Urban Dispersion Research Articles

The model SIRANE for atmospheric urban pollutant dispersion; PART II, validation of the model on a real case study

Abstract We analyse the performance of the model SIRANE by comparing its outputs to field data measured within an urban district. SIRANE is the first urban dispersion model based on the concept of street network, and contains specific parametrical law to explicitly simulate the main transfer mechanisms within the urban canopy. The model validation is performed by means of field data collected during a 15 days measurement campaign in an urban district in Lyon, France. The campaign provided information on traffic fluxes and cars emissions, meteorological conditions, background pollution levels and pollutant concentration in different location within the district. This data set, together with complementary modelling tools needed to estimate the spatial distribution of traffic fluxes, allowed us to estimate the input data required by the model. The data set provide also the information essential to evaluate the accuracy of the model outputs. Comparison between model predictions and field measurements was performed in two ways. By evaluate the reliability of the model in simulating the spatial distribution of the pollutant and of their time variability. The study includes a sensitivity analysis to identify the key input parameters influencing the performance of the model, namely the emissions rates and the wind velocity. The analysis focuses only on the influence of varying input parameters in the modelling chain in the model predictions and complements the analyses provided by wind tunnel studies focussing on the parameterisation implemented in the model. The study also elucidates the critical role of background concentrations that represent a significant contribution to local pollution levels. The overall model performance, measured using the Chang and Hanna (2004) criteria can be considered as ‘good’ except for NO and some of BTX species. The results suggest that improvements of the performances on NO require testing new photochemical models, whereas the improvement on BTX could be achieved by correcting their vehicular emissions factors.

Effect of hilly urban morphology on dispersion in the urban boundary layer

Air flow and dispersion in the atmospheric surface layer are strongly affected by terrain and buildings. Through Large-eddy simulation (LES) with a three-dimensional immersed boundary method (IBM) atmospheric boundary layer flow in a hilly urban area was simulated to study turbulence and dispersion properties in and above the urban canopy. Five different domains were designed to simulate flow over an infinite sequence of hills (defined by the Witch of Agnesi and having a maximum slope of 0.26), buildings on flat terrain and buildings on the Witch of Agnesi hills (hill height to building height ratios 3/2 and 9/4). Shear stress and velocity variance above the urban canopy were smaller for the small hill with buildings compared to building array on flat terrain. Shear stress increased with the hill height for hills with buildings. For hills with buildings turbulence kinetic energy (TKE) in the urban canopy increased dramatically upwind of the hillcrest and fell below the canopy level TKE for the flat urban case in the lee of the hill. Canyon ventilation at the sub-canopy level was two to three times larger for the hilly urban case compared to the flat case, but air exchange through the top of urban canyons was not greatly affected by the hill. Our study demonstrates that urban dispersion models with the ability to handle terrain and bluff obstacles in the domain are necessary to simulate important flow features and dispersion in hilly urban environments.

The model SIRANE for atmospheric urban pollutant dispersion; part I, presentation of the model

In order to control and manage urban air quality, public authorities require an integrated approach that incorporates direct measurements and modelling of mean pollutant concentrations. These have to be performed by means of operational modelling tools, that simulate the transport of pollutants within and above the urban canopy over a large number of streets. The operational models must be able to assess rapidly a large variety of situations and with limited computing resources. SIRANE is an operational urban dispersion model based on a simplified description of the urban geometry that adopts parametric relations for the pollutant transfer phenomena within and out of the urban canopy. The streets in a city district are modelled as a network of connected street segments. The flow within each street is driven by the component of the external wind parallel to the street, and the pollutant is assumed to be uniformly mixed within the street. The model contains three main mechanisms for transport in and out of a street: advection along the street axis, diffusion across the interface between the street and the overlying air flow and exchanges with other streets at street intersections. The dispersion of pollutants advected or diffused out of the streets is taken into account using a Gaussian plume model, with the standard deviations σ y and σ z parameterised by the similarity theory. The input data for the final model are the urban geometry, the meteorological parameters, the background concentration of pollutants advected into the model domain by the wind and the emissions within each street in the network. ► SIRANE simulates pollutant dispersion in urban atmosphere at the district scale. ► Flow and dispersion within the urban canopy are modelled by means of a street network approach. ► The dispersion in the external atmosphere is modelled by a Gaussian model.

