Elevated Point Source Research Articles

A model for dispersion and deposition of radioisotopes in the convective planetary boundary layer

A two region model for atmospheric dispersion and deposition of fine particles and gases, such as I 131 or Cs 137, from a continuous elevated point source has been established. A Gaussian model is used for the region just downstream of the source, referred to as region I, whereas a gradient transfer approach has been used for the next region, referred to as region II. In modelling region I, the concept of the ‘convective velocity scale’ has been applied to the lateral and vertical dispersion parameters. In region II, a numerical model has been developed to solve the diffusion equation. The time dependent three dimensional diffusion equation is reduced to a two dimensional one by the method of moments. It is further split into two one-dimensional equations, one each for the vertical diffusion and the horizontal advection parts, and solved by implicit and upwind finite difference methods, respectively. The physical process of dry deposition, and various diffusivity and wind velocity profiles for different atmospheric stability conditions have been incorporated into the model. Allowance is made for an inversion layer through which a small amount of leakage may occur. In order to ensure that the computer code performs its functions in the manner intended and the results can be used with confidence, a systematic verification and validation activity has been carried out. The latter included graphical displays of the time depencence of the concentration field and its dependence on selected parameters as well as a comparison with other models. The application of the model is shown to be suitable for assesing the mesoscale transport of released material.

A Semi-Lagrangian Spectral Algorithm for Pollutant Transport and Diffusion Studies

Abstract A semi-Lagrangian model was developed based on the atmospheric transport-diffusion equation, employing the semi-Lagrangian technique incorporated with the spectral method. The numerical algorithms of the semi-Lagrangian technique and the spectral method for solving the transport-diffusion equation, subjected to boundary conditions, are described. The model simulated the transport and diffusion of pollutants from an elevated point source. The accuracy of the algorithms was revealed by numerical tests compared with analytical solutions in cases of simple profiles of the mean wind and the eddy diffusivity. To demonstrate the feasibility of the algorithms for studying air pollution in the atmospheric boundary layer, the model, using realistic profiles of the mean wind and the eddy diffusivity, simulated the distributions of pollutant concentrations under various atmospheric stabilities. As will be shown, the algorithms are capable of producing excellent results, making them attractive and promising f...

Integrated pollutant concentration distribution over area surrounding elevated point source

Air pollutant dispersion from a continuous elevated point source in the atmosphere for a combination of varying meteorological parameters is studied in the frame of a Gaussian plume model. The Gaussian basic formula for the concentration is reformulated in polar coordinates. The study derives a general formula for the annual or seasonal integrated concentration over sixteen angular sectors surrounding the source. A computer program is developed to determine the frequency occurrence of the wind velocity and direction together with the stability classes in order to evaluate the integrated pollutant concentration as a function of the distance from the source in the different angular sectors. The results have been applied to the area surrounding the reactor of the Nuclear Research Center at Inshas-Egypt.

Simulations of dispersion in complex terrain using a higher-order closure model

A three-dimensional higher-order closure dispersion model is used to simulate dispersion from an elevated point source in an area with complex terrain in the Colorado River Valley (southwestern U.S.). The central part of the domain is characterized by the valley surrounded by ridges typically 1000 m high. The mean and turbulence values predicted by a higher-order closure fluid dynamic model are input to the dispersion model. The model uses a polar coordinate system with its origin at the point source. In the first part of the study the dispersion calculations were evaluated against surface and aircraft measurements of SO 2 concentrations. The simulated SO 2 concentrations agree reasonably well with both the measured surface and aircraft data. In the second part of the study the dispersion model reveals the highly complex structure and evolution of the plume in this type of topographic forcing. It is shown that the plume is quite frequently divided into two separate parts because the wind field diverges over this complex terrain. Furthermore:, the model is able to simulate the asymmetry and tilting of the plume caused by wind shear and local upslope flows.

Atmospheric Tracer Concentrations from Elevated Source in Urban Core

An understanding of building wake effects is critical to the correct simulation of near-field dispersion of air pollutants that can be entrained in the wake cavity. Downwash of effluent plumes from elevated sources due to building wake effects can raise atmospheric concentrations of the effluent gas near ground level close to the source. Maximum observed ground level concentrations can be due to downwash under certain circumstances. In this field experiment 51 SF[sub 6] samplers were deployed to measure the near-field tracer concentrations due to an elevated point source of heated effluent under stable to neutral atmospheric conditions in urban core. Over 2,000 hourly samples were collected and analyzed. Isopleths of SF[sub 6] hourly concentrations indicate a complicated relationship between concentration and the individual and aggregate position, as well as between concentration and the geometry of surrounding obstacles.

Lagrangian Particle Simulation of Tracer Dispersion in the Lee of a Schematic Two-Dimensional Hill

Abstract Spray, a 3D Lagrangian particle model for the simulation of complex flow dispersion, is presented. Its performance is tested against the Environmental Protection Agency wind tunnel concentration distribution of passive tracer released from elevated point sources, located in the lee region of a two-dimensional schematic hill, in a neutrally stratified boundary layer. Based on the measured values of the first two moments of the turbulent flow velocity, the mean fields are computed over a regular grid using a mass-consistent model, whereas the turbulence structure is simply interpolated. From these fields, trajectories of tracer particles are computed using a linear formulation of the Langevin equation, with a correlated, skewed forcing. The self-consistence test (well-mixed condition), aimed at maintaining an initially well-mixed particle distribution uniform in time, has shown satisfactory results in the region under study. The computed concentration field turns out to be in good agreement with th...

Characteristics of worst‐case maximum air concentrations

The properties of the Gaussian plume equation have led many industrial designers to plan their stack system so that ambient pollutant concentrations are reduced to levels considered safe from the standpoint of undesirable effects. This paper demonstrates the characteristics of worst‐case maximum air concentrations due to an elevated point source diffusing over flat terrain. The critical wind speed, critical plume rise and critical downwind distance are obtained from the Gaussian plume model. The analysis is based on the assumption that dispersion parameters σy and σz are functions of downwind distance and atmospheric stability. The proposed methodology should be especially useful in applications in design of stacks, air quality management and air pollution episode control planning.

Statistical characteristics of concentration fluctuations in dispersing plumes in the atmospheric surface layer

Measurements have been made of concentration fluctuations in a dispersing plume from an elevated point source in the atmospheric surface layer using a recently developed fast-response photoionization detector. This detector, which has a frequency response (−6 dB point) of about 100 Hz, is shown to be capable of resolving the fluctuation variance contributed by the energetic subrange and most of the inertial-convective subrange, with a reduction in the fluctuation variance due to instrument smoothing of the finest scales present in the plume of at most 4%.

Probability distributions of concentration fluctuations of a weakly diffusive passive plume in a turbulent boundary layer

Results are presented from an experimental investigation of turbulent dispersion of a saline plume of large Schmidt number (Sc=830) in a turbulent boundary-layer shear flow simulated in a laboratory water channel. The dispersion measurements are obtained in a neutrally buoyant plume from an elevated point source over a range of downstream distances, where both plume meandering and fine-structure variations in the instantaneous plume are important. High-resolution measurements of the scalar fluctuations in the plume are made with a rake of conductivity probes from which probability distributions of concentration at various points throught the plume are extracted from the time series.

Three‐dimensional analytical model of atmospheric dispersion of pollutant in a stable boundary layer

A three‐dimensional atmospheric dispersion model of a heavy admixture emitted from an elevated continuous point source is presented. Appropriate boundary conditions have been employed to model the effects of various removal mechanisms like deposition, settling and leakage of pollutants. This model takes care of variation of mixing layer height with heat flux, geostrophic drag by terrain and several meteorological parameters. A realistic form of variable eddy diffusivity is used in this model for a stable atmospheric condition. It has been found that the effect of settling of large particles is to reduce pollutant concentration always far away (horizontally as well as vertically) from the source. Whereas, in the lower part of the boundary layer near the source its effect is to increase concentration for small values of settling velocity and to decrease for larger values. The present model, being three dimensional in nature, is more suitable to assess the impact of point source emission.

Analysis of wind tunnel dispersion data over two-dimensional obstacles

An analysis of wind tunnel data of dispersion from elevated point sources over a flat floor and gently sloping, 2-D hills is performed. (The data were obtained by Khurshudyan et al., 1981.) Formulas for the mixing lengths and the Lagrangian time scales are tested, suitable for use in various dispersion models. Some expressions for the vertical first moments of the plume concentration distribution suitable for shear flow (Hunt, 1985) are also tested. Then, a normalization is suggested, based on the source mean flow and turbulence parameters, for the ground-level concentration maximum value and position along the plume centerline. Using this normalization, the maximum position is almost constant, regardless of source height variation and of whether the hill is present or not, at least for source positions upstream or over the top. The maximum values allow the determination of normalized terrain amplification factors TAFN, which are shown to be in most cases closer to one than the corresponding TAF obtained without normalization.

Can the Variation in Samara Mass and Terminal Velocity on an Individual Plant Affect the Distribution of Dispersal Distances?

