Air quality modelling

Modelling is a set of scientific methods used to represent the complicated processes that govern meteorology and the behaviour of pollutants in the atmosphere.

Modelling can identify the sources that contribute to poor air quality and can project air-quality changes for different 'what if' scenarios.

For example, what would happen to air quality if a new highway, industrial or housing development were built? Modelling can help us make decisions on how to maintain and improve air quality.

The provincial government's involvement in air quality modelling includes:

• Conducting modelling to support air quality management decisions

• Providing modelling guidance

• Conducting reviews and making decisions on the acceptability of dispersion modelling submitted by consultants to the Ministry of Environment and Climate Change Strategy in support of permit applications and environmental assessment

• Collecting meteorological and air quality data in support of air quality modelling

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Dispersion modelling

An air quality dispersion model is a series of equations that mathematically describe the behaviour of pollutants in the air.

It provides a cause and effect link between the emissions into the air and the resulting air pollution concentrations.

Dispersion models are used in different applications, but have traditionally been used for air quality assessments in support of decisions regarding approvals and permits for regulated sources.

This 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 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 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. All models are a representation of reality, which means we cannot expect a model to provide perfect results. In some situations, the model may predict the actual concentrations very well, but not in others.

There are many reasons for this. Since our understanding of the details is not complete, the equations that describe this process are not complete. Models require inputs (emissions, weather and geography). If these inputs are of poor quality, then the model output will also be of poor quality.

Weather information is a critical model input. This is available from the many meteorological stations operated throughout the province by the Ministry of Environment and Climate Change Strategy, 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:

• The correct model is selected for a given situation
• All the inputs have been checked for completeness and accuracy
• 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 and Climate Change Strategy 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.

Dispersion modelling guidelines

Given the importance of dispersion models and their popularity, the B.C. government has developed several documents that can be used to guide their use in air quality analyses. These documents were developed in consultation with dispersion modelling experts.

Taken together, these documents help establish best practices and promote consistency and clarity of modelling requirements in B.C.

They were produced to help ensure that dispersion modelling studies developed for the B.C. government are:

• Appropriate for the needs of the application
• Correctly and consistently applied
• Appropriate for producing information to support air quality management decisions

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Guideline Documents and Tools for Air Quality Modelling in B.C.

Document or Tool	Description and Usage
A Primer on the British Columbia Air Quality Dispersion Modelling Guideline (PDF, 485KB)  Updated: October 2021	Provides background information on air quality dispersion and modelling concepts and additional information about the modelling guidelines. This is a useful document for all audiences.
B.C. Air Quality Dispersion Modelling Guideline (PDF, 2MB) Updated: August 2022	Provides recommendations on dispersion models and their application. This document is intