Air Pollution Modeling and Its Application II

It is now recognized that a variety of chemical transformations occurs in plumes. The types of transformations, their rates of occurrence, and the magnitudes of the resultant pollutant concentrations depend on the chemical composition of a plume and of the ambient air entrained in a plume, the elevation and temperature of a plume, and the meteorological conditions (including solar intensity) experienced by a plume. It is postulated that these transformations contribute to atmospheric haziness and discoloration, atmospheric acidity, and extended sulfate and ozone concentrations in rural areas. In addition, it is postulated that transformations in plumes contribute to brief, elevated ground-level concentrations of nitrogen dioxide.

Experiments have been carried out in a windtunnel of a chemically reacting plume. A NO-plume in an O₃-environment has been studied and the conversion from NO to NO₂ has been determined.Based on the results from these experiments and existing field studies, a critical review of chemically reacting plume models has been carried out.It is shown that in principle these models should take into account the effect of inhomogeneous mixing and the correlation between concentration fluctuations.

A study is being made of pollutant transport and transformation within a large number of major industrial and urban plumes. The North Sea to the west of The Netherlands was selected as the sampling area for the field experiments conducted during 1979 – 1981.

The problem of surface depletion of a Gaussian plume has been formulated by Horst (1977). As in most models of deposition, the deposition flux, F(x,y) = v_d C(x,y,z_d), at a given point in computed as the deposition velocity v_d times the pollutant concentration, C(x,y,z_d), at a particular reference height z_d ; however, in Horst’s model C(x,y,z) is the concentration due to the initial, unabsorbed plume minus the sum of concentration deficits due to all upwind deposition. Thus, absorption is treated through the simultaneous ground reflection and dispersion of real particles plus the ground-level creation and dispersion of “anti-particles.”

Mathematical models used to simulate photochemical urban air pollution are generally based upon the solution of the atmospheric diffusion equation, which describes the rate of change of chemical species in each cell of a three-dimensional grid-mesh. In such models, point source emissions are assumed to be immediately dispersed into a grid cell. This approximation generally leads to an overestimation of plume dispersion and thus considerably affects the accurate treatment of photochemical kinetic processes.A model has been developed that offers a realistic treatment of large point source emissions in urban areas. This Plume-Airshed Reactive-Interacting System (PARIS) is based upon the embedment of one or more reactive plume models in a conventional airshed model. These plume models describe the chemistry and dynamics of large point source plumes, including their interaction with the ambient environment. Plume trajectories are calculated from the gridded wind field used for the airshed model, and plume dispersion is computed according to the local temperature lapse rate. As the plume size becomes comparable with the grid cell size, the plume material is mixed into a grid of the airshed model with the background airshed material.This paper presents some results of PARIS model simulations performed for the St. Louis urban area; in addition, the performance of the model is analyzed and implications regarding its use in the simulation of air quality are discussed.

A comprehensive chemical model of gas-phase plume/ambient interactions using simple dispersion terms has been developed. Simulations have been carried out for an idealised source emitting into ambient air masses with different histories with particular reference to the formation of acidic species. Diurnal and seasonal effects together with the influence of ambient air composition and plume expansion rate have been investigated for dispersion times up to 19 hours.The model predictions indicate that in general, greater chemical conversion of plume components occurs in material emitted in the late afternoon and overnight than in material emitted in the morning, and that chemical reactions are faster in Summer than in Winter. However, the nett quantitative environmental effect of the plume is a complex function of the physical and chemical parameters which cannot be determined a priori and further modelling studies are required before universally applicable simplifications to the chemical scheme can be made.In addition to the general studies, the model has been modified to simulate, in an idealised manner, a specific case study in which a labelled power plant plume was tracked and analysed by instrumented aircraft on a trajectory from the U.K. over the North Sea. For these experiments conducted on a clear sunny day, satisfactory agreement between measured values for the important gaseous plume constituents and model predictions are obtained.

