultrafine nanowires (≈22 nm) and Mn3O4 nano-octahedra assembled with scaffold nanofibers formed dual-network structures with high gas permeabilities, superior redox properties, and abundant surface acidities. The resultant M2TZ (2-MnOx/Zr-TiO2) catalytic membranes exhibited exceptional NH3 selective catalytic reduction (NH3-SCR) activity (T90 = 215 °C) with a gas hourly space velocity of 75,000 h−1, as well as outstanding SO2 and H2O tolerance and long-term stability. Meanwhile, owing to the puffy hierarchical nanonet structure, the M2TZ catalytic membranes also show a high filtration efficiency of 99.99% for PM0.3 removal with a low air resistance of 125 Pa, high air permeability of 1259 m3 m−2 h−1 kPa−1, and satisfactory PM capture at 300 °C. This work may shed light on the design of advanced multifunctional membranes for air pollutant purification.

A trait-based investigation into evergreen woody plants for traffic-related air pollution mitigation over time

This study investigated influences of leaf traits on particulate matter (PM) wash-off and (re)capture (i.e., net removal) over time. Leaf samples were taken before and after three rainfall events from a range of 10 evergreen woody plants (including five different leaf types), which were positioned with an optical particle counter alongside a busy road. Scanning electron microscopy was used to quantify the density (no./mm2), mass (μg/cm2), and elemental composition of deposited particles. To enable leaf area comparison between scale-like leaves and other leaf types, a novel metric (FSA: foliage surface area per unit branch length) was developed, which may be utilised by future research. Vehicle-related particles constituted 15 % of total deposition, and there was a notable 50 % decrease in the proportion of tyre wear particles after rainfall. T. baccata presented the lowest proportion (11.1 %) of vehicle-related particle deposition but the most consistent performance in terms of net PM removal. Only four of the 10 plant specimens (C. japonica, C. lawsoniana, J. chinensis, and T. baccata) presented effective PM wash-off across all particle size fractions and rainfall intensities, with a generally positive relationship observed between rainfall intensity and wash-off. Mass deposition was more significantly determined by particle size than number density. Interestingly, larger particles were also less easily washed off than smaller particles. Some traits typically considered to be advantageous (e.g., greater hairiness) may in fact hinder net removal over time due to retention under rainfall. Small leaf area is one trait that may promote both accumulation and wash-off. However, FSA was found to be the most influential trait, with an inverse relationship between FSA and wash-off efficacy. This finding poses trade-offs and opportunities for green infrastructure design, which are discussed. Finally, numerous areas for future research are recommended, underlining the importance of systems approaches in developing vegetation management frameworks.

The influence of particle oxidation catalyst (POC) mode on emissions reduction of a turbo-charging non-road diesel under wide operating conditions

In the present study, the influence of particle oxidation catalyst (POC) on emissions reduction of a non-road diesel engine is experimentally investigated under wide operating conditions. First, the diesel engine with four POC-aftertreatments (other reactors are kept unchanged) is tested under eight operating conditions non-road steady cycle (NRSC). Then, the diesel engine emissions with best POC-aftertreatment combination and without aftertreatment are comparatively tested under universal conditions. The experiments show that under NRSC, POC-3 which has Pt catalysts with Al2O3 support, asymmetrical channels layout and closed outlet form structure has the best overall exhaust purification, especially for nitrogen oxides (NOx) and particulate matter (PM). The PM conversions efficiency of POC-1, POC-2, POC-3 and POC-4 is 15.3%, 51.7%, 60.9% and 58.8%, respectively under NRSC. The contribution rates of emissions analysis results under NRSC indicated that a single POC mode cannot cover the whole engine operating range. Under universal characteristic test, the brake specific fuel consumption (BSFC) of original engine is a little lower than that of POC-3 in the whole engine operating conditions due to higher exhaust back pressure of POC-3. The engine with POC-3 has very good exhaust conversion efficiencies, the average conversion efficiency of NOx, PM, carbon monoxide (CO), and hydrocarbons (HC) is 96.1%, 51.9%, 77.8%, and 95.6%, respectively. The PM capture efficiency of POC-3 at low load of whole speed range and all load of medium speed are more than 50% compared to other operating conditions. There is little effect on carbon dioxide (CO2) emissions by POC.

