Airshed Models

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Modeling may be necessary to estimate the changes in ambient air quality, both locally and at a distance, caused by a particular set of emissions. Modeling can be appropriate for new plants and for modifications to existing plants. This note provides guidance on some models that may be useful in the context of typical World Bank Group projects.

Introduction

Air quality is an issue of increasing concern in many countries, and projects which either introduce new sources of emissions or are designed to reduce emissions both require careful analysis to quantify the effects as far as possible. For many sources this will typically require mathematical modeling of the changes in ambient concentrations which result from the new emissions. There are a small number of widely used models and these are reviewed in this note.

Air quality modeling can be a complex task and the objectives need to be clear. The costs of a study can range from $10,000 to $500,000, depending on the complexity of the situation and the level of detail required, but there are many cases where costs are at the lower end of this scale. The simplest approach uses a point source dispersion model which estimates the ground-level concentrations of pollutants of interest at some distance (typically from hundreds of meters to tens of kilometers) from a point source. More complicated models allow the examination of multiple sources, including area sources (i.e. non-point). For an area containing a number of point and non-point sources, an air quality model could be constructed which includes all of the sources in the area, but in practice such models are rare because of the costs of development and the data required to make the model a realistic tool.

This document examines the most commonly used air quality dispersion models as they may be used for assessing the impact of key pollutants (SO2, Nox and particulates) from point sources on air quality. (Far-field dispersion and acid rain deposition are governed by different principles and utilize different types of models which are discussed elsewhere in this Handbook.)

Although thermal power plants are often singled out as major polluting sources, nearly all industrial facilities, especially those with short stacks, have the potential to cause localized areas of unacceptable air quality. In addition, urban areas can act as diffuse sources of air pollution, particularly where poor quality fuels are burned in household stoves. Cases with multiple point sources or with area sources (or both) can often be modeled using simplifying assumptions or by integrating the impacts of individual sources.

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