Evaluation of intake fractions for different subpopulations due to primary fine particulate matter (PM2.5) emitted from domestic wood combustion and traffic in Finland

Domestic wood combustion and traffic are the two most significant primary fine particulate matter (PM2.5) emission source categories in Finland. We estimated emission–exposure relationships for primary PM2.5 emissions from these source categories using intake fractions (iF), which describes the fraction of an emission that is ultimately inhaled by a target population. The iFs were calculated for four different emission source subcategories in Finland in 2000: (1) domestic wood combustion in residential buildings, (2) domestic wood combustion in recreational buildings, (3) traffic exhaust and wear emissions, and (4) traffic resuspension emissions. The iFs were estimated for both total population and for subpopulations with different gender, age, and educational status. Primary PM2.5 emissions were based on the Finnish Regional Emission Scenario model and the dispersion of particles was calculated using the Urban Dispersion Modeling system of Finnish Meteorological Institute. Both emissions and dispersion were estimated on a 1 km spatial resolution. The iFs for primary PM2.5 emissions from (1) residential and (2) recreational buildings were 3.4 and 0.6 per million, respectively. The corresponding iF for (3) traffic exhaust and wear and (4) traffic resuspension emissions were 9.7 and 9.5 per million, respectively. The differences in population-weighted outdoor concentrations were significant between subpopulations with different educational status so that people with higher education were exposed more to traffic-related PM2.5.

Urban Dispersion Modelling and Experiments in the Daytime and Nighttime Atmosphere

An analytical model of atmospheric dispersion in urban areas in both daytime and nighttime conditions is presented. The model is based on a Gaussian formulation where the horizontal and vertical diffusion coefficients are determined according to analytical theories. The model is validated with dispersion measurements from field experiments conducted in Oklahoma City, Salt Lake City, St. Louis and London, U.K. The theory is in good agreement with the data for both daytime and nighttime conditions. The data support the conclusion that the magnitude of the nighttime stratification in the urban atmosphere is weak; however, its effects on dispersion are not negligible. The predicted existence of two distinct dispersion regimes, in the near and in the far field, is also confirmed by the data. The good collapse of the data suggests that urban dispersion is governed by the characteristic length scales of atmospheric boundary-layer turbulence, rather than urban canopy length scales that are more likely to affect dispersion only in the vicinity of the source.

Accelerating urban fast response Lagrangian dispersion simulations using inexpensive graphics processor parallelism

Owing to the potential consequences associated with accidental or deliberate releases of chemical or biological agents in urban areas, fast response urban dispersion models must rapidly provide solutions that can be easily analyzed by researchers and emergency responders. In this paper, we describe a novel application of an existing Lagrangian dispersion modeling system to achieve real-time simulation and visualization of an urban plume that a user can interact with in a virtual environment (VE) through the utilization of commodity graphics hardware, utilizing the highly parallel computational capabilities available on graphics processing units (GPU). GPUs have quickly developed from video game technology to open up new avenues for enhancing simulation performance and visualization of engineering and science applications. For computer graphics applications, GPUs provide highly parallel and inexpensive data paths for processing geometry and pixels, but for simulation these parallel paths are exploited for solving general problems. In this paper, a newly developed dispersion model (GPU Plume) is tested against an analytical solution, a CPU implementation of the Lagrangian dispersion model and wind tunnel data for dispersion around a single building. GPU Plume is shown to provide results that are similar in accuracy to the CPU model, but with computation times that are up to two orders magnitude smaller. In addition, challenges associated with the implementation of Lagrangian dispersion models onto the GPU architecture are discussed in this paper.