A multilayer and single layer regional scale models have been used in a short term assessment during two frontal storms in October, 1977. A comparison with observed values has been made of results of the model assessments for SO₄ air concentrations using an in-cloud SO₄= wet deposition conversion, an in-rain conversion and no in-cloud or in-rain conversion in the multilayer model. Also an assessment using in-rain conversion using a single layer model was compared with observations. The results were mixed in that the multilayer model with no in-cloud or in-rain conversion yielded the best SO₄= air concentration patterns while the multilayer model using in-cloud conversion yielded the best fit to observed wet deposition of SO₄=. Further work is necessary since only 35% of the SO₂ emission in emissions northeast United States were used in this study.

The washout of Hydrogen fluoride, emitted by a continuous point source has been considered as being an absorption process.According to Hales (1972), the fundamental equations describing this process are developed and are discussed more in detail for 2 special cases; namely the irreversible and reversible washout. In both cases the influence on the fluoride concentration in a raindrop of stack height, distance from the source and raindrop radius has been discussed. Moreover, the mean concentration during a real rainshower has been calculated.From the previous theoretical study a computer program has been developed in order to simulate the fluoride washout around a brickworks in Belgium. This program is able to predict very well the experimental washout values obtained from a precipitation sampling network set up in a radious of 2 km. around these brick works and using a sampling period of 2 weeks.

The EKMA is a Lagrangian photochemical air quality simulation model that calculates ozone from its precursors: nonmethane hydrocarbons (NMHC) and nitrogen oxides (NO_x). This study evaluated the performance of the EKMA when it is used to estimate the maximum ozone concentration that can occur in an urban area and its environs. The evaluation was conducted using data for five U.S. cities. This paper reports the results for St. Louis, Missouri.A novel statistical evaluation procedure was developed to measure the accuracy of the EKMA ozone estimates. The accuracy parameter is defined as the ratio of observed to estimated ozone. Associated with this ratio is an accuracy probability, which is defined as the probability that the ratio lies within a predefined percent (e.g., ±20 percent) of unity, a unit value of the ratio denoting perfect agreement between observation and prediction. Equations were derived that express the ratio as a function of NMHC and NO_x. The evaluation procedure thus uses NMHC and NO_x as inputs to calculate the accuracy probability of the EKMA ozone estimate. The full range of accuracy probabilities associated with the EKMA ozone estimates is displayed in graphical form on the NMHC-NO_x plane.

Comparisons of modelled and perceived urban visual air quality are presented for an average visual air quality day in the winter of 1981 for Denver, Colorado. The field study design for capturing human judgments of visual air quality and the three dimensional visual air quality simulation model are outlined. The comparisons illustrate the feasibility as well as the difficulties associated with predicting human judgments of visual air quality with an air quality simulation model.The research for this project was facilitated by the Center for Research on Judgment and Policy, Institute of Behavioral Science, University of Colorado. The project was supported in part by BRSG Grant RR07013-14 awarded by the Biomedical Research Support program, Division of Research Resources, NIH.

A model for mesoscale hazard evaluation, along a trajectory, with time/distance varying meteorological conditions, has been developed. For release periods longer than one hour, multiple superimposed trajectories and meteorological conditions are utilized.The centreline concentration for each hour’s release is assumed to move along a trajectory, calculated from actual wind measurements or climatological average data, The model utilizes the Gaussian distribution formulae for the calculation of the concentration at preselected grid points near the ground. The wind speed and stability category may vary continuously along the trajectory, At these grid points the instantaneous (one hour average) and the total integrated concentrations are calculated. Ground deposition is calculated assuming a constant settling velocity.The final results are given as radiation fields and total integrated doses, computed by the methods suggested by WASH-1400 and the ICRP-26 report.Three experiments are presented in the work and the main advantages of the present method for the chosen cases are discussed.

The heightened awareness among scientists in Europe and North America of the opotential dangers to the ecosystem posed by the input of acidic substances to lakes, soil, and vegetation through the atmospheric medium has spurred the development and application of models to describe the long range transport of pollutants. Most long range transport models are intended for use in characterizing regional patterns over long averaging period, thus allowing significant simplifications in the level of detail required for the treatment of transport an dispersion.

