Pollutants In Street Canyons Research Articles

A UAV-based method for efficiently measuring the spatial pattern of multiple air pollutants in street canyons

Street canyons are fundamental units of urban spaces where individuals engage in social activities. The air quality within street canyons directly impacts the health and behavior of residents. Therefore, it is crucial to comprehensively assess and depict the actual distribution of multiple air pollutants (SO2, CO, NO2, O3, PM2.5, and PM10) in street canyons, for both investigation and validation purposes. However, an efficient and accurate method for doing so is rarely available. To address this issue, indoor and field measurements were conducted to determine the response times of these pollutants in various polluted environments, identify the optimal stay duration at individual sampling points, and optimize the sampling layout. A method based on Unmanned Aerial Vehicle (UAV) was developed to efficiently measure the spatial pattern of pollutants in street canyons and verified through field application. The results showed that the actual response times of pollutants in heavily and slightly polluted environments were less than 30 s. The recommended stay duration at an individual sampling point in an outdoor environment was determined to be 2 min, with uncertainty of 2 %. For the sampling layout, it is recommended to take 12 samples to obtain a spatial pattern of a street canyon. These samplings should be arranged at the air inlet, middle, and air outlet along the middle of the street canyon, as well as at their bottom, 20 %–30 %, 50 %–60 %, and top in the vertical direction, respectively. The UAV-based method can efficiently complete the sampling process in just 24 min.

Impact of urban tree arrangement on pedestrian exposure to the size-fractional particulate matter in a city boulevard

Trees act as natural filters that mitigate roadside air pollution. However, the filtration impact of different tree arrangements on traffic pollutants with different particle diameters has rarely been analysed in real street canyon environments. To quantify how roadside tree arrangements impact pedestrian exposure to particle number concentrations (PNCs) of different diameters (0.25–32 μm), in situ field measurements were carried out in a boulevard-type street canyon in the city of Xi'an, China. This study analysed the experimental data of PNCs collected along segments of a pedestrian lane under four typical tree arrangements: open space without trees, a sparse-spaced tree arrangement, a medium-spaced tree arrangement, and a dense-spaced tree arrangement in a street canyon. Our results reveal that the effect of tree arrangement on PNCs depended on the particle diameter. In general, trees can significantly reduce coarse PNC (particles with diameters >2.5 μm) but not the fine PNC. Quantitative analysis showed that a medium-spaced tree arrangement, in which tree crowns are adjacent to each other but do not overlap, is the most capable of reducing PNC, followed by a sparse-spaced tree arrangement, while a the dense-spaced tree arrangement has the least impact. The attenuation effect of trees on the PNCs increased with increasing particle diameter. Moreover, the presence of trees altered the local microclimate, which also affected how exposure to PNCs changed. Our empirical findings further highlight the complexity of how trees affect particulate pollutants in street canyons and provide timely insights for enhancing tree-planning management in cities from the perspective of air quality improvement.

Numerical Analysis of the Effects of Different Window-Opening Strategies on the Indoor Pollutant Dispersion in Street-Facing Buildings

The idling of automobiles at street intersections can lead to pollutant accumulation which impacts the health of residents in street-facing buildings. Previous research focused on pollutant dispersion within street canyons and did not consider the coupling of indoor and outdoor pollutants. This paper employs the computational fluid dynamics (CFD) method to simulate the dispersion characteristics of vehicle emission pollutants in street canyons, primarily investigating the indoor and outdoor pollutant dispersion patterns under various window opening configurations (single-sided ventilation, corner ventilation, and different positions of the glass under corner ventilation). Additionally, the study considers the impacts of the aspect ratio and ambient wind speed. Studies have shown that corner ventilation is effective in reducing indoor pollutant levels. When the two window glass positions are far away from the center of the intersection, the average CO mass fraction in the single-sided ventilation room is reduced by 87.1%. The average indoor CO mass fraction on the leeward side decreases with the increasing wind speed and aspect ratio. At a wind speed of 8 m/s, the average indoor CO mass fraction on the leeward side decreases to 2.45 × 10−8. At an aspect ratio of 2, the indoor CO mass fraction on the leeward side decreases with increasing floors before stabilizing at approximately 4.77 × 10−9. This study suggests optimal window opening strategies to reduce indoor pollutant levels in street-facing buildings at street intersections, offering guidance to indoor residents on window ventilation practices.

