Study Of Pollutant Dispersion Research Articles

Urban Pollutant Dispersion on a Turbulent Fluid Model

In 2019, more than 90% of the world’s population was living in places where pollutantconcentrations exceeded the air-quality guidelines. Anthropogenic activity has been the main cause of climate change in the last 20 years, especially in urban centres where industrial activities and motorized traffic mainly occur. This article presents a study of air pollutant dispersion by computational fluid dynamics modelling using OpenFOAM for urban street canyon simulations. The aim of this study was to investigate the influence of vegetation on natural ventilation, dispersion and CO2 mitigation. The behavior of pollutants was modeled by the Reynolds-averaged Navier–Stokes equations (RANS) and the k−ε fluid turbulence model. The results indicate that aerodynamic effects are more important at lower pollutant wind velocities, as they reduce turbulent dispersion and allow a higher reduction in the concentration of air pollutants. In addition, the direction of the wind, relative to the main axis of the street, had a significant impact on the results found. In general, in the presence of trees, perpendicular winds can lead to higher concentration of pollutants, while parallel winds allow improve air quality, considerably reducing those pollutant concentrations.

Development of a new dynamic smagorinsky model by an artificial neural network for prediction of outdoor airflow and pollutant dispersion

Development of a new dynamic smagorinsky model by an artificial neural network for prediction of outdoor airflow and pollutant dispersion

Numerical simulation studies of the flow field and pollutant diffusion around street canyons under different thermal stratifications

In previous studies of pollutant dispersion around street canyons, researchers have found that pollutants do not dissipate easily under stable conditions, but they have tended to study only under weak stability conditions and relatively little research has been done on strong stability conditions. In this study, we mainly used 3-D computational fluid dynamics (CFD) technology to investigate the influence of various Richardson numbers (Rib) on the flow field structure and pollutant dispersion around an ideal street canyon under stable stratification. The results show that under stable stratification, different Rib values have significant effects on the flow field, turbulent field, and concentration field around the street canyon. The main effects are that as Rib increases, the vortices inside the street canyon gradually become larger and move downward, the length of the attachment point behind the buildings first increases and then gradually decreases, and the normalized turbulent kinetic energy (TKE/uH2) gradually decreases. The vertical diffusion range of the plume decreases, the horizontal transport distance increases, and the maximum concentration value at the center of the plume gradually increases. This research result may provide new scientific basis for the prediction and control of urban air pollution.

Evaluation of fast fluid dynamics with different turbulence models for predicting outdoor airflow and pollutant dispersion

Fast fluid dynamics (FFD) could provide informative and efficient airflow and concentration simulation. The commonly used turbulence model in FFD was Re-Normalization Group (RNG) k-ε turbulence model which solved two transport equations to obtain eddy viscosity. To reduce this part of time and further improve computing speed, this investigation implemented no turbulence model, Smagorinsky model and dynamic Smagorinsky model which calculated eddy viscosity without solving equation in FFD in an open-source program, OpenFOAM. By simulating several outdoor cases of varying complexity and comparing with experiment and CFD, this study assessed the accuracy and computing efficiency of FFD with four turbulence models. Compared with CFD, FFD greatly improved the computing speed without reducing accuracy. The simulation of FFD without turbulence model was fast but inaccurate. FFD with Smagorinsky model increased the computing speed while ensuring the same accuracy as RNG k-ε turbulence model. FFD with dynamic Smagorinsky model provided accurate results with high efficiency. Computation errors arose mainly from inaccurate prediction of turbulence dispersion. The computing cost was associated with the number of transport equations and calculation method of model coefficient. This investigation recommended the use of FFD with dynamic Smagorinsky model for outdoor airflow and pollutant dispersion studies.

