7 - Atmospheric dispersion of pollutants

Summary

Summary

Atmospheric dispersion and chemistry is a complex subject, for which this chapter offers only a brief introduction, with focus on a special class of models that are appropriate for damage cost calculations. Such models can be relatively simple, because damage costs involve long-term averages over large areas. Gaussian plume models, suitable for the local zone, are described in some detail and equations are provided for a specific version to allow the reader to carry out calculations. Further from the source, the removal of pollutants from the atmosphere becomes important and is crucial for regional modeling. The removal rates can be expressed in terms of a velocity that we call the depletion velocity, a quantity that accounts for dry and wet deposition and, for reactive pollutants, chemical transformation. To illustrate key features of regional modeling, we develop a simple model and compare it with results from the EMEP model. We present several methods of estimating depletion velocities. We also develop a simple model for an approximate calculation of impacts and damage costs due to air pollution. It is called the “uniform world model” (UWM), because it is exact in the limit where the depletion velocity and the receptor density are uniform. We have validated the model by about 200 comparisons with detailed site-specific calculations using the EcoSense software of the ExternE projects in Europe, Asia and the Americas. For emissions from stacks of 50 m or more, detailed calculations agree with the simplest version of the UWM, within a factor of two in most cases. We provide modifications for site, stack height and receptor distribution that greatly improve the accuracy and applicability of the UWM. The UWM is very relevant for policy applications because it yields representative results for typical situations, rather than for one specific site.