B.C. Air Quality

Dispersion Modelling

What is an Air Quality Dispersion Model?

An air quality dispersion model is a series of equations that mathematically describe the behaviour of pollutants in the air. It provides a cause-effect link between the emissions into the air and the resulting air pollution concentrations. Dispersion models have been used in many different applications, but have traditionally been used for air quality assessments in support of decisions regarding approvals and permits for regulated sources.

This simple but powerful ability means that dispersion models can be used to examine scenarios that would otherwise be too expensive, difficult or destructive to do in the real world. For example, models can be used to answer difficult questions such as:

  • Which emission sources are causing the air quality to be poor?
    A model can identify sources that are the greatest contributors to poor air quality, and thus provide air quality managers some idea of which sources should receive the highest priority to control.
  • Should a proposed factory be allowed to be built?
    A model can determine whether a proposed new source will cause an air quality problem, and thus help in decisions on project approval.
  • How close should a residential area be allowed to encroach on a chemical refinery?
    A model can delineate risk zones from an uncontrolled release of a poisonous gas and thus help in land use and planning decisions.

How Dispersion Models Work

Dispersion models are based on our understanding of how pollutants travel and disperse in the air. Equations, which represent the science of this behaviour, are used to calculate the downwind air concentrations that result from a source that emits something into the air.

Some models have basic equations, and other more complex models include more detailed equations. However, all models are a representation of reality (not reality itself); we cannot expect a model to provide perfect results. In some situations the model may predict the actual concentrations very well, and in others it may not.

The reasons for this are many. Since our understanding of the details of the behaviour of air pollution is not complete, it follows that the equations which describe this process are not complete. In addition, models require inputs (emissions, weather and geography). If these inputs are of poor quality, then the model output will also be of poor quality (i.e. the model will not magically correct poor input).

Weather information is a critical model input. This is available from the many meteorological stations operated throughout the province by the Ministry of Environment, as well from other sources such as Environment Canada airport sites.

Interpreting the output and assessing how much confidence decision-makers should place in the results involves considerable expertise and experience. Those involved in dispersion modelling need to make sure that the correct model is selected for a given situation, that all the inputs have been checked for completeness and accuracy, and that testing has been done to ensure that the model has be used correctly. Furthermore, modellers need to be skilled in providing guidance to decision-makers in how the output should be used.

Air quality data collected by the Ministry of Environment is used to determine how well a model performs by comparing model predictions to actual observations. This provides some level of confidence in the model, while helping to identify any parts of the model that need to be improved.

Guidelines for Dispersion Modelling

Given the importance of dispersion models and their growing use, the Government of British Columbia has developed the British Columbia Air Quality Dispersion Modelling Guideline in consultation with dispersion modelling experts. The Guideline provides recommendations on models accepted for use in BC and how they should be applied.

This will ultimately help in promoting consistency and establishing good practices so that models will be applied in a way that provides the information needed to inform decision-making. A primer is also available which is intended to help non-experts understand the key messages of the Guideline: A Primer on the British Columbia Air Quality Dispersion Modelling Guideline (PDF).


Related Links:

Reports and Publications — Modelling