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It is often necessary for operators of industrial processes, developers, or regulatory authorities to quantify the air pollution impact of an existing or proposed scheme (e.g. a power station or a road) for the purposes of licensing or planning. A predictive tool such as an atmospheric dispersion model can be useful in this respect, as it provides a means of calculating air pollution concentrations near ground level in the vicinity of the emitting source, given information about the emissions and the prevailing meteorology. It is the concentration of the pollutant near ground level that is important in determining whether there are any potential ill effects, e.g. on human health; air quality standards and guidelines are set in terms of concentration values, not source rates.The aim of this chapter is to provide details on what types of model are available, how they should be used, and their limitations. The information required to run dispersion models, the type of results that are produced, and how these can be interpreted are described. The case studies illustrate how the results of various dispersion models can be used in an air quality assessment and highlight some of the problems that are encountered in the modeling approach. A detailed technical description of the theory of atmospheric dispersion is provided in Chapter 10.Dispersion models take a number of forms, from simple nomograms and spreadsheets to sophisticated computer programs. It is important that the model is appropriate for the complexity of the study (DoE 1995a). Factors such as model accuracy and validation need to be taken into account (see Case study 5). It is also important to assess the availability of both meteorological and emissions data; some models do not require detailed meteorological data because they make broad assumptions about the prevailing local climate, while others may require quite detailed information on meteorology and emissions. EMISSION SOURCES RECOGNIZED BY ATMOSPHERIC DISPERSION MODELSThe three main sources of air pollution modeled are:1 Emissions from vehicle exhausts on roads, which generally make the greatest contribution to air pollution in urban areas.2 Controlled industrial, commercial, and domestic emissions from chimneys. 3 Fugitive emissions (e.g. leakages from industrial plant, or particulate matter from mineral extraction schemes; see Chapter 9). These sources may be divided into three main categories that are recognized by dispersion models: 1 Point sources. These are individual chimneys. The simpler models can treat only one stack at a time (EA 1998), though more sophisticated

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