Narrow Street Canyons Research Articles

Development of point sampling technology for identifying high-emitting vehicles in narrow and deep street canyons

Side road measurements of fine particle (FP) concentrations were performed in two narrow and deep street canyons in the historic center of Naples using point sampling technology (PS). The FP concentration in the range of 20–103 nm was measured using a condensation particle number counter (CPC) with a 1 Hz frequency. Video recording was used to detect passing vehicles classified into three categories: motorcycles, cars, and light-duty vehicles. The two monitored streets were adjacent, with similar geometrics and orientation but different high- and low-traffic levels. FP concentration time series data were processed to identify concentration peaks generated by the exhausts of passing vehicles. A sensitivity analysis study was carried out to determine how data processing parameters could affect high-emitting vehicle (high emitter) identification. Once the optimum parameters were defined, the contribution of high emitters exhaust emissions to local air quality was assessed. The results showed that PS technology offered viable opportunities for application in narrow street canyons where deploying more common crossroad and top-down monitoring can be troublesome. However, some issues remain to be solved to accurately identify high emitterss in high-traffic cases. PS technology can be useful for developing effective air quality management policies in historical centres.

A predictive analysis of thermal stress in a densifying urban business district under summer daytime conditions in a Mediterranean City

A new urban master plan in the metropolitan area of Tel Aviv, Israel calls for the extreme densification of an already intensively-built business district. This study assesses the existing daytime thermal stress experienced by pedestrians under the Mediterranean climatic conditions of the region during the summer season, and predicts future changes under various development scenarios. Microclimatic measurements were used to calculate the Index of Thermal Stress (ITS) under present conditions, and possible future scenarios were modeled by hypothetically altering the urban geometry and subjecting the results to a series of sensitivity analyses. Findings reveal that the net impact of taller buildings is a reduction in pedestrian thermal stress on clear summer days, since the deep shading of narrow street canyons lowers the radiant heat exposure – outweighing the weaker convective cooling that is expected due to higher air temperature and lower wind speed. Nonetheless, outdoor summer conditions remain thermally stressful during daytime hours, and there is a need for appropriate urban design strategies to further mitigate urban warming. Additional shading improved thermal comfort at certain times and street orientations, but available space for street trees is limited. This emphasizes the need for creative solutions, such as building-integrated green infrastructure.

Isolating the impacts of urban form and fabric from geography on urban heat and human thermal comfort

Public health risks resulting from urban heat in cities are increasing due to rapid urbanisation and climate change, motivating closer attention to urban heat mitigation and adaptation strategies that enable climate-sensitive urban design and development. These strategies incorporate four key factors influencing heat stress in cities: the urban form (morphology of vegetated and built surfaces), urban fabric, urban function (including human activities), and background climate and regional geographic settings (e.g. topography and distance to water bodies). The first two factors can be modified and redesigned as urban heat mitigation strategies (e.g. changing the albedo of surfaces, replacing hard surfaces with pervious vegetated surfaces, or increasing canopy cover). Regional geographical settings of cities, on the other hand, cannot be modified and while human activities can be modified, it often requires holistic behavioural and policy modifications and the impacts of these can be difficult to quantify. When evaluating the effectiveness of urban heat mitigation strategies in observational or traditional modelling studies, it can be difficult to separate the impacts of modifications to the built and natural forms from the interactions of the geographic influences, limiting the universality of results. To address this, we introduce a new methodology to determine the influence of urban form and fabric on thermal comfort, by utilising a comprehensive combination of possible urban forms, an urban morphology data source, and micro-climate modelling. We perform 9814 simulations covering a wide range of realistic built and natural forms (building, roads, grass, and tree densities as well as building and tree heights) to determine their importance and influence on thermal environments in urban canyons without geographical influences. We show that higher daytime air temperatures and thermal comfort indices are strongly driven by increased street fractions, with maximum air temperatures increases of up to 10 and 15 °C as street fractions increase from 10% (very narrow street canyons and/or extensive vegetation cover) to 80 and 90% (wide street canyons). Up to 5 °C reductions in daytime air temperatures are seen with increasing grass and tree fractions from zero (fully urban) to complete (fully natural) coverage. Similar patterns are seen with the Universal Thermal Climate Index (UTCI), with increasing street fractions of 80% and 90% driving increases of 6 and 12 °C, respectively. We then apply the results at a city-wide scale, generating heat maps of several Australian cities showing the impacts of present day urban form and fabric. The resulting method allows mitigation strategies to be tested on modifiable urban form factors isolated from geography, topography, and local weather conditions, factors that cannot easily be modified.

