Wind tunnel measurements of concentration fluctuations in an urban street canyon

A wind tunnel study was performed to examine some turbulent characteristics and statistical properties of the concentration field developing from the steady release of a tracer gas at street level in a canyon amidst urban roughness. The experiment was conducted with the approaching wind direction perpendicular to the street axis and, with a street width to building height aspect ratio equal to one. Concentration time series were recorded at 70 points within the test street cross-section and above. Mean concentrations, variances and related turbulent quantities, as well as other statistical quantities including quantiles were computed. Concentration spectra and autocorrelation functions were also examined. The emphasis is put here on the results concerning mean concentrations and the variance of concentration fluctuations. The main objective of this paper is to put forward potential benefits of the experimental approach taken in this study. Through a simple and already widely studied configuration it is aimed to show how, for modelling purposes, this approach can help improving our understanding of the mechanisms of dipersion of pollution from car exhausts in built-up areas and, with further measurements, how it could assist in drawing specifications for siting monitoring networks.     

Modeling reactive pollutant dispersion in an urban street canyon

Reactive pollutant dispersion in an urban street canyon with a street aspect ratio of one is numerically investigated using a computational fluid dynamics (CFD) model. The CFD model developed is a Reynolds-averaged Navier–Stokes equations (RANS) model with the renormalization group (RNG) k–ε turbulence model and includes transport equations for NO, NO₂, and O₃ with simple photochemistry. An area emission source of NO and NO₂ is considered in the presence of background O₃ and street bottom heating (ΔT=5 °C) with an ambient wind perpendicular to the along-canyon direction. A primary vortex is formed in the street canyon and the line connecting the centers of cross-sectional vortices meanders over time and in the canyon space. The cross-canyon-averaged temperature and reactive pollutant concentrations oscillate with a period of about 15 min. The averaged temperature is found to be in phase with NO and NO₂ concentrations but out of phase with O₃ concentration. The photostationary state defect is small in the street canyon except for near the roof level and the upper downwind region of the canyon and its local minimum is observed near the center of the primary vortex. The budget analysis of NO (NO₂) concentration shows that the magnitude of the advection or turbulent diffusion term is much larger (larger) than that of the chemical reaction term and that the advection term is largely balanced by the turbulent diffusion term. On the other hand, the budget analysis of O₃ concentration shows that the magnitude of the chemical reaction term is comparable to that of the advection or turbulent diffusion term. The inhomogeneous temperature distribution itself affects O₃ concentration to some extent due to the temperature-dependent photolysis rate and reaction rate constant.     

UV Fourier transform measurements of tropospheric O3, NO2, SO2, benzene, and toluene.

Using the differential optical absorption spectroscopy (DOAS) technique and a Fourier transform spectrometer, NO2, SO2, O3, benzene. and toluene were measured during three measurement campaigns held in Brussels in 1995, 1996, and 1997. The O3 concentrations could be explained as the results of the local photochemistry and the dynamical properties of the mixing layer. NO2 concentrations were anti-correlated to the O3 concentrations, as expected. SO2 also showed a pronounced dependence on car traffic. Average benzene and toluene concentrations were, respectively 1.7 ppb and between 4.4 and 6.6 pbb, but high values of toluene up to 98.8 ppb were observed. SO2 concentrations and to a lesser extent, those of NO2 and 03, were dependent on the wind direction. Ozone in Brussels has been found to be influenced by the meteorological conditions prevailing in central Europe. Comparisons with other measurements have shown that 03 and SO2 data are in general in good agreement, but our NO2 concentrations seem to be generally higher.     

A multi-approach monitoring of particulate matter, metals and PAHs in an urban street canyon

For the first time until now, the results from a prediction model (Atmospheric Dispersion Modelling System (ADMS)-Road) of pollutant dispersion in a street canyon were compared to the results obtained from biomonitors. In particular, the instrumental monitoring of particulate matter (PM10) and the biomonitoring of 14 polycyclic aromatic hydrocarbons (PAHs) and 11 metals by Quercus ilex leaves and Hypnum cupressiforme moss bags, acting as long- and short-term accumulators, respectively, were carried out. For both PAHs and metals, similar bioaccumulation trends were observed, with higher concentrations in biomonitors exposed at the leeward canyon side, affected by primary air vortex. The major pollutant accumulation at the leeward side was also predicted by the ADMS-Road model, on the basis of the prevailing wind direction that determines different exposure of the street canyon sides to pollutants emitted by vehicular traffic. A clear vertical (3, 6 and 9 m) distribution gradient of pollutants was not observed, so that both the model and biomonitoring results suggested that local air turbulences in the street canyon could contribute to uniform pollutant distribution at different heights.     

