Air Quality In Street Canyons Research Articles

Enhancing ventilation in street canyons using adjustable roof-level wind flow deflectors

Traffic emission impacts the air quality at both the road surface and in near road buildings. Previous research has examined the benefit of using roof-level wind catchers to reduce concentration along building facades in urban environments. Although roof-level interventions are more effective in improving the street canyon air quality in its entirety, localized re-distributions that detrimentally affect specific regions in and around the street canyons are unavoidable. In the current work, a deflector system that can adjust its position and orientation to changing ambient conditions is introduced to ensure that no particular region perpetually experiences compromised air quality. At optimal roof-level positions, the wind deflectors resulted in a local maximum and minimum of pollution removal through mean flow-induced fluxes and overall canyon concentrations respectively. In this study, the potential of the wind deflectors was first demonstrated using 2D Computational Fluid Dynamics (CFD) investigations. A maximum reduction in overall canyon concentration of 2.84 fold was predicted when the deflector was placed 2 m from the leeward walls. Subsequently, the benefit of the dynamic nature of the intervention and the efficacy of the same in a more realistic 3D city-type environment was demonstrated by considering two different pollution source conditions. The wind deflectors performed modestly for the Cross Road Pollution (CRP) source model by reducing 7%, 11% and 13% of CO exposure on the leeward wall, upwind side wall and downwind side wall without affecting the windward wall of the target street canyon. Whereas for the Side Road Pollution (SRP) source model, it reduced 91%, 32% and 34% on the same with a 17% reduction on the windward wall of the target street canyon. Finally, the concept of an adjustable deflector system was demonstrated to mitigate prolonged high exposure for building occupants exposed to changing traffic emission sources via all the surrounding building facades and at the ground.

The effect of noise barriers on viaducts on pollutant dispersion in complex street canyons

• The viaduct-noise barrier can remarkably influence the flow structure. • Using field synergy theory to analyze noise barrier-viaduct system. • Noise barrier-viaduct reduced the CO concentration in pedestrian breathing area. The noise reduction effect of noise barriers has been extensively studied, but the effect on pollutant dispersion remains unclear. A computational fluid dynamics (CFD) simulation is conducted to investigate the effects of different heights, lengths, and types of noise barriers and different wind speeds on pollutant dispersion in street canyons with viaducts. The field synergy theory of the convective mass transfer process is used for quantitative analysis of pollutant dispersion in street canyons. The results show that as the height and length of the noise barrier increase, the pollutant dispersion capacity decreases. As the wind speed increases, the rate of decrease in the average CO concentration declines. The effect of the wind speed on the synergistic improvement of the speed and concentration gradient vectors differs for different types of noise barriers. The performance follows the order: fully-closed noise barrier > left noise barrier > right noise barrier > semi-closed noise barrier. The different noise barrier types significantly impact the flow field and pollutant dispersion and reduce the CO concentration to varying degrees, except for the fully-closed type. The average CO concentration in the pedestrian breathing zone is reduced by a maximum of 55.85% on the leeward side and by 53% on the windward side, indicating that an appropriate noise barrier on the viaduct reduces noise pollution and improves the air quality in street canyons, especially in the pedestrian breathing zone.

The right tree for the right street canyons: An approach of tree species selection for mitigating air pollution

Trees planted in street canyons affect the dispersion and deposition of air pollutants primarily through their aerodynamic and deposition effects to change the air quality. However, the potential of trees to improve air quality is uncertain as it depends on a combination of tree forms (species), street canyon geometry and local meteorological conditions. We developed a method to select tree forms with appropriate morphology in specific street canyons based on Computational Fluid Dynamics (CFD) simulation, aiming to promote the dispersion and concentration reduction of pollutants at pedestrian height. In this method, we combined the leaf area density (LAD), crown diameter (CD), tree height (H), and trunk height (TH) as representative metrics for various tree forms. The effects of street canyon aspect ratio, wind direction, and wind speed on the potential of trees to regulate pollutant concentrations were comprehensively considered. In a case study in Wuhan, China, 15 tree forms and corresponding 36 tree species adapted to different street canyons were identified using the validated ENVI-met model. Our research offers practical recommendations and effective tools for professionals and policymakers to select tree species that can enhance the air quality in street canyons.