On the application and grid-size sensitivity of the urban dispersion model CAIRDIO v2.0 under real city weather conditions

Abstract. There is a gap between the need for city-wide air-quality simulations considering the intra-urban variability and mircoscale dispersion features and the computational capacities that conventional urban microscale models require. This gap can be bridged by targeting model applications on the gray zone situated between the mesoscale and large-eddy scale. The urban dispersion model CAIRDIO is a new contribution to the class of computational-fluid dynamics models operating in this scale range. It uses a diffuse-obstacle boundary method to represent buildings as physical obstacles at gray-zone resolutions in the order of tens of meters. The main objective of this approach is to find an acceptable compromise between computationally inexpensive grid sizes for spatially comprehensive applications and the required accuracy in the description of building and boundary-layer effects. In this paper, CAIRDIO is applied on the simulation of black carbon and particulate matter dispersion for an entire mid-size city using a uniform horizontal grid spacing of 40 m. For model evaluation, measurements from five operational air monitoring stations representative for the urban background and high-traffic roads are used. The comparison also includes the mesoscale host simulation, which provides the boundary conditions. The measurements show a dominant influence of the mixing layer evolution at background sites, and therefore both the mesoscale and large-eddy simulation (LES) results are in good agreement with the observed air pollution levels. In contrast, at the high-traffic sites the proximity to emissions and the interactions with the building environment lead to a significantly amplified diurnal variability in pollutant concentrations. These urban road conditions can only be reasonably well represented by CAIRDIO while the meosocale simulation indiscriminately reproduces a typical urban-background profile, resulting in a large positive model bias. Remaining model discrepancies are further addressed by a grid-spacing sensitivity study using offline-nested refined domains. The results show that modeled peak concentrations within street canyons can be further improved by decreasing the horizontal grid spacing down to 10 m, but not beyond. Obviously, the default grid spacing of 40 m is too coarse to represent the specific environment within narrow street canyons. The accuracy gains from the grid refinements are still only modest compared to the remaining model error, which to a large extent can be attributed to uncertainties in the emissions. Finally, the study shows that the proposed gray-scale modeling is a promising downscaling approach for urban air-quality applications. The results, however, also show that aspects other than the actual resolution of flow patterns and numerical effects can determine the simulations at the urban microscale.

Numerical investigation of the effects of environmental conditions, droplet size, and social distance on interpersonal droplet transmission in a deep urban street canyon

This study investigated the interpersonal droplet transmission between a healthy and an infected person in a deep and narrow street canyon using Computational Fluid Dynamics (CFD) simulation. The CFD simulations modelled various droplet sizes (Dp), background wind speeds (Uref), relative humidity (RH), and social distances (D) to estimate their effects on interpersonal droplet transmission. The results revealed noticeably opposite effects of these factors. For example, small background wind moved droplets upward and suspended them in the air for a longer time while high wind speeds distributed droplets in the street canyon with few of them retained in the air. Relative humidity had a trifling impact on dispersing small droplets (10μm, 25μm, 50μm), whereas it significantly modified the dispersion of large droplets, especially in small background wind speeds. Furthermore, small droplets travelled longer distances in dry air and were either deposited on the surrounding buildings’ walls or suspended in the air. In contrast, larger droplets in moist air rapidly deposited on the ground or the infected person’s body. In dry air, 45% of large droplets were inhaled or suspended in the air, exposing pedestrians to contaminated droplets. Large social distances significantly diluted the small droplets but increased the infection risk from large droplets because of the complex interaction of the ambient airflow and the gravity. It is recommended to keep social distances of 2 m and 4 m for pedestrians in deep urban street canyons in Windy condition and Calm-Wet condition, respectively.