Previous studies at SRI International (Johnson et al., 1978 ; Bhumralkar et al., 1979 and 1981) developed a puff trajectory model for regional-scale air pollution modeling.★ The SRI model exists in several forms : The simpler form (EURMAP-1) can be used to obtain long-term averages ; the more complicated form (EURMAP-2 ) is used for short-term pollution episodes.

During the 1960’s and early 1970’s observations of increasing acidity of lakes in southern Norway and in the Adirondacks of New York gave an indication that pollutants were being transported great distances from their point of origin. The long transport times were sufficient to allow chemical transformations to occur, adding further complexity. Thus in the early 1970’s the phenomenon of long-range transport of air pollution began to be examined with increasing interest. Studies of transport, transformation and deposition mechanisms have been undertaken and attempts have been made to model these processes. Eliassen (1980) has presented a review of these modelling efforts.

Since there is now a greater awareness of the chronic effects of exposure to low levels of atmospheric pollutants, the long-range transport of atmospheric material is receiving increasing attention in recent years. Furthermore, there is the concern about the effects of successor species which come about by, for instance, chemical transformation or radioactive decay of the primary species during transport in the atmosphere. Concentrations of successor species may build up slowly and consequently transport processes may need to be studied over long distances.

An air pollution transport model is developed. It will be applied in an area which covers the Netherlands and (parts of) the surrounding countries (500 × 500 km2). Transport is described by 1$$\partial c/\partial t + u\partial c/\partial x + v\partial c/\partial y = \partial /\partial z(K\partial c/\partial z) + S.$$

In accordance with the U.S.-Canada Memorandum of Intent on Transboundary Air Pollution, Work Group 2 and its Modeling Subgroup are charged with describing the transport and transformation of air pollutants from their source regions to final deposition, especially wet sulfur deposition in sensitive ecological regions. The Modeling Subgroup has organized a comprehensive inter-comparison and evaluation of eight regional air quality simulation models for eastern North America. The initial inter-comparison and evaluation was based on transfer matrix elements for 11 source regions and 9 targeted sensitive areas. So far, no model has emerged as clearly superior or inferior to the others from the first of three rounds of model evaluation. The first round of evaluation has primarily served to reveal (1) the deficiences in the monitoring data bases, (2) the need for some changes in input parameters for some of the models, and (3) the need to use at least one more year of independent data for model evaluation. The future plans for completing the comprehensive inter-comparison and evaluation are described.

For the quantitative interpretation of the measurement results of the Dutch national air pollution monitoring network two retrospective numerical models were developed. Based on the same meteorology both models only differ in numerical treatment of air pollution transport. In the Eulerian GRID-model advection is simulated by the pseudo-spectral advection scheme as given by Christensen and Prahm (1976), over a 32×32 grid with gridpoint-distances of 15 km, covering the 450×450 km2 surroudings of the Netherlands. In the Lagrangian PUFF-trajectory model the 15×15 km2 source areas are aggregated to puffs which are advected according to the same wind fields as used in the GRID-model.

It is now a current trend for scientists engaged in studies connected with atmospheric pollution to try to link closer and closer the respective knowledges in the conventional pollution and in the meteorology research domains. For a great number of problems, this is widely due to the very strong relationship between thermodynamic properties of the atmosphere and the physicochemical behaviour of atmospheric pollutants.

No general procedure is available for calculation of parcel trajectories that demonstrates consistent accuracy over a broad range of conditions. Many techniques exist (Pack et al., 1978; Hoecker, 1977; Peterson, 1966), but most have limitations such as being site-specific, being applicable under only certain meteorological conditions or times of day, relying on the synoptic-scale rawinsonde network for wind data or being applicable to only specific levels in the atmosphere. Peterson (1966) provides particularly interesting comparisons of trajectories calculated using a number of standard diagnostic techniques. In general, procedures that are relatively successful in some situations, provide poor trajectory estimates in other situations. It is also difficult to determine a priori which procedure will be best in a given case. Thus, there is a need for a general and reliable technique for atmospheric transport computations.

