Plume dispersion model

FIG. 26-54 Horizontal dispersion coefficient for Pasquill-Gifford plume model, Reprinted ffomD. A. Ct owl and J. F. Louvar, Chemical Process Safety, Fundamentals with Applications, Z.9.90, p. 138. Used hy permission of Ft entice Hall)... 

Air Pollution Dispersion Application of air dispersion modeling principles and EPA tools to assessing environmental impacts from stack and area releases of pollutants Dispersion theory Gaussian plume model Ground-level concentrations Worst case scenarios Air quality impact assessments Stationary source emissions... 

The earliest and still widely used dispersion model to compute pollutant concentration profiles is the Gaussian plume model for single or multiple source pollution problems. Box-type model techniques, which can take into account nonlinear interactions among different species arising from chemical reactions, have been used in longer-range dispersion computations. 

The Gaussian Plume Model is the most well-known and simplest scheme to estimate atmospheric dispersion. This is a mathematical model which has been formulated on the assumption that horizontal advection is balanced by vertical and transverse turbulent diffusion and terms arising from creation of depletion of species i by various internal sources or sinks. In the wind-oriented coordinate system, the conservation of species mass equation takes the following form ... 

Software for dispersion modeling uses Gaussian plume model. Phe system calculates concentration or deposition values for inputed time periods. May be used in conjunction with "Breeze Air."... 

The jet-plume model only simulates vertical jets. Terrain is assumed to be flat and unobstructed. Application is limited to surface roughness mush less than the dispersing layer. User experti.se is required to ensure that the selected runtime options are self-consistent and actually reflect the physical release conditions. Documentation needs improvement there is little guidance... 

The modeling package, delivered to the EPA, includes nationwide data bases for emissions, dispersion meteorology, and population patterns. These data are used as input for a Gaussian plume model for point sources and a box model for urbanwide area sources. Prototype modeling is used for point sources that are too numerous to define individually. Building wake effects and atmospheric chemical decay are addressed. 

Two types of neutrally buoyant vapor cloud dispersion models are commonly used the plume and the puff models. The plume model describes the steady-state concentration of material released from a continuous source. The puff model describes the temporal concentration of material from a single release of a fixed amount of material. The distinction between the two... 

Figure 5-10 Dispersion coefficients for Pasquili-Gifford plume model for rural releases.

Figure 5-11 Dispersion coefficients for Pasquill-Gifford plume model for urban releases.

Passive puff or plume In addition to the restriction on plumes discussed above, there is an along-wind dispersion time scale given by td = 2Gjur where Gx is evaluated at the endpoint distance xe. The release can usually be considered a plume if ts > 2.5 fd, where ts is the source time scale defined above, and the release can be considered a puff if td > ts. For td< ts< 2.5 td, neither puff nor plume models are entirely appropriate the predicted concentration is considered the largest of the puff and plume predictions. 

The dispersive contributions due to plume spreading (given by and Z ) are statistically independent of the contributions due to plume meandering (given by 5 and 5 ). The fluctuating plume model is depicted in Fig. 3. 

Weber, A. H. (1976). Atmospheric Dispersion Parameters in Gaussian Plume Modeling, EPA-600/4-76-030A. U.S. Environ. Prot. Agency, Washington, D.C. 

Pheromone propagation by wind depends on the release rate of the pheromone (or any other odor) and air movements (turbulent dispersion). In wind, the turbulent diffusivity overwhelms the diffusion properties of a volatile compound or mixture itself. Diffusion properties are now properties of wind structure and boundary surfaces, and preferably termed dispersion coefficients. Two models have dominated the discussion of insect pheromone propagation. These are the time-average model (Sutton, 1953) and the Gaussian plume model. 

The Gaussian plume model estimates the average pheromone flux by multiplying the measured odor concentration by mean wind speed, using the following formula (Elkinton etal, 1984). Everything is the same as in the Sutton model, except that ay and az, respectively, replace the terms Cy and Cz of the Sutton model. Dispersion coefficients are determined for each experiment separately. 

In this expression, 3 represents the increase factor of vertical diffusion due to the plume. Gaussian plume or dispersion models are based on standard deviations of the plume dimensions (crx, cry, oz). These represent a measure of the diffusive capacity of the atmosphere. They are dependent on the turbulence conditions of the atmosphere, the vertical temperature gradient (which helps to establish atmospheric turbulence in the vertical direction) and the transporting distance. 

Consequence modeling, for the purposes of the illustrations given in this chapter, means the prediction of ambient atmospheric concentrations using models for quantifying the release of fluids from containment, and the formation of vapor and liquid aerosol plumes using dispersion models. 

One commonly used suite of models that is based on Gaussian plume modeling is the Industrial Source Complex (ISC) Dispersion Models (US EPA, 1995). This suite includes both a short-term model (ISCST), which calculates the hourly air pollutant concentrations in an area surrounding a source, as well as a long-term model (ISCLT), which calculates the average air pollutant concentrations over a year or longer. ISCLT uses meteorological data summarized by frequency for 16 radial sectors (22.5° each) this data format is referred to as a stability array (STAR). Within each sector of STAR, joint frequencies of wind direction, wind speed, and atmospheric stability class are provided. 

The emphasis in chemical agent modeling appears to be on using plume models to predict the spread and concentration levels of a chemical release. However, the accuracy of such predictions is highly dependent on knowledge of the precise location and magnitude of the chemical release, the physical characteristics of the plume (e.g., the initial particle-size distribution), and detailed knowledge of the stochastic nature of local atmospheric dispersion. In reality, these parameters are likely to be poorly known in any cleverly executed asym-... 

Use the Gaussian dispersion equation for the estimation of downwind ambient particulate concentrations. To account for particle settling, a titled plume model may be used, in which H is decreased to account for the vertical settling motion of the particles over a travel distance x. 

Brown, M.J., Arya, S.P., and Snyder, W.H., 1992. Vertical dispersion from surface and elevated releases an investigation of a non-Gaussian plume model, J. Appl. Meteorol., 32, pp. 490-505. 

Sykes, R.L, Lewellen, W.S., and Parker, F.S., 1986. A Gaussian plume model of atmospheric dispersion based on second order closure, J. Climate. Appl. Meteorol. 25, pp. 322-331. 

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