Characterization of Urban Greening in a District of Lecce (Southern Italy) for the Analysis of CO2 Storage and Air Pollutant Dispersion

This paper is devoted to the assessment of urban greening effects on two important ecosystem services, i.e., air quality and CO2 storage, including the corresponding economic impacts in a real urban area, i.e., a district located in the Mediterranean city of Lecce (southern Italy). Two tools were employed, i-Tree Canopy and the computational fluid dynamics (CFD) microclimate model ENVI-met. i-Tree Canopy allowed fully determining the land-cover percentage on the basis of different ground cover classes and obtaining an estimate of annual values of CO2 storage, air pollutant removal, and economic benefits in the presence of urban greening. The estimate in i-Tree Canopy considered only the amount of greening; therefore, air pollutant removal estimates were only potential. As the vegetation was located in street canyons, its interaction with local meteorology and urban geometry strictly affected the dispersion of nitrogen oxides (NOx) (taken here as an example) as obtained from ENVI-met simulations. In ENVI-met, both deposition/absorption and aerodynamic effects were considered, and local increases in concentration were found in the district. The analysis of results obtained from different tools (one complex (CFD model) and the other simple (i-Tree model)) showed the error associated with the simple model in the computation of impacts if the interaction among the vegetation characteristics, the meteorological conditions, and the urban geometry was neglected; however, it also uncovers a novel approach for comprehensively characterizing a given area in terms of its vegetation cover, CO2 storage, and economic benefits, as well as local effects on air quality. This study is set in a broader context aimed at assessing the air quality in urban canopies of Mediterranean areas characterized by the presence of narrow street canyons where pollutants can accumulate due to ineffective air exchange with the above atmosphere.

Bus commuter exposure and the impact of switching from diesel to biodiesel for routes of complex urban geometry

We report on commuters’ exposure to black carbon (BC), PM2.5 and particle number (PN, with aerodynamic diameter, da, in the range 0.01 <da< 1.0 μm) collected on-board diesel- and biodiesel-fuelled buses of the Bus Rapid Transit (BRT) system of the city of Curitiba, Brazil. Particulate concentrations measured at high sampling rates allowed the capture of fine gradients along the route and the comparison of in-cabin air pollution on buses of different technologies.Of all metrics, BC showed the largest discrepancies, with mean concentrations of 20.1 ± 20.0 μg m−3 and 3.9 ± 26.0 μg m−3 on diesel- and biodiesel-fuelled buses, respectively. Mean PM2.5 concentrations were similar (31.6 ± 28.5 μg m−3 and 29.0 ± 17.8 μg m−3), whilst mean PN concentrations were larger on the biodiesel buses (56,697 ± 26,800 # cm−3vs. 43,322 ± 32,243 # cm−3). The results are in line with studies on biodiesel emission factors that reported lower BC mass but more particles with smaller diameters. Our hypothesis is that different emission factors of diesel and biodiesel engines reflected in differences of in-cabin particulate concentrations. We found that the passenger exposure during the bus commutes was affected not only by the fuel used but also by the street geometry along the route, with segments with canyon configurations resulting in peak exposure to particulates. The results suggest that i) switching from diesel to biodiesel may help abate commuters’ exposure to BC particles on-board buses of the BRT system, whilst it would need to be complemented with after-treatment technologies to reduce emissions; ii) further reductions in exposure (to peaks in particular) could be achieved by changing bus routes to ones that avoid passing through narrow urban street canyons.

