Atmospheric Dispersion Modelling
Dispersion modelling is a mathematical tool used to predict the effect of emissions to atmosphere, from stacks for instance, on ambient pollutant levels.
This information, together with knowledge of background pollutant levels from other sources, is used to assess the environmental impact of a particular process.
Examples of the use of such assessments include the preparation for the Environment Agency or local authorities of applications for process authorisation; calculation of stack heights; investigation of the potential impact of installing equipment to reduce atmospheric emissions from a process, or assistance with the attribution of specific pollution incidents to the source of the pollution.
Tata Steel Research, Development & Technology’s Swinden Technology Centre uses a modern, commercial dispersion modelling package - ADMS - based on research into the physics of the lower layers of the atmosphere.
A recent report commissioned by the Environment Agency recommended the use of ADMS or an equivalent model for the prediction of ground level pollutant concentrations under conditions which are likely to give rise to the greatest environmental impact.
Dispersion modelling is not an exact science, since atmospheric turbulence is unpredictable, but validation of the ADMS model against a number of sets of experimental data has given encouraging results; on average, the difference between predicted and observed maximum ground level concentrations was rarely greater than a factor of two for the conditions in the Environment Agency study.
This is good performance for a dispersion model and is better than achieved in the same study by the older, more empirically based models previously available.
The inputs to the dispersion model come under three headings:
- Source e.g. stack height, waste gas volume and pollutant emission rates
- Local environment, e.g. hills and buildings
- Weather, e.g. wind speed and direction, amount of cloud cover
Source data may be obtained from measurement exercises on existing plant or from design information for proposed new plants or equipment.
The local environment will be defined through the use of site plans and maps of the local area.
Where hills influence dispersion, digital height data from the Ordnance Survey may be used. The weather data may be entered for specific hours when an incident occurred, or long-term data may be obtained from the Meteorological Office to evaluate annual average concentrations