Influence of cetane improvers on the air quality in an urban street canyon

The concentration distributions of NO_x, PM, HC and CO in an urban street canyon have been estimated using a two-dimensional air quality numerical model based on the k– turbulent model and the atmospheric convection diffusion equation when various cetane improvers were used in diesel fuels. A wind vortex can be found within the street canyon, and the pollutants emitted from the bottom of the street canyon tend to follow the course of the wind field, moving circularly. The addition of cetane improvers can improve the air quality in a street canyon, all of the pollutants were found to decrease with increasing centane number.     

Large eddy simulation of shading effects on NO2 and O3 concentrations within an idealised street canyon

A large eddy simulation (LES) model that accounts for chemical reactions between oxides of nitrogen and ozone has been used to investigate the effect of local shading within an idealised street canyon on pollutant concentrations. It has shown that local shading can have a substantial impact on kerbside concentrations (>6 ppb difference for some situations presented) and that this may need to be taken into account to set up numerical model runs as well as sampling sites. A sensitivity study has been performed to investigate the effect of various governing parameters. A strong influence was found for the actual reduction of the photolytic rate constant within the shaded areas. A near linear relationship appeared between the reduction and the effect on pollutant concentrations. The chemical regime above and within the street canyon (determined by background concentrations aloft and emission rates at the ground) was also shown to be of high importance. The geometrical layout of the shading within the canyon and the wind speed in the canyon was shown to affect the spatial distribution of the shading effect rather than its overall magnitude.     

Description of pollutant dispersion in an urban street canyon using a two-dimensional lattice model

The pollutant dispersion in a street canyon has been described in this work by using an isothermal two-dimensional lattice model coupled to the Smagorinsky sub-grid scale model. The influence of the ratio between the height of the upstream and downstream canyon walls, as well as the gap distance between them on the flow pattern, was analyzed considering the situations of ‘open country’ or isolated street canyon and ‘urban roughness’ in which the influence of an urban fabric was considered. The model determined the trajectories of a large number of passive tracer particles released in the computational domain, making it easy to visualize the flow regimes established in each case. The results agreed with the observations reported from the experiments showing a strong influence on the flow inside the canyon exerted by the upstream landscape configuration.     

Three-dimensional modeling of air flow and pollutant dispersion in an urban street canyon with thermal effects

Effects of excess ground and building temperatures on airflow and dispersion of pollutants in an urban street canyon with an aspect ratio of 0.8 and a length-to-width ratio of 3 were investigated numerically. Three-dimensional governing equations of mass, momentum, energy, and species were modeled using the RNG k-epsilon turbulence model and Boussinesq approximation, which were solved using the finite volume method. Vehicle emissions were estimated from the measured traffic flow rates and modeled as banded line sources, with a street length and bandwidths equal to typical vehicle widths. Both measurements and simulations reveal that pollutant concentrations typically follow the traffic flow rate; they decline as the height increases and are higher on the leeward side than on the windward side. Three-dimensional simulations reveal that the vortex line, joining the centers of cross-sectional vortexes of the street canyon, meanders between street buildings and shifts toward the windward side when heating strength is increased. Thermal boundary layers are very thin. Entrainment of outside air increases, and pollutant concentration decreases with increasing heating condition. Also, traffic-produced turbulence enhances the turbulent kinetic energy and the mixing of temperature and admixtures in the canyon. Factors affecting the inaccuracy of the simulations are addressed.     