Since the conventional straight-line Gaussian diffusion model is only applicable to environmental exposure calculations under constant meteorological diffusion conditions and as the so-called advanced diffusion models are, as a rule,not suited because they are not (or insufficiently) calibrated and validatedthe required input parameters such as vertical wind and temperature profiles are difficult to obtaina classification according to diffusion categories is problematicthe diffusion and weather march statistics required for important ranges of application are lackingthe evaluation requires a sophisticated numerical integration, we have extended the Gaussian diffusion model to cover non-stationary diffusion conditions in a so-called volume source model, which permits calculation of the environmental exposure due to released pollutant puffs taking account of the change in wind velocity, wind direction, diffusion category and precipitation intensity during transport, using customary diffusion parameters and the diffusion as well as weather march statistics available for many sites.

In a previous paper (Racher et al.,1), we have applied to the mid-Rhine valley a mass-consistent wind field model based on a variational technique for solving the incompressible continuity equation originally developed by Sherman2. Taking advantage of this variational approach which directly inserts the collected experimental information into the integration procedure itself, we depart from reference 2 mainly on three points. First, we use a terrain-following vertical coordinate (σ-coordinate), thus avoiding the problems of intersecting the topography. Secondly, as will be seen in next paragraph, we have generalized the boundary conditions so as to include the effects of inflow and outflow across the boundaries. And thirdly, we have designed an initialization procedure (Roussel3, Mac Lain4) which aims at retaining most of the meteorological information collected on an irregularly-distributed network of stations. This is an important point which deserves further discussion, since the final relaxed solution is rather strongly dependent upon the initial first-guess wind field. Our initialization scheme avoids too much smoothing in the measured wind field through local expansion of the solution for each horizontal wind component in terms of a polynomial basis function within every triangle obtained by joining the stations. The values thus obtained are then redistributed over the grid points of a regular grid covering the domain under study. In doing so, we retain some of the fine-scale atmospheric structures reflected into the local wind measurements. These structures involve radiative and thermal as well as dynamic effects at many spatial and temporal scales: the adjustment implied through the continuity equation bears only upon the wind components, though accomodating in part the non-dynamic effects which contribute to them. One can assume that a larger number of constraint equations would lead to a reduction in the crucial role played by the initialization procedure.

A most important application of air pollution modeling is the prediction of the changes in the airborne concentrations and surface deposition of pollutants as a function of changes in source emissions. Both the evaluation of the impact of new sources and of the effectiveness of control strategies are issues that are addressed by model calculations. Of particular interest, for some time in Europe and more recently in North America, is the incorporation of pollutants into precipitation, the so-called “acid rain” problem. One aspect of this problem which emphasizes the need for accurate source-receptor relationship is the concern over the long-range transport of pollutants from sources in one political jurisdiction to precipitation in another jurisdiction; see, for example, the following papers: Johnson, et al. (1978), Bhumralkar, et al. (1981), and Shannon (1981), which use two-dimensional trajectory methods.

The modeling of regional-scale transport and deposition of atmospheric pollutants is complicated by the considerable uncertainties involved in atmospheric process parameterizations. Beyond the uncertainties involved in parameterizing dispersion over relatively short distances (say less than 50 km) is the need to estimate dispersion over long time scales, usually including multi-diurnal cycles. Moreover, other atmospheric processes, usually ignored, must be considered. Dry deposition plays an important role as a removal process over long time scales and must be parameterized in a physically realistic manner. The removal of pollutants by wet deposition is an extremely complex process parameterized at best to be a simple non-linear function. Chemical conversion of primary pollutants to secondary is also poorly understood but has been parameterized for some pollutants, notably sulfur dioxide to sulfate, as a first-order chemical conversion. The purpose of this paper is to examine the sensitivity of individual trajectory model runs to variations in model parameterizations. This information is intended to help define the confidence in which we can hope to characterize the effects of anthropogenic emissions on receptor locations far downwind of the source.

In modeling the distributions of atmospheric pollutants, the study of the effects of the topography on the pollutants is of vital importance. As the terrain is flat, the Gaussian dispersion model is the basic method used to calculate pollutant concentrations. However, the Gaussian dispersion models are found to overpredict the concentration over mountainous terrain (Start, et. al., 1977). In order to better understand dispersion over complex terrain, the sophisticated dynamic model based on second-order closure schemes (Yamada, 1979) has been constructed. Due to the complexity of dynamic model, an alternative approach with the use of statistical method for analyzing the dynamic relationship between the geostrophic wind, vertical heat flux and surface wind over complex terrain was proposed (Kau et. al., 1981). The results were encouraging. It was found that the correlations between the calculated and observed surface wind speed were high for all time periods of the day and night. The surface wind speed was dependent primarily on the slope wind, cross isobaric angle, surface thermal stability and geostrophic wind. The wind direction was dependent primarily on the geostrophic wind direction, aspect angle of the topography, upcanyon direction and cross isobaric angle. In this paper, extending the works of Kau et. al., (1982) intend to predict the pollutant concentration over complex terrain.

The program MODIS (Moment Distribution) is designed to extend the Gaussian plume model up to the range of about 102 km. Numerical experiments with stationary solutions are reported to show reliability and accuracy (essentially dependent on the grid Peclét number) of the results. The influence of the Coriolis force on the ground centre line of the plume is shown to be considerably smaller than estimates based on analytical models.

High ground level concentrations are very often observed for plumes dispersing in convective boundary layers. As has been pointed out by Lamb (1978, 1979), Willis and Deardorff (1975, 1978) and Venkatram (1980), conventional Gaussian plume methods are inadequate for treating dispersion inside a convective boundary layer. This is because, convective boundary layers consists of large scale eddies, the updrafts and downdrafts. Transport processes due to these eddies cannot be expressed by simple gradient transfer hypothesis for which the Gaussian plumes are valid.

Emissions from fugitive sources can be estimated from observed concentrations around an industry. The mathematical technique designed and calibrated for this application is a K-analytical diffusion model. The influence of buildings upon the nearby concentration field is also analysed, by a numerical two dimensional model for wind and concentration fields; this allows to locate monitoring stations to avoid these effects.

It has been recognized in recent years that models developed to describe the dispersion of neutrally buoyant plumes or clouds ire inadequate to describe negatively buoyant heavy vapor clouds formed, for example, by accidental spills of volatile liquids. Subsequently, numerous models have been proposed to describe heavy vapor clouds, often with nonexistent or very limited comparison with experimental data, beyond those used to adjust or “calibrate” model parameters. We compare here several heavy vapor dispersion models on a nearly consistent basis against the same sets of experimental data.

Regional scale models are important tools in the study of the atmospheric cycles of trace gases. For example, it is apparent that only after the regional cycle of a pollutant is understood can an efficient and cost-effective control strategy be developed. However, the observed distribution of a pollutant in the atmosphere results from complex interactions between the source distribution, the transport by the mean winds (both horizontally and Vertically), the mixing by turbulent diffusion, the generation or depletion by chemical interaction with other trace species, and the removal by physical interaction with surfaces (dry deposition) and by encounter with a dispersed liquid phase (wet removal). Regional scale models provide a means of studying these complex processes.

The dispersion of aerosols by turbulent diffusion can be modelled on a computer by following the paths of a group of particles, whose actual turbulent velocities are simulated by random movements. This particle simulation is a useful model of turbulent diffusion — as far as statistical averages are considered -, if the most important properties of the actual physical process are reproduced on the computer.

The analysis of the dispersion of pollutants issuing from sources near the ground into the atmospheric boundary layer has traditionally been accomplished via so-called Gaussian plume models. It is assumed that the flow field properties are invariant in the lateral and downwind directions which, along with other simplifying assumptions, allows the concentration field to be described by a Gaussian form; simplicity being one of the models’main attractions. Such models may be modified to take account of the duration of release and the frictional drag of the underlying surface (Clarke 1979). Dispersion may be treated by ‘K’ profile models (for example Maul (1977)) but it is difficult to specify the diffusivity profile from routine meteorological measurements.

