Street Canyon Research Articles

Regression-based sensitivity analysis and multi-objective optimisation of energy performance and thermal comfort: Building envelope design in hot arid urban context

Previous research has established that energy demand of buildings in street canyons are affected by the surrounding urban physical characteristics. This suggests that it is possible to optimise the energy performance of buildings by acting on the physical urban parameters that potentially influence their overall energy consumption. Unfortunately, few studies have addressed this issue. Precisely, improving buildings thermal behaviour and energy demand pattern throughout sensitivity-based optimisation by focusing on the building’s envelope design without neglecting the effect of urban context, remains a relatively under-explored research topic. The present study attempts to fill this gap by implementing a combined method based on sensitivity analysis and multi-objective optimisation of energy demand and thermal comfort for an existing residential building subjected to hot arid climatic conditions. The proposed method utilises Latin hypercube sampling with multivariate multiple linear regression to identify the relative importance of design parameters on building performance criteria, namely, the annual energy demand and the integrated summer thermal discomfort. Non-dominated sorting genetic algorithm II (NSGA-II) is then applied to simultaneously optimise the investigated building performance criteria with regards to the identified relevant envelope parameters. The results of sensitivity analysis indicate that window characteristics, thermal insulation, and roof reflectance are the most influential factors. The optimisation results, in turn, show promising improvement potential with a wide range of design solutions that can ensure better energy performance as well as better thermal comfort. The annual energy demand could be significantly improved by 13.2% up to 49.5% while discomfort degrees could be decreased by over 26%.

High exposure of ultrafine particles at Guangzhou bus stops and the impact of urban layout

Urban bus stops have been identified as hotspots for particulate matter due to intense vehicle activities, including acceleration from standstill, this poses a risk of short-term exposure to high levels of pollution for residents who wait for buses during their daily commute. In order to provide constructive and scientific recommendations for the placement of bus stops from the perspective of regional architectural layout, we conducted a comparative observational experiment in Guangzhou to investigate the exposure to ultrafine particle (UFP) at bus stations with different building layouts. Our study found that there was a significant interaction between the urban layout and vehicle activities of bus stations with 99% confidence level. The average level of UFP pollution at the street canyon bus stations was 48.2 103ptccm−1 in the afternoon. Pedestrians waiting at the bus stations built in the street canyon could be exposed to excessive UFP levels (higher than 30.0103ptccm−1) during 82.2% of the waiting time. Compared to the park, the UFP concentration at the bus station in the canyon was found to be more responsive to changes in traffic volume, and the diffusion of UFP was more dependent on meteorological conditions, according to the results of grey relational analysis. However, near the park, there was no significant difference in UFP levels with and without the bus station. Based on our findings, we recommend that the department of urban planning and design consider creating a more open diffusion environment when setting up bus stops to reduce the risk of exposure to high level of UFP pollution.

Spatial and temporal variations of microclimate and outdoor thermal comfort in informal settlements of warm humid Dar es Salaam, Tanzania

In developing countries, urbanization is dominated by the growth of informal settlements which represents 40–80% of major cities. The challenges brought up by the growth of informal settlements spans from social-economic to environmental. Previously, upgrading of the informal settlements focused on social-economic aspects such as provision of necessary services for the residents, whereas the quality of the outdoor thermal environment has not received much attention. This paper entails to investigate the potential of upgrading the outdoor thermal environment in informal settlements in the warm humid city of Dar es Salaam, Tanzania through examining the influence of addition of trees with different Leaf Area Index (LAI) and incremental increase of buildings heights. The study uses simulation as a method for analysis of the warm season and calculates the Physiological Equivalent Temperature (PET) as a thermal index. Results show substantial improvement of both microclimate and outdoor thermal comfort. Incremental increase of buildings heights in a street canyon to 12, 18, and 24 m leads to the reduction of PET by 2.5, 2.8, and 3.8 °C respectively at 2:00 p.m. Similarly, applying LAI's of 2, 4, and 6 m2/m2 leads to reduction of the mean radiant temperature by 7.9, 10.1, and 12.2 °C; while PET was reduced by 3.9, 4.7, and 5.6 °C respectively at 2:00 p.m. Nonetheless, upgrading of informal settlements shows marginal influence on the reduction of air temperature. Despite the noted thermal improvement in the studied area, the thermal comfort limits of the warm season were difficult to reach. The findings suggest that addition of vegetation is the economically most effective way for upgrading thermal conditions in informal urban fabric areas.