Atmospheric dispersion and urban planning: An interdisciplinary approach to city modeling

• Block typology is a key element to represent real-like urban geometries. • Idealized arrays underestimate real densities; simplified arrays overestimate them. • Derived arrays based in a real region presented proximity to a real city representation. • The frontal area density can lead to a misinterpretation of a region density. • The use of urban indices provides a more precise density representation. Geometry modeling is a common approach in pollutant dispersion studies. Block typology is a key element for representing geometries closer to real city environments. However, urban pollutant modeling studies and urban planning processes have different approaches regarding block typology and applied metrics. Therefore, the objective of this work is to compare urban block typologies and urban metrics used in literature studies with those found in real cities. The methodology combined a literature review with an empirical analysis of sample areas in selected cities. The results showed that more than 50 % of the studies applied idealized building arrays. Nonetheless, the idealized array tends to underestimate real densities, often misrepresenting urban planning indices. On the other hand, derived geometry reduces modeling complexity and increases the applicability of studies in urban planning. Based on our findings, we suggest an urban block parameterization derived from real urban areas (representative of the densest cities in Asia, Europe, and America). This study selects five block typologies derived from actual cities (single block, detached buildings, courtyard, inner courtyards, and row buildings) with estimated values of the floor area ratio (FAR) and surface coverage (SC) that, when combined, provide a more precise representation of density.

Field synergy analysis of pollutant dispersion in street canyons and its optimization by adding wind catchers

The microenvironment, which involves pollutant dispersion of the urban street canyon, is critical to the health of pedestrians and residents. The objectives of this work are twofold: (i) to effectively assess the pollutant dispersion process based on a theory and (ii) to adopt an appropriate stratigy, i.e., wind catcher, to alleviate the pollution in the street canyons. Pollutant dispersion in street canyons is essentially a convective mass transfer process. Because the convective heat transfer process and the mass transfer process are physically similar and the applicability of field synergy theory to turbulence has been verified in the literature, we apply the field synergy theory to the study of pollutant dispersion in street canyons. In this paper, a computational fluid dynamics (CFD) simulation is conducted to investigate the effects of wind catcher, wind speed and the geometry of the street canyons on pollutant dispersion. According to the field synergy theory, Sherwood number and field synergy number are used to quantitatively evaluate the wind catcher and wind speed on the diffusion of pollutants in asymmetric street canyons. The results show that adding wind catchers can significantly improve the air quality of the step-down street canyon and reduce the average pollutant concentrations in the street canyon by 75%. Higher wind speed enhances diffusion of pollutants differently in different geometric street canyons.

Numerical studies of passive and reactive pollutant dispersion in high-density urban models with various building densities and height variations

Abstract Vehicular pollutant exposure in the near-road buildings of high-density urban areas has been rarely studied. This paper investigates the impacts of high-density building morphology on passive (CO) and reactive (NOx-O3) pollutant dispersion in three-dimensional (3D) urban-like models, e.g. medium (street width W = 30 m,λp = 0.25, λf = 0.6) and compact (W = 18 m,λp = 0.39,λf = 0.94) layouts with uniform (H = 72 m) or various heights (H1 = 48 m,H2 = 96 m). Personal intake fraction P_iF and its spatially-averaged value for the entire building wall ( W) are utilized for exposure assessment. Some meaningful findings are proposed: 1) When wind blows through neighborhood-scale (~1 km) urban models, it decelerates rapidly and then reaches a flow balance where pedestrian-level velocity and pollutant exposure vary depending on building densities and height variations. 2) Uniform-height cases attain overall W of ~0.3–1.3 ppm in target streets. The compact layout (λf = 0.94) experiences weaker vertical exchange where downward helical flows transport more pollutants to windward wall than medium-type (λf = 0.6), inducing two-order larger windward-side W (~1 ppm) than leeward-side (~0.01 ppm). 3) Building height variations obtain greater velocity and smaller W (~0.2–1.0 ppm) in front of taller buildings than those behind them ( W~2.6–4.2 ppm). 4) No matter with or without height variations, densifying building arrays (λf = 0.6 to 0.94) raises windward-side W and mitigates leeward-side W, but overall W for both sides of target streets only change slightly. 5) With present NOx-O3 photochemistry (source emissions NO:NO2 = 10:1, background [O3] = 20 ppbv), NO2 exposure is 40%–230% larger than passive CO exposure, but NO is near to CO. This study implies the near-road pollutant exposure in high-density urban models is sensitive to building morphology which should be carefully designed.

