The dispersion models vary depending on the mathematics used to develop the model, but all require the input of data that may include:. The plots of areas impacted may also include isopleths showing areas of minimal to high concentrations that define areas of the highest health risk. The isopleths plots are useful in determining protective actions for the public and responders. The atmospheric dispersion models are also known as atmospheric diffusion models, air dispersion models, air quality models, and air pollution dispersion models.
Discussion of the layers in the Earth's atmosphere is needed to understand where airborne pollutants disperse in the atmosphere.
Guidelines for Use of Vapor Cloud Dispersion Models, Second edition
The layer closest to the Earth's surface is known as the troposphere. The lowest part of the troposphere is called the atmospheric boundary layer ABL or the planetary boundary layer PBL and extends from the Earth's surface to about 1. The air temperature of the atmospheric boundary layer decreases with increasing altitude until it reaches what is called the inversion layer where the temperature increases with increasing altitude that caps the atmospheric boundary layer.
The upper part of the troposphere i. The ABL is of the most important with respect to the emission, transport and dispersion of airborne pollutants. The part of the ABL between the Earth's surface and the bottom of the inversion layer is known as the mixing layer.
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Almost all of the airborne pollutants emitted into the ambient atmosphere are transported and dispersed within the mixing layer. Some of the emissions penetrate the inversion layer and enter the free troposphere above the ABL. In summary, the layers of the Earth's atmosphere from the surface of the ground upwards are: the ABL made up of the mixing layer capped by the inversion layer; the free troposphere; the stratosphere; the mesosphere and others.
Many atmospheric dispersion models are referred to as boundary layer models because they mainly model air pollutant dispersion within the ABL. To avoid confusion, models referred to as mesoscale models have dispersion modeling capabilities that extend horizontally up to a few hundred kilometres. It does not mean that they model dispersion in the mesosphere.
The technical literature on air pollution dispersion is quite extensive and dates back to the s and earlier. One of the early air pollutant plume dispersion equations was derived by Bosanquet and Pearson.
Sir Graham Sutton derived an air pollutant plume dispersion equation in [ 2 ] which did include the assumption of Gaussian distribution for the vertical and crosswind dispersion of the plume and also included the effect of ground reflection of the plume. Under the stimulus provided by the advent of stringent environmental control regulations , there was an immense growth in the use of air pollutant plume dispersion calculations between the late s and today. A great many computer programs for calculating the dispersion of air pollutant emissions were developed during that period of time and they were called "air dispersion models".
The above equation not only includes upward reflection from the ground, it also includes downward reflection from the bottom of any inversion lid present in the atmosphere. The sum of the four exponential terms in g 3 converges to a final value quite rapidly.
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The two most important variables affecting the degree of pollutant emission dispersion obtained are the height of the emission source point and the degree of atmospheric turbulence. The more turbulence, the better the degree of dispersion. The resulting calculations for air pollutant concentrations are often expressed as an air pollutant concentration contour map in order to show the spatial variation in contaminant levels over a wide area under study.
In this way the contour lines can overlay sensitive receptor locations and reveal the spatial relationship of air pollutants to areas of interest. Briggs first published his plume rise observations and comparisons in That was followed in by his classical critical review of the entire plume rise literature, [ 8 ] in which he proposed a set of plume rise equations which have become widely known as "the Briggs equations". Subsequently, Briggs modified his plume rise equations in and in Briggs considered the trajectory of cold jet plumes to be dominated by their initial velocity momentum, and the trajectory of hot, buoyant plumes to be dominated by their buoyant momentum to the extent that their initial velocity momentum was relatively unimportant.
Although Briggs proposed plume rise equations for each of the above plume categories, it is important to emphasize that "the Briggs equations" which become widely used are those that he proposed for bent-over, hot buoyant plumes. In general, Briggs's equations for bent-over, hot buoyant plumes are based on observations and data involving plumes from typical combustion sources such as the flue gas stacks from steam-generating boilers burning fossil fuels in large power plants. A logic diagram for using the Briggs equations [ 3 ] to obtain the plume rise trajectory of bent-over buoyant plumes is presented below:.
The above parameters used in the Briggs' equations are discussed in Beychok's book. Wikimedia Foundation. Bibliography of atmospheric dispersion modeling — Each of the books listed in this Bibliography of atmospheric dispersion modeling includes the author s , the publication date, the title, the edition, by whom published, and the ISBN or ISSN where available. List of atmospheric dispersion models — Atmospheric dispersion models are computer programs that use mathematical algorithms to simulate how pollutants in the ambient atmosphere disperse and, in some cases, how they react in the atmosphere.
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Check copyright status Cite this Title Guidelines for use of vapor cloud dispersion models. Center for Chemical Process Safety. Edition 2nd ed. Subjects Atmospheric diffusion -- Mathematical models. Hazardous substances -- Environmental aspects -- Mathematical models. Vapors -- Mathematical models. Contents Machine derived contents note: Preface. Background and Objectives. Input Data Required. Source Emission Models. Dispersion Models.
Evaluation of Models with Field Data. Summary of Seven Worked Examples. Appendix A.