Numerical study on the impact of traffic lane adjustments and low boundary walls on pedestrian exposure to NO2 in street canyons

Mitigating the adverse effects of air pollution, especially on human health, is one of the greater contemporary challenges for cities. Street canyons have herein been identified as bottleneck areas in urbanized environments. Focusing on the necessity of fast-response interventions, strategies to control source-receptor pathways (e.g. implementing low boundary walls (LBWs)) are gaining interest. A potential strategy which is greatly overlooked is the adjustment (reduction or displacement) of traffic lanes in order to increase the distance between source (traffic) and recipient (pedestrians). Within our study, computation fluid dynamics (CFD) is used to simulate the impact of alternations to traffic lanes (whether or not combined with LBWs) on the pedestrian exposure to NO2 for a specific case-study (Belgiëlei, Antwerp) under two prevailing wind directions. The average differences in NO2 concentrations for the entire pedestrian area ranged between +1.0 % to −3.6 %. On specific locations, reduction up to −8.0 % were reached. In case of perpendicular winds, a lateral displacement of all traffic lanes towards the windward facade including LBWs was found most beneficial to reduce pedestrian exposure. LBWs also showed to be efficient in reducing potential adverse effects of lane displacement under less frequent wind directions.

Spatial–Temporal Federated Transfer Learning with multi-sensor data fusion for cooperative positioning

With the development of advanced embedded and communication systems, location information has become a crucial factor in supporting context-aware or location-aware intelligent services. Among these services, modern Intelligent Transportation System (ITS) has the strictest requirements for real-time, accurate, and private location data. In this study, a Spatial-Temporal Federated Transfer Learning (ST-FTL) framework is designed and introduced to achieve more precise cooperative positioning with multi-sensor data fusion while protecting the location data privacy in urban ITS. Specifically, a three-layer FTL architecture is constructed to enhance the prediction accuracy on GPS positioning errors for vehicles in different regions especially when facing missing local data in some specific scenarios (e.g., urban canyons), in which Transfer Learning (TL) is incorporated to optimize the initialization of global model with faster convergence but less communication cost in Federated Learning (FL). A multi-attribute based spatial-temporal clustering algorithm is developed to facilitate the finding of more appropriate source domains similar to the target domain in a density-based scheme, while a convolutional-gated unit is newly designed to further filter out the useless features and add new features based on the convolution operations and gating mechanism, resulting in more effective global model initialization and weight aggregation from cross-region selection. A multi-sensor data fusion model is built in local to improve the prediction accuracy on positioning errors, in which an improved time-aware asymmetric attention mechanism is involved to selectively adjust the weight importance of the incorporated GPS and Inertial Measurement Unit (IMU) data in a more targeted fusion process. Additionally, an improved Siamese network structure, which breaks the traditional dual network structure and only adopts one single network structure, is leveraged to realize the lightweight data augmentation based on the better utilization of historical similar data from local vehicles themselves, when suffering from insufficient training data. Experiments and evaluations based on two different public datasets demonstrate the outstanding performance of our proposed model and method in achieving superior prediction accuracy and faster convergence for cooperative positioning compared with other state-of-the-art positioning methods in urban ITS.

Time-evolving Impact of Trees on Street Canyon Microclimate

Nowadays, cities are frequently exposed to heatwaves, worsening outdoor thermal comfort and increasing cooling energy demand in summer. Urban forestry is seen as one of the viable and preferable solutions to combat extreme heat events and urban heat island (UHI) in times of climate change. While many cities have initiated tree-planting programs in recent years, the evolving impact of trees on street microclimate, in a time span of up to several decades, remains unclear. This study focuses on the cooling effects of a common type of street trees, linden trees, in five age groups, i.e., 10-20, 20-30, 30-40, 40-60, and 60-100 years old, while the diversity of trees is present in nature, that is, various species and crown geometries, The leaf area index (LAI) and leaf area density (LAD) of linden trees vary nonlinearly as they grow, peaking at different ages. Computational fluid dynamics (CFD) simulations were performed for an idealized street canyon with linden trees of various age groups. Turbulent airflow, heat and moisture transport, shortwave and longwave radiation, shading, and transpiration were fully coupled and solved in OpenFOAM. Meteorological data, including air temperature, wind speed, moisture, and shortwave radiation of the heatwave in Zurich (June 2019), were applied as boundary conditions. The results show that young trees in the age group of 10-20 years old provide little heat mitigation at the pedestrian level in an extreme heat event. Optimal heat mitigation by trees is observed for the group of 30-60 years old linden trees. Finally, the potential impact of growing trees on air ventilation in streets is evaluated.