Numerical investigation of the impact of urban trees on O3–NOx–VOCs chemistry and pollutant dispersion in a typical street canyon

Urban greening is one of major factors that influences flow, turbulence and air quality in street canyons. This paper aims to investigate the impact of urban trees on O3–NOx–VOCs (ozone -nitrogen oxides - volatile organic compounds) chemistry and pollutant dispersion in street canyons by Computational Fluid Dynamics (CFD). The Atmospheric Photolysis calculation framework (i.e., APFoam), which includes complex O3–NOx–VOCs chemistry into CFD, is employed to carry out the numerical simulations. The validation of the APFoam modelling results has been completed prior to further modelling works, including turbulent airflow, pollutant dispersion, and photochemical reactions. The influence of aerodynamic effects, biogenic VOC (BVOCs) emission and dry deposition of urban trees on air quality in a typical two-dimensional (2D) street canyon with aspect ratio H/W = 1 (where H is the building height and W the street width) are thoroughly examined. Moreover, the source contribution on ozone (O3) creation and the human health risk are also analyzed. Results show that, inside the street canyon, aerodynamic effects of trees have a greater impact on photochemical pollutant concentrations than BVOCs emission and dry deposition, the latter showing the smallest impact. In particular, the aerodynamic effects cause a wind reduction by 35%–45% at pedestrian level and subsequently an increase of nitrogen monoxide (NO) and nitrogen dioxide (NO2) concentrations by 95% and 66% near the ground, respectively, and an O3 concentration decrease by 35%. Further, the BVOCs emitted from trees, the vehicle VOCs and the background VOCs contribute 15%, 67%, and 9% to O3 production, respectively. These findings further suggest that the APFoam is an effective and promising tool which allows us to investigate the influencing mechanisms of trees on photochemical pollutant dispersion and urban air quality for the purpose of developing sustainable urban policy.

Impact of urban viaducts on the vertical distribution of fine particles in street canyons

In the urbanization process, the viaduct has been regarded as an effective way to relieve metropolis traffic pressures in the urban area. However, due to the double emission source from elevated- and ground-road vehicles, the viaduct-built environment inevitably deteriorates the street canyon's air quality. Besides, the emitted air pollutants are trapped in such street canyons for extended periods. With this consideration, we used measured data to explore the impact of the viaduct on the vertical distribution of PM2.5 in street canyons. First, the experiments were conducted in a residential building near the inner ring of a viaduct in Shanghai, China. Next, the vertical distribution of PM2.5 and its diurnal variation were revealed based on these data. The results indicated the bimodal distribution patterns in the vertical profiles of PM2.5 mass concentrations, with the lower peak at height of 5 m under the viaduct and the upper peak at height of 10 m above the noise barrier on the viaduct. Further, the impact of the viaduct on the vertical distribution of PM2.5 was investigated. Results from both observation and simulation are consistent, revealing that 21–23% of PM2.5 was trapped beneath the viaduct, and the noise barrier causes the PM2.5 to decrease by 4–5%. The traffic source on the viaduct contributes 20–40% of the PM2.5. This study revealed the dispersion patterns of PM2.5, and the impact of the viaduct on it, which could be beneficial to optimizing the viaduct and the noise barrier to release traffic-related air pollutants in street canyons.

The impact of street canyon morphology and traffic volume on NO2 values in the street canyons of Antwerp

Air pollution remains a major environmental and health concern in urban environments, especially in street canyons that show increased pollution levels due to a lack of natural ventilation. Previous studies have investigated the relationship between street canyon morphology and in-canyon pollution levels. However, these studies are typically limited to the scale of a single street canyon and city-wide assessments on this matter are scarce. In 2018, NO2 concentrations were measured in 321 street canyons in the city of Antwerp (Belgium) as part of the large-scale citizen-science project “CurieuzeNeuzen”. In our research, this data was used to study the correlation between morphological indices (e.g. aspect ratio (AR), lateral aspect ratio (LAR), presence of trees) and the traffic volumes on a city-wide scale. The maximum hourly traffic volume (TVmax) and AR correlated significantly with the measured NO2 values, making them useful indicators for air quality in street canyons. For street canyons with AR > 0.65, a TVmax of 300 vehicles/hour was found as a threshold value to guarantee acceptable air quality. No significant correlations were found for the other parameters. Finally, a number of typical street canyon types were defined, which can be of fundamental interest for further research and spatial policy making.