Hydrodynamic Modeling and Simulation of Water Residence Time in the Estuary of the Lower Amazon River

Studies about the hydrodynamic behavior in the lower Amazon River remain scarce, despite their relevance and complexity, and the Water Residence Time (Rt) of this Amazonian estuary remains poorly unknown. Therefore, the present study aims to numerically simulate three seasonal Rt scenarios based on a calibrated hydrodynamic numerical model (SisbaHiA) applied to a representative stretch of the lower Amazon River. The following methodological steps were performed: (a) establishing experimental water flow in natural channels; (b) statistically test numerical predictions (tidal range cycles for different hydrologic periods); and (c) simulating velocity fields and water discharge associated with Rt numerical outputs of the hydrodynamic model varied from 14 ≤ Rt ≤ 22 days among different seasonal periods. This change has shown the significant influence of hydrologic period and geomorphological features on Rt. Rt, in its turn, has shown significant spatial heterogeneity, depending on location and stretch of the channels. Comparative analyses between simulated and experimental parameters evidenced statistical correlations higher than 0.9. We conclude that the generated Rt scenarios were consistent with other similar studies in the literature. Therefore, they depicted the applicability of the hydrodynamics to the conservation of the Amazonian aquatic ecosystem, as well as its relevance for biochemical and pollutant dispersion studies, which still remain scarce in the literature.

Notes on atmospheric pollution dispersion studies at the AEC

Notes on atmospheric pollution dispersion studies at the AEC

Impact of source shape on pollutant dispersion in a street canyon in different thermal stabilities

In previous studies of pollutant dispersion around and inside urban street canyons, several kinds source shape assumptions were employed. However, the impact of source shape on pollutant transport in urban street canyon is barely concerned. As a determinant of pollutant distribution, the shape effects of pollutant source should be analyzed carefully. In this study, source shape effects on street canyon pollutant dispersion are investigated using computational fluid dynamics (CFD) approach based on an idealized street canyon model in different thermal stabilities. The line shape source, band shape source and surface shape source assumptions are adopted to represent the widely used source shape patterns. The prediction accuracy of this numerical model is validated against wind tunnel measurement and LES results and is proved to be acceptable. Numerical simulations are performed under neutral and unstable thermal stratifications. Pollutant distribution inside the street canyon and its variation with thermal stability are examined. The results demonstrate that pollutant source shape can significantly alter the distribution of pollutant concentration in the whole street canyon. The most uneven distribution occurs under the surface shape source condition, where more than 60% pollutant is located in the leeward wall zone, but only 19% pollutant is found in the windward wall zone. The surface source assumption contributes to more dispersed distribution. Moreover, different source shape assumptions lead to distinct characteristics of pollutant residue in the street canyon, especially in the near-ground-zone. Also, the variation of pollutant residue with the thermal stability is greatly influenced by the source shape configuration. The source shape effect should not be neglected in future street canyon environment studies. The new findings present in this study can improve the understanding of pollutant dispersion characteristics in urban street canyons, and benefit architects, city-planners and policy makers to achieve optimized solution for urban air pollution. • Evaluation of the source shape effects on pollutant dispersion in a street canyon in different thermal stabilities. • Pollutant source shape can significantly alter the distribution of pollutant concentration in the whole street canyon. • The surface source assumption contributes to more dispersed distribution. • Different source shape assumptions lead to distinct characteristics of pollutant residue in the street canyon. • The variation of pollutant residue with the thermal stability is greatly influenced by the source shape configuration.

