Effect of urban morphology on wind condition in idealized city models

Wind conditions in urban environments are important for a number of reasons. They can serve to transport air pollutants out of the urban environment and to moderate urban microclimatic conditions if satisfactory, yet can compromise pedestrian comfort and safety if not. We aim to study experimentally and numerically the effects of urban morphology (e.g., overall city form (skyline), street orientation, and street configuration) on wind conditions in cities. This report considers our initial investigations of two idealized city forms that are coincidentally similar to ancient Roman cities that were organized on one or two primary streets – a main north–south street, the cardus maximus, and a secondary east–west street, the decumanus maximus – and contained within a well-defined perimeter.We first consider round and square city models with one main street set parallel to the approaching wind and a secondary street producing an intersection at city centre. Not surprisingly, wind conditions in the two city models are dissimilar due to their shape differences. We then consider a long rectangular city model with a fully developed steady flow region along the main street. If the main street of the round city model is narrow, the parallel approaching wind cannot blow through the entire street and a penetrating inflow exists at the leeward opening. For the round city model with two crossing streets, a slightly non-parallel wind to the main street generates a stronger wind level in the entire street volume.     

孤立与非孤立城市街道峡谷内污染物扩散

通过求解二维不可压N-S方程、k-ε方程及污染物对流扩散方程,模拟了孤立街道峡谷与非孤立街道峡谷内的流场及交通污染物浓度场.计算结果与风洞试验结果总体趋势一致.非孤立街道峡谷内污染物壁面浓度要大于孤立街道峡谷内的壁面浓度.通过计算街道峡谷建筑屋顶高度处的垂直方向污染物通量,说明了湍流扩散是污染物扩散出街道峡谷的主要原因,其污染物通量总为正,而平均流通量可以为负.非孤立街道峡谷由于平均流流动和湍流流动的总扩散通量减少,造成污染物在街道峡谷内集聚,从而理论上解释了非孤立街道峡谷与孤立街道峡谷污染扩散的差别.     

A comprehensive experimental databank for the models verification of urban car emission dispersion

A summary presentation is made of representative samples from a comprehensive experimental databank on car exhaust dispersion in urban street canyons. Physical modelling, under neutral stratification conditions, was used to provide visualisation, pollutant concentration and velocity measurements above and inside test canyons amidst surrounding urban roughness. The study extended to two different canyon aspects ratios, in combination with different roof configurations on the surrounding buildings. To serve as a reliable basis for validation and testing of urban pollution dispersion codes, special emphasis was placed in this work on data quality assurance.     

Flow and dispersion in an urban cubical cavity

Flow and dispersion in an urban cubical cavity are numerically investigated using a Reynolds-averaged Navier–Stokes equations (RANS) model with the renormalization group (RNG) k–? turbulence closure model. The urban cubical cavity is surrounded by flank walls that are parallel to the streamwise direction, called end-walls, as well as upstream and downstream walls. A primary vortex and secondary vortices including end-wall vortices are formed in the cavity. Because of the end-wall drag effect, the averaged mean-flow kinetic energy in the cavity is smaller than that in an urban street canyon that is open in the along-canyon direction. A trajectory analysis shows that the end-wall vortices cause fluid particles to move in the spanwise direction, indicating that flow in the cavity is essentially three-dimensional. The iso-surfaces of the Okubo–Weiss criterion capture cavity vortices well. The pollutant concentration is high near the bottom of the upstream side in the case of continuous pollutant emission, whereas it is high near the center of the primary vortex in the case of instantaneous pollutant emission. To get some insight into the degree of pollutant escape from the cavity according to various meteorological factors, extensive numerical experiments with different ambient wind speeds and directions, inflow turbulence intensities, and cavity-bottom heating intensities are performed. For each experiment, we calculate the time constant, which is defined as the time taken for the pollutant concentration to decrease to e^?1 of its initial value. The time constant decreases substantially with increasing ambient wind speed, and tends to decrease with increasing inflow turbulence intensity and cavity-bottom heating intensity. The time constant increases as the ambient wind direction becomes oblique. High ambient wind speed is found to be the most crucial factor for ventilating the cavity, thus improving air quality in an urban cubical cavity.     