Towards a model evaluation protocol for urban scale flow and dispersion models

This paper reports on the stages forming a model evaluation protocol for urban flow and dispersion models proposed within the COST Action 732 on “Quality Assurance and Improvement of Micro-Scale Meteorological Models”. It discusses the different components forming model evaluation with emphasis on validation and implementation of the protocol for the test case Mock Urban Setting Test (MUST). The protocol was proposed with building-resolving models in mind, but integral models have also been included. The suggested approach can be used for further micro-scale model evaluation and for the standardisation of their applications.

The Measure of Land Consumption Caused by Urban Planning

The phenomenon of land consumption is connected with different territorial criticalities, such as the impoverishment of very fine agricultural land, urban dispersion, the spatial and ecological fragmentation, the hydrogeological upheaval and so on [1]. Attention to land consumption is by now consolidated and the search for suitable modalities for its measurement gets more and more stronger. Restraint and examination of land consumption are overriding goals which have to be shared at all levels of urban and territorial planning in the context of policies of subsidiarity and interinstitutional cooperation. The intrinsic relationships between the zoning and physical structure of urban environments suggest that the mechanisms through which planners can evaluate the environmental consequences of existing zoning ordinances and improve the scientific basis of future decision making in order to mitigate the negative effects of urban development should be in place [2]. This essay aims to identify the indicators of measurement of land consumption caused by instruments of urban planning in order to support as well during a strategic environmental evaluation polices of territorial government aiming at models of sustainable urban development.

Evaluations of urban atmospheric transport and dispersion models using data from the Joint Urban 2003 field experiment

We have evaluated the performance of several urban Atmospheric Transport and Dispersion (AT&D) models by comparing model predictions to tracer gas concentrations measured during the Joint Urban 2003 field experiment in Oklahoma City, USA. These models include the Urban Canopy, Urban Dispersion Model (UDM), and Micro-SWIFT/SPRAY (MSS) modes within the HPAC modelling suite, QUIC-URB/QUIC-PLUME models, and the MESO/RUSTIC models. We discuss some of the results of these comparisons, including relative model performance according to bias and scatter metrics, differences in model behaviour for predictions of daytime vs. nighttime releases, and operational considerations such as runtime differences.

Quality of life: a reappraisal

PurposeWith this reappraisal, the purpose of this paper is to present a reflexion on and discussion of the concept of quality of life (QL) with the intention of delimiting its meaning and application within the scope of the research project entitled “Costs and benefits of urban dispersion on a local scale”.Design/methodology/approachThe concept of QL contains a significant degree of complexity and multidimensional variables, in addition to the dynamic nature inherent in all social phenomena. The application of this concept at a local level and within the context of the Portuguese socio‐territorial reality requires rethinking the concept through the different authors and approaches, in order to delineate the research process, and guarantee its operationalisation, selecting the social indicators than can serve this purpose, with the intent of gaining a clearer understanding of QL as perceived and evaluated by the people and groups living in various dispersed urban areas.FindingsFrom the readings of literature in the field, one can understand the importance of choosing the relevant domains when analysing and measuring QL. As with the choice of indicators, in order to be able to measure the QL, simultaneously, at a local level, the choice of indicators and the delimitation of units of analysis are also fundamental in order to be able to obtain the comparison and real measure of quality of life and not the contingencies of specific contextual characteristics.Originality/valueThe study aims to open a new research perspective in the field of social sciences, more specifically in the areas related to QL and urban dispersion.

Measuring and Assessing Urban Sprawl: What are the Remaining Options for Future Settlement Development in Switzerland for 2030?

Transformation of land use in and around European cities is proceeding as fast as never before, and urban sprawl is a reality in Europe. This process is coming along with significant landscape changes that can even lead to the loss of landscape identity. Is it possible to find indications of which regions are prone to urban sprawl in order to curtail undesired future settlement developments in time? To answer this question we used settlement development scenarios for Switzerland, and analysed their spatial implications using a set of four metrics, which allow for comparing the degree of urban sprawl in different regions. Two aspects were explored: (1) by how much settlement development could potentially increase in Switzerland, and (2) the suitability of the metrics as indicators for characterizing and assessing the development of urban sprawl. The results show that overall in Switzerland the urban permeation and dispersion of settlement areas is likely to increase (in all scenarios), but to different degrees. However, the results differ very much between the various types of settlement and between the cantons, and even a decrease in urban dispersion is possible. In combination with scenarios of settlement growth, the metrics provide useful evidence on regional characteristics such as the overall pressure of settlement development and likely transformations of the respective settlement types that should be taken into account in spatial development concepts. There is a need for calibration of the indicators on a regional level to define specific thresholds to limit urban sprawl.