Study on mitigation of automobile exhaust pollution in an urban street canyon: Emission reduction and air cleaning street lamps

Affected by the rapid growth of the number of fuel motor vehicles, the automobile exhaust pollution in the street canyon is becoming more and more serious. Different from the pollution source control technology usually used for outdoor air pollution control, an air cleaning street lamp is designed in this work. A 3D steady numerical model with line source pollution is established to simulate the air flow and pollutant diffusion in street canyons and validated. Then, the distributions of pollutants in 90 scenarios of street canyons are numerically simulated. The results show that wind direction and wind speed have great influence on pollutants diffusion in street canyons. Air quality in street breathing zones is improved with the reduction of emission; however, the air quality inside the street canyon is still worse than that outside the street canyon. Scenario with air cleaning street lamps with four 45° downward air outlets is shown to work better in improving air quality. In most of the studied cases, air cleaning street lamps are found to be more effective than reducing emissions from automobiles. A better method to improve the air quality in street canyons is to combine emission reduction measures and air cleaning street lamps. For the high-pollution breathing zone under perpendicular, oblique and parallel approaching wind directions, by this better method the mean PM concentration decreases by 14.38%, 7.68% and 17.78%, respectively; the mean NOx concentration decreases by 24.92%, 21.33% and 29.03%, respectively; the mean CO concentration decreases by 12.77%, 7.45% and 15.05%, respectively.

The mitigation strategy of automobile generated fine particle pollutants by applying vegetation configuration in a street-canyon

Urban vegetations are widely used as one of the mitigation approaches to combat the public health threat from air particulate matter (PM) pollution for urban residents. However, vegetation effect at the points of interest (e.g. leeward wall, windward wall, pedestrian-level) in street-canyon from different vegetation configurations on the air quality is still not well established. We, therefore, used the numerical simulation approach to evaluate vegetation effect (VE) for different vegetation configurations (VCs) (e.g. tree or tree-shrub plantings in two sides and either side of windward or leeward) with several tree species on the traffic-originated PM pollutants in a street-canyon under a perpendicular wind. The total VE varied from −4.0% to 20.6% while pedestrian-level VE from −3.5% to 15.4% depending on different VCs. Cypress species have better total VE varying from 3.5% to 11.5% and pedestrian-level VE from 4.8% to 10.9% than the other species for same VC due to its higher deposition velocity. For those only trees used cases, the best VEs (pedestrian-level VE: 3.3%–10.9%; total VE: 2.1%–11.5%) were found on the leeward side planting where is closer to the higher polluted domain and had less obstructions for wind movement. We found that two sides planting of enhanced tree-shrub configuration by Cypress in the street-canyon was an optimal strategy to improve the total VE by 19.3%–20.6% and pedestrian-level VE 14.1%–15.4%, as well as mitigate the highly concentrated PM in the street center. The VE for the leeward wall was significantly correlated with aerodynamic parameter (CdLAD) (P < 0.001) while VEs for the windward wall and pedestrian level with deposition parameter (LADvd) (P < 0.001). Clearly, street-canyon air quality can be improved by making good use of pressure loss coefficient of vegetation to alter pollutants distribution and selecting vegetation with high deposition velocity to filter more pollutants. Our research provides insights for urban planners and designers to develop the best management practices of urban forestry.

Impact of rooftop stack configuration on 2D street canyon air quality

We experimentally investigated the impact of rooftop stack position on the pollutant entrapment within 2D street canyon configurations. Analyses were performed in a water channel loop system by using the image analysis techniques: FTV (Feature Tracking Velocimetry) and LIF (Light Induced Fluorescein) respectively for measuring velocity and concentration. The set-up consists in an array of 20 identical buildings with aspect ratio equal to one (ARB = B/H, where H is the height of the eaves and B is the building width), mimicking an idealized 2D urban canopy. They were equally spaced with a unitary canyon aspect ratio (defined as ARC = W/H; where W is the distance between buildings facades), and varied in shape by using flat and gable roofs with pitch α = 45°, with different chimney positions and heights, for a total number of 9 investigated configurations. Results demonstrate that the presence of gable roof significantly varies the flow in the shear layer, which, in combination with different chimney position and height, lead to not trivial effects on pollutants dispersion.