Air intake contamination by building exhausts: tracer gas investigation of atmospheric dispersion models in the urban environment

The establishment of a safe distance between sources of pollution and air intakes is based on a complex exercise that should take into account several wind, physical, and topographical factors. To estimate the maximum concentrations of the pollutants as a function of the distance from the emission source, some heating, ventilation, and air conditioning (HVAC) system designers use the atmospheric dispersion models suggested by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). Two of these models, the Halitsky and Wilson-Chui-Lamb models, have been developed and evaluated mainly with laboratory data. There have been relatively few evaluations with full-scale field data. The objective of this study, carried out on a building in downtown Montreal, Quebec, Canada, was to compare the measured concentrations of a tracer gas emitted by an exhaust stack with those predicted by these models. The results indicate that the Halitsky model gives lower than actual dilution, while the Wilson-Chui-Lamb model generally gives acceptable estimates, with occasional over-estimations of the dilution.     

Characteristics of puff dispersion in an urban environment

Instantaneous releases of sulfur hexafluoride tracer were carried out as part of the Joint Urban 2003 field campaign in Oklahoma City. Data from 10 fast-response tracer samplers were used to examine the crosswind and along-wind spread of the tracer, the decay of tracer concentrations, and the retention of tracer within approximately 1 km of the release locations. The time variation of the median values of the tracer concentrations, normalized by the peak value observed at a given sampler, could be approximately described by an exponential decay with characteristic decay times on the order of 1–2 min. The longer times were found for early morning releases and the shorter times were associated with later morning or afternoon releases, suggesting that atmospheric stability or the depth of the mixed layer may affect puff dispersion even in urban environments. The median retention times required for 99% of the exposure to be realized at a given location were found to be correlated reasonably well with the median decay times. These characteristic time scales should be regarded as lower limits for concentration decay because the analysis excluded a number of anomalous cases in which the decaying concentrations exhibited an extended tail that indicated a very slow ventilation rate. The median values of the along-wind dispersion parameter σ_x grouped into downwind distance ranges can be described by a linear variation with distance with an initial “hold up” contribution due to building effects of about 30–45 m, but there are considerable variations about this relationship. Downwind 0.5–1 km from the release point the lateral puff dispersion (σ_y) was roughly 70% of the along-wind dispersion.     

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.     

Large-eddy simulation for flow and dispersion in urban streets

Large-eddy simulations (LES) with our recently developed inflow approach (Xie, Z.-T., Castro, I.P., 2008a. Efficient generation of inflow conditions for large-eddy simulation of street-scale flows. Flow Turbul. Combust., vol. 81(3), pp. 449–470.) have been used for flow and dispersion within a genuine city area – the DAPPLE site, located at the intersection of Marylebone Rd and Gloucester Pl in Central London. Numerical results up to second-order statistics are reported for a computational domain of 1.2 km (streamwise) × 0.8 km (lateral) × 0.2 km (in full scale), with a resolution down to approximately one meter in space and one second in time. They are in reasonable agreement with the experimental data. Such a comprehensive urban geometry is often, as here, composed of staggered, aligned, square arrays of blocks with non-uniform height and non-uniform base, street canyons and intersections. Both the integrative and local effect of flow and dispersion to these geometrical patterns were investigated. For example, it was found that the peaks of spatially averaged u_rms, v_rms, w_rms and <u′w′> occurred neither at the mean height nor at the maximum height, but at the height of large and tall buildings. It was also found that the mean and fluctuating concentrations in the near-source field is highly dependent on the source location and the local geometry pattern, whereas in the far field (e.g. >0.1 km) they are not. In summary, it is demonstrated that full-scale resolution of around one meter is sufficient to yield accurate prediction of the flow and mean dispersion characteristics and to provide reasonable estimation of concentration fluctuations.     

A simple urban dispersion model tested with tracer data from Oklahoma City and Manhattan

A simple urban dispersion model is tested that is based on the Gaussian plume model and modifications to the Briggs urban dispersion curves. An initial dispersion coefficient (σ_o) of 40 m is assumed to apply in built-up downtown areas, and the stability is assumed to be slightly unstable during the day and slightly stable during the night. Observations from tracer experiments during the Joint Urban 2003 (JU2003) field study in Oklahoma City and the Madison Square Garden 2005 (MSG05) field study in Manhattan are used for model testing. The tracer SF₆ was released during JU2003 near ground level in the downtown area and concentrations were observed at over 100 locations within 4 km from the source. Six perfluorocarbon tracer (PFT) gases were released near ground level during MSG05 and sampled by about 20 samplers at the surface and on building roofs. The evaluations compare concentrations normalized by source release rate, C/Q, for each sampler location and each tracer release, where data were used only if both the observed and predicted concentrations exceeded threshold levels. At JU2003, for all samplers and release times, the fractional mean bias (FB) is about 0.2 during the day (20% mean underprediction) and 0.0 during the night. About 45 –50% of the predictions are within a factor of two (FAC2) of the observations day and night at JU2003. The maximum observed C/Q is about two times the maximum predicted C/Q both day and night. At MSG05, for all PFTs, surface samplers, and release times, FB is 0.14 and FAC2 is about 45%. The overall 60 min-averaged maximum C/Q is underpredicted by about 40% for the surface samplers and is overpredicted by about 25% for the building-roof samplers.     

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.     

Heterogeneous Fenton degradation of ofloxacin catalyzed by magnetic nanostructured MnFe2O4 with different morphologies

Environmental Science and Pollution Research - Magnetic nanostructured MnFe2O4 with different morphologies, synthesized via chemical co-precipitation and hydrothermal method, was assayed as...     

Estimating micrometeorological inputs for modeling dispersion in urban areas during stable conditions

We examine the performance of three methods to estimate the surface friction velocity and the Monin–Obukhov (MO) length in stable conditions. Estimates from these methods are compared with measurements made at two urban sites: the Wilmington site located in the middle of an urban area, and the VTMX site located on a sloping, smooth area in Salt Lake City. The first method uses the mean wind at a single height (Single U or SU), the second uses the wind speed at a single level and the temperature difference between two levels (U delta T or UDT), and the third method uses two levels of wind speed and temperature (delta U delta T or DUDT). The performance of the SU and UDT methods in estimating u_* are comparable. The SU method yields better estimates of the MO length than the UDT method does. The DUDT method performs poorly in estimating both u_* and L. The major conclusions of this study are that (1) measurements of mean winds and temperatures at one or two levels at an urban location can provide adequate estimates of micrometeorological variables required in modeling dispersion in the stable boundary layer, and (2) methods based on using differences in temperatures and velocities between two levels can provide unreliable estimates of these variables because these differences can be overwhelmed by inevitable uncertainties in the measurement of mean variables.     

Polychlorinated dibenzo-p-dioxin and dibenzofuran in urban air of an Andean city

Particle-bound polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) in ambient air were monitored together with particulate matter less than 10 μm (PM₁₀) at three sampling sites of the Andean city of Manizales, Colombia; during September 2009 and July 2010. PCDD/Fs ambient air emissions ranged from 1 fg WHO-TEQ m⁻³ to 52 fg WHO-TEQ m⁻³ in particulate fraction. The PM₁₀ concentrations ranged from 23 μg m⁻³ to 54 μg m⁻³. Concentrations of PM₁₀ and PCDD/Fs in ambient air observed for Manizales - a medium sized city with a population of 380 000 - were comparable to concentrations in larger cities. The highest concentrations of PCDD/Fs and PM₁₀ found in this study were determined at the central zone of the city, characterized by public transportation density, where diesel as principal fuel is used. In addition, hypothetical gas fractions of PCDD/Fs were calculated from theoretical K_p data. Congener profiles of PCDD/Fs exhibited ratios associated with different combustion sources at the different sampling locations, ranging from steel recycling to gasoline and diesel engines. Taking into account particle and gas hypothetical fraction of PCDD/Fs, Manizales exhibited values of PCDD/Fs equivalent to rural and urban-industrial sites in the southeast and center of the city respectively. Poor correlation of PCDDs with PM₁₀ (r = −0.55 and r = 0.52) suggests ambient air PCDDs were derived from various combustion sources. Stronger correlation was observed of PCDFs with PM₁₀. Poor correlation between precipitation and reduced PM₁₀ concentration in ambient air (r = −0.45) suggested low PM₁₀ removal by rainfall.     