Experimental study of airflow and pollutant dispersion in cross-ventilated multi-room buildings: Effects of source location and ventilation path

Abstract Indoor air quality (IAQ) problem and associated health risks, determined by the indoor air pollutant, are of great concern in modern society. This paper presents a detailed experimental study of pollutant dispersion and airflow characteristics in a multi-room building under cross ventilation condition. A series of experiments were conducted in a scaled multi-room chamber which represents a slap-shaped building. A wind wall system was developed to simulate the cross-ventilated environment. The windows and doors were used as cross ventilation openings, enabling three ventilation path test scenarios. Tracer gas technique was employed to reveal the pollutant dispersion features in the chamber under different conditions. Both the air velocities and tracer gas concentrations at selected key positions in the chamber were measured. The mean concentration values were analyzed to investigate the influence of source location and ventilation path on the pollutant dispersion. This study shows that the pollutant dispersion and airflow characteristics can be strongly influenced by the source location and the ventilation path under approximately the same inflow conditions, and the differences were quantitatively presented. The results give useful insights into the airflow pollutant dispersion process in cross-ventilated multi-room buildings and provide a valuable database for the validation of CFD simulations.

A CFD modelling study of reactive pollutant dispersion in an urban street canyon

This study numerically explores the diffusion rules of pollutants in an idealized urban street canyon where photochemical reactions are present in the neighbourhood. The airborne pollutants dispersion and reaction in street canyons are further investigated in this study. There is a relatively tense traffic pollutants release in urban neighborhoods, including nitrogen oxides and hydrocarbons. Photochemical reaction occurs at strong solar radiation, forming photochemical smog and other environment issues. We selected typical O3 + NO→O2 + NO2 reactions. Effects of the pollutant source location on pollutant dispersion and reaction are also investigated. Numerical simulations were conducted on chemically activity reactive air nitrogen oxides in urban street canyons with W (street canyon wide)/H (building height) = 1. Provided three different locations from the line source street canyon bottom exhaust nitric oxide diffusion and react with ozone in the free stream. The chemical reaction has a significant impact on the concentration of pollutants in the valley volume.

Numerical study of flow characteristics and pollutant dispersion using three RANS turbulence closure models

The flow characteristics and pollutant dispersion around building are investigated numerically using three RANS turbulence closure models, i.e. Standard k–ɛ model, Realizable k–ɛ and RNG k–ɛ model. Two physical configurations are considered in this study: a single isolated obstacle and two obstacles forming an idealized street canyon. Numerical simulations are performed with OpenFOAM® source code which utilise the finite volume method to solve the transport equations of mass, momentum, turbulent kinetic energy and turbulent dissipation rate. When numerical results were compared to wind tunnel experiments, it was found that the RNG k–ɛ turbulence model yielded the best agreement with experimental data. Moreover, the coefficients of the RNG k–ɛ turbulence model were adapted to better predict the flow characteristics in the urban boundary layer. The results obtained using the appropriate model show good agreement with experimental measurements for velocity flow profile and turbulent kinetic intensity. The prediction of pollutant dispersion reveals that in addition to the Reynolds number, the emission location plays an important role in the pollutant concentration distribution.

Mass flux analysis of 137Cs plumes emitted from the Fukushima Daiichi nuclear power plant

The flow vectors of radioactive cesium-137 (137Cs) plume emitted from the Fukushima Daiichi nuclear power plant in March 2011 were quantitatively depicted by a mass flux analysis in this study. 137Cs plumes were calculated by an Eulerian dispersion model with a 3-km horizontal resolution. The vertically column-integrated mass flux was consistent with the flow approximation based on ground surface 137Cs observations, even though there were some discrepancies that were caused by differences in the wind direction between the ground surface and the dominant plume layer. These discrepancies were explained by combining the use of the ground surface horizontal mass flux with the column-integrated mass flux. The mass flux analysis clearly provided an illustration of 137Cs dominant stream locations, directions, and depositions by reducing high-dimensional model outputs into a lower-dimensional plot. Mass flux (i.e. the product of the mass density and wind velocity) has often been used in dynamic meteorology but has not been used as frequently in atmospheric chemistry or pollutant dispersion studies. However, the concept of mass flux is a robust alternative for conventional validation approaches that only utilize a time series of pollutant concentrations. Mass flux analyses can be used further in atmospheric chemistry as a quantitative visualization tool to track the emission, advection, dispersion, and deposition of atmospheric constituents.