Cooling Tower Plume and Drift Concentration in presence of Urban Canyon

This paper presents a computational analysis of the cooling tower plume and drift emitted by mechanical draught cooling towers situated in urban areas. The data from the Wang X (2019) and Briggs plume lift height formula are used to prove results obtained from computational Modeling of cooling tower drift and it was found to be effective. The simulation setup uses a multiphase mixture model for plume analysis and DPM for analysis of drift in the Ansys fluent. The CFD simulation results are accurate within 50 to 100 m with the Briggs plume lift height formula. The effect of wind coming from urban canyons on the cooling tower results in decreased plume lift height and increase drift concentration and deposition rate on the ground.

Urban Pollutant Dispersion on a Turbulent Fluid Model

In 2019, more than 90% of the world’s population was living in places where pollutantconcentrations exceeded the air-quality guidelines. Anthropogenic activity has been the main cause of climate change in the last 20 years, especially in urban centres where industrial activities and motorized traffic mainly occur. This article presents a study of air pollutant dispersion by computational fluid dynamics modelling using OpenFOAM for urban street canyon simulations. The aim of this study was to investigate the influence of vegetation on natural ventilation, dispersion and CO2 mitigation. The behavior of pollutants was modeled by the Reynolds-averaged Navier–Stokes equations (RANS) and the k−ε fluid turbulence model. The results indicate that aerodynamic effects are more important at lower pollutant wind velocities, as they reduce turbulent dispersion and allow a higher reduction in the concentration of air pollutants. In addition, the direction of the wind, relative to the main axis of the street, had a significant impact on the results found. In general, in the presence of trees, perpendicular winds can lead to higher concentration of pollutants, while parallel winds allow improve air quality, considerably reducing those pollutant concentrations.

Interaction of Urban Configuration, Temperature, and De Facto Population in Seoul, Republic of Korea: Insights from Two-Stage Least-Squares Regression Using S-DoT Data

Climate change exacerbates thermal experiences in urban environments, affecting the frequency of social activities in public spaces. As climate change is expected to have a greater influence on thermal comfort, effective integration of climatic knowledge and urban design is required. However, there is a lack of knowledge regarding urban configurations that are resistant to temperature and promote urban vitality. This study aimed to explore the correlation between urban configuration, thermal environment, and urban vitality. We categorized the urban configuration of Seoul and analyzed the urban configuration type that impacts urban vitality and temperature. We used the number of the de facto population to measure urban vitality. The two-stage least-squares (2-SLS) model was used to address endogeneity concerns related to configuration, temperature, and de facto population. This study shows that de facto population is influenced by both urban configuration type and temperature. Effective design strategies for maintaining de facto population while responding to climate change include a combination of small blocks and high height-to-width ratios (H/W). In contrast, open-space urban configurations negatively impact de facto population. In high-density and high-H/W areas, de facto population increased due to shading effects but decreased when the critical value was exceeded. Urban configurations with high density and deep urban canyons have greater de facto population in winter than in summer.

Particle-size resolved aerosol levels of total and extractable platinum in urban and rural regions of Western Europe

In this study, we determined the levels of total and extractable platinum in size-resolved atmospheric aerosol (PM) samples obtained from roadside and background urban sites in each of six European cities (Amsterdam, Frankfurt, London, Milan, Stockholm, and Thessaloniki), as well as at three rural sites in northern, central and southern Europe. A Hi-Vol sampler was used to collect PM in three size classes (PM>7, PM7-3, PM3) which were characterized for total platinum and extractable platinum, using a suite of physiologically relevant extraction fluids – alveolar lung fluid (ALF), gambles saline, 0.07M HCl, and MQ (high purity, 18MΩ, water). The resulting large data set of Pt in ambient aerosols is one of the most extensive developed to date.Order-of-magnitude differences in air concentrations of total platinum were observed between urban sites, ranging from 2.4 to 45.6 pg/m3. Relatively consistent and low concentrations (0.9–4.9 pg/m3) of total platinum were observed at the rural background sites. Roadside canyon sites in London and Thessaloniki exhibited the highest concentrations. Levels of total platinum at urban background sites were similar across the cities (2.4–14.2 pg/m3) and were approximately ½ the levels measured at urban roadside/canyon sites. When total platinum concentrations were normalized to PM mass the range in air concentrations was substantially reduced to just 4-fold (mean of 129 ng/g at rural background sites to 532 ng/g, London/Thessaloniki). The majority of the total platinum was present in the fine (PM3) fraction, however the contribution of coarse particles (>7 μm and 7-3 μm) to total platinum was also very significant (>30% at nearly all sites).Median site class extractable platinum concentrations (total suspended particulate, TSP) were all below 1 pg/m3 except for the urban canyon sites in London and Thessaloniki (1.8–6.8 pg/m3). Rural background concentrations of extractable platinum (TSP) were especially low (0.05 pg/m3 in MQ and 0.13–0.25 pg/m3 in the other solvents). For the TSP, the MQ extractable fraction was 2.5–5.5% of total platinum across site classes and 6.1–21.1% across site classes for the other three solvents. Except for the most contaminated urban canyon sites (London and Thessaloniki) the differences in platinum extracted by the three non-MQ solvents was not significant when site class medians are compared; thus, 0.07M HCl may be used to address the putatively bioaccessible fraction of platinum in aerosol particulate matter.