On the Flow Past an Array of Two-Dimensional Street Canyons Between Slender Buildings

The flow above idealized, two-dimensional series of parallelepipedal buildings is examined with the aim of investigating how the building width to height aspect ratio affects the turbulence in the roughness sublayer and the ventilation of the underlying street canyons. We compare the case of buildings with a squared section with a configuration with slender buildings both in the case of unit canyon width to height aspect ratio and in the case of AR = 2. The former corresponds to skimming flow and the latter to wake-interference regime. Measurements are performed in a water channel, measuring velocity on a vertical mid-plane using a particle image velocimetry technique. The mean flow, its second-order turbulence statistics, the exchange fluxes, and the integral time scales investigated, with results showing that slender buildings enhance turbulence production and yield longer integral time scales in the region just above the building roof. The combined analysis of the turbulence production fields and the snapshots of the flow during sweep and ejection events demonstrate that the shear layer between the canyon and the external flow is significantly more unstable with slender buildings, mainly because the damping effect of the vertical velocity fluctuations from the flat roof of the upwind building is substantially missing. Consequently, a larger (downstream) portion of the interface is prone to the direct interaction of the external flow structures. The higher turbulence intensity promotes the ventilation at the canyon interface. In summary, the present experiments show that the effect of the reduced building aspect ratio is particularly significant when the urban canopy consists of narrow canyons. The result is of interest since narrow street canyons are typically bounded by slender buildings in the urban texture of the old European city centres.

A Numerical Study of the Temperature Reduction by Water Spray Systems within Urban Street Canyons

To reduce energy demand (both fossil fuel and renewable energy) for cooling the urban heat island environment, some solutions have been studied. Among these methods, the water spray system is considered more flexible due to its dynamic controls. This study investigated the cooling effect of water spray systems in the street canyon under different aspect ratios and high relative humidity environments using a computational fluid dynamics model. This model was validated with water channel and wind tunnel experiments. The results showed that the most effective cooling area was the area just under the spray nozzles. However, in a narrow street canyon, people in the middle of the street may feel the cooling effect because of the dispersion and accumulation of the cooled air. Our simulations demonstrated that air under the nozzles was saturated and this revealed that under drier conditions the water spray systems will have higher cooling performance. We also found that using large water droplets created a wider cooling area in the middle of the street canyon, and this phenomenon was not changed much if the nozzle height was increased from 2.5 m to 3.5 m.

Channel Measurement and Modeling for 5G Urban Microcellular Scenarios

In order to support the development of channel models for higher frequency bands, multiple urban microcellular measurement campaigns have been carried out in Berlin, Germany, at 60 and 10 GHz. In this paper, the collected data is uniformly analyzed with focus on the path loss (PL) and the delay spread (DS). It reveals that the ground reflection has a dominant impact on the fading behavior. For line-of-sight conditions, the PL exponents are close to free space propagation at 60 GHz, but slightly smaller (1.62) for the street canyon at 10 GHz. The DS shows a clear dependence on the scenario (median values between 16 and 38 ns) and a strong distance dependence for the open square and the wide street canyon. The dependence is less distinct for the narrow street canyon with residential buildings. This behavior is consistent with complementary ray tracing simulations, though the simplified model tends to overestimate the DS.

Numerical modelling of mean radiant temperature in high-density sub-tropical urban environment

Outdoor thermal comfort has been a widely concerned issue in tropical and subtropical cities. In order to assess the conditions of outdoor thermal comfort, quantitative information on different spatial and temporal scales is required. This paper employs a numerical model (SOLWEIG – SOlar and LongWave Environmental Irradiance Geometry) to examine the spatial and temporal variations of mean radiant temperature (Tmrt), as an indicator of radiant heat load and outdoor heat stress in high-density sub-tropical urban environment in summer. The SOLWEIG model is found to simulate the six-directional shortwave and longwave radiation fluxes as well as Tmrt very well. Simulation results show that urban geometry plays an important role in intra-urban differences in summer daytime Tmrt. Open areas are generally warmer than surrounding narrow street canyons. Street canyons are sheltered from incoming direct solar radiation by shading of buildings while open areas are exposed to intense solar radiation, especially along the sunlit walls where high Tmrt is observed due to reflected short-wave radiation and long-wave radiation emitted from the sunlit building walls. The present study confirms that there are great potential in using urban geometry to mitigate high radiant heat load and daytime heat stress in the compacted urban environment. In high-density subtropical cities where high daytime Tmrt causes severe thermal discomfort in summer, dense urban structures are able to mitigate the extremely high Tmrt and improve outdoor thermal comfort. However, the shading strategy has to be cautious about air ventilation in such a dense urban environment.

