An Integrated Approach to Street Canyon Pollution Modelling

An integrated method for the prediction of the spatial pollution distribution within a street canyon directly from a microscopic traffic simulation model is outlined. The traffic simulation package Paramics is used to model the flow of vehicles in realistic traffic conditions on a real road network. This produces details of the amount of pollutant produced by each vehicle at any given time. The authors calculate the dispersion of the pollutant using a particle tracking diffusion method which is superimposed on a known velocity and turbulence field. This paper shows how these individual components may be integrated to provide a practical street canyon pollution model. The resulting street canyon pollution model provides isoconcentrations of pollutant within the road topography.     

Field Study of Wind and Traffic to Test a Street Canyon Pollution Model

In the U.K., local authorities have new duties to review and assess air quality. Dispersion models are important tools in this process. The performance of a street canyon model, AEOLIUS, in calculating carbon monoxide (CO) concentrations in urban areas is discussed. A field experiment was conducted in a busy street canyon in Leek, Staffordshire. Wind speed and direction were measured at three heights adjacent to the street. The canyon's CO concentrations and traffic counts were recorded. Predicted concentrations of CO, calculated using AEOLIUS, were compared with the observed values. The concept of a roof-top wind is discussed, as are the consequences of using wind measurements from outside the town. Choice of wind measurement location and height of the anemometer above the canyon had a pronounced effect on calculating the roof-top wind. Two methods of deriving a street level wind speed from a roof-top wind speed gave results that differ by up to a factor of two. AEOLIUS had variable skill at predicting CO concentrations depending on the roof-top wind direction: possible reasons for this variability are explored. A sensitivity study of the model showed that vehicle emissions have the greatest impact on predicted concentrations. Implications for local air quality management are discussed.     

Computational Fluid Dynamics Modelling of the Pollution Dispersion and Comparison with Measurements in a Street Canyon in Helsinki

A measuring campaign was conducted in a street canyon (Runeberg St.) in Helsinki in 2003–2004. The concentrations of NO _x , NO₂, PM₁₀ and PM_2.5 were measured at street level and at roof level at an urban background location. This study utilises the data measured from 1 Jan to 30 April, 2004, when wind speed and direction measurements were also conducted on-site at the roof level. The computational fluid dynamics model ADREA-HF was used to compute the street concentrations, and the results were compared with the measurements. The predictions for the selected cases agreed fairly well (within < 25 % for 15 min average values) with the measured data, except for two cases: a windward flow in case of a low wind speed, and a moderate southerly flow parallel to the street canyon. The main reasons for the differences of predictions and measurements are the negligence of traffic-induced turbulence in the modelling and an under-prediction of ventilation of urban background air from a crossing street. Numerical results are presented for various example cases; these illustrate the formation of the vortices in the canyon in terms of the wind direction and speed and the influence of the characteristics of the flow fields on the concentration distributions.     

OSPM - A Parameterised Street Pollution Model

For many practical applications, as e.g. in support of air pollution management, numerical models based on solution of the basic flow and dispersion equations are still too complex. Alternative are models that are basically parameterised semi-empirical models making use of a priori assumptions about the flow and dispersion conditions. However, these models must, be thoroughly tested and their performance and limitations carefully documented. The Danish Operational Street Pollution Model (OSPM) belongs to this category of parameterised models. In the OSPM, concentrations of exhaust gases are calculated using a combination of a plume model for the direct contribution and a box model for the recirculating part of the pollutants in the street. Parameterisation of flow and dispersion conditions in street canyons was deduced from extensive analysis of experimental data and model tests. Results of these tests were used to further improve the model performance, especially with regard to different street configurations and a variety of meteorological conditions.     

Comparison of Numerical Street Dispersion Models with Results from Wind Tunnel and Field Measurements

Numerical dispersion models developed and validated in different European countries were applied to data sets from wind tunnel and field measurements. The comparison includes the Danish Operational Street Pollution Model (OSPM) and the microscale flow and dispersion model MISKAM. The latter is recommended for application in built-up areas in the draft of the new German guideline VDI 3782/8. In a first step the models were applied to simplified street configurations. Different parameters as length and height of adjacent buildings and the angle of the incoming flow were varied. The results were compared to recent wind tunnel measurements. In a second step the models were applied to two extensively investigated field data sets from Jagtvej, Copenhagen and G ttinger Straße, Hannover. Intensified and more transparent and accessible validation procedures would be helpful for the thorough user.     