Comparative study of measured and modelled number concentrations of nanoparticles in an urban street canyon

This study presents a comparison between measured and modelled particle number concentrations (PNCs) in the 10–300 nm size range at different heights in a canyon. The PNCs were modelled using a simple modelling approach (modified Box model, including vertical variation), an Operational Street Pollution Model (OSPM) and Computational Fluid Dynamics (CFD) code FLUENT. All models disregarded any particle dynamics. CFD simulations have been carried out in a simplified geometry of the selected street canyon. Four different sizes of emission sources have been used in the CFD simulations to assess the effect of source size on mean PNC distributions in the street canyon. The measured PNCs were between a factor of two and three of those from the three models, suggesting that if the model inputs are chosen carefully, even a simplified approach can predict the PNCs as well as more complex models. CFD simulations showed that selection of the source size was critical to determine PNC distributions. A source size scaling the vehicle dimensions was found to better represent the measured PNC profiles in the lowest part of the canyon. The OSPM and Box model produced similar shapes of PNC profile across the entire height of the canyon, showing a well-mixed region up to first ≈2 m and then decreasing PNCs with increased height. The CFD profiles do correctly reproduce the increase from road level to a height of ≈2 m; however, they do not predict the measured PNC decrease higher in the canyon. The PNC differences were largest between idealised (CFD and Box) and operational (OSPM) models at upper sampling heights; these were attributed to weaker exchange of air between street and roof-above in the upper part of the canyon in the CFD calculations. Possible reasons for these discrepancies are given.     

Exploring the processes governing roadside pollutant concentrations in urban street canyon

This paper describes an in-depth analysis to investigate the huge variation in the measured roadside air-pollutant concentrations of carbon monoxide and nitrogen dioxide in terms of the traffic flow levels, the orientation of the street to the prevailing wind, the wind speed, temperature and barometric pressure. The work has attempted to develop generic parameters that can be applied to other urban areas. However, in the absence of a measure of congestion at the site in Palermo (Italy), the methodological approach proposed used the simultaneous noise measurements, in units of decibels (B), to help parameterise a generic congestion indicator in terms of the traffic flow. The potential transferability of the approach was demonstrated for a site in Marylebone Road, London (UK), given the similarity of the two study sites, canyon shape, traffic characteristics and road orientation. The results showed that, within the range of data available, noise levels could be used as a proxy for flow change on the shoulders of the peak hour and hence congestion and a generic relationship with factors statistically significant at 99 % confidence allows roadside concentrations due to traffic to be estimated with a regression coefficient of R ²?=?0.73 (R?=?0.85). The research demonstrates that whilst there are indeed underlying relationships that can explain the roadside concentrations based on traffic and meteorological conditions, evidence is presented that confirms the complexity of the physical and chemical processes that govern roadside concentrations.     

Street level versus rooftop concentrations of submicron aerosol particles and gaseous pollutants in an urban street canyon

Gaseous air pollutants and aerosol particle concentrations were monitored in an urban street canyon for two weeks. The measurements were performed simultaneously at two different heights: at street level (gases 3 m, aerosol particles 1.5 m) and at a rooftop 25 m above the ground. The main objective of the study was to investigate the vertical changes in concentrations of pollutants and the factors leading to the formation of the differences. The physical parameters controlling the concentration gradients (e.g. the flow and micrometeorology) were not directly measured and the conclusions of the study rely mostly on the high time resolution concentration measurements. It was concluded that dilution and dispersion decreases the concentrations of pollutants emitted at street level by a factor of roughly 5 between the two sampling heights. However, for some compounds the chemical reactions were seen to be of more importance when the vertical gradient is formed. In order to determine the processes leading to gradients in aerosol particle concentrations the photochemical formation of submicrometer aerosol particles was investigated using a theoretical expression based on the measured data. It was clearly seen that most of the particles originate from traffic in the vicinity of the measurement site. Also a few events were detected which might have been due to local gas-to-particle conversion.     

街道峡谷机动车尾气污染扩散模拟与影响因子分析研究

为预测和分析街道峡谷污染物浓度,研究了街道峡谷污染物浓度影响因子.利用重庆市交通干线街道峡谷两侧NOx浓度的监测数据,验证了街道峡谷机动车尾气污染扩散模型--OSPM模型.风速转换系数修正后的OSPM模型的模拟值与实测值的R达0.862 58;风场因子验证了风速转换系数修正后的OSPM模型能较好地模拟重庆市街道峡谷的污染物浓度,一定程度上能满足环境空气质量评价要求.同时,通过分析OSPM模型的影响因子,提出了控制街道峡谷机动车尾气污染状况的建议.     