LiDAR mini-matching positioning method based on constraint of lightweight point cloud feature map

In the global navigation satellite positioning denial environment, the matching localization based on a high-definition map is a well-recognized method to assist autonomous vehicles in achieving continuous, stable, and reliable navigation and positioning in complex scenes. However, due to the complexity, density, and richness of the typical high-definition point cloud map, there are high technical thresholds in the process of localization production, application, and maintenance. As a result, referring to the idea of traverse survey in geodesy, we design a priori lightweight point cloud feature map structure based on road mileage nodes as the main line to increase the flexibility of map segmentation, splicing, retrieval, and communication transmission. At the same time, an efficient and practical mini-matching positioning method is proposed. Finally, using the real datasets of various urban canyon scenes for verification, the proposed method can achieve stable sub-meter level localization.

Application of roadside air purifiers in urban street canyons: A pilot-scale study in Hong Kong

The implementation of roadside air purifiers has emerged as an effective active control measure to alleviate air pollution in urban street canyons. However, technical questions raised under real conditions remain challenging. In this study, we conducted a pilot-scale investigation involving seven units of self-designed roadside air purifiers in an urban street canyon in Hong Kong. The air cleaning effects were quantified with an air quality sensor network after rigorous quality control. The removal efficiencies of Nitrogen dioxide (NO2), Fine suspended particulates (PM2.5), Carbon monoxide (CO), and Nitric oxide (NO) were determined by comparing with simultaneously measured ambient concentrations, with hourly average efficiencies of 14.0 %–16.9 %, 3.5–10.0 %, 11.9 %–18.7 %, and 19.2 %–44.9 %, respectively. Generally, the purification effects presented variations depending on the ambient pollutants' levels. Higher ambient concentrations of NO2, PM2.5, CO correlated with increased purification effects, while NO presented the opposite trend. The influence of interval distance combined with spatial distribution indicated the operation of purifiers will induce local NO2 attenuation even at an interval distance of four meters. Statistical analysis delivered evidence the air cleaning ability exhibited optimal performance when relative humidity level is ranged from 70 % to 90 %, aligning with the prevailing conditions in Hong Kong. Additionally, improved purification effects were observed at the downwind direction, and their performance was enhanced when the wind speed exceeded 2.5 m/s. Moreover, we estimated the operational lifetime of the air purifiers to be approximately 130 days, offering crucial information regarding the filter replacement cycle. This work serves as a pioneering case study, showcasing the feasibility and deployment considerations of roadside air purifiers in effectively controlling air pollution in urban environments.

The impacts of urban canyons morphology on daylight availability and energy consumption of buildings in a hot-summer Mediterranean climate

Proper sunlight access has a positive impact on the physical and mental health of citizens, as well as on reducing energy consumption. This capacity may be limited by inappropriate urban morphology, and urban canyons and buildings are deprived of proper daylight. The purpose of this research is to study the current solar potential on the surface of the canyons and buildings and the effect of urban morphology parameters on the amount of each of the main variables of the research in Ilam, Iran. First, solar radiation, daylight illuminance, and air temperature were measured at different points of the studied canyons for three days. After that, the studied area was modeled with Rhino-Grasshopper software, and simulation was done using Ladybug Tools and UMI plugin. The results showed that the amount of daylight available in most canyons is low. Less than 30 % of the canyon’s surfaces benefited from 6 h of direct sunlight during the measurement. The simulation results for the 62 studied buildings showed that the highest amount of daylight is related to buildings with a height of more than 10 m. 56 % of the buildings had an annual solar radiation of less than 500 mw/m2. The roofs of the buildings received the most radiation, accounting for 72 % of the total radiation. By examining the parameters of urban morphology, it was found that the most influential parameter on the amount of solar radiation was the floor area, and the height of the buildings had the most influence on energy consumption.