CityFlux perfluorocarbon tracer experiments

Abstract. In June 2006, two perfluorocarbon tracer experiments were conducted in central Manchester UK as part of the CityFlux campaign. The main aim was to investigate vertical dispersion in an urban area during convective conditions, but dispersion mechanisms within the street network were also studied. Paired receptors were used in most cases where one receptor was located at ground level and one at roof level. One receptor was located on the roof of Portland Tower which is an 80 m high building in central Manchester. Source receptor distances in the two experiments varied between 120 and 600 m. The results reveal that maximum concentration was sometimes found at roof level rather than at ground level implying the effectiveness of convective forces on dispersion. The degree of vertical dispersion was found to be dependent on source receptor distance as well as on building height in proximity to the release site. Evidence of flow channelling in a street canyon was also found. Both a Gaussian profile and a street network model were applied and the results show that the urban topography may lead to highly effective flow channelling which therefore may be a very important dispersion mechanism should the right meteorological conditions prevail. The experimental results from this campaign have also been compared with a simple urban dispersion model that was developed during the DAPPLE framework and show good agreement with this. The results presented here are some of the first published regarding vertical dispersion. More tracer experiments are needed in order to further characterise vertical concentration profiles and their dependence on, for instance, atmospheric stability. The impact of urban topography on pollutant dispersion is important to focus on in future tracer experiments in order to improve performance of models as well as for our understanding of the relationship between air quality and public health.

Urban permeation of landscapes and sprawl per capita: New measures of urban sprawl

A B S T R A C T Urban sprawl (dispersed urban development) has increased at alarming rates in Europe and North America over the last 50 years. Quantitative data are urgently needed in monitoring systems for sustainable development. However, there is a lack of reliable measures of urban sprawl that take into account the spatial configuration of the urban areas (not just total amount). This paper introduces four new measures of urban sprawl: degree of urban dispersion (DIS), total sprawl (TS), degree of urban permeation of the landscape (UP), and sprawl per capita (SPC). They characterize urban sprawl from a geometric point of view. The measures are related through TS = DIS ! urban area, UP = TS/size of the landscape studied, and SPC = TS/number of inhabitants. The paper investigates the properties of the new measures systematically using 13 suitability criteria which were derived from a clear definition of urban sprawl as discussed in a previous paper. The scale of analysis is specified by the so-called horizon of perception. Second, the new measures are applied to threeexamplesfromSwitzerland.Subsequently,themeasuresarebrieflycomparedtoothermeasuresof urban sprawl from the literature. We demonstrate that UP is an intensive and area-proportionately additivemeasure andissuitableforcomparingurban sprawlamong regionsofdifferingsize, whileSPCis most appropriate when comparing sprawl in relation to human population density. The paper also provides practical advice for calculating the new measures. We conclude that the new method is more suitable than previous methods to quantify the indicator ‘‘urban sprawl’’ in monitoring systems as this method distinguishes the phenomenon of urban sprawl from its various causes and consequences. This article is part II of a set of two papers.

A new urban boundary layer and dispersion parameterization for an emergency response modeling system: Tests with the Joint Urban 2003 data set