高层建筑群对街谷内颗粒物扩散特性的影响

为了获得城市冠层内高层建筑群的高度变化对城市颗粒物污染的作用情况,采用大涡模拟方法研究了不同高层建筑群的街谷形状因子对街谷内空气流动与污染物扩散规律的影响。结果表明：在高层建筑群上方形成一个顺时针旋涡,旋涡中心位于城市峡谷内靠近高层建筑群背风处;随着街谷形状因子的增大,高层建筑群的滞留效应增强,导致高层建筑物上方的剪切层湍动能增强;当形状因子为2.5时,湍动能达到1.9 m2·s-2,此时城市街谷内可吸入颗粒物的稀释扩散条件变差;在涡旋和气流夹带作用下,可吸入颗粒物浓度在垂直方向上分布具有明显的分层现象,大量可吸入颗粒物聚集于低建筑迎风面底部。不同街谷形状因子下街谷内空气流动与污染物扩散规律的探明将为有关部门制定相应规划提供参考。     

Elemental composition of airborne particulate matter in the multi-impacted urban area of Thessaloniki,Greece

Ambient concentrations of PM₁₀ and their elemental composition in the multi-impacted area of Thessaloniki, N. Greece is presented in this study. High concentrations of PM₁₀ were observed, with 80% exceedances of the proposed daily limit of 50 μg m⁻³. The elemental concentrations were similar to the levels observed in moderately polluted urban areas. Spatial and temporal variation of particle mass and elemental concentrations as well as the influence of meteorological conditions were examined. The examined elements were characterised with respect to their origin from natural or anthropogenic emissions on the basis of crustal and marine enrichment factors. Factor analysis of elemental composition pattern and elemental ratios in PM₁₀ were also used to identify possible pollution source-types.     

SO2 dosages in an urban area

Five winter-semesters of data of SO₂ concentration at three sites in the urban area of Milan were analyzed to generate statistics of dosage which describe the dosage distributions, the temporal structure, the dependence upon concentration thresholds and the six-month-average concentration. By considering the joint distributions of dosages and their durations it is found that the basic ambiguity between dosages and their durations can be resolved since for each dosage value only a limited range of durations is observed. It is also found that the six-month-average concentration has a dominating role on the statistics of dosages; previous works have reported a similar influence on concentration statistics, durations and extreme values. For most of the statistics it has been possible to develop simple and accurate empirical models.     

PM10 regional transport pathways in Thessaloniki,Greece

In this study, the most dominant regional transport pathways for the city of Thessaloniki, Greece were identified and linked to air quality issues with respect to particulate matter (PM). Using air mass trajectories, cluster analysis techniques and PM10 measurements of a background-urban station of the greater Thessaloniki area during 2001–2004, it was found that north-eastern and southern flows were the most frequent in appearance with high potential to influence the city of Thessaloniki, especially when coinciding with biomass burning or Saharan dust events correspondingly. These incidents appeared to occur mostly during summer adding to a PM10 monthly mean up to 10 μg m^?3. High concentrations of surface PM10 related to north-eastern flows were in most cases accompanied with high aerosol columnar optical depths implying that particulate matter transport from the North-East was multi-layered. South-southwesterly flows originating from N. Africa, though less frequent, seemed to affect decisively Thessaloniki's aerosol budget especially during transition seasons. These flows were related with an increase of the monthly PM10 average up to 20–30 μg m^?3 for the time period studied. Finally, northerly flows were found to transport rather clean air masses that did not seem to contribute to the air quality deterioration of the city.     

针对街道峡谷内部气流场的参数化研究

为了建立一个能够快速且可靠地模拟街区尺度交通污染物三维分布的模型以应对实时在线模拟的需求,将城区的最小单位假定为两侧建筑物等高的长街区(街道峡谷),基于CFD模型对街道峡谷内部气流场的模拟结果,对街道峡谷内不同区域的气流(U、V、W 3个分量)与街区几何比例(高宽比HW和长宽比LW)、背景风(u和v 2个分量)的数值关系进行了回归分析。分析结果显示,分量U在非靠近边界的区域与街区几何比例和背景风线性相关性较好;分量V则在靠近地面或顶面的区域;分量W则是在靠近墙面的区域。利用回归分析得到的数值关系构建的参数化方法可以计算得到近似于CFD模型模拟结果的街道峡谷内部气流场。在此基础上,基于高斯模型建立了一个参数化模型,并以黑炭气溶胶BC为例对参数化模型进行评估,评估结果表明,参数化模型具有一定的实用价值。     