Reducing Exposures to Traffic-Related Air Pollution in Urban Areas: Regional Planning, Neighborhood Design, and Individual Behavior

The assessment of exposure to traffic-related air pollution has seen advances along various dimensions. Air pollution dispersion models with fine spatial resolution and ability to reflect near-road air quality in street canyons, have made possible the development of exposure surfaces associated with strategic long-range scenarios affecting land-use and transportation in urban areas. Meanwhile, personal monitoring and GPS-enabled applications have motivated the development of a wide range of tools intended to inform users of their own exposure and ways to reduce it. While scientific evidence points to the success of these tools in identifying measures, at the policy or personal level, able to reduce air pollution exposures, there much left to learn about the impact of these tools on human behavior. How do policy makers use scenario-based information on air quality to formulate policy decisions? And how do individuals respond to air quality information provided to them? Are there individuals more inclined to respond to exposure reduction advice in a positive manner? This presentation will detail how high resolution air pollution data has informed a range of interventions to reduce traffic-related air pollution exposures in Canadian cities, from clean routes applications to large investments in transportation infrastructures and urban design. New evidence will also be presented from a stated-preference experiment examining how air pollution information impacts individual behavior.

An investigation on the effect of street morphology to ambient air quality using six real-world cases

Street canyons are vulnerable to air pollution mainly caused by vehicle emissions, which are therefore closely related to pedestrians’ health. Previous studies have showed that air quality in street canyons is associated with street morphology, though the majority of them have focused on idealized street models. This paper attempts to investigate the relationship of street morphology to air quality for 6 irregular real-world cases selected from America, Europe, and China, i.e. Manhattan, Paris, Barcelona, Berlin, London and Nanjing. Each street is analyzed as a set of slices to propose a couple of morphology indices for quantitatively assessing the actual street morphology. Pollutant transport rate of mean flows and turbulent diffusion, net escape velocity and age of air are obtained from computational fluid dynamics (CFD) simulations to assess the ventilations and pollutant dispersion within street canyons with a parallel approaching wind. The results show that the street morphology characteristics, including the street width, lateral openings and intersections, are closely related to the air flows in street canyons. The air quality improves with a decreasing aspect ratio of central street owing to a larger vertical exchange through the street roof, which suggests an open central street is of better air quality. The lateral openings and intersections of streets have important effects on the air flows in street canyons, and the effects are particularly pronounced when the street widths are similar. The street continuity ratio indicates street continuity. It relates to the openings and the symmetry of a street and impacts on the air flows and pollutant dispersion through the lateral openings of the central street. The street spatial closure ratio is determined by the street continuity ratio and the aspect ratio of the central street. When the aspect ratio of central street is not excessively high, higher values of street continuity ratio and spatial closure ratio can lead to a stronger channel flow in street canyons and improve the air quality. The octagon intersections are favorable for air flowing through the lateral openings and improve the channel flows. The oblique intersections can also greatly improve the street ventilations, mainly due to the enhanced air flows through the lateral openings and the increased turbulent diffusion through the street roofs.

A review of air pollutant diffusion models for urban street

Street is an important place for urban residents, and the air quality in streets is closely related to residents' health. Air pollutant diffusion model is an often used measure to simulate and evaluate air quality in street canyon in recent years, but the problems existing in model application need to be clarified. This review selected four air pollutant diffusion models, i.e., ENVI-met, FLUENT, MISKAM and OSPM, which were most popular in street canyon studies. Through comparisons on the model mechanisms, the operational processes, the temporal-spatial resolutions and so on, their differences in model scopes and modeling capabilities in street canyon air quality simulation were discriminated, the model deficiencies were pointed out according to temporal scale, modeling, weather handling, turbulent simulation and photochemical pollution, and several model optimization measures were suggested in terms of street canyon study. Moreover, through reviewing the case stu-dies of the four models, the shortcomings in model application were concluded, and some new techniques were highlighted for future application studies such as laser radar to improve input data accuracy, and the importance of considering urban heat island and urban pollutant island simultaneously was emphasized in evaluating street environment quality.