Reusable and Transparent Impaction‐Based Filter with Micro Apertured Multiscale Polymeric Stencil for Particulate Matter Capture

AbstractAir pollution by particulate matter (PM) in the air including PM1.0, PM2.5, and PM10, which are categorized by particle size, is a critical global environmental issue, harming the climate, ecosystems, and human health. Especially, ultrafine dust including PM1.0 and PM2.5 poses significant human health risks. Commercial fabric‐based filters effectively trap PMs but cause high‐pressure drop and limited filter capacity and reusability. Electrospun nanofiber filters address some issues but have low mechanical strength, toxic exposure risks, long fabrication times, and restrained reusability. Herein, a reusable and transparent impaction‐based PM filter using a UV‐curable polymeric stencil with micro apertures is proposed. The polymeric stencil filters achieve high filter efficiency (68–94%), superior filter capacity, and low‐pressure drop (<64 Pa). The polymeric stencil filters can be easily cleaned with water or ethanol and remain stable under extreme temperatures (−196 to 450 °C) with slight shrinkage (0–7%). The polymeric stencil filters can be broadly utilized for not only industrial, indoor, and vehicle filters but also transparent and flexible facial health masks.

The influence of plant species, leaf morphology, height and season on PM capture efficiency in living wall systems

Green infrastructure (GI) is already known to be a suitable way to enhance air quality in urban environments. Living wall systems (LWS) can be implemented in locations where other forms of GI, such as trees or hedges, are not suitable. However, much debate remains about the variables that influence their particulate matter (PM) accumulation efficiency. This study attempts to clarify which plant species are relatively the most efficient in capturing PM and which traits are decisive when it comes to the implementation of a LWS. We investigated 11 plant species commonly used on living walls, located close to train tracks and roads. PM accumulation on leaves was quantified by magnetic analysis (Saturation Isothermal Remanent Magnetization (SIRM)). Several leaf morphological variables that could potentially influence PM capture were assessed, as well as the Wall Leaf Area Index. A wide range in SIRM values (2.74–417 μA) was found between all species. Differences in SIRM could be attributed to one of the morphological parameters, namely SLA (specific leaf area). This suggest that by just assessing SLA, one can estimate the PM capture efficiency of a plant species, which is extremely interesting for urban greeners. Regarding temporal variation, some species accumulated PM over the growing season, while others actually decreased in PM levels. This decrease can be attributed to rapid leaf expansion and variations in meteorology. Correct assessment of leaf age is important here; we suggest individual labeling of leaves for further studies. Highest SIRM values were found close to ground level. This suggests that, when traffic is the main pollution source, it is most effective when LWS are applied at ground level. We conclude that LWS can act as local sinks for PM, provided that species are selected correctly and systems are applied according to the state of the art.

Is there a downside to plant ecological services in the city? Influences of particulate matter on the two-spotted spider mite (Tetranychus urticae) foraging on the small-leaved lime in urban conditions

The aim of this research was to examine how particulate matter (PM) pollution affects the life history of the two-spotted spider mite (TSSM), Tetranychus urticae (Trombidiformes: Tetranychidae), in modelled urban conditions. For this purpose, experimental populations of TSSM were cultured on the foliage of small-leaved lime (Tilia cordata) contaminated with PM at intensities corresponding to differing city zones such as a park, a busy road and an industrial area. The control samples in the study were washed, unpolluted leaves. The spider mite was selected as a model organism due to its cosmopolitan distribution, broad host spectrum, resistance to a variety of pesticides and food-intake mode involving cell-content sucking, while T. cordata is widely planted in cities and has demonstrated a considerable capability for PM capture. Data on the longevity and mortality of particular instars and on female fecundity at different pollution levels were collected and statistically evaluated. Concentrations of PM typical for roads and industrial city zones significantly reduced total female fecundity (avg. 53.9 and 55.9 eggs/female, respectively, vs 79.2 in control), which entailed a slower population increase, while the survival rate of particular developmental instars (P = 0.52) and fertility curves (P = 0.19) remained unchanged. The presence of PM caused physiological effects in the mites, despite the lack of direct consumption of the pollutant by adult and juvenile instars. Considering the incomparable resilience of TSSM to unfavourable environmental factors, it is predicted that the detrimental influence of PM on other representatives of urban arthropods may be even more severe. The results suggest that there is a need for further investigations into the ecological ramifications of air purification provided by urban green spaces.

Study on CO emission during DOC assisted DPF regeneration

Diesel particulate filters is the most effective post-treatment device used to reduce diesel exhaust particulate emissions. As the amount of particulate matter captured in the diesel particulate filter continues to increase, the exhaust pressure of the diesel engine gradually increases, thereby affecting the power performance of the diesel engine. Therefore, when the particulate matter capture amount reaches a certain value, the diesel particulate matter filter needs to be regenerated. Regeneration of diesel particulate filters will produce secondary pollution, of which CO is one of the most important secondary pollution. In this paper, the CO generation mechanism and influencing factors in the regeneration process of diesel particulate filters are analyzed. The effects of exhaust pressure and exhaust flow on CO emission during regeneration are measured through experiments. The results show that when the exhaust back pressure is greater than 80kPa, the CO solubility in the exhaust gas increases rapidly, and decreases linearly with the increase of exhaust flow during regeneration. Therefore, in order to reduce the solubility of CO in tail gas during regeneration of diesel particulate filters, the exhaust back pressure shall be less than 80kPa and the exhaust flow shall be as high as possible.

Study on the Effects of Wet Flue Gas Desulfurization on Particulate Matter Emission from Industrial Coal-Fired Power Plants

This study aimed to investigate the effects of wet flue gas desulfurization (WFGD) on particulate matter (PM) emissions in coal-fired power plants (CFPPs) using an electrical low-pressure impactor (ELPI). The investigation was conducted on five industrial CFPPs of various loads in China to clarify the influence factors of WFGD on PM10 emissions. After WFGD, the proportion of PM2.5 to PM10 in the outlet flue gas increases, which showed that the WFGD system is selective in the capture of PM, with a significant effect on the capture of large particle sizes. The investigation found that four spray layers have a better effect on the capture of particles than two spray layers. Additionally, the investigation also found that unit load is not the main factor affecting the efficiency of PM10 capture by WFGD. Instead, the factors affecting the capture efficiency of PM10 by WFGD are the inlet flue gas temperature and the dust concentration. Relatively higher inlet flue gas temperature and lower inlet dust concentration will both result in higher emission of PM0.1~1 from the WFGD outlet. These findings suggest that a matched integration of WFGD and CFPP is essential for ultra-low PM emission control and green industry development.

Electrospun heterojunction nanofibrous membranes for photoinduced enhancement of fine particulate matter capture in harsh environment

Particulate matter (PM) pollution has posed a severe threat to the environment and human health. Especially for submicron-level fine particulate PM0.3, it is still difficult to achieve high-efficiency filtration. Herein, we loaded TiO2/g-C3N4 (TCN) heterojunction photocatalyst on polystyrene/polyacrylonitrile (PS/PAN) multiscale nanofiber membranes by a simple one-step blending electrospinning technique. Due to the strong long-range electrostatic force of the built-in electric field, the composite membrane has a remarkably enhanced filtration effect under light. Even under harsh conditions with a high PM0.3 concentration of more than 1,800,000 m−3 and a high flow rate of 25 L/min, it still has a high filtration efficiency of greater than 98.5%, an increase of about 20% compared with dark conditions, and the pressure drop is only low to 94 Pa. More importantly, the electrostatic field can be regenerated with light, and this preparation strategy can be effectively extended to other photocatalyst heterojunctions.