Dispersion of Pollutants in Street Canyon under Traffic Induced Flow and Turbulence

A 3-D Eulerian-Lagrangian approach to moving vehicles is presented that takes into account the traffic induced flow rate and turbulence. The method is applied to pollutants dispersion in a street canyon. The approach is based on CFD calculations using Eulerian approach to the continuous phase and Lagrangian approach to the "discrete phase" of moving objects - vehicles. A commercial CFD code StarCD was used into which the Lagrangian model was integrated. As an example a street canyon is taken into consideration. It has the length of 50 m and the aspect ratio of 1.27. The speed of wind was assigned values of 4, 7 and 12 m/s at the altitude of 300 m. The total height of the domain is 115 m. In the study different traffic situations are considered, namely one-way and two-way traffic with different traffic rates per lane. The predictions show that different traffic situations affect pollutants dispersion in the street canyon and that there are also differences in the pollutants dispersion in case of one- and two-way traffic.     

Carbon Monoxide Concentration in a Street Canyon of Buenos Aires City (Argentina)

The analysis of three years of 8-h CO concentration values registered in a deep street canyon downtown shows high frequency of values that exceed WHO health protection guidelines. An inverse relationship between opposing percentiles of the distributions of CO concentrations and mean wind speed could be found. Data also showed a variation of mean CO values with prevailing wind direction. The averaged concentration value obtained when the sampler probe is on the leeward side is lower than the obtained when it is on the windward wall. A preliminary explanation of this feature may be related to the advection of polluted air from a high traffic density area nearby.     

Dispersion in Urban Environments; Comparison of Field Measurements with Wind Tunnel Results

A wind tunnel study was performed to determine the dispersion characteristics of vehicle exhaust gases within the urban canopy layer. The results were compared with those from a field monitoring station located in a street canyon with heavy traffic load. The agreement found was fair. In the second part of the paper it is shown how wind tunnel data can be utilized to supplement and thereby enhance the value of field data for model validation purposes. Uncertainty ranges were quantified which are inherent to mean concentration values measured in urban streets.     

Turbulent Ventilation of a Street Canyon

A selection of turbulence data corresponding to 185 days of field measurements has been analysed. The non-ideal building geometry influenced the circulation patterns in the street canyon and the largest average vertical velocities were observed in the wake of an unbroken line of buildings. The standard deviation of vertical velocity fluctuations normalised by the ambient wind speed was relatively insensitive to ambient wind direction and sensor position, and it was usually larger than the corresponding 1-hour average velocity. Cross-correlations of spatially separated velocity measurements were small, and this suggests that most of the velocity fluctuations were fairly local and not caused by unsteady street vortices. The observed velocities scaled with the ambient wind speed except under low-wind conditions.     

乌鲁木齐市一次冷空气过境天气对空气污染的影响分析

根据对2006年11月20～28日乌鲁木齐市出现冷空气过境天气过程的天气形式和主要气象要素及空气污染变化情况进行的分析,结果表明:冷空气过境天气前后,诸多气象要素发生了变化,冷空气导致逆温层的破坏和降水过程使各项污染物均得到有效的清除.通过此分析可为类似天气的空气质量预报提供思路.     

Air Quality in Norwich, UK Multi-Scale Modelling to Assess the Significance of City, County and Regional Pollution Sources

Norwich is the eastern most city in the United Kingdom. Despite a population of only 100,000 and very little local industry, studies have shown that the city experiences levels of nitrogen dioxide, ozone, particulates and sulphur dioxide exceeding the UK Air Quality Standards. Because of Norwich's situation within a large, predominantly rural area a large non-resident workforce is one factor that contributes to large, often very congested traffic flows. The city's location close to the European mainland also exposes it to polluted airmasses transported from the continent, especially in the case of particulates and ozone. In order to assess the relative contributions of local and regional sources, data from rural and urban monitoring sites are to be used in conjunction with ADMS-Urban and the UK Meteorological Office's NAME model.     