EXTRACTING URBAN MORPHOLOGY FOR ATMOSPHERIC MODELING FROM MULTISPECTRAL AND SAR SATELLITE IMAGERY

Abstract. This paper presents an approach designed to derive an urban morphology map from satellite data while aiming to minimize the cost of data and user interference. The approach will help to provide updates to the current morphological databases around the world. The proposed urban morphology maps consist of two layers: 1) Digital Elevation Model (DEM) and 2) land cover map. Sentinel-2 data was used to create a land cover map, which was realized through image classification using optical range indices calculated from image data. For the purpose of atmospheric modeling, the most important classes are water and vegetation areas. The rest of the area includes bare soil and built-up areas among others, and they were merged into one class in the end. The classification result was validated with ground truth data collected both from field measurements and aerial imagery. The overall classification accuracy for the three classes is 91 %. TanDEM-X data was processed into two DEMs with different grid sizes using interferometric SAR processing. The resulting DEM has a RMSE of 3.2 meters compared to a high resolution DEM, which was estimated through 20 control points in flat areas. Comparing the derived DEM with the ground truth DEM from airborne LIDAR data, it can be seen that the street canyons, that are of high importance for urban atmospheric modeling are not detectable in the TanDEM-X DEM. However, the derived DEM is suitable for a class of urban atmospheric models. Based on the numerical modeling needs for regional atmospheric pollutant dispersion studies, the generated files enable the extraction of relevant parametrizations, such as Urban Canopy Parameters (UCP).

A numerical study of reactive pollutant dispersion in street canyons with green roofs

Roof greening is a new technique for improvement of outdoor thermal environment which influences air quality through its impacts on thermal and flow field. In order to examine effects of green roofs on reactive pollutant dispersion within urban street canyons, a computational fluid dynamics (CFD) model was employed which contained NO-NO2-O3 photochemistry and energy balance models. Simulations were performed for street canyons with different aspect ratios (H/W) of 0.5, 1.0, and 2.0 such that leaf area density (LAD) of green roofs changed. It was found that roof greening led distribution of pollutants to alter for H/W = 0.5 and 1.0 cases in such a manner that their averaged concentrations had small variations as LAD changed. However, by increasing LAD in H/W = 2.0, ventilation efficiency of nitrogen oxides increased since the flow was enhanced within the canyon. Additionally, averaged concentration of ozone in H/W = 2.0 increased with increasing LAD, owing to downward flux of ozone at roof level. Results show that roof greening is a good strategy which can be used in order to improve air quality and thermal environment, especially within deep street canyons.

Characterization of valley flows within two confluent valleys under stable conditions: observations from the KASCADE field experiment

Wind patterns in main and tributary valley systems undergo diurnal variations that are very dependent on the valley's dimensions. Characterizing the down‐valley winds is important for pollutant dispersion studies. It is also a challenge for numerical simulations. This paper uses KASCADE (KAtabatic winds and Stability over CAdarache for Dispersion of Effluents) observations to describe down‐valley flow development and characteristics in two intersecting, shallow valleys of different sizes located in the pre‐alpine region of southeast France. The Durance Valley and its tributary valley, the Cadarache Valley, are different in depth (200 and 100 m depth, respectively), slope (0.2 and 1.2°), width (5–8 and 1–2 km) and length (>50 and 6 km). Both down‐valley flows were dominant features for the period of measurements. The distinct valley scales lead to different characteristics: the smaller Cadarache down‐valley wind is primarily thermally driven and attains wind speeds of up to 1–4 m s−1 with its jet nose at around 30 m in height above ground. The flow at that height initiates 2 h after sensible heat flux turnover, a time‐scale which relates to a linear wave solution for along‐valley winds. The Durance down‐valley wind involves larger scales –for both terrain features and large‐scale circulation –and is related to a mountain‐to‐plain circulation. Therefore its onset occurs with a delay of ∼6 h after sunset. It reaches speeds of 4–8 m s−1 and a depth of 175–225 m above ground. Its highest occurrence is just after sunrise. Between the two valley winds, a shear layer is observed, the origin of which could be a combination of a Cadarache valley flow return current and a Durance down‐valley wind stream redirected by orography.