Due to an increase of activities over recent years with the transport and storage of liquefied gaseous fuels (e.g. liquefied natural gas), it has become necessary to make careful assessments of the environmental risks associated with such operations. For example, the collision of a liquefied natural gas (LNG) carrying tanker can conceivably release a large amount of potentially flammable methane vapor into the atmosphere. Verified numerical models capable of predicting the gravitational spread and atmospheric dispersion processes following an LNG spill can provide qualitative and quantitative estimates of the motion of such a combustible vapor cloud. Solutions from these models can also be used as initial conditions for other models which treat the deflagation and detonation processes if accidental ignition of the NG/air mixture occurs. LLNL is involved in experimental and model verification studies for the Department of Energy with programs which encompass each aspect of the potential scenarios. Some of the major unresolved questions regarding LNG research are described in Mott et al. (1981). In this paper, we focus attention on the modeling of the vapor dispersion phase only.

To predict the dynamic behavior of trace contaminants in given three-dimensional and time-dependent flow fields, a transport equation must be solved of the general form 1$$ \frac{\partial c}{\partial t}+u\frac{\partial c}{\partial x}+v\frac{\partial c}{\partial y}+w\frac{\partial c}{\partial z}=\frac{\partial }{\partial x}\left( {{K}_{x}}\frac{\partial c}{\partial x} \right)+\frac{\partial }{\partial y}\left( {{K}_{y}}\frac{\partial c}{\partial y} \right)+\frac{\partial }{\partial z}\left( {{K}_{z}}\frac{\partial c}{\partial z} \right) $$ where c is the mean concentration of a chemically inert contaminant, u,v, and w are the given components of the mean wind field, and K_x, K_y and K_z are given eddy diffusivities. The concentration field may be three-demensional and time-varying due to the three-demensional and time-dependent flow field and, in general, the eddy-diffusivities are also functions of position and time. In a wind field not varying in time, the concentration field may still be time-dependent due to unsteady source emission. In many cases, the turbulent transport in the main wind direction, say x-direction, may be neglected in comparison to the advective transport in the same direction, and it is sufficient to solve the less general form 2$$ \frac{\partial c}{\partial t}+u\frac{\partial c}{\partial x}+v\frac{\partial c}{\partial y}+w\frac{\partial c}{\partial z}=\frac{\partial }{\partial y}\left( K\frac{\partial c}{\partial y} \right)+\frac{\partial }{\partial z}\left( {{K}_{z}}\frac{\partial c}{\partial z} \right) $$ The x-direction is a so-called “marching”-direction, and significant savings of computing time are possible if (2) is solved instead of (1). Therefore, in this paper, we will consider the numerical solution of equation (2).

The releases of pollutants to the atmosphere may be divided into three groups according to the length of the release: (i) Instantaneous release of a puff, (ii) Continuous (or quasicontinuous) release which creates an (quasi) infinite plume, (iii) The large group of intermediate cases of finite releases, which produce finite plumes. Many different methods exist, enabling the calculation of the concentration of the pollutant and the hazards resulting from its release. The methods range from simple gaussian models to very elaborated three dimensional numerical models. The major effort until the present time, concentrated in finding solutions to the first two groups of cases. The case of a steady wind direction and speed is fairly easy to handle, even with simple hand calculations, for the case of a puff as well as a (finite or infinite) plume, e.g. Slade (1968), Turner (1970).

Since long it has been recognized, that even a small density surplus can have a significant effect on the motion of a dense fluid. Probably the first to report experimental data on this subject was W. Schmidt (1911). This author describes a number of laboratory experiments with cold air. These experiments show, that a gravity current is formed, when an amount of cold air is released on the ground. This current attains a velocity relative to the ambient lighter fluid, that is of the order $$\sqrt {g'H}$$, where $$g' = g\Delta \rho /\rho$$ is the reduced gravity, Δρ the density difference, ρ the density and H the depth of the current. Schmidt related his experimental results to the outflow of cold air in the atmosphere.