A new urban parameterization for a fast-running dispersion prediction modeling system suitable for emergency response situations is introduced. The parameterization represents the urban convective boundary layer in the dispersion prediction system developed by the National Atmospheric Release Advisory Center (NARAC) at Lawrence Livermore National Laboratory. The performance of the modeling system is tested with data collected during the field campaign Joint Urban 2003 (JU03), held in July 2003 in Oklahoma City, Oklahoma. Tests were performed using data from three intense operating periods held during daytime slightly unstable to unstable conditions. The system was run in operational mode using the meteorological data that would be available operationally at NARAC to test its effectiveness in emergency response conditions. The new parameterization considerably improves the performance of the original modeling system, by producing a better degree of pattern of correspondence between predictions and observations (as measured by Taylor diagrams), considerably reducing bias, and better capturing directional effects resulting in plume predictions whose shape and size better resemble the observations (via the measure of effectiveness). Furthermore, the new parameterization shows similar skills to urban modeling systems of similar or greater complexity. The parameterization performs the best at the three JU03 sensor arcs (1, 2, and 4 km downwind the release points), with fractional bias values ranging from 0.13 to 0.4, correlation values from 0.45 to 0.71, and centered root-mean-square error being reduced more than 50% in most cases. The urban parameterization has been tested with grid increments of 125, 250, 500 and 1000 m, performing best at 250 and 500 m. Finally, it has been found that representing the point source by a Gaussian distribution with an initial spread of particles leads to a better representation of the initial spread induced by near-source buildings, resulting in lower bias and improved correlation in downtown Oklahoma City.

No boundaries: exurbia and the study of contemporary urban dispersion

The dream of a house in the country, at the seashore, or near a ski hill, is one shared by many in North American society. But the environmental and social impacts of the realization of this dream by an increasing number of people has created crises and conflict for many communities. The concept of exurbia has traditionally been used to describe settlement patterns simultaneously dispersed from the city yet also connected to urban networks. This paper reviews scholarship across disciplines including geography, ecology, sociology, and political ecology. Exurbia is here proposed to be strengthened as a powerful conceptual approach to capture and discuss the complex processes producing this phenomenon. Previous scholarship has produced excellent but largely disconnected work on the periurban zone around cities, exurban settlement processes, tensions between exurbanites and other rural residents, environmental impacts and habitat fragmentation. Future work on exurbia holds a great deal of promise to think about cultural values supporting the processes that produce these landscapes, working across scales from local to global using interdisciplinary and multi-method study.

Spatial structure and mobility patterns: towards a taxonomy of the Italian urban systems

- Urban spatial organization has become a wide field of research in the last years, since it is thought to be an important determinant of the city's performance from many points of view. Nevertheless, Italian urban spatial organization has not been studied in depth and a general description of the Italian urban territory is lacking. The aim of this work is to build a taxonomy of the Italian cities where the latter are conceptualised as agglomeration of contiguous municipalities on the basis of their patterns of spatial organization and commuting-to-work mobility. For this purpose, three preliminary steps had to be carried out. First of all, the major Italian urban systems have been identified, following a functional approach that is based on the principle of maximum self-containment of commuters' flows. Secondly, original indicators have been built to gain a better understanding of cities' spatial organization and of their patterns of mobility. Thirdly, the relation between these two dimensions has been investigated through a multivariate statistical analysis. The results of the analysis show that variables of spatial organization especially urban dispersion and of mobility patterns are closely related and that cities might be aggregated in five groups, ranging from the most compact and transitoriented cities to the most dispersed and car-oriented ones.

Short‐range urban dispersion experiments using fixed and moving sources

AbstractFour perfluorocarbon tracer dispersion experiments were carried out in central London, United Kingdom in 2004. These experiments were supplementary to the dispersion of air pollution and penetration into the local environment (DAPPLE) campaign and consisted of ground level releases, roof level releases and mobile releases; the latter are believed to be the first such experiments to be undertaken. A detailed description of the experiments including release, sampling, analysis and wind observations is given. The characteristics of dispersion from the fixed and mobile sources are discussed and contrasted, in particular, the decay in concentration levels away from the source location and the additional variability that results from the non‐uniformity of vehicle speed. Copyright © 2009 Royal Meteorological Society

Overview of the Brooklyn Traffic Real-time Ambient Pollutant Penetration and Environmental Dispersion (B-TRAPPED) study: theoretical background and model for design of field experiments