Modelling of Fluid Flow and Pollutant Dispersion in a Street Canyon

A two-dimensional steady state numerical simulation has been carried out for a typical street canyon ventilated by a cross-wind. The PHOENICS package from CHAM was used to solve for the air flow above and within the street canyon. The k-epsilon turbulence model was used for turbulence modelling and pollutant sources were added at ground level over the road but not over the pavements. Results for the air flow showed the formation of a longitudinal vortex within the street canyon, as found by other researchers. Pollutant concentrations were predicted with the highest values occurring at the leeward walls of the upwind buildings, and the lowest values on the windward walls of the downwind buildings. The accuracy of these simulations was examined by comparing the predicted results with field observations. Reasonable agreement was obtained, confirming the difference between concentrations on the leeward and windward walls. The results show that the dispersion characteristics can be simulated in terms of structural configurations.     

乌鲁木齐市室内环境空气污染成因及防治对策

为进一步掌握室内环境空气污染状况,对乌鲁木齐市毛坯房、公共场所和居民住宅的室内环境空气质量现状进行了监测统计,并调查了居民的健康状况,分析了室内空气污染的原因。监测结果表明,乌鲁木齐市新装修房屋室内环境污染较严重。对此提出了不同污染状况的防治对策。     

遂宁市2013年大气污染特征及成因

根据遂宁市近年的环境空气监测数据,系统研究了2013年大气污染特征和2007—2013年的变化趋势,分析了颗粒物的组成和主要来源,并与周边地区大气环境监测结果进行了横向比较。研究结果表明,2013年遂宁市环境空气质量达标率为88.1%,大气中SO2、NO2和PM10的年均质量浓度分别为31、30、94μg/m3,空气质量达到二级标准,但劣于往年水平,大气中首要污染物是PM10,夏季和秋季的空气质量明显优于冬季和春季;遂宁市空气质量的变化趋势与周边地区一致,呈现出一定的区域性污染特征,但明显优于周边地区;进一步从5个方面阐述了遂宁市2013年空气质量下降的原因;根据遂宁市大气污染现状,提出了大气防控措施和建议。     

2013年6月江苏一次大气污染过程分析

研究了2013年6月在江苏范围内的一次污染天气过程。6月10—22日期间,江苏省13个省辖城市均出现不同程度的轻度污染以上天气,个别城市甚至达到重度或严重污染,根据后向轨迹模型、颗粒物离子浓度的分析可见,这是一次局地秸秆焚烧所引起,加之不利的气象条件所形成的区域污染过程,根据监测结果详细阐述此次污染的变化特征及成因机制,并分析提出相应的对策及建议。     

The performance evaluation of WinOSPM model for urban street canyons of Nantes in France

Air quality modelling is primarily the quantative approach. It is more difficult as it demands input data accuracy, uncertainties and the efficient methodologies to judge the extent of models accuracy. As a result, model validation has to be regarded as an integral part of the modelling process. Furthermore, models are often validated on a limited number of testcases therefore, appropriate evaluation procedure must be implemented to ensure these models will be applicable for various conditions. The study presented here was carried out to evaluate theWinOSPM (Preliminary version of windows based Operational Street Pollution Model) for air pollutants viz. CO, NO, NO₂, NO_x and C₆H₆ for three street canyons of Nantes (France) and for the three base years 1999, 2000, and 2001. Each street canyon selected for this study has typical and unidentical features. The rue de Strasbourg and Boulevard Victor Hugo have many building exceptions whereas rue Crébillon has not any. Application of the model above to the three street canyons revealed that WinOSPM could be used in the case when measurements are not available. This was justified from the results at rue Crébillon. The special interest was in the benzene modelled values as its content in fuel has been targeted to reduce to 1% for the years 2000 and onwards (from its 5% until the year 1999). The 50 to 70% reduction in the benzene concentrations is found for both the years i.e. in 2000 and 2001. This has further justified that air quality models are useful and interesting tools in optimising emission reduction strategies. Moreover, it is also the new pollutant added to the measurement campaign of Air Pays de la Loire (APL) for the city of Nantes. For benzene weekly averages are estimated from the hourly-modelled values for all the streets and compared with that of measurements. They are found in excellent agreement with each others. For other pollutants annual means and percentiles were compared. The statistical analysis was done to evaluate the models performance using index of agreement and correlation coefficient. The index of agreement (d) and correlation coefficient (r) for all the streets show that estimated concentration levels are in good agreement with that of measurements. From the index of agreements, it can be inferred that model has very less potential for errors. The models sensitivity to building-exceptions was also tested for the rue de Strasbourg. Results did not reflect this feature very well. It is perceived that the influence of this feature might have been suppressed in averaging the annual hourly values. This influence is apparently seen in hourly average time series variations. Finally, WinOSPM model was found a simple but very useful model. It could very well represent the detailed flow and dispersion conditions in urban streets.     

