Study of traffic-related pollutant removal from street canyon with trees: dispersion and deposition perspective

Sources and environmental impacts of volatile organic components in a street canyon: Implication for vehicle emission

Street canyons serve as a representative environment that directly reflects the impact of vehicular emissions. Volatile organic compounds (VOCs) sampling during an O3 pollution event and a PM2.5 pollution episode was conducted at an urban site and a street canyon in Zhengzhou, China. It has been determined that street canyons suffer from more severe particle and NOx pollution than the urban site. Additionally, O3 has been identified as a significant or emerging pollutant in street canyon environments. In terms of VOCs, the street canyon exhibits 1.4 and 1.1 times higher total VOC concentrations compared to the urban site during the O3 and PM2.5 pollution episodes, respectively. In the street canyon location, there was a slight increase in the proportion of alkanes and aromatics, while the proportions of oxygenated VOCs and halogenated hydrocarbons decreased. Source apportionment analysis reveals that street canyons were more susceptible to the accumulation of VOCs from coating solvent, liquid petroleum gas (LPG), and gasoline additives. Consequently, the environmental impacts of VOCs originating from coating solvent and LPG were more pronounced in the street canyon location compared to the urban site. The trends of NOx concentration indicate that future continuously stricter vehicle emission standards and control policies can further reduce vehicle exhaust emissions and more attention needs to be focused on the reduction of non-exhaust emissions (i.e., coating solvent) and LPG vehicles.

The impact of dynamic traffic and wind conditions on green infrastructure performance to improve local air quality

Road traffic represents the dominant source of air pollution in urban street canyons. Local wind conditions greatly impacts the dispersion of these pollutants, yet street trees complicate ventilation in such settings. This case study adopts a novel modelling framework to account for dynamic traffic and wind conditions to identify the optimal street tree configuration that prevents a deterioration in air quality. Measurement data from a shallow to moderately deep street canyon (average 0.5 H/W aspect ratio and four lanes of 1-way traffic) in Dublin, Ireland was used for model calibration. The computational fluid dynamics (CFD) models were used to examine scenarios of dynamic traffic flows within each traffic lane with respect to its impact on local PM2.5 concentrations on adjacent footpaths, segmenting air quality monitoring results based on different wind conditions for model calibration. The monitoring campaign identified higher PM2.5 concentrations on the leeward (north) footpath, with average differences of 14.1 % (2.15 μg/m3) for early evening peaks. The modelling results demonstrated how street trees negatively impacted air quality on the windward footpath in parallel wind conditions regardless of leaf area density (LAD) or tree spacing, with mixed results observed on the leeward footpath in varying traffic flows and wind speeds. Perpendicular wind direction models and high wind speed exacerbated poor air quality on the windward footpath for all tree spacing models, while improving the air quality on the leeward footpath. The findings advise against planting high-LAD trees in this type of street, with a minimum of 20 m spacing for low-LAD trees to balance reducing local air pollution and ventilation capacity in the street. This study highlights the complexities of those in key decision-marking roles and demonstrates the need to adopt a transparent framework to ensure adequate modelling evidence can inform tree planting in city streets.

Alternative scenarios for urban tree surveys: Investigating the species, structures, and diversities of street trees using street view imagery

Grasping information about street trees can assist urban environmental managers in quantifying and evaluating their costs and ecological benefits. Street view imagery has the potential for urban street tree surveys. However, few studies have been conducted on the inventory of street tree species, size structures and diversity based on street view imagery at the urban scale. In this study, we tried to conduct a survey of street trees in urban areas of Hangzhou using street view images. First, we constructed a size reference items system and determined that using it for street view measurements of street trees was comparable to field measurements results (R2 = 0.913-0.987). On this basis, we investigated the distribution characteristics and differences of street trees in Hangzhou using Baidu Street View and found that Cinnamomum camphora was the dominant tree species in Hangzhou (46.58 %), and the high proportion made urban street trees susceptible to ecological hazards. In addition, surveys conducted separately in various urban districts revealed that the diversity of street trees in new urban areas was smaller and less uniform. Additionally, as the gradient got further away from the city center, the street trees are smaller, the diversity first increased and then decreased, and the evenness gradually decreased. This study analyzes the use of Street View to investigate the distribution of species, size structure, and diversity of urban street trees. The use of street view imagery will simplify the collection of data on urban street trees and provide urban environmental managers with a foundation for strategy development.

Numerical simulation of the influence of building-tree arrangements on wind velocity and PM2.5 dispersion in urban communities

Airflow behavior and outdoor PM_2.5 dispersion depend significantly on the building-tree layouts and orientation towards the prevailing wind conditions. To investigate this issue, the present work evaluates the aerodynamic effect of different building-tree layouts on the outdoor PM_2.5 dispersions in the urban communities of Shijiazhuang City, China. The adopted numerical CFD technique was based on the standard k-ε model and the Disperse Phase Model (DPM). For this study, ten different building-tree arrangements were conceptualized and all these configurations were simulated by using Ansys Fluent software to quantify the implications on the outdoor PM_2.5 dispersion due to their presence. The results have shown that: (1) a wide building interval space could benefit the air ventilation and thus decrease PM_2.5 concentrations, however, this effectiveness is highly influenced by the presence of the trees; (2) the trees on the leeward side of a building tend to increase the local wind velocity and decrease the pedestrian-level PM_2.5 concentrations, while those on the windward side tend to decrease the wind velocity. The small distance with trees in the central space of the community forms a wind shelter, hindering the particle dispersion; and (3) the configuration of parallel type buildings with clustered tree layouts in the narrow central space is most unfavorable to the air ventilation, leading to larger areas affected by excessive PM_2.5 concentration.

Effect of transport infrastructure development on selected components of the environment of inner-city river valley and the possibility of its revitalization (Lublin, Poland)

The study covered an urban river valley, strongly transformed due to the transport infrastructure development. The paper evaluates changes in spatial management of the valley section passing through the city centre that occurred during the past two centuries (long-term and short-term phenomena) as well as their effect on selected environmental components. The basic spatial analyses were carried out with the use of specialized software, cartographic materials and photographic and descriptive documentation of the studied area (archival data). The most unfavourable changes in the potential of the valley environment occurred over the past few decades. They are manifested in a considerable deterioration of landscape values as well as the quantity and quality of water resources. Relationship was identified between water quality and intensity of motor vehicle traffic near the river. Despite the progressing urbanisation of the valley, revitalisation procedures can be still carried out and attractive space in the city centre can be created. In order to demonstrate the reasonableness of the presented concept, the paper suggests that urban succession should be incorporated as a term covering time trends and accumulated transformations. It can be helpful in analysing and determining the directions of development in disputable situations.

Optimization of Ecosystem Services of Shanghai Urban–Suburban Street Trees Based on Low-Carbon Targets

Road traffic carbon emissions are an important cause of global warming, and street trees play an important role in regulating road carbon emissions. During urbanization, major differences in the planting management modes and growth status of the street trees in urban–suburban gradient may exist, leading to significant differences in the low-carbon values of the street trees in urban–suburban gradient. Based on this, this study took two typical urban–suburban gradient zones in Shanghai as an example to analyze the changes in the characteristics of street tree species, planting density, tree sizes, and low-carbon contribution with urban and rural changes, and proposed strategies for optimizing the low-carbon contribution of urban street trees. The results showed that, from the inner ring to the outer ring and the suburban ring, the proportion of London plane tree gradually changed from 82% to 11%, and the proportion of the camphor tree gradually changed from 9% to 70%; the average DBH of the trees gradually decreased from 28.81 to 23.74 cm. The number of plantings per unit road length gradually increased, and the number of plantings per unit area gradually decreased; therefore, the average low-carbon contribution of urban–suburban street trees is not significant, but the low-carbon contribution of upper street trees per unit area is higher, and suburban unit street trees have a higher low-carbon contribution. Finally, this article proposes different optimization strategies for future urban micro-renewal and suburban new-city construction.

Street Trees for Bicyclists, Pedestrians, and Vehicle Drivers: A Systematic Multimodal Review

Multimodal Complete Streets have emerged as a prominent aspiration of urban planning to ensure safe access for all users of streets including pedestrians, bicyclists, motorists, and transit users. Concurrently, municipal leaders are pursuing ambitious tree planting initiatives. These co-arising trends are potentially good news, as trees are important elements of livable cities and Complete Streets. Yet, street trees may have different health and safety benefits and disbenefits for various circulation modes. To advance a multimodal approach to research and practice, we undertook a systematic literature review with goals to (1) identify the scholarly literature addressing links between street trees, human health, and safety for pedestrians, bicyclists, and vehicle drivers; (2) depict the principal disciplines, themes, and conceptual scope of this research; and (3) discuss the implications for urban planning and design practice and research. This review drew upon 13 scholarly databases and yielded 63 relevant articles spanning 15 countries, of which 49 constituted original research. The systematic analysis covers eight research categories. Findings show exponential growth in related scholarship over the past two decades, especially for pedestrians. Journals oriented toward interdisciplinary planning and public health and safety are leading this rise, and benefits far outweigh disbenefits. Yet, there are multimodal tensions especially as it relates to the role of street trees in relationship to drivers and pedestrians. Implications for research and practice are discussed, with an eye towards governance, design, and equity.