Fast response measurements of the dispersion of nanoparticles in a vehicle wake and a street canyon

The distributions of nanoparticles (below 300 nm in diameter) change rapidly after emission from the tail pipe of a moving vehicle due to the influence of transformation processes. Information on this time scale is important for modelling of nanoparticle dispersion but is unknown because the sampling frequencies of available instruments are unable to capture these rapid processes. In this study, a fast response differential mobility spectrometer (Cambustion Instruments DMS500), originally designed to measure particle number distributions (PNDs) and concentrations in engine exhaust emissions, was deployed to measure particles in the 5–1000 nm size range at a sampling frequency of 10 Hz. This article presents results of two separate studies; one, measurements along the roadside in a Cambridge (UK) street canyon and, two, measurements at a fixed position (20 cm above road level), centrally, in the wake of a single moving diesel-engined car. The aims of the first measurements were to test the suitability and recommend optimum operating conditions of the DMS500 for ambient measurements. The aim of the second study was to investigate the time scale over which competing influences of dilution and transformation processes (nucleation, condensation and coagulation) affect the PNDs in the wake of a moving car. Results suggested that the effect of transformation processes was nearly complete within about 1 s after emission due to rapid dilution in the vehicle wake. Furthermore, roadside measurements in a street canyon showed that the time for traffic emissions to reach the roadside in calm wind conditions was about 45 ± 6 s. These observations suggest the hypothesis that the effects of transformation processes are generally complete by the time particles are observed at roadside and the total particle numbers can then be assumed as conserved. A corollary of this hypothesis is that complex transformation processes can be ignored when modelling the behaviour of nanoparticles in street canyons once the very near-exhaust processes are complete.     

PM10 emission factors for non-exhaust particles generated by road traffic in an urban street canyon and along a freeway in Switzerland

Recent studies have shown clear contributions of non-exhaust emissions to the traffic related PM10 load of the ambient air. These emissions consist of particles produced by abrasion from brakes, road wear, tire wear, as well as vehicle induced resuspension of deposited road dust. The main scope of the presented work was to identify and quantify the non-exhaust fraction of traffic related PM10 for two roadside locations in Switzerland with different traffic regimes. The two investigated locations, an urban street canyon with heavily congested traffic and an interurban freeway, are considered as being typical for Central Europe. Mass-relevant contributions from abrasion particles and resuspended road dust mainly originated from particles in the size range 1–10 μm. The results showed a major influence of vehicle induced resuspension of road dust. In the street canyon, the traffic related PM10 emissions (LDV: 24 ± 8 mg km^?1 vehicle^?1, HDV: 498 ± 86 mg km^?1 vehicle^?1) were assigned to 21% brake wear, 38% resuspended road dust and 41% exhaust emissions. Along the freeway (LDV: 50 ± 13 mg km^?1 vehicle^?1, HDV: 288 ± 72 mg km^?1 vehicle^?1), respective contributions were 3% brake wear, 56% resuspended road dust and 41% exhaust emissions. There was no indication for relevant contributions from tire wear and abrasion from undamaged pavements.     

Trend analysis of urban NO2 concentrations and the importance of direct NO2 emissions versus ozone/NOx equilibrium

The annual air quality standard of NO₂ is often exceeded in urban areas near heavy traffic locations. Despite significant decrease of NO_x emissions in 1986–2005 in the industrial and harbour area near Rotterdam, NO₂ concentrations at the urban background remain at the same level since the end of the nineties. Trend analysis of monitoring data revealed that the ozone/NO_x equilibrium is a more important factor than increasing direct NO₂ emissions by traffic. The latter has recently been identified as an additional NO₂ source due to the introduction of oxy-catalytic converters in diesel vehicles and the growing number of diesel vehicles. However, in Rotterdam over the period 1986–2005 direct NO₂ emissions by road traffic only increased 3–4%. Due to the importance of the ozone/NO_x equilibrium, it is concluded that local NO_x emissions in Rotterdam need substantial reduction to achieve lower NO₂ urban background levels. This is a relatively costly abatement strategy and, therefore, a “hotspot” approach aiming at reducing NO_x emissions by local traffic measures is more effective to meet European air quality standards.