Characterization of PM10 atmospheric aerosol at urban and urban background sites in Fuzhou city, China

Background

PM₁₀ aerosol samples were simultaneously collected at two urban and one urban background sites in Fuzhou city during two sampling campaigns in summer and winter. PM₁₀ mass concentrations and chemical compositions were determined.

Methods

Water-soluble inorganic ions (Cl^?, NO ₃ ^? , SO ₄ ^2? , NH ₄ ⁺ , K⁺, Na⁺, Ca²⁺, and Mg²⁺), carbonaceous species (elemental carbon and organic carbon), and elements (Al, Si, Mg, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Br, and Pb) were detected using ion chromatography, thermal/optical reflectance, and proton-induced X-ray emission methods, respectively.

Results

PM₁₀ mass concentrations, as well as most of the chemical components, were significantly increased from urban background to urban sites, which were due to enhanced anthropogenic activities in urban areas. Elements, carbonaceous species, and most of the ions were more uniformly distributed at different types of sites in winter, whereas secondary ion SO ₄ ^2? , NO ₃ ^? , and NH ₄ ⁺ showed more evident urban-background contrast in this season. The chemical mass closure indicated that mineral dust, organic matters, and sulfate were the most abundant components in PM₁₀. The sum of individually measured components accounted for 86.9?C97.7% of the total measured PM₁₀ concentration, and the discrepancy was larger in urban area than in urban background area.

Conclusion

According to the principal component analysis?Cmultivariate linear regression model, mineral dust, secondary inorganic ions, sea salt, and motor vehicle were mainly responsible for the PM₁₀ particles in Fuzhou atmosphere, and contributed 19.9%, 53.3%, 21.3%, and 5.5% of PM₁₀, respectively.     

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.     

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.     

CFD model simulation of dispersion from chlorine railcar releases in industrial and urban areas

To assist in emergency response decisions and planning in case of releases of pressurized liquefied chlorine from railroad tank cars in industrial sites and cities, the FLACS Computational Fluid Dynamics (CFD) model has been used to simulate the transport and dispersion of the dense chlorine cloud. Two accident locations are studied: an actual railcar accident at an industrial site in Festus, MO, and a hypothetical railcar accident at a rail junction in the Chicago urban area. The results show that transport of a large dense gas release at ground level in an industrial site or large city could initially extend a hundred meters or more in the upwind and crosswind directions. The dense cloud may follow terrain drainage, such as river channels. Near the source, the obstacles tend to slow down the dense gas cloud and may constrain it and cause increased concentrations. Farther downwind, the obstacles may cause enhanced mixing and dilution once the cloud has grown larger. In some cases, significant amounts of cloud mass may become “trapped” in obstacle wakes for many minutes after the main cloud has passed. Although the CFD model can account for the details of the flow and dispersion much better than standard widely-used simple dense gas models, many similarities are found among the various models in their simulated variations with downwind distance of the maximum cloud centerline concentration.     

The potential of statistical state space models in urban ozone forecasting

State space models for tropospheric urban ozone prediction are introduced and compared with linear regression models. The linear and non-linear state space models make accurate short-term predictions of the ozone dynamics. The average prediction error one hour in advance is 7 μg/m³ and increases logarithmically with time until it reaches 26 μg/m³ after 30 days. For a given sequence of solar radiation inputs, predictions converge exponentially with a time scale of 8 hours, so that the model is insensitive to perturbations of more than 150 μg/m³ O₃. The slow increase of the prediction error in addition to the uniqueness of the prediction are encouraging for applications of state space models in forecasting ozone levels when coupled with a model that predicts total radiation. Since a radiation prediction model will be more accurate during cloud-free conditions, in addition to the fact that the state space models perform better during the summer months, state space models are suitable for applications in sunny environments.