The Brooklyn traffic real-time ambient pollutant penetration and environmental dispersion (B-TRAPPED) study was a multidisciplinary field research project that investigated the transport, dispersion, and infiltration processes of traffic emission particulate matter (PM) pollutants in a near-highway urban residential area. The urban PM transport, dispersion, and infiltration processes were described mathematically in a theoretical model that was constructed to develop the experimental objectives of the B-TRAPPED study. In the study, simultaneous and continuous time-series PM concentration and meteorological data collected at multiple outdoor and indoor monitoring locations were used to characterize both temporal and spatial patterns of the PM concentration movements within microscale distances (<500 m) from the highway. Objectives of the study included (1) characterizing the temporal and spatial PM concentration fluctuation and distribution patterns in the urban street canyon; (2) investigating the effects of urban structures such as a tall building or an intersection on the transport and dispersion of PM; (3) studying the influence of meteorological variables on the transport, dispersion, and infiltration processes; (4) characterizing the relationships between the building parameters and the infiltration mechanisms; (5) establishing a cause-and-effect relationship between outdoor-released PM and indoor PM concentrations and identifying the dominant mechanisms involved in the infiltration process; (6) evaluating the effectiveness of a shelter-in-place area for protection against outdoor-released PM pollutants; and (7) understanding the predominant airflow and pollutant dispersion patterns within the neighborhood using wind tunnel and CFD simulations. The 10 papers in this first set of papers presenting the results from the B-TRAPPED study address these objectives. This paper describes the theoretical background and models representing the interrelated processes of transport, dispersion, and infiltration. The theoretical solution for the relationship between the time-dependent indoor PM concentration and the initial PM concentration at the outdoor source was obtained. The theoretical models and solutions helped us to identify important parameters in the processes of transport, dispersion, and infiltration. The B-TRAPPED study field experiments were then designed to investigate these parameters in the hope of better understanding urban PM pollutant behaviors.

Characterizing urban surface cover and structure with airborne lidar technology

Urban and landscape planners are becoming increasingly aware of the potential of light detection and ranging (lidar) technology to produce height and structural information over large geographic areas in both an economic and time-efficient fashion. In urban environments where the structural complexity is high, for example, lidar is seen as a critical and innovative dataset to improve the characterization of both vegetation and building attributes. Using a small-footprint, first- and last-return lidar dataset of Vancouver, Canada, we demonstrate the potential to derive a suite of attributes important for describing the interactions of the urban surface and atmosphere in weather forecasting, air pollution, and urban dispersion modelling. Two levels of attributes were defined. First, primary attributes such as building shape, size, and location and tree classification were calculated. Building extent and size were computed using an object-based approach based on connectivity and height rules. The classification of tree crown areas was derived from the location of last-return data, filtered to remove the incidence of last returns caused by the interaction of the lidar beam with building edges, and height rules. Validation showed that building areas derived from lidar compared well with aerial photography estimates (r2 = 0.96, p < 0.001, n = 98). The percentage difference between estimates was equal to 16% (n = 83) when buildings were discriminated from the surrounding features. However, the percentage difference between estimates increased to 35% (n = 98) when commission errors were considered, as lidar often overestimated building areas due to closely spaced buildings (gaps less than 1–2 m) not being separated. Similarly, the height and area of lidar-extracted trees were highly correlated with field-based measurements (r2 = 0.84 and 0.76, respectively, p < 0.001, n = 50). Once these primary attributes were derived, we demonstrate the extraction of a number of secondary attributes including building mean height, normalized building volume, building wall surface area, and interelement spacing. Of significance, this research has shown that lidar can provide spatially detailed estimates of urban structure and cover which characterize the aerodynamic and energetic properties of urban areas.

Air quality assessment in Bologna by an urban dispersion model

An urban dispersion model (ADMS?Urban) was run to calculate PM10 and NO2 concentration during a one-year period, on a district of Bologna, Italy. Moreover, the exposure of 333 children was estimated using model outputs corrected with observations. The model was able to correctly simulate the long-term statistical properties of roadside observed concentrations (such as annual mean and frequency distribution), but it failed in reproducing instantaneous values. The proposed methodology is of easy implementation, and it could be useful to identify areas with high pollutant levels, to estimate population exposure and to evaluate the benefit of pollution reduction policies.