Influence of tree planting pattern coupled with wall thermal effect on pollution dispersion within urban street canyon

Among the factors influencing the flow field and dispersion characteristics of pollutants in street canyons, trees not only act as obstacles that impede canyon ventilation but also contribute to non-uniform temperature distributions on its walls through transpiration and shading effects. The coupling effects, however, have not been given adequate attention. This study employed numerical simulations validated by wind-tunnel experiments to investigate the coupling effects of tree planting patterns (crown density Cd, planting density Pd) and wall heating conditions on airflow and pollutant diffusion within urban street canyons. Results show that the variations in Cd or Cd of the tree planting configurations studied have no significant impact on flow structure and pollutant dispersion in the isothermal street canyons. When Pd is 0.25, all three monitoring planes exhibit relatively high net escape velocity (NEV*) and notably reduced overall pollutant concentrations, indicating that this planting configuration is optimal for reducing pollution levels under leeward wall heating. When the windward wall is heated, the pollutant concentration is reduced more significantly under Pd of 0.25 and 0.75 while NEV* on the windward side is about 48. The average NEV* is minimized to approximately 21 at a Cd of 80 % when crown density or planting density is increased for all wall heating scenarios. Moreover, the other configurations of tree planting had little impact on ventilation and dispersion of pollutants in street canyons. The findings can offer technical guidance for the optimal design of urban green facilities to improve local microclimate environment and air quality.

ASSESSMENT OF THE INFLUENCE OF THE INTENSITY OF TRAFFIC FLOWS ON THE STATE OF THE STREET AND ROAD NETWORK OF THE CITY

The article is devoted to the assessment and forecasting of the level of atmospheric air pollution in street canyons in Kyiv by controlling the parameters of traffic flow. Today, road transport is the main anthropogenic source of environmental pollution. The specificity of air pollution by traffic flows lies in their ground location, close proximity to people's homes, which leads to the accumulation of pollutants near the ground surface - in the breathing zone of people. The article analyzes the main approaches to modeling the dispersion of pollutants in the atmosphere from motor vehicles. It is advisable to assess the level of urban air pollution in homogeneous elements of the urban area - street canyons. Based on the urban street canyons model (OSPM), the level of pollution in the street canyons of the Pechersk district of Kyiv by the main harmful substances contained in car exhaust gases is determined. The critical intensity of traffic flows for each of the street canyons at which the level of pollution by the corresponding harmful substance reaches the maximum permissible value was determined. Thus, the obtained results of the study allow us to quickly predict the level of air pollution in the roadside space of cities and prevent environmentally hazardous situations by controlling the parameters of the traffic flow.

Effects of solar radiation and tree planting on photochemical reaction kinetics in urban street canyon

The investigation of the impact of tree planting on photochemical reaction kinetics and dispersion characteristics of reactive pollutants in street canyons under actual solar radiation remains a pertinent subject for research. The numerical models were employed and validated through wind-tunnel experiments to investigate the dynamic and thermodynamic effects of tree planting (LAD = 4.77) on turbulent flow, heat transfer, and NOx-O3 photochemical reaction in urban street canyons. The effects of solar radiation at three local solar times (LST0900, LST1200 and LST1500) and three background O3 concentrations (20 ppb, 100 ppb and 200 ppb) were considered. The ozone depletion rate (dO3) was defined to measure the photochemical reaction equilibrium. The results show that the presence of trees can offset the impact of varying LST solar radiation and promote greater stability in flow structures within street canyons. Specifically, a stable clockwise main vortex is observed in the upper part, while multiple smaller vortices form more regularly in the lower part. Compared with no trees, planting trees can effectively reduce the temperature by about 5 K in the lower part of street canyon through the shading and transpiration cooling effects. In conjunction with its dynamic effects, the tree planting can also effectively reduce the average concentration of pollutants before the reaction. It results in a predominant accumulation of NOx near the pollution source at the bottom center, thereby mitigating their detrimental impacts on both windward and leeward sides. Additionally, under the same background O3 concentration, tree planting can also influence the equilibrium process of photochemical reactions and enhance their reverse reaction leading to ozone depletion. The findings of this study contribute to a deeper understanding of the impact of tree greening on the local microclimate within urban street canyons.