We examined the contribution of seed mass variation to (1) variation in terminal velocity and (2) variation in dispersal distances for species with winged seeds. Coefficients of variation of terminal velocity were usually on the order of 0.13. These values were too low to have a significant effect on distance variation because of the much larger magnitude of the turbulent intensities of the wind in the vertical and horizontal directions. This assertion was tested with a micrometeorological model of dispersal from an elevated point source within a forest canopy. The coefficient of variation of terminal velocity was necessarily low because of (1) the aerodynamic specification that terminal velocity be proportional to the square root of mass and (2) the design problems inherent in a negative correlation between mass and planform area. We conclude that there is unlikely to be selection for mass variation to reduce sibling densities and therefore intersib competition.

On a numerical model of dispersion of chemically reactive pollutants from a point source

A finite‐difference scheme is developed for the solution of one‐dimensional time‐dependent atmospheric diffusion equation with gravitational settling and chemical reaction terms. The coefficient of eddy diffusivity is taken to be functions of vertical distance. The pollutants are emitted from an elevated point source over flat terrain and are assumed to be confined within the mixing height of the atmosphere. The stability of the atmosphere is characterised by Monin‐Obukhov length parameter. Gravitational settling and ground absorption have also been considered in the present model. It is also assumed that pollutants undergo chemical reaction in the atmosphere. The ground level and source level concentrations of the pollutants are also calculated for different representative atmospheric conditions and the results are discussed in detail.

Maximum ground level concentration Of air pollutant

The emission of an air pollutant from an elevated point source according to Gaussian plume model has been presented. Au. elementary theoretical treatment for both the highest possible ground-level concentration and the downwind distance at which this maximum occurs for different stability classes has been constructed. The effective height release modification was taken into consideration. An illustrative case study, namely, the emission from the research reactor in Inchas, has been studied.
 The results of these analytical treatments and of the derived semi-empirical formulae are discussed and presented in few illustrative diagrams.

Contaminant dispersion from an elevated point source

A previously developed scheme for the solution of the convective-diffusive equation is used to generate a three-dimensional contaminant concentration field and the results compared with experimental values.

On plume meandering under stable stratification

Under stable stratification and light winds large horizontal wind direction fluctuations have been observed quite often leading to meandering of pollutants plume downwind of elevated point sources. Due to this plume meandering long term concentration levels can be reduced by a factor of 4 or more, compared to straight plume conditions. Some possible mechanisms causing plume meandering under stable conditions are discussed, including gravity waves and vortices with horizontal or vertical axis.

Unsteady-state dispersion of a reactive gaseous species from an elevated time-dependent point source forming secondary species

The dispersion of a reactive gaseous species emitted from an elevated time-dependent point source undergoing a first-order chemical reaction removal process such as rainout/washout and forming secondary species has been studied by solving unsteady-state three-dimensional diffusion equation by prescribing different forms of fluxes at the source. By using the method of images, the solutions have also been obtained when the source is located at (0, 0, h s) and the ground is acting as a reflecting plane. The effects of removal processes on the dispersion of primary and secondary species have been discussed.

Transport of Sulphur dioxide with deposition and wash out in wet and dry regions

Here we have investigated theoretically the average integrated ground level concentration of sulphur dioxide in wet and dry regions due to the emission from an elevated point source. The profiles of concentration distribution have been obtained in wet and dry regions on various rates of rainfall.

Recurrence of high concentration values in a diffusing, fluctuating scalar field

We propose a simple model for esimating the average number of occurrences per unit time η(c 0) that a threshold concentration c 0 is exceeded. It is based on the joint probability density of the observed concentration c(t) and its time derivative ċ(t) under the assumption that c(t) is a stationary time series; this assumption leads to the hypothesis that c(t) and ċ(t) are statistically independent. Adopting plausible forms of the frequency distributions of c and ċ, we apply the model to diffusion from an infinite area source and from an elevated point source, both in the neutral boundary layer, and obtain simple results for η(c 0) and the average duration of one excursion above c 0 as functions of c 0, the mean and the standard deviation of the concentration, and surface-layer variables.

A climatological model of dispersion in an inhomogeneous boundary layer

A non-Gaussian long term model (KAPPAG-LT) for atmospheric dispersion simulations is presented. The model is based on a two-dimensional analytic solution of the advection-diffusion equation for a shear flow. The vertical inhomogeneity of the atmospheric boundary layer is accounted for by power laws for wind velocity and eddy diffusivity profiles. The performance of the KAPPAG-LT model in estimating annual ground level mean concentrations is compared to that of the EPA Regulatory model ISCLT for the cases of an elevated point source and an area source. A preliminary evaluation against SO 2 data from a heavily industrialized area is presented.