Impact of urban street canyon architecture on local atmospheric pollutant levels and magneto-chemical PM10 composition: An experimental study in Antwerp, Belgium

As real-life experimental data on natural ventilation of atmospheric pollution levels in urban street canyons is still scarce and has proven to be complex, this study, experimentally evaluated the impact of an urban street canyon opening on local atmospheric pollution levels, during a 2-week field campaign in a typical urban street canyon in Antwerp, Belgium. Besides following up on atmospheric particulate matter (PM), ultrafine particles (UFPs) and black carbon (BC) levels, the magneto-chemical PM₁₀ composition was quantified to identify contributions of specific elements in enclosed versus open street canyon sections. Results indicated no higher overall PM, UFP and BC concentrations at the enclosed site compared to the open site, but significant day-to-day variability between both monitoring locations, depending on the experienced wind conditions. On days with oblique wind regimes (4 out of 14), natural ventilation was observed at the open location while higher element contributions of Ca, Fe, Co, Ni, Cu, Zn and Sr were exhibited at the enclosed location. Magnetic properties correlated with the PM₁₀ filter loading, and elemental content of Fe, Cr, Mn and Ti. Magnetic bivariate ratios identified finel-grained magnetite carriers with grain sizes below 0.1 μm, indicating similar magnetic source contributions at both monitoring locations. Our holistic approach, combining atmospheric monitoring with magneto-chemical PM characterization has shown the complex impact of real-life wind flow regimes, different source contributions and local traffic dynamics on the resulting pollutant concentrations and contribute to a better understanding on the urban ventilation processes of atmospheric pollution.

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

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

Reduction of particulate matter concentrations by local removal in a building courtyard: Case study for the Delhi American Embassy School

Exposure to particulate matter (PM) is strongly linked to human morbidity and mortality, where higher exposure entails higher all-cause daily mortality and increased long-term risk of cardiopulmonary mortality. The objective of this study is to demonstrate how and to what extent the local removal of PM_2.5 can lead to reduced exposure for the children and teachers in the naturally ventilated courtyard of the American Embassy School (AES) high school building in Delhi. The study is performed by computational fluid dynamics (CFD) with the 3D steady Reynolds-averaged Navier-Stokes (RANS) equations in combination with the realizable k-ε turbulence model on a very high resolution grid. First, CFD validation is performed using wind-tunnel experiments of the flow pattern in and above a generic single street canyon. Next, the case study is conducted where four commercially available electrostatic precipitation (ESP) units are installed at different positions inside the courtyard and the resulting performance is evaluated. PM_2.5 dispersion is modeled with an Eulerian advection-diffusion equation. It is shown that the best ESP positions yield overall volume-averaged PM_2.5 concentration reductions up to 34.1% in the courtyard's corridors, demonstrating the proposed mitigation strategy to be effective. Perspectives for further reduction of the PM concentrations and the related reduction of health risks are discussed.

Buoyant wind-driven pollutant dispersion and recirculation behaviour in wedge-shaped roof urban street canyons

The present study investigated the buoyant wind-driven pollutant plume dispersion and recirculation behaviour inside urban street canyons formed by buildings with wedge-shaped roofs. Numerical modelling was performed using a computational fluid dynamics (CFD) large eddy simulation (LES). Street canyon models with a strongly buoyant fire source located on the street and environmental winds perpendicular to the canyon were developed using the fire dynamics simulator (FDS). The complex interaction of buoyancy and wind, as well as their combined effects on the pollutant plume dispersion, was simulated inside the urban street canyon. The results showed that the flow pattern of pollutant plume dispersion inside the street canyon with increasing wind speed for different roof inclination angles could be divided into three regimes, including a recirculation regime, a quasi-recirculation regime and a non-recirculation regime. The pollutant levels in the street canyon, as indexed by carbon monoxide (CO) concentration, increased under the recirculation regime. For the quasi-recirculation regime, however, the leeward buildings primarily suffered from the higher pollutant levels. The critical wind speed needed to trigger recirculation was analysed for various roof inclination angles. A correlation was proposed to predict the critical wind speed of various wedge-shaped roof angles for recirculation regime and quasi-recirculation regimes.

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.

Multi-scale comparison of the fine particle removal capacity of urban forests and wetlands

As fine particle (FP) pollution is harmful to humans, previous studies have focused on the mechanisms of FP removal by forests. The current study aims to compare the FP removal capacities of urban forests and wetlands on the leaf, canopy, and landscape scales. Water washing and scanning electron microscopy are used to calculate particle accumulation on leaves, and models are used to estimate vegetation collection, sedimentation, and dry deposition. Results showed that, on the leaf scale, forest species are able to accumulate more FP on their leaf surface than aquatic species in wetlands. On the canopy scale, horizontal vegetation collection is the major process involved in FP removal, and the contribution of vertical sedimentation/emission can be ignored. Coniferous tree species also showed stronger FP collection ability than broadleaf species. In the landscape scale, deposition on the forest occurs to a greater extent than that on wetlands, and dry deposition is the major process of FP removal on rain-free days. In conclusion, when planning an urban green system, planting an urban forest should be the first option for FP mitigation.