英国大气污染控制及行动措施

通过对英国大气污染历史及环境现状的对比，总结英国大气污染控制措施，为我国大气污染防治提供经验。     

Application of CFD Methods for Modelling in Air Pollution Problems: Possibilities and Gaps

Different urban air pollution problems deal with complex structure of air flows and turbulence. For such problems the Computer Fluid Dynamics (CFD) methods become widely used. However, this approach despite a number of advantages has some problems. Experience of use of CFD tools for development of models and suggestions of their applications for a local scale air pollution over a complex terrain and stable stratification are discussed in this paper, including: Topography and complex geometry: choose of the co-ordinate system and computer grid; Turbulence closure for air pollution modelling: modified k- model for stable stratified ABL; Boundary conditions for vertical profiles of velocity for stable-stratified atmosphere; Effects of the radiation and thermal budget of inclined surfaces to dispersion of pollutants; Artificial sources of air dynamics and circulation.Some examples of CFD applications for air pollution modelling for a flat terrain, mountainous area, mining open cast and indoor ventilation are discussed. Modified k- model for stably-stratified ABL is suggested. Due to the isotropic character of the k- model a combination of it in vertical with the sub-grid turbulence closure in horizontal can be more suitable for ABL. An effective scheme of boundary conditions for velocity profiles, based on the developed similarity theory for stable-stratified ABL, is suggested. Alongside with the common studies of atmospheric dispersion, the CFD methods have also demonstrated a good potential for studying anthropogenic and artificial-ventilation sources of air dynamic and circulation in local-scale processes of air pollution.     

京津冀区域大气重污染过程特征初步分析

基于为京津冀区域和城市环境空气质量预报和空气重污染预警业务提供必要基础参考资料和区域重污染发生发展规律认识的需求,应用现有空气监测网2013—2014年度京津冀区域13个城市空气质量监测数据,分析了该区域2013—2014年空气质量整体情况和污染过程的季节变化规律、污染范围,统计了两年间31次区域范围大气重污染过程,并根据污染过程的空气质量变化特点和大气环流形势,着重对31次重污染过程中均压场天气型污染开展分析。结果表明,2013—2014年京津冀区域空气污染形势严峻,全年约有六成日数受颗粒物污染影响;京津冀区域空气污染南北差异显著,有自北向南逐步加重的特点,南部污染严重城市对区域污染贡献巨大,石家庄、保定、邢台、邯郸4城市将PM_(10)、PM_(2.5)年均浓度分别拉升31、16μg/m~3;2013—2014年京津冀区域大范围重污染过程集中发生在秋冬季,两季的污染过程对区域两年PM_(10)、PM_(2.5)平均浓度分别拉升27、21μg/m~3;京津冀区域均压场天气型污染可细分为臭氧型均压场和颗粒物型均压场。当秋冬季出现较小气压梯度、西南小风、逆温层等均压场天气型时,容易造成区域颗粒物污染过程;而春末、夏季出现均压场天气型时,容易造成O_3污染。     

浅析沙尘天气对和田市大气污染的影响及对策

通过和田市的沙尘天气和空气质量污染特征分析表明，环境空气的主要污染物是TSP、降尘。指出沙尘天气是导致和田市空气重度污染的主要原因，并就此提出具体的防治对策和建议。