A Simulation Study of the Impact of Urban Street Greening on the Thermal Comfort in Street Canyons on Hot and Cold Days

The urban heat island effect has become a widely concerning issue worldwide. Many researchers have made great efforts to improve the summer thermal comfort of urban street canyons by optimizing street greening. Relatively less research has focused on how to improve the thermal comfort of street canyons by optimizing street greening during cold days. Many researchers have proposed strategies to improve the summer thermal comfort of street canyons using road greening. This may have a significant negative impact on the winter thermal comfort of street canyons due to the lack of consideration of the impact on hot and cold days simultaneously, especially when the road green space is mainly composed of evergreen tree species. We aimed to explore the impacts of urban street greening on thermal comfort on hot and cold days at the same time. We used Zhutang West Road in Changsha, China, as an example and built six different models to explore the impacts of the street vegetation types, number of street trees, tree heights, crown widths, and Leaf Area Index on the thermal comfort of the street canyon. In addition, we also considered the impact of different building features and wind directions on the thermal comfort of the street canyon. We employed ENVI-met (version 5.5.1) to simulate different urban street greening models. The results show that the model with a high tree canopy density, tall trees, large and dense crowns, and sufficient building shade has good thermal comfort on hot days (the average physiological equivalent temperature (PET) is 31.1 °C for the study period) and bad thermal comfort on cold days (the PET is 13.3 °C) when it is compared with the other models (the average PETs are 36.2 °C, 31.5 °C, 41.5 °C, 36.2 °C, and 35.5 °C, respectively, on hot days and for other models). In addition, the model with a very large number of short hedges has a positive impact on thermal comfort during hot days (the PET is 31.1 °C). The PET value of another comparable model which does not have hedges is 31.5 °C. Even if the model with a small building area has good ventilation, the small building shade area in the model has a more obvious impact and the model has relatively good thermal comfort during cold days (the PET is 14.2 °C) when it is compared to models with bigger building areas (the PET is 13.9 °C). In summer, when the wind is parallel to the direction of the street canyon, the wind speed in the street canyon is high and the model has relatively good thermal comfort (the PET is 35.5 °C) compared with another model which has different wind direction and lower wind speed at the street canyon (the PET is 36.2 °C). In winter, when the wind is perpendicular to the direction of the street canyon, buildings and trees have a strong windproof effect and this is beneficial to the improvement of thermal comfort (the PET is 15.3 °C for this model and 13.9 °C for another comparable model). This research lays a solid foundation and encourages people to think about the impact of building and tree composition and configuration on the thermal comfort of street canyons during hot and cold days simultaneously.

Simulating Microscale Urban Airflow and Pollutant Distributions Based on Computational Fluid Dynamics Model: A Review.

Urban surfaces exert profound influences on local wind patterns, turbulence dynamics, and the dispersion of air pollutants, underscoring the critical need for a thorough understanding of these processes in the realms of urban planning, design, construction, and air quality management. The advent of advanced computational capabilities has propelled the computational fluid dynamics model (CFD) into becoming a mature and widely adopted tool to investigate microscale meteorological phenomena in urban settings. This review provides a comprehensive overview of the current state of CFD-based microscale meteorological simulations, offering insights into their applications, influential factors, and challenges. Significant variables such as the aspect ratio of street canyons, building geometries, ambient wind directions, atmospheric boundary layer stabilities, and street tree configurations play crucial roles in influencing microscale physical processes and the dispersion of air pollutants. The integration of CFD with mesoscale meteorological models and cutting-edge machine learning techniques empowers high-resolution, precise simulations of urban meteorology, establishing a robust scientific basis for sustainable urban development, the mitigation of air pollution, and emergency response planning for hazardous substances. Nonetheless, the broader application of CFD in this domain introduces challenges in grid optimization, enhancing integration with mesoscale models, addressing data limitations, and simulating diverse weather conditions.