Did we choose the best one? A new site selection approach based on exposure and uptake potential for waste incineration.

Ecological problems arising after the construction and operation of a waste incineration plant generally originate from incorrect decisions made during the selection of the location of the plant. The main objective of this study is to investigate how the selection method for the location of a new municipal waste incineration plant can be improved by using a dispersion modelling approach supported by geographical information systems and multi-criteria decision analysis. Considering this aim, the appropriateness of the current location of an existent plant was assessed by applying a pollution dispersion model. Using this procedure, the site ranking for a total of 90 candidate locations and the site of the existing incinerator were determined by a new location selection practice and the current place of the plant was evaluated by ANOVA and Tukey tests. This ranking, made without the use of modelling approaches, was re-evaluated based on the modelling of various variables, including the concentration of pollutants, population and population density, demography, temporality of meteorological data, pollutant type, risk formation type by CALPUFF and re-ranking the results. The findings clearly indicate the impropriety of the location of the current plant, as the pollution distribution model showed that its location was the fourth-worst choice among 91 possibilities. It was concluded that the location selection procedures for waste incinerators should benefit from the improvements obtained by the articulation of pollution dispersion studies combined with the population density data to obtain the most suitable location.

Sensitivity of PBL schemes of the WRF-ARW model in simulating the boundary layer flow parameters for their application to air pollution dispersion modeling over a tropical station

Mesoscale atmospheric circulations play an important role in the transport of air pollution and local air quality issues. The planetary boundary layer (PBL), the thermo-dynamical structure and the flow field play an important role in air pollution dispersion. Hence, the PBL parameters over Nagpur, India are simulated using the ARW v. 3.6.1 mesoscale model. High-resolution simulations are conducted with triple nested domains having a horizontal resolution of 27, 9 and 3 km, as well as 27 vertical levels by using the 1 . 1º NCEP Final Analysis meteorological fields for initial and boundary conditions. Eight fair-weather days in winter and summer (January and April 2009) with no significant synoptic activity were chosen for the study. Sensitivity experiments of the ARW model were conducted with two non-local (Yonsei University [YSU], and Asymmetric Convective Model v. 2 [ACM2]) and three local turbulence kinetic energy (TKE) closure (Mellor-Yamada Nakanishi and Niino Level 2.5 PBL [MYNN2], Mellor-Yamada-Janjic [MYJ], and quasi-normal scale elimination [QNSE]) turbulence diffusion parameterizations, to study the evolution of PBL parameters and the thermodynamical structure during the study period. After validation of the simulated parameters with the available in-situ data, it was revealed that the non-local PBL scheme YSU, followed by local scheme MYNN2, could able to capture the characteristic variations of surface meteorological variables and the thermodynamical structure of the atmosphere. The present results suggest that the PBL schemes, namely YSU and MYNN2, performed better in representing the boundary-layer parameters and are useful for air pollution dispersion studies.

A Three-Dimensional Analytical Solution for the Study of Air Pollutant Dispersion in a Finite Layer

We present a closed-form analytical solution for the advection–diffusion equation, where the planetary boundary layer is divided into subdomains, where in each subdomain averaged values of eddy diffusivity and wind speed are assumed. The solution procedure combines an appropriate auxiliary eigenvalue problem with mathematical induction. A transcendental equation giving the eigenvalues for any numbers of subdomains is also developed. Convergence of the solution is numerically verified. The solution is used to evaluate the model against the Copenhagen experiment and computed results are in agreement with experimental ones.