1. Topic

2. Introduction

Models are used to estimate concentrations of pollutants at ground level in areas where there is insufficient monitoring. All models can only be as accurate are the data put into them, (see topic on What data is required by atmospheric dispersion models). By their very nature there are areas of uncertainty so they should be used as a guide but with caution.

3. Discussion

There are three main types of pollution source:

· Road traffic: Emissions from vehicle exhaust are a major contribution to urban pollution. In some areas, evaporative emissions of hydrocarbons e.g. from fuel tanks, may also need to be considered.

· Industrial, commercial and domestic emissions: These are controlled releases from stacks or chimneys.

· Fugitive emissions: These are leakages from industrial processes or particles from quarrying which are released in an uncontrolled fashion. These sort of emissions are very difficult to measure so may not be easily modelled.

The sources can be divided into four types of release to the atmosphere which can be used by the models:

· Point sources: These are individual chimney stacks. The simpler models can only deal with one at a time but the more advanced can model several simultaneously.

· Line sources: Traffic along roads are modelled as straight line segments. The simpler models will only deal with one road segment but the more advanced will deal with a whole city network with several hundred different sources each of which can have different vehicles or characteristics applied to them. Some models can deal with canyons bridges and underpasses.

· Area sources: A group of point or line sources can be treated as an area source. Fugitive emissions from and industrial area or car parks can be treated in this way. They could be used for modelling background sources across a city as a grid pattern, for example, the emissions from areas of domestic housing.

· Volume sources: These could include area sources with vertical depth, e.g. emissions from an airport taking into account aircraft take-off and landing.

Most atmospheric pollutants are released as buoyant gasses such as sulphur dioxide from stacks or oxides of nitrogen from vehicles. Most models calculate the dispersion of these hot plumes. Some of the more sophisticated models can take into account deposition of pollutants from the atmosphere by rain washout or the gravitational settling of particles. Some models are also designed to calculate the chemical reactions which may occur during transportation, e.g. NO to NO₂ and ozone photochemistry. These are known as secondary pollutants as they are formed in the atmosphere and not always at source. Pollutants such as NO₂ and PM₁₀ can have primary and secondary sources so a model may have to cope with these.

4. Recommendation / Conclusion

Models must be fit for purpose. Identify the pollutants of concern and the air quality objectives associated with them. Select a model which is appropriate. See Topic What data is required by atmospheric dispersion models.

5. Examples / Further Reading

6. Additional Documents / Web Links

· LAQM.TG1(00) - Review and assessment: Monitoring air quality http://www.defra.gov.uk/

· LAQM.TG2(00) - Review and assessment: Estimating emissions http://www.defra.gov.uk/

· LAQM.TG3(00) - Review and assessment: Selection and use of dispersion models http://www.defra.gov.uk/

· LAQM.TG4(00) - Review and assessment: Pollutant specific guidance http://www.defra.gov.uk/

Last Updated

25th January 2005