The impact of roadside trees on traffic released PM10 in urban street canyon: Aerodynamic and deposition effects

Tree planting has the potential to affect local air quality by both aerodynamic and deposition effects. However, most of the previous studies have failed to take deposition effect into account, which may lead to overestimation in pollution level. This study evaluates the aerodynamic and deposition effects of roadside trees on street canyon air quality and assesses the sensitivity of these effects to different tree parameters by CFD approach. The dispersion of traffic released PM10 in an idealized isolated street canyon are investigated. The results show that, for aerodynamic effect, pressure loss coefficient is a major factor on near-wall mean concentrations, up to +54% and −39% at leeward and windward, respectively. For deposition effect, leaf area density and deposition velocity are dominant factors. When compared with cases having the same pressure loss coefficient, the deposition effect of cases with large leaf area density and high deposition velocity reaches as much as −39% at leeward and −89% at windward, while for low deposition velocity, deposition effect is not distinguishable. Overall, the results suggest that both aerodynamic and deposition effects could be major impacts in street canyon air quality, and that vegetation parameters should be carefully handled when assessing street canyon pollution level.

Wind driven natural ventilation and pollutant dispersion in the dense street canyons: Wind Opening Percentage and its effects

Street canyon could represent a fundamental element where urban transports and dwellers are crowded, and it has been investigated concerning on its ambient air environment and wind penetrating buildings. Natural ventilation could regulate built and street canyon air environment, whereas it doesn’t produce carbon during its process. Former published discussions straightforwardly modeled the street canyons as semi-enclosed models established well from urban grounds. However, this simplified model greatly deviates from the real situation. Meanwhile, natural ventilation across the windows in the elevation of buildings have not been widely investigated.In the present work, multi-scale flow phenomena across street canyons and built air motions are fully numerically simulated, regarding of effects of window opening percentages on the wind flow, energy and species transports. The results indicate that the exchange of air current between the upside and bottom of street canyons gradually enhances with the enlargement of window opening percentage, which also gradually dilutes the pollutant concentration in our target street canyon, bypassing those pollutants into the adjacent regions. Ventilation pattern inside the upper room is apparently different from that in the ground floor. In addition, window opening percentage tends to have more impacts on the pollutants distribution around leeward buildings. Present work can contribute to ameliorating air quality in street canyons by utilizing natural ventilation.

Impacts of cross-ventilation on the air quality in street canyons with different building arrangements

The effect of cross-ventilation in buildings on air quality within street canyons is evaluated by employing computational fluid dynamic (CFD) models. The impacts of the airflow entering the canyons through the upstream building windows on flow field and pollutant distribution were investigated in both regular and staggered canyons, and the influences of window-opening-percentage (WOP) for different street width to building height aspect ratios (AR) was studied. The numerical results show that the airflow through the windows of the upstream building can destroy the primary vortex and leads to a new flow pattern within the canyon. When the WOP is increased from 0 to 10%, the ability of the flow for pollutant dilution and dispersion can be improved. This causes the pollutant concentrations decreased by 23% to 27% in the street. And these reductions are greater for staggered arrangements than that for regular arrangements. The influences of the WOP on air flow inside the canyon decrease with increasing the AR. A limiting aspect ratio exists, beyond which the flow structure and the pollutant concentration in the canyon will not be changed no matter what the WOP is.

A CFD modeling study in an urban street canyon for ultrafine particles and population exposure: The intake fraction approach

Air quality in street canyons is of major importance, since the highest pollution levels are often encountered in these microenvironments. The canyon effect (reduced natural ventilation) makes them “hot spots” for particulate pollution contributing to adverse health effects for the exposed population. In this study we tried to characterize the influence of UFP (ultrafine particle) emissions from traffic on population exposure in an urban street canyon, by applying the intake fraction (iF) approach. One month long measurements of UFP levels have been monitored and used for the need of this study. We applied a three dimensional computational fluid dynamic (CFD) model based on real measurements for the simulation of UFP levels. We used infiltration factors, evaluated on a daily basis for the under study area, to estimate the indoor UFP levels. As a result the intake fraction for the pedestrians, residents and office workers is in the range of (1E-5)–(1E-4). The street canyon is mostly residential justifying partially the higher value of intake fraction for residents (1E-4). The above iF value is on the same order of magnitude with the corresponding one evaluated in a relative street canyon study. The total iF value in this microenvironment is one order of magnitude higher than ours, explained partially by the different use and activities. Two specific applications of iF to assess prioritization among emission sources and environmental justice issues are also examined. We ran a scenario with diesel and gasoline cars and diesel fueled vehicle seems to be a target source to improve overall iF. Our application focus on a small residential area, typical of urban central Athens, in order to evaluate high resolution iF. The significance of source–exposure relationship study in a micro scale is emphasized by recent research.