Influence of Building Height Variation on Air Pollution Dispersion in Different Wind Directions: A Numerical Simulation Study

Due to the rapid advancement of urbanization, traffic–related pollutants in street canyons have emerged as the primary source of PM2.5, adversely impacting residents’ health. Therefore, it is necessary to reduce PM2.5 concentrations. In this study, a three–dimensional steady–state simulation was conducted using Computational Fluid Dynamics (CFD). Three representative wind directions (θ = 0°, 45°, and 90°, corresponding to parallel, oblique, and perpendicular winds) and five different building height ratios (BHR = 0.25, 0.5, 1, 2, and 4) were used to explore the effect of building height variations on PM2.5 dispersion within street canyons. The results indicated that wind direction significantly influenced PM2.5 dispersion (p < 0.001). As θ increased (θ = 0°, 45°, and 90°), PM2.5 concentration in the canyon increased, reaching the most severe pollution under perpendicular wind. Building height variations had a minor impact compared to wind direction, but differences in PM2.5 concentration were still observed among various BHRs. Specifically, under parallel wind, the influence of BHR on PM2.5 dispersion was relatively small as compared to oblique and perpendicular winds. For oblique wind, PM2.5 concentrations varied based on BHR. Street canyons composed of low–rise or multi–story buildings (BHR = 0.25 or 4) slightly increased PM2.5 concentrations within the canyon, while the lowest PM2.5 concentration was observed at a BHR of 0.5. Under perpendicular wind, symmetrical (BHR = 1) and step–down canyons (BHR = 2 and 4) exhibited comparable peak concentrations of PM2.5, whereas step–up canyons (BHR = 0.25 and 0.5) showed relatively lower concentrations.

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 Tree Height on PM2.5 Concentration in Street Canyons: A Simulation Study

With the rapid development of cities and the rapid increase in automobile ownership, traffic has become one of the main sources of PM2.5 pollution, which can be reduced by road greening through sedimentation, blocking, adhesion, and absorption. Using the method of combining field monitoring and ENVI-met simulation, the influence of the tree height on the PM2.5 concentration on both sides of the city streets was discussed. The influence of tree height on PM2.5 under five conditions was analyzed, including 10 m tall trees (i), 15 m tall trees (ii), alternating distribution of 15 and 10 m tall trees (iii), 5 m tall trees (iv), no trees on either side of the road (v). The results show that: Roadside trees can increase the concentration of PM2.5 in the narrow space of street canyons. However, without roadside trees, PM2.5 from traffic sources is not reduced in time, it is more easily spread to the distance. When the height of the roadside trees is 5 m and their crown widths are smaller than those of other trees, there is a relatively wide space between them. Compared with the higher roadside tree models with larger crown widths, the concentration of PM2.5 on the roadway and the downwind sidewalk is relatively low. In the three models (i–iii) with tree height above or equal to 10 m, the PM2.5 concentration around the trees do not show regular change with the change in tree height. Due to the tree height of 10 and 15 m, the crown width is large enough, and the alternate distribution of tree height of 15 and 10 m fails to make the PM2.5 concentration in the models lower than the models with tree height of 15 m or 10 m. The reasonable height of roadside trees in street canyons helps improve the wind circulation to promote the diffusion of PM2.5 pollution. There is no optimal height of roadside trees for PM2.5 pollution in street canyons, thus it is necessary to select the height reasonably according to the specific situation.

CFD simulations of wind flow and pollutant dispersion in a street canyon with traffic flow: Comparison between RANS and LES

• CFD simulations for a street canyon with moving traffic are systematically validated. • Among the five RANS models, SKE has the best performance against wind tunnel data. • All RANS models overestimate pollutant concentration near moving traffics. • LES outperforms the RANS simulation in predicting mean velocity and concentration. • Pollutant concentrations are strongly affected by turbulent diffusion fluxes. The dispersion of traffic pollutants in street canyons takes place under the joint influence of the urban morphology and traffic-induced air motions. Computational fluid dynamics (CFD) are increasingly used to explore air quality problems in cities, however, numerical setup guidance for studies considering moving traffics remains missing. This study presents a systematic evaluation and comparison of steady Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulation (LES) in reproducing the wind flow and pollutant dispersion in a street canyon under traffic flow using the quasi-steady method. The RANS simulations are performed with five turbulence models, and the standard k-ε (SKE) turbulence model is found to yield the best agreement with the wind-tunnel data. The LES simulation with the wall-adapting local eddy-viscosity subgrid-scale model outperforms all RANS models in simulating the mean velocity and mean pollutant concentration, and can accurately predict the turbulent velocity. The counter-gradient turbulent mass flux captured by LES reveals that the gradient-diffusion hypothesis used in steady RANS fails in the regions near moving traffics, which strongly affects the predicted mean concentration. The findings of this study can support future CFD studies on pollutant dispersion in urban environments.

Assessment of air pollution in cities by road transport in projects for managing the metropolis ecological state

The article is devoted to the study of the pollution level in the city streets by road transport. Purpose. The aim of the work is to operative assess the concentration of pollutants in street canyons in projects for managing the ecological state of the metropolis. Research Methodology. The article used statistical analysis, mathematical modeling. Scientific novelty. A model for assessing the level of air pollution in city street canyons is proposed. On the basis of this model, the concentration of pollutants in the street canyons of the Pechersky district of Kiev was determined, taking into account the daily dynamics of the traffic flows intensity. Conclusions. The research results can be used in the operational forecasting of the pollution level of the roadside space ecosystems, which will allow timely, by controlling the parameters of the traffic flow, to prevent critical situations in which the concentration of pollutants exceeds the maximum permissible values. Key words: management, transport, pollution assessment, traffic flow, design, modeling of pollution fields.

A Review on the Dispersion and Distribution Characteristics of Pollutants in Street Canyons and Improvement Measures

The air quality in a street canyon seriously affects the exposure level of pollutants for pedestrians and is directly related to the indoor air quality (IAQ) of surrounding buildings. In order to improve the street canyon environment, it is necessary to clarify the distribution and dispersion characteristics of pollutants. Through field tests, wind tunnel experiments, and numerical simulation, the current research studied the nature of pollutants in street canyons and provided some improvement measures. This paper comprehensively introduces the characteristics of pollutants in street canyons and reviews past studies on the following parts: (a) the dispersion principle and main impact factors of pollutants in street canyons, (b) the spatial and temporal distribution of pollutants in street canyons, (c) the relationship between pollutants in street canyons and indoor air quality, and (d) improvement measures of the street canyon environment. The dispersion of pollutants is dominated by the air exchange between the street canyon and the upper atmosphere, which is strengthened when the wind speed is high or when the temperature in the street canyon is obviously higher than the surrounding area. The heat island effect is beneficial for pollutant dispersion, while the inversion layer has a negative influence. Dense buildings mean lower pollutant diffusion capacity, which causes pollutants to easily gather. Pollutants tend to accumulate on the leeward side of buildings. The concentration of pollutants decreases with the increase of height and drops to the background level at a height of several hundred meters. The temporal distribution of pollutants in street canyons varies in diurnal, weekly, and annual periods, and the concentration peaks in the winter morning and summer evening. Besides, pollutants in street canyons have a significant influence on IAQ. To improve the street canyon environment, green belts and other facilities should be reasonably set up in the streets. Future research should pay attention to comprehensive test data, solving disagreement conclusions, and quantitative evaluation of the various impact factors on pollutants, etc.

Understanding the effects of roadside hedges on the horizontal and vertical distributions of air pollutants in street canyons

Built-up environments limit air pollution dispersion in street canyons and lead to complex trade-offs between green infrastructure (GI) usage and its potential to reduce near-road exposure. This study evaluated the effects of an evergreen hedge on the distribution of particulate matter (PM1, PM2.5, PM10), black carbon (BC) and particle number concentrations (PNCs) in a street canyon in West London. Instrumentation was deployed around the hedge at 13 fixed locations to assess the impact of the hedge on vertical and horizontal concentration distributions. Changes in concentrations behind the hedge were measured with reference to the corresponding sampling point in front of the hedge for all sets of measurements. Results showed a significant reduction in vertical concentrations between 1 and 1.7 m height, with maximum reductions of –16% (PM1 and PM10) and –17% (PM2.5) at ∼1 m height. Horizontal concentrations revealed two zones between the building façade and the hedge, with opposite trends: (i) close to hedge (within 0.2 m), where a reduction of PM1 and PM2.5 was observed, possibly due to dilution, deposition and the barrier effect; and (ii) 0.2–3 m from the hedge, showing an increase of 13–37% (PM1) and 7–21% (PM2.5), possibly due to the blockage effect of the building, restricting dispersion. BC showed a significant reduction at breathing height (1.5 m) of between –7 and –50%, followed by –15% for PNCs in the 0.02–1 µm size range. The ELPI + analyser showed a peak of ∼30 nm. The presence of the hedge led to a ∼39 ± 32% decrease in total PNCs (0.006–10 µm), suggesting a greater removal in different modes, such as a 83 ± 12% reduction in nucleation mode (0.006–0.030 µm), 74 ± 15% in ultrafine (≤0.1 µm), and 34 ± 30% in accumulation mode (0.03–0.3 µm). These findings indicate graded filtering of particles by GI in a near-road street canyon environment. This insight will guide the improved design of GI barriers and the validation of microscale dispersion models.

