ABSTRACT
The objective of modelling is to use a tool that can reproduce some aspect of an environmental system with reasonable accuracy. Many physico–chemical processes can affect flow and transport, and not all of them may be well known. Models vary considerably in their complexity, and may take account of any of the following processes taking place in the atmosphere. Many of the terms introduced here will be developed in later chapters.
Emissions — type and scale.
Plume rise — associated with assisted draught or buoyant emissions.
Transport (advection) by the wind.
Vertical and/or lateral and/or horizontal diffusion caused by turbulence.
Reflections from the ground and/or from the top of the mixing layer.
Transformations of pollutants by physical or chemical processes.
Gravitational settling of particulate matter.
Dry deposition, ie uptake of substances on to soil and other surfaces.
Wet deposition such as rain out and wash out of substances in clouds, rain, snow and fog etc.
Topographical effects through variations in land surface and entrainment in the lee of buildings.
Variations in diffusion with height.
Variations in emissions and meteorology over time and distance.
Random variations (ie stochastic processes).
Mathematical expressions can be derived to represent diese atmospheric processes. Consequently, there is an enormous range of possible atmospheric dispersion models available for the prediction of air quality. However, the environmental characteristics may not be well established, and so the conceptualization is often difficult and hard to systematize.