The effect of urban geometry on mean radiant temperature under future climate change: a study of three European cities.

Future anthropogenic climate change is likely to increase the air temperature (T(a)) across Europe and increase the frequency, duration and magnitude of severe heat stress events. Heat stress events are generally associated with clear-sky conditions and high T(a), which give rise to high radiant heat load, i.e. mean radiant temperature (T(mrt)). In urban environments, T mrt is strongly influenced by urban geometry. The present study examines the effect of urban geometry on daytime heat stress in three European cities (Gothenburg in Sweden, Frankfurt in Germany and Porto in Portugal) under present and future climates, using T(mrt) as an indicator of heat stress. It is found that severe heat stress occurs in all three cities. Similar maximum daytime T(mrt) is found in open areas in all three cities despite of the latitudinal differences in average daytime T(mrt). In contrast, dense urban structures like narrow street canyons are able to mitigate heat stress in the summer, without causing substantial changes in T(mrt) in the winter. Although the T(mrt) averages are similar for the north-south and east-west street canyons in each city, the number of hours when T(mrt) exceeds the threshold values of 55.5 and 59.4 °C-used as indicators of moderate and severe heat stress-in the north-south canyons is much higher than that in the east-west canyons. Using statistically downscaled data from a regional climate model, it is found that the study sites were generally warmer in the future scenario, especially Porto, which would further exacerbate heat stress in urban areas. However, a decrease in solar radiation in Gothenburg and Frankfurt reduces T(mrt) in the spring, while the reduction in T(mrt) is somewhat offset by increasing T(a) in other seasons. It suggests that changes in the T(mrt) under the future scenario are dominated by variations in T(a). Nonetheless, the intra-urban differences remain relatively stable in the future. These findings suggest that dense urban structure can reduce daytime heat stress since it reduces the number of hours of high T(mrt) in the summer and does not cause substantial changes in average and minimum T(mrt) in the winter. In dense urban settings, a more diverse urban thermal environment is also preferred to compensate for reduced solar access in the winter. The extent to which the urban geometry can be optimized for the future climate is also influenced by local urban characteristics.

Spatial variability of frontal area index and its relationship with urban heat island intensity

In densely urbanized regions, the local climate is greatly influenced by the urban morphology, including interactions between buildings, space, and human activities. The Kowloon Peninsula in Hong Kong, with some of the greatest urban population densities in the world, represents an extreme case of the influence of the built environment on climate, with high-rise buildings, narrow street canyons, and little green space. In this study, the building frontal area index (FAI), a parameter for estimating aerodynamic resistance of the urban surface as a predictor of wind ventilation, was derived from a three-dimensional building database. The relationship between FAI and urban heat island intensity (UHII) from an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite image was calculated at different scales. The highest correlation (r = 0.574, n = 4900) was obtained at 100 m resolution, which suggests that the optimum operational scale of FAI is 100 m resolution for the study area, i.e. the scale and size at which FAI impacts on the urban climate. The presence/trend of any higher correlation at different resolutions was tested using the nonparametric Mann–Kendall test, and the results show that the statistic Z-value generated from the test is smaller than the hypothesis significance levels of 90%, 95%, and 99%; thus, the hypothesis of having a higher correlation at any scale other than 100 m resolution is rejected. Planning authorities may use the FAI generated at 100 m resolution for designing wind ventilation corridors across Hong Kong at this scale, especially when temperature and air quality in the inner city are of major concern. However, for applications to other cities with different standard morphologies, the FAI–UHII relationship should be re-evaluated.