Research progress on air pollutant distribution in urban street canyons

Urban street canyon is one of the most important characteristics and spatial forms of cities. It is one of the most frequently used public spaces in cities, with the most serious automobile exhaust pollution and the largest population density. The unreasonable space configuration and internal composition might decrease self-purification of urban ventilation but increase local air pollutant concentration. Here, we reviewed the impacts of street canyon morphology, street trees, vehicle flow and meteorological factors on the distribution of air pollutants in street canyons. We scrutinized the relevant methods of numerical simulation, wind tunnel experiments, and field monitoring on the distribution and diffusion of air pollutants in street canyons. We recommended that future research should concentrate on the impacts of various parameters on the distribution and diffusion of air pollutants based on the field monitoring data. Meanwhile, further research should develop optimization strategies for street canyon design which is conducive to the dispersion of air pollutants, and put forward scientific support and optimization scheme for the controlling of air pollutants from the perspective of urban planning and pattern optimization.

Comprehensive evaluation of an advanced street canyon air pollution model

ABSTRACT A street canyon pollution dispersion model is described which accounts for a wide range of canyon geometries including deep and/or asymmetric canyons. The model uses up to six component sources to represent different effects of street canyons on the dispersion of road traffic emissions. The final concentration is a weighted sum of the component concentrations dependent on output point location; canyon geometry; and wind direction relative to canyon orientation. Conventional approaches to modeling pollution in street canyons, such as the “Operational Street Pollution Model” (OSPM), do not account for canyons with high aspect ratios, pavements, and building porosity, so are not applicable for all urban morphologies. The new model has been implemented within the widely used, street-level resolution ADMS-Urban air quality model, which is used for air quality assessment and forecasting in cities such as Hong Kong where high-rise buildings form deep and complex street canyons. The new model is evaluated in relation to measured pollutant concentration data from the “Optimisation of modelling methods for traffic pollution in streets” (TRAPOS) project and routine measurements from 42 monitoring sites in London. Comparisons have been made between modeling using the new canyon model; a simpler approach to canyon modeling based on the OSPM formulation; and without any inclusion of canyon effects. The TRAPOS dataset has been used to highlight the model’s ability to replicate the dependence of concentration on wind speed and direction, and also to show improved model performance for the prediction of high concentration values, which is particularly important for model applications such as planning and assessment. The London dataset, in which the street canyons are less well defined, has also been used to demonstrate improved model performance for this advanced approach compared to the simpler methods, by categorizing the measurement locations according to site type (background, near-road, and strong canyon). Implications: Currently available air dispersion models do not allow for a number of geometric features that influence air dispersion within street canyon environments. The new advanced street canyon model described in this paper accounts for: emissions from each road carriageway separately; canyon asymmetry; canyon porosity; and pavements. The extensive model evaluation presented shows that the new model demonstrates good performance, better than more basic approaches in which the complex geometries that define “canyons” are neglected.

Impacts of the Tree Canopy and Chemical Reactions on the Dispersion of Reactive Pollutants in Street Canyons

Traffic-related air pollution in street canyons can cause health problems for pedestrians. In order to clarify the behavior of reactive pollutants, such as NOx and O3, in street canyons, a computational fluid dynamics (CFD) model coupled with a chemistry model and tree canopy model was developed, and then, a set of numerical experiments were performed to investigate the impacts of chemical reactions and aerodynamic effects of trees planted in a canyon. The results were compared with the observation data. Through the results of the numerical experiments designed to simulate a realistic urban street canyon, it was found that chemical reactions have a dominant impact on the NO/NO2 ratio and O3 concentration. While the tree canopy had little impact on the NO/NO2 ratio, it had a moderate impact on the flow field in the canyon and the amount of NOx and O3 in the canyon. In accordance with the aerodynamic effects of tree canopies, the local NOx concentration in the experiments increased and decreased by up to 51% and 11%, respectively. The current findings of this study demonstrate the utility of the proposed model for conducting air quality investigations in urban areas.