Capture and migration of particulate matter in activated carbon sintering flue gas purification system based on ultra-low emission target

By using alkali metal K as the tracer component of sintered ash, the particulate matter (sintered ash and activated carbon powder) in the industrial two-stage moving bed purification project of sintered flue gas was studied. First, the distribution of K element in the gas phase and each link of the purification system was investigated. The material flowing direction as well as the trapping and releasing mechanism of sintered ash in the purification system were analyzed by BET, XRD, and SEM-EDS techniques. The migration and release rule of activated carbon (AC) powder from moving bed to gas phase was investigated, and the contribution of AC powder to flue gas particles and the key release link were clarified. The results showed that the sintered ash capture rate in the primary adsorption tower of AC was as high as 99.7%, while 65.92% of the sintered ash was captured in the middle chamber of the primary adsorption tower. Sintered ash is trapped by AC at 0–0.5 mm on the surface, which suggested the main trapping mechanisms are inertial collision and interception diffusion settling. The trapped sintered ash is recycled through the system with the AC, and eventually 99.9% ash is discharged with the sieved carbon powder from the desorption tower. The percentage of charcoal powder in the exit flue gas particulate matter is 55.45% and the release of activated charcoal powder to the gas phase in the secondary adsorption tower. Which is the key link affecting the exit flue gas particulate matter concentration. This study elucidates the capture and migration law of particulate matter in AC flue gas purification system, and provides a strong basis for the improvement of technology to achieve stable ultra-low emission of particulate matter from AC system.

Electrospinnability analysis of natural cellulose nanofibers for use in high-efficiency particulate matter capture based on rheological behaviors

To aid the development of air filters with high-efficiency fine particulate matter (PM) capture properties, the electrospinnabilities of natural cellulose nanofibers, which considerably affect filter performance, were investigated via rheological behavior analysis. The cellulose nanofibers were prepared by mixing trifluoroacetic acid (to dissolve cellulose) and 1,2-dichloroethane (to improve electrospinnability) with various types of cellulose. The rheological behavior of the precursor solution was analyzed according to mixing time via large amplitude oscillatory shear studies. The electrospinnability of the cellulose precursor solution was investigated based on the flow-induced structural anisotropy characteristics and the storage-loss moduli plot obtained by varying the frequencies. The precursor solution with hardwood pulp (HP) exhibited the highest specific surface area and the lowest combined content of hemicellulose and lignin, and thus, the optimal electrospinnability. The electrospun HP nanofiber sample with an HP content of 1.5 wt% displayed the optimal electrospinnability and the highest specific surface area of 1021 m2/g. It also exhibited high-efficiency PM capture characteristics, such as a PM2.5 capture efficiency and quality factor and an air permeability of 98.99%, 0.27, and 309 cm3/cm2 s, respectively. This study suggests the potential for the development of next-generation green nanofiber air filters using cellulose nanofibers.

Seasonal Variations of Particulate Matter Capture and the Air Pollution Tolerance Index of Five Roadside Plant Species

Particulate matter (PM) is the most dangerous type of air pollutant and is harmful to human health. Plants can be used as a biofilter to remove PM from the atmosphere and improve air quality. In this study, we used the air pollution tolerance index and four leaf traits of five different plant species commonly used in landscaping in Korea to determine which plants are best suited to remove PM from the atmosphere in roadside areas in spring, summer, and autumn. We found that the PM concentrations in the atmosphere impacted the amount of PM accumulated in the plants, with increased PM accumulation during periods of increased environmental PM levels on the roadside. Euonymus japonicus, and Euonymus alatus accumulated the highest amount of PM and had the highest tolerance levels to air pollution. Thus, these species could be suitable for use in areas with high PM concentrations to improve air quality. We also found that shrubs were more effective in accumulating PM than trees and recommend that shrubs and trees be used together to further increase the amount of PM removed from the atmosphere in urban areas.