Influence of elevated walkways on outdoor thermal comfort in hot-humid climates based on on-site measurement and CFD modeling

The elevated walkway is commonly designed to facilitate pedestrian movement and alleviate traffic conflicts. Its shading effect is supposed to benefit outdoor thermal comfort on hot days, especially in hot and humid regions, but relevant research has been scarce. To validate this hypothesis, we conducted on-site measurements on an elevated walkway and ground sidewalks in Hong Kong during summer and winter. The parametric simulation study was performed to investigate the effects of the elevated walkway, street aspect ratio (H/W), walkway width (Wew), and sidewall type on outdoor thermal comfort in ideal urban street canyons through large eddy simulation and RayMan modeling. Results indicate that the elevated walkway increases the ground-level physiological equivalent temperature (PET) by up to 2.7 °C and causes a 2−17 °C lower walkway-level PET value. Ground-level and walkway-level PET values initially increase and then decrease with rising H/W but positively correlate with Wew. Compared to the open sidewall, the semi-hermetic sidewall decreases the ground-level PET value slightly (below 0.8 °C) but raises the walkway-level PET value (below 1.3 °C). This study confirms the efficacy of elevated walkways in improving pedestrian thermal comfort in hot weather, contributing to shade strategies for heat stress mitigation and thermal comfort improvement.

Sky-view images aided NLOS detection and suppression for tightly coupled GNSS/INS system in urban canyon areas

Although the global navigation satellite system (GNSS) and inertial navigation system (INS) integration framework has many advantages compared to the standalone GNSS and INS systems, its accuracy can be severely degraded in urban canyon areas, due to non-line-of-sight (NLOS) signals. Detection and suppression of NLOS signals are the key to improving the location accuracy of urban areas. Therefore, this paper proposes a method for NLOS detection using the improved region growing algorithm on sky-view images captured by a fish-eye camera. After obtaining NLOS detection results, this paper constructs a weighted model with an adaptive scale factor to suppress the influence of NLOS signals. Experimental results show that the proposed method can effectively improve the performance of NLOS detection and suppression. On this basis, a tightly coupled GNSS/INS integration system based on an extended Kalman filter is developed and tested on the vehicle equipment. Results show that the proposed method outperforms the traditional GNSS single point positioning (SPP) and tightly coupled GNSS SPP/INS integration on positioning accuracy. The root mean square error can be reduced by 33.7% and 24.6% in the north and east directions respectively. It indicates that the tightly coupled GNSS SPP/INS integration aided by the fish-eye camera has certain research value and potential on positioning in urban canyon areas.

Definition of a maximum threshold of direct solar radiation exposure for pedestrians of diverse walking abilities

Since pedestrians are impacted by solar radiation differently, urban designers must evaluate solar radiation exposure of pedestrian paths adopting an inclusive approach. This paper proposes a maximum threshold of direct solar radiation exposure for pedestrians based on activity, user profile and environmental conditions, defined as the difference between the energy consumption before feeling exhausted and the energy cost of walking. Two users of diverse walking abilities, a young adult and an elderly person with mobility impairment, were characterised by metabolic activity, walking speed and maximum energy capacity. Based on the theoretical framework, the energy budget of young adults to cope with thermal stress was set as three times higher than for the elderly. This framework was used to quantify the contribution of direct solar radiation to energy balance and then classify walkability during clear-sky summer hours; the term ‘walkable’ referred to environmental conditions allowing users to walk without feeling exhausted. The methodology was tested on an open area and an urban canyon in Milan; applicability by urban designers was key in developing a simplified way to evaluate shading needs. This approach could be applied to evaluate solar radiation exposure of pedestrian paths adopting diverse user experiences as an evaluation criterion.

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.

Spatial variability of fine particle number concentration in an urban area: The effect of aspect ratio and vehicular traffic

The condensation particle number counting (CPC) technique was used to measure the number concentration of fine particles (FPs) from 20 to 1000 nm in several monitoring campaigns inside an urban area, with the aim of studying the spatial variability inside the urban area at pedestrian level and its correlation with aspect ratio H/W (height of buildings/street width) and local traffic. The effect of aspect ratio on the FP concentrations was studied in the range of 0 ≤ H/W ≤ 4.04. To study the effect of local traffic on FP concentrations, measurements were carried out in adjacent and with similar geometrics (building height, street width, and aspect ratio) pedestrian and traffic-streets. Vertical profiles were also studied measuring FP concentration in a deep street canyon at heights from 1 to 12 m. The study shows a significant impact of local traffic on FP concentrations (traffic streets 2.7 times higher than pedestrian streets). With the increase of the value of the aspect ratio, FP concentrations exceed European averages. Additionally, It was observed a clear decrease in FP concentrations with height in deep street canyons. These results emphasize the importance of aspect ratio and local traffic when assessing the impact of FPs pollution.