The ensemble average concentration due to an Instantaneous Point Source of an inert pollutant (<C_IPS>) is given by the probability density of pollutant particle displacement G(x,t| x_s,O), where x_s is the source location and x is the particle location after a time t from the release.

This study is related to the general simulation of gas and particles dispersion in and around a manufactory. The physical and the mathematical models used are described ; the results of both experiments are commented and qualitatively compared to the field data. Some technical solutions to tentatively reduce the pollution level due to this type of industry are also analysed.

A full scale dispersion experiment was carried out in Vienna, Austria, from June to November 1979 to investigate the effects of emissions from a 25 m urban parking garage on ambient carbon monoxide concentrations. A comparison of concentrations measured in the building cavity with those yielded by standard formulas from the literature is made.

Within the framework of the Belgian National Research and Development Program Environment-Air of the Ministry of Science Policy a detailed investigation of the environmental behaviour of Sb-particulates emitted by a nonferrous metal industry situated in an open and flat region has been carried out over a period of approximately two years (1979–1980). The main purpose of this testcase was the development and verification of appropriate methodologies to deal with the different aspects of the impact of nonferrous metal industries upon the environment. The specific choice of Sb as testcase is due to the fact that the involved factory is the only Sb-emitter in the region, so that problems with background-levels were avoided, that the factory was willing to cooperate and that no direct health hazards existed.

In recent years the Electric Power Research Institute (EPRI) has engaged in an extensive experimental project, the Plume Model Validation (PMV) project, designed to assess the ability of existing atmospheric dispersion models in predicting ground level impacts from tall, buoyant sources associated with electric power generating facilities. The PMV project has four primary objectives: Establish by statistically rigorous procedures the accuracy and uncertainty of ground-level concentrations predicted by available plume dispersion models.Assess model performance over a range of meteorological and source conditions at a given site, and determine the transferability of plume model performance from one site to another.Create and store extensive databases on observed power plant plume behavior and make these data available to the scientific community. Three terrain types are planned for evaluation: plains, moderately complex and complex.Develop and validate improved plume models.

Since the passage of the Clean Air Act Amendments in August 1977, air quality simulation models have seen increased use for projecting air pollutant impacts of proposed sources for comparison with National Ambient Air Quality Standards (NAAQS) and Prevention of Significant Deterioration (PSD) increments. The U.S. Environmental Protection Agency has issued guidelines on air quality modeling (U.S. Environmental Protection Agency, 1978).

Gent, a moderate sized city of 300.000 habitants in the north of Belgium, is situated at the southern end of the sea-channel Gent-Terneuzen. On both sides of the channel, over a length of 15 km, an industrial zone has grown with a variety of activities such as electricity generation, oil refineries, metallurgical and chemical plants. The region around the city, world-famous for its flowers and plants, counts many greenhouses whereof the heating brings considerable quantities of SO₂ in the ambient air. Within the framework of the National R & D Programme Environment-Air, a detailed inventory of the emissions in this region has been made by several groups of researchersl.

The objective of this study is to develop and validate a plume downwash model using site-specific plant operating data, meteorological data, and sulfur dioxide monitoring data at a multi-source power generating complex in Michigan. The base model selected in U.S. Environmental Protection Agency’s Gaussian-Plume Multiple Source Air Quality Algorithm (RAM). This model is modified to include plume downwash algorithms developed by Briggs and Huber. During the course of the model validation study, the accuracy and reliability of the modified base model are assessed by alternating various modeling parameters and algorithms, such as (a) Pasquill-Gifford, Brookhaven and Huber dispersion coefficients; (b) Briggs trajectory and final plume rise formulae; (c) treatment of plume rise enhanced dispersion suggested by Pasquill and Irwin; (d) treatment of plume partial inversion penetration by Briggs; etc.The evaluation of model performance focuses on assessing the validity and accuracy of model predictions in comparison with the field measurements. Emphases are placed on the adequacy of physical representation of various segments that comprise the model for assessing its validity, and various statistical comparisons between predicted and measured concentrations for assessing its accuracy. An improved plume downwash model has been developed as a result of this study.