The combining effect of the roof shape, roof-height non-uniformity and source position on the pollutant transport between a street canyon and 3D urban array

Although most people live in cities, the pollutant transport in so-called street canyons is still not well understood. The main reason is the complexity of the problem, where the roof shape, roof height and position of the pollutant source within a 3D urban array may play a significant role. This paper tries to improve the understanding and investigates the pollutant transport in six different types of street canyons using large-eddy simulations (LES). All street canyons were part of four different 3D urban arrays and differed in the roof shape and height. The results demonstrate that roof shape, roof-height non-uniformity, and the source position have a crucial impact on the advective and turbulent pollutant transport between the studied canyons and urban arrays. Both the flat roofs and roof-height variability enhance the lateral transport between the street canyons and intersections irrespective of the studied cases. This is crucial for the evaluation of the street canyon pollution if one changes the position of the source within the non-uniform urban array. Although the differences in the pollutant transport between the flat and pitched roofs mitigate in the cases of the variable roof heights, they are still appreciable.

Assessment of pollutant dispersion in urban street canyons based on field synergy theory

Vehicle emissions are an important factor in the deterioration of air quality in urban street canyons. To improve the air quality in urban street canyons, it is necessary to have a deep understanding of the mechanism of pollutant dispersion within the urban boundary layer. The transport process of pollutants in street canyons is essentially a convective mass transfer process. In this paper, a computational fluid dynamics (CFD) simulation is conducted to investigate the effects of wind speed, the setting of the viaduct, and the roof shapes of the street canyons on pollutant dispersion. According to the field synergy theory, Sherwood number and field synergy number are used to evaluate the three factors on the diffusion of pollutants in street canyons quantitatively. The results show that the Sherwood number decreases with increased viaduct construction height and decreased wind speed, which is compatible with the observed growth of pollutant fraction in the street canyon. When the pollution source on the viaduct is considered, a less substantial influence of construction height on pollutant diffusion is observed. Among the four types of roofs, upward pitched roofs resulted in the highest pollution, with a pollution concentration degree of 1.6–2.3 times that of flat roofs, followed by double pitched roofs, with a pollution concentration degree of 1.2–2 times that of a flat roof. By the method of analogy analysis and numerical simulation, the field synergy theory has been extended from applications in heat transfer in small-scale confined spaces to those in urban street canyon pollution simulation in large-scale open spaces.

Field synergy analysis of pollutant dispersion in street canyons and its optimization by adding wind catchers

The microenvironment, which involves pollutant dispersion of the urban street canyon, is critical to the health of pedestrians and residents. The objectives of this work are twofold: (i) to effectively assess the pollutant dispersion process based on a theory and (ii) to adopt an appropriate stratigy, i.e., wind catcher, to alleviate the pollution in the street canyons. Pollutant dispersion in street canyons is essentially a convective mass transfer process. Because the convective heat transfer process and the mass transfer process are physically similar and the applicability of field synergy theory to turbulence has been verified in the literature, we apply the field synergy theory to the study of pollutant dispersion in street canyons. In this paper, a computational fluid dynamics (CFD) simulation is conducted to investigate the effects of wind catcher, wind speed and the geometry of the street canyons on pollutant dispersion. According to the field synergy theory, Sherwood number and field synergy number are used to quantitatively evaluate the wind catcher and wind speed on the diffusion of pollutants in asymmetric street canyons. The results show that adding wind catchers can significantly improve the air quality of the step-down street canyon and reduce the average pollutant concentrations in the street canyon by 75%. Higher wind speed enhances diffusion of pollutants differently in different geometric street canyons.

The effect of exhaust emissions from a group of moving vehicles on pollutant dispersion in the street canyons

Moving vehicles are the primary emission source of road pollution and the vehicle-induced turbulence (VIT) affecting the flow field in the street canyon. This paper used the dynamic mesh updating method to simulate the air pollution under the real situation of vehicle movement and exhaust emission in the street canyon. The dispersion characteristics of vehicular exhaust under various vehicle arrangements were analyzed. The personal intake fraction (P_IF) was introduced as an index to analyze the impact of factors such as vehicle speed and ambient wind velocity on the pollution exposure level of pedestrians. Based on the general assumptions in the study of street canyon pollution that vehicle-induced turbulence (VIT) is a continuous turbulence source and vehicle exhaust pollutants is a uniform pollution source, the conditions of the hypothesis are preliminarily explored. The research indicates that VIT can be regarded as a continuous turbulence source when vehicles maintain a “safe distance” on the road and form a stable traffic flow. Moreover, according to the range of dispersion of exhaust pollution, a “healthy distance” was proposed to guide driver to avoid direct impact of exhaust emitted from the vehicles in front.