Effects of electric vehicles on air quality in street canyons

Effects of electric vehicles on air quality in street canyons

Air Quality Modeling of Santhepet Street Canyon of Mysore City Using FLUENT

Automobile transport has become an inherent part of human life, and the adverse effect like air pollution is becoming more drastic. Vehicular traffic is leading to deterioration of air quality in the streets of Mysore city in recent years. It is very important to assess the impact of traffic related air pollution to take administrative decisions for improving air quality. In the present study, an attempt has been made to study the street canyon air quality by monitoring the concentration of pollutants like Suspended Particulate Matter (SPM), Sulfur dioxide (SO2) and Nitrogen dioxide (NO2) in Santhepet street canyon of Mysore city. Pollutants are monitored for 8 hours using instrument High Volume Air Sampler (HVAS) at two sampling stations in the canyon. During sampling period, the meteorological condition such as temperature, wind speed and humidity are found to be 33 0 C, 3-4km/hr and 50% respectively. The pollutants SPM, SO2 and NO2 concentration at sampling station 1 are 943.89, 15.73 and 1319.57 µg/m 3 and concentration of pollutants in sampling station 2 are 520.69, 13.93 and 350.87µg/m 3 respectively. Computational Fluid Dynamics (CFD) is a tool which assists in modeling the airflow and dispersion of pollutants among complex urban street canyons. FLUENT is the most widely used commercial CFD code for a range of flow applications. An attempt has been made to study the dispersion of the pollutants inside an urban street canyon using FLUENT.

Effects of building roof greening on air quality in street canyons

Building roof greening is a successful strategy for improving urban thermal environment. It is of theoretical interest and practical importance to study the effects of building roof greening on urban air quality in a systematic and quantitative way. In this study, we examine the effects of building roof greening on air quality in street canyons using a computational fluid dynamics (CFD) model that includes the thermodynamic energy equation and the transport equation of passive, non-reactive pollutants. For simplicity, building roof greening is represented by specified cooling. Results for a simple building configuration with a street canyon aspect ratio of one show that the cool air produced due to building roof greening flows into the street canyon, giving rise to strengthened street canyon flow. The strengthened street canyon flow enhances pollutant dispersion near the road, which decreases pollutant concentration there. Thus, building roof greening improves air quality near the road. The degree of air quality improvement near the road increases as the cooling intensity increases. In the middle region of the street canyon, the air quality can worsen when the cooling intensity is not too strong. Results for a real urban morphology also show that building roof greening improves air quality near roads. The degree of air quality improvement near roads due to building roof greening depends on the ambient wind direction. These findings provide a theoretical foundation for constructing green roofs for the purpose of improving air quality near roads or at a pedestrian level as well as urban thermal environment. Further studies using a CFD model coupled with a photochemistry model and a surface energy balance model are required to evaluate the effects of building roof greening on air quality in street canyons in a more realistic framework.

Experimental simulation of air quality in street canyon under changes of building orientation and aspect ratio

To assist validation of numerical simulations of urban pollution, air quality in a street canyon was investigated using a wind tunnel as a research tool under neutral atmospheric conditions. We used tracer gas techniques from a line source without buoyancy. Ethylene (C(2)H(4)) was used as the tracer gas. The street canyon model was formed of six parallel building rows of the same length. The flow and dispersion field was analyzed and measured using a hot-wire anemometer with split fiber probe and fast flame ionization detector. The diffusion flow field in the boundary layer within the street canyon was examined at different locations, with varying building orientations (θ=90°, 112.5°, 135° and 157.5°) and street canyon aspect ratios (W/H=1/2, 3/4 and 1) downwind of the leeward side of the street canyon model. Results show that velocity increases with aspect ratio, and with θ>90°. Pollutant concentration increases as aspect ratio decreases. This concentration decreases exponentially in the vertical direction, and decreases as θ increases from 90°. Measured pollutant concentration distributions indicate that variability of building orientation and aspect ratio in the street canyon are important for estimating air quality in the canyon. The data presented here can be used as a comprehensive database for validation of numerical models.