Predicting velocities and turbulent momentum exchange in isolated street canyons

A simplified way of studying the transport of mass and momentum through dense neighborhoods is to consider the flow field as a combination of a mainly horizontal flow along street canyons and a vertical flow into and out of street canyons. In this paper, we derive a simple, semi-analytical model for calculating the expected wind speeds in narrow, isolated, idealized street canyons of uniform height that are aligned with and at an angle to the wind through the introduction of a non-dimensional parameterization of the vertical turbulent exchange of horizontal momentum between the urban canopy layer (UCL) and the urban boundary layer (UBL). The model is closed and evaluated using computational fluid dynamics (CFD) in a generic way. The RMSE of the normalized bulk velocity in the street canyon in the direction of the freestream flow is 1.8 × 10−4, 2.3 × 10−2 and 9.4 × 10−2 for variations in façade roughness, aspect ratio and canyon orientation, respectively.We argue that there is a need for a fast, simple methodology to assess the impact of urban form on neighborhood microclimate, especially in dense neighborhoods, and specifically for use in the early phases of design. This methodology should be able to predict areas in a neighborhood that are prone to low wind speeds or weak mass and momentum exchange rates with the UBL above because these areas may be particularly susceptible to pollutant retention and the urban heat island (UHI) effect. Current methodologies are often inadequate for this purpose because they use computationally intensive techniques to solve for flow through a neighborhood and often require a strong technical background to support their use. The work in this paper addresses this problem for an idealized, narrow street canyon, and we discuss the need for parameterizations for urban form that are relevant to mass and momentum exchange rates to extend this model beyond a single street canyon. Finally, we discuss how this work could be further developed into generalized planning guidelines and incorporated into a comprehensive urban planning methodology that aims to passively mitigate the UHI effect and pollutant retention through the clever design of urban form.

Outdoor airflow analysis and potential for passive cooling in the traditional urban context of Dubai

The main aim of the study is to investigate passive cooling performance in traditional urban contexts in the hot humid climate of the city of Dubai. Three cases were simulated for Al-Ras area with laminar and turbulent wind flow depending on Computational Fluid Dynamics (CFD) methodology. The laminar case was firstly run to study the general wind behavior around buildings and at the pedestrian level. The other two cases were turbulence modeling in both winter and summer seasons. The results were merely discussed and analyzed in terms of passive cooling via natural ventilation and its impact on human comfort. Narrow street canyons (4 m and less) can accelerate wind speed passing through it, resulting in a better passive cooling performance but sometimes in creating eddies if there are lots of bending angles. When the wind speed is higher (5 m/s), wind can reach deeper inside the traditional narrow streets providing better potential for thermal comfort. Most locations (49–57% of the studied area) inside the traditional urban context (street canyons aspect ratio, AR = 2–0.67) have wind speeds that range from light breeze to gentle breeze (according to Beaufort scale); which has the potential to provide natural cooling with around 5–8.5 °C lower temperature comfort sensation with basic assumption of 1.3 metabolic rate (MET) and 0.4 insulating value of summer clothing (CLO).

Potential changes in outdoor thermal comfort conditions in Gothenburg, Sweden due to climate change: the influence of urban geometry

AbstractQuantitative information about outdoor thermal comfort, on various temporal and spatial scales, is required to design better cities and mitigate heat problems not only in warm but also in temperate climates. The overall objective of this study is to explore the augmentation of global/regional climate changes by urban features such as geometry in a compact mid‐rise high‐latitude city (Gothenburg). The magnitude of spatial and temporal variations of intra‐urban mean radiant temperatures (Tmrt) is quantified using the SOLWEIG (SOlar and LongWave Environmental Irradiance Geometry) model. Hourly time resolution, statistically downscaled meteorological data, based on the ECHAM5‐GCM under the A1B emission scenario is used to simulate changes in Tmrt and physiologically equivalent temperature (PET) at the 2080–2099 time horizon.Results show that urban geometry causes large intra‐urban differences in Tmrt, on hourly, daytime and yearly time scales. In general, open areas are warmer than adjacent narrow street canyons in summer, but cooler in winter. According to the ECHAM5‐based scenario, the daytime Tmrt will increase by 3.2 °C by the end of this century. This is 0.4° more than simulated increase in air temperature (2.8 °C) and is mainly a result of decreases in summer cloudiness. Occasions of strong/extreme heat stress are expected to triple. This equates to 20–100 h a year, depending on geometry. Conversely, the number of hours with strong/extreme cold stress decreases by 400–450 h. Furthermore, the number of hours with no thermal stress increases by 40–200 h a year.The study confirms the potential for using geometry to mitigate daytime thermal stress. A densely built structure mitigates extreme swings in Tmrt and PET, improving outdoor comfort conditions both in summer and in winter. Furthermore, it highlights the importance of including information on either Tmrt or thermal comfort in climate scenarios to describe the combined effects of changes in multiple climate variables and to more realistically measure the impact on humans. Copyright © 2010 Royal Meteorological Society