This paper presents an overview of some recent dispersion modelling work carried out at Warren Spring Laboratory (WSL), Department of Industry, and describes the more important features and results rather than giving a detailed description of each of the applications. The use of long and short period Gaussian plume models to London and Glasgow is discussed and the results are compared with measurements carried out in these areas. The use of acoustic sounder (SODAR) data to provide input data of particular use in dispersion modelling calculations of hourly average concentrations is also described.

Atmospheric dispersion experiments are carried out in Copenhagen under neutral and unstable conditions in order to study the atmospheric dispersion process in a built-up area. The tracer sulphurhexafluoride is released without buoyancy from a meterological instrumented tower at a height of 115 m, and then collected at ground-level positions in up to three crosswind series of tracer-sampling units, positioned 2–6 km from the point of release. In addition to standard meteorological profile measurements along the tower, the three-dimensional wind fluctuations at the height of release are measured. Characteristic dispersion parameters are estimated from the measured tracer concentrations (averaging time 1 hour), and compared to the dispersion parameters that can be calculated from the atmospheric parameters by various methods. Some of these methods (based on a stability-index) have been in common use for a long time, other (based on wind-fluctuation measurements) reflect recent research. The wind-fluctuation based methods turned out to compare most favourably with the results from the experiments. Based on these experiments a half-empirical model is devised for the prediction of the lateral and vertical dispersion parameters, σ_y and σ_Z, for elevated point sources in an urban area under neutral to unstable conditions.

Until recently, the ability to provide objective evaluations of air quality models has suffered from: The scarcity of adequate test data bases.The lack of a standardized statistical protocol for model evaluation.

Because many practical problems involve air flow patterns and turbulence characteristics over complicated topographies, it is most interesting to be able to explore the capabilities offered by physical modeling of the atmospheric boundary layer, either in a wind tunnel or in a water tank.

For electrical production from nuclear energy, the existing cooling systems release their heat into the atmosphere as a combination of latent and sensible heat, such devices need a very important water flow. And the increasing consumption of electrical power implies new elaborating of plants more powerful but not heavy in water because it becomes more and more difficult to find appropriate sites near river or sea. For this reason, the forthcoming cooling tower will have to emit only waste dry heat. Such released heat over a small area can be intense. Thus there is legitimate concern about the effects of such heat source on natural atmospheric processes and the possible initiation of atmospheric processes which otherwise would not occur.

In this paper, the theory of plume rise from stacks with scrubbers is critically evaluated. The significant moisture content of the scrubbed plume upon exit leads to important thermodynamic effects during plume rise which are unaccounted for in the usual dry plume rise theories. For example, under conditionally unstable atmospheres, a wet scrubbed plume treated as completely dry acts as if the atmosphere were stable whereas in reality, the scrubbed plume behaves instead as if the atmosphere were unstable. Even the use of moist plume models developed for application to cooling tower plume rise are not valid since these models employ (a) the Boussinesq approximation, (b) use a number of additional simplifying approximations which require small exit temperature differences between tower exit and ambient, and (c) are not calibrated to stack data.Although these two theories are often used to predict plume rise from stacks with scrubbers, both theories contain unacceptable assumptions. This paper details the invalid approximations made in each theory. The direction and magnitude of the important errors are estimated for these models when applied to scrubber stack plumes.

Because stable plumes tend to have less impact at ground level, the behavior of plumes from elevated sources in stable air has not attracted the same interest that plume behavior in unstable air has. However, some aspects of stable-plume behavior hold great interest. For example, concentrations in the plume remain high for long periods of time, which increases the importance of chemical reactions taking place among plume constituents or between constituents and the ambient air. The nature of the stable plume also has its effect during periods of fumigation, when the high concentrations within the plume are mixed to ground level by the onset of unstable conditions.