Numerical modeling on air quality in an urban environment with changes of the aspect ratio and wind direction

Due to heavy traffic emissions within an urban environment, air quality during the last decade becomes worse year by year and hazard to public health. In the present work, numerical modeling of flow and dispersion of gaseous emissions from vehicle exhaust in a street canyon were investigated under changes of the aspect ratio and wind direction. The three-dimensional flow and dispersion of gaseous pollutants were modeled using a computational fluid dynamics (CFD) model which was numerically solved using Reynolds-averaged Navier–Stokes (RANS) equations. The diffusion flow field in the atmospheric boundary layer within the street canyon was studied for different aspect ratios (W/H?=?1/2, 3/4, and 1) and wind directions (θ?=?90°, 112.5°, 135°, and 157.5°). The numerical models were validated against wind tunnel results to optimize the turbulence model. The numerical results agreed well with the wind tunnel results. The simulation demonstrated that the minimum concentration at the human respiration height within the street canyon was on the windward side for aspect ratios W/H?=?1/2 and 1 and wind directions θ?=?112.5°, 135°, and 157.5°. The pollutant concentration level decreases as the wind direction and aspect ratio increase. The wind velocity and turbulence intensity increase as the aspect ratio and wind direction increase.     

Assimilating concentration observations for transport and dispersion modeling in a meandering wind field

Assimilating concentration data into an atmospheric transport and dispersion model can provide information to improve downwind concentration forecasts. The forecast model is typically a one-way coupled set of equations: the meteorological equations impact the concentration, but the concentration does not generally affect the meteorological field. Thus, indirect methods of using concentration data to influence the meteorological variables are required. The problem studied here involves a simple wind field forcing Gaussian dispersion. Two methods of assimilating concentration data to infer the wind direction are demonstrated. The first method is Lagrangian in nature and treats the puff as an entity using feature extraction coupled with nudging. The second method is an Eulerian field approach akin to traditional variational approaches, but minimizes the error by using a genetic algorithm (GA) to directly optimize the match between observations and predictions. Both methods show success at inferring the wind field. The GA-variational method, however, is more accurate but requires more computational time. Dynamic assimilation of a continuous release modeled by a Gaussian plume is also demonstrated using the genetic algorithm approach.     

Fluctuations in wind direction at Venice,related to the origin of the air masses

At Venice, sited in the middle of a lagoon facing the Adriatic sea, the wind and the atmospheric stability are characterized by Alps-sea interactions and by cold outbreaks through particular gaps of the Alps chain. As a consequence, radiosonde launchings show thermal profiles different from those usually observed over the mainland and also the fluctuations of the wind direction vary with the origin and dynamics of the air masses. In this paper the relationships between thermal and dynamic stability in terms of standard deviation of the wind direction σ_θ are discussed, since they are of particular interest in managing industrial emissions. The σ_θ shows a poor diurnal trend, especially during winter; the seasonal trend is slightly more marked. However, it is possible to divide theσ_θ into stability classes, draw stability roses and relate the latter to the local dynamic climatology and in particular to the different origins of the air masses.     

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.     

A vegetation modeling concept for Building and Environmental Aerodynamics wind tunnel tests and its application in pollutant dispersion studies

A new vegetation modeling concept for Building and Environmental Aerodynamics wind tunnel investigations was developed. The modeling concept is based on fluid dynamical similarity aspects and allows the small-scale modeling of various kinds of vegetation, e.g. field crops, shrubs, hedges, single trees and forest stands. The applicability of the modeling concept was validated in wind tunnel pollutant dispersion studies. Avenue trees in urban street canyons were modeled and their implications on traffic pollutant dispersion were investigated. The dispersion experiments proved the modeling concept to be practicable for wind tunnel studies and suggested to provide reliable concentration results. Unfavorable effects of trees on pollutant dispersion and natural ventilation in street canyons were revealed. Increased traffic pollutant concentrations were found in comparison to the tree-free reference case.     

Turbulence and dispersion modeling near highways

Traffic-induced turbulence plays a dominant role in the dispersion of pollutants near highways. The formulations for velocity deficit and turbulence in vehicle wakes, developed from theoretical and physical modeling studies of Eskridge and his colleagues at US EPA about 20 years ago, are discussed. The vehicle wake parameterizations incorporated in ROADWAY-2, a near-highway pollutant dispersion model, and its evaluation results are described. The first field measurements of velocities and turbulence in the vehicle wake, using a towed array of 3-D sonic anemometers, are analyzed, and the results are presented and discussed. Specific recommendations are made for additional work in field measurements, laboratory studies, and mathematical model development and evaluation.     

Application of air pollution dispersion modeling for source-contribution assessment and model performance evaluation at integrated industrial estate-Pantnagar

Source-contribution assessment of ambient NO₂ concentration was performed at Pantnagar, India through simulation of two urban mathematical dispersive models namely Gaussian Finite Line Source Model (GFLSM) and Industrial Source Complex Model (ISCST-3) and model performances were evaluated. Principal approaches were development of comprehensive emission inventory, monitoring of traffic density and regional air quality and conclusively simulation of urban dispersive models. Initially, 18 industries were found responsible for emission of 39.11 kg/h of NO₂ through 43 elevated stacks. Further, vehicular emission potential in terms of NO₂ was computed as 7.1 kg/h. Air quality monitoring delineates an annual average NO₂ concentration of 32.6 μg/m³. Finally, GFLSM and ISCST-3 were simulated in conjunction with developed emission inventories and existing meteorological conditions. Models simulation indicated that contribution of NO₂ from industrial and vehicular source was in a range of 45-70% and 9-39%, respectively. Further, statistical analysis revealed satisfactory model performance with an aggregate accuracy of 61.9%.     

The variations in the concentrations of airborne particulate matter with wind direction and wind speed in Denmark

A statistical investigation of the connection between wind direction and wind speed and concentrations of airborne particulate matter at different places in Denmark is briefly described. The results show that the mean concentration levels over the whole country are highest in case of southerly to southeasterly winds. In general, the mean concentrations are decreasing with increasing wind speed for most wind directions, but in southeasterly winds the mean concentrations are higher for high wind speeds than for low wind speeds.     

Turbulent diffusion modelling FQR wind flow and dispersion analysis

Within the framework of searching for relatively simple, reliable and universal eddy viscosity /diffusivity models, a new three dimensional general non-isotropic model is proposed applicable to any domain complexity and any atmospheric stability conditions. The model utilizes the transport equation for turbulent kinetic energy but introduces a new approach in effective length scale estimation based on the flow global characteristics and local atmospheric stability. The model is discussed in detail and predictions are given for flow field and boundary layer thickness. The results are compared with experimental data with satisfactory results.     

Pore-scale modeling of dispersion in disordered porous media

We employ a direct pore-level model of incompressible flow that uses the modified moving particle semi-implicit (MMPS) method. The model is capable of simulating both unsteady- and steady-state flow directly in microtomography images of naturally-occurring porous media. We further develop this model to simulate solute transport in disordered porous media. The governing equations of flow and transport at the pore level, i.e., Navier-Stokes and convection-diffusion, are solved directly in the pore space mapped by microtomography techniques. Three naturally-occurring sandstones are studied in this work. We verify the accuracy of the model by comparing the computed longitudinal dispersion coefficients against the experimental data for a wide range of Peclet numbers, i.e., 5×10(-2)相似文献   

Inherent uncertainty in air quality modeling

This paper summarizes the discussions of a working group that was charged with the task of examining inherent uncertainty in air quality modeling. The major topics of the paper are:

• 1.1. Definition of inherent uncertainty in air quality models;
• 2.2. Determination of inherent uncertainty;
• 3.3. Role of inherent uncertainty in model evaluation.

The concepts introduced here are illustrated through a numerical simulation with Gifford's fluctuating plume model.     

Temporal behavior of continuous releases in flows with wind direction reversal

By analytical and numerical analysis of solutions of the advection-diffusion equation for continuous line-sources in highly simplified models of breeze circulation with wind direction reversal in space and/or in time, it is shown that, because of the continuous accumulation of recycled pollutants over a restricted space area around the source location, the ground level concentration may not tend to a bounded, time-asymptotic distribution as in the case of unidirectional wind speed. Moreover, it is shown that the along-wind distribution may present a bimodal configuration with large peak values which increase monotonically and tend to combine for asymptotic times. These features cannot be reproduced by any model based on a steady-state formula, like the conventional Gaussian model.     

A routine to compute mean and standard deviation of fluctuating wind direction

A computer routine schematic, developed for use with micro-processor to compute mean wind direction and standard deviation of fluctuations in wind direction, is described. The main feature of the routine is overcoming of the ‘gap’ problem which arises due to discontinuity in the potentiometer of the wind vane.     

Effects of wind direction on coarse and fine particulate matter concentrations in southeast Kansas

Field data for coarse particulate matter ([PM] PM10) and fine particulate matter (PM2.5) were collected at selected sites in Southeast Kansas from March 1999 to October 2000, using portable MiniVol particulate samplers. The purpose was to assess the influence on air quality of four industrial facilities that burn hazardous waste in the area located in the communities of Chanute, Independence, Fredonia, and Coffeyville. Both spatial and temporal variation were observed in the data. Variation because of sampling site was found to be statistically significant for PM10 but not for PM2.5. PM10 concentrations were typically slightly higher at sites located within the four study communities than at background sites. Sampling sites were located north and south of the four targeted sources to provide upwind and downwind monitoring pairs. No statistically significant differences were found between upwind and downwind samples for either PM10 or PM2.5, indicating that the targeted sources did not contribute significantly to PM concentrations. Wind direction can frequently contribute to temporal variation in air pollutant concentrations and was investigated in this study. Sampling days were divided into four classifications: predominantly south winds, predominantly north winds, calm/variable winds, and winds from other directions. The effect of wind direction was found to be statistically significant for both PM10 and PM2.5. For both size ranges, PM concentrations were typically highest on days with predominantly south winds; days with calm/variable winds generally produced higher concentrations than did those with predominantly north winds or those with winds from "other" directions. The significant effect of wind direction suggests that regional sources may exert a large influence on PM concentrations in the area.     

Accounting for averaging time in air pollution modeling

This paper examines the validity of a commonly used expression to estimate concentrations for averaging times that are much smaller than that corresponding to the model estimate. These “peak” concentrations, which can be several times larger than the model estimate, are required in applications such as odor assessment. We show that this expression cannot be justified, and that information on short-term concentrations is only contained in the probability distribution of time-averaged concentrations. The paper proposes a simple model of concentration time series to examine the effect of averaging time on concentration probability distributions. The results from the model are compared with previous theoretical and experimental studies.     

A puff pollutant dispersion model with wind shear and dynamic plume rise

A puff diffusion model, which includes wind shear and dynamic plume rise, is developed for numerical prediction of pollutant concentrations under unsteady and non-uniform flow conditions. The plume from a continuous source is treated as a series of puffs emitted successively from the source. Each puff is represented by a set of six tracer particles, which define the size, shape and location of the puff. Initially these particles are located at the surface of the source, on arbitrarily chosen orthogonal axes. The location of the particles is computed at each time step by taking into account advection, eddy diffusion, wind shear and entrainment of ambient air during plume rise. The concentration distribution of each puff is determined by fitting an ellipsoid to the cluster of the six particles and assuming a three-dimensional Gaussian distribution, with standard deviation equal to the half-lengths of the principal axes of the ellipsoid. The concentration at a point of interest is obtained by summing the contributions from nearby puffs. The effect of wind shear on the pollutant concentration is investigated by use of a typical wind shear encountered in the atmosphere. The results show that, at 600 m downstream from the source, the present model gives concentrations a factor of 2 higher and lower at one standard deviation below and above the plume center, respectively, than that of conventional models in which no wind shear is considered. The plume-rise formulation is calibrated against the observations compiled by Briggs and the model is used to predict the trajectory of a plume observed by Slawson and Csanady. Excellent agreement between the prediction and the observation can be achieved if an appropriate eddy diffusivity is chosen.     

Box models versus Eulerian models in air pollution modeling

Box models are widely used in air pollution modeling. They allow the use of simple computational tools instead of the simulation of 3D Eulerian grid models, given by a large set of partial differential equations. We investigate here the theoretical justification of such box models. The key point is the comparison with the underlying Eulerian model describing the dispersion of pollutants in the atmosphere. We restrict the study to a vertical monodimensional case for more clarity. The main result is that the nonlinearity of the chemical kinetics, which is a characteristic feature of chemistry, induces the loss of accuracy.     

Measurement and three-dimensional modeling of airflow and pollutant dispersion in an undersea traffic tunnel

Airflow and pollutant dispersion in a cross-harbor traffic tunnel were experimentally and numerically studied. Concentrations of the gaseous pollutants CO, NOx, and total hydrocarbons (THC) at three axial locations in the tunnel, together with traffic flow rate, traffic speed, and types of vehicle were measured. Three-dimensional (3D) turbulent flow and dispersion of air pollutants in the tunnel were modeled and solved numerically using the finite volume method. Traffic emissions were modeled accordingly as banded line sources along the tunnel floor. The results reveal that cross-sectional concentrations are nonuniformly distributed and that concentrations rise with downstream distance. The piston effect of vehicles alone can provide 9-23% dilution of air pollutants in the tunnel, compounded to a 23-74% dilution effect according to the ventilation condition.     

Numerical simulations of air pollutant dispersion in a stratified planetary boundary layer

A numerical model is described for computing pollutant concentration distributions downwind from a source. It is based on the three-dimensional dispersion equation governing the time-dependent advective and diffusive transport of air pollutants and is solved numerically by a mixed Lagrangian-Eulerian finite-difference scheme. The model includes the vertical wind shear, the turning of the actual wind, and vertical variations of the vertical eddy diffusivity. In this paper the model is used to simulate the pollutant dispersion process in a stratified planetary boundary layer. The vertical profiles of horizontal mean wind and vertical eddy diffusivities are calculated numerically from a planetary boundary layer model. The influence of the ground roughness and the atmospheric stability on the pollutant distribution is investigated. The results indicate that both parameters essentially determine the air pollutant dispersion process.