Vertical transport of accumulation mode particles between two street canyons and the urban boundary layer

Abstract Concentrations and turbulent fluxes of accumulation mode particles were measured during the 2004–2005 ‘Canopy and Aerosol Particle Interaction in Toulouse Urban Layer’ project (CAPITOUL) at the top of two intersecting street canyons and in the urban boundary layer (UBL) in Toulouse, France. Particle numbers were strongly affected by boundary layer depth and showed limited sensitivity to local emissions. Differences in the diurnal patterns of particle numbers were observed between the finer fraction (0.3–0.4 μm) and coarser fraction (1.6–2.0 μm) of accumulation mode particles, indicating different processes of formation, evolution and transportation may be dominant. Highest particle numbers were observed in the narrow street canyon which had more limited local emissions and comparatively small particle fluxes. However, the improved ventilation rate in the wider canyon was also associated with the downward mixing of particles into the street canyon from the UBL. The results from this study clearly illustrate the temporal and spatial variability of particle numbers and fluxes in the urban atmosphere.

A model for vehicle-induced non-tailpipe emissions of particles along Swedish roads

Abstract One of the most important parameters that controls the suspension of road dust particles in the air is road surface moisture. This is calculated every hour from a budget equation that takes into account precipitation, evaporation and runoff. During wet conditions a road dust layer is built up from road wear which strongly depends on the use of studded tyres and road sanding. The dust layer is reduced during dry road conditions by suspension of particles due to vehicle-induced turbulence. The dust layer is also reduced by wash-off due to precipitation. Direct non-tailpipe vehicle emissions due to the wear and tear of the road surface, brakes and tyres are accounted for in the traditional way as constant emission factors expressed as mass emitted per vehicle kilometre. The model results are compared with measurements from both a narrow street canyon in the city centre of Stockholm and from an open highway outside the city. The model is able to account for the main features in the day-to-day mean PM10 variability for the street canyon and for the highway. A peak in the PM10 concentration is typically observed in late winter and early spring in the Nordic countries where studded tyres are used. This behaviour is due to a combination of factors: frequent conditions with dry roads, high number of cars with studded tyres and an accumulated road dust layer that increases suspension of particles. The study shows that using a constant emission factor for PM10 or relating PM10 emissions to NOx cannot be used for prediction of day-to-day variations in PM10 concentrations in the traffic environments studied here. The model needs to describe variations in dust load, wetness of the road and how dust suspension interacts with these processes.

Some aspects of air pollution in Buenos Aires city

This paper presents three aspects of air pollution in the city of Buenos Aires (Argentina). First, we describe the main features of air pollution climatology in the city: the characteristics of wind flow, atmospheric stability and mixing heights. Then, we present the results of the application of DAUMOD and ISCST3 atmospheric dispersion models to calculate spatial and temporal distributions of carbon monoxide and nitrogen oxides background concentrations. Finally, we present the main features of carbon monoxide concentrations observed in a narrow street canyon located downtown.

A Laboratory model for the flow in urban street canyons induced by bottom heating

Water tank experiments are carried out to investigate the convection flow induced by bottom heating and the effects of the ambient wind on the flow in non-symmetrical urban street canyons based on the PIV (Particle Image Visualization) technique. Fluid experiments show that with calm ambient wind, the flows in the street canyon are completely driven by thermal force, and the convection can reach the upper atmosphere of the street canyon. Horizontal and vertical motions also appear above the roofs of the buildings. These are the conditions which favor the exchange of momentum and air mass between the street canyon and its environment. More than two vortices are induced by the convection, and the complex circulation pattern will vary with time in a wider street canyon. However, in a narrow street canyon, just one vortex appears. With a light ambient wind, the bottom heating and the associated convection result in just one main vortex. As the ambient wind speed increases, the vortex becomes more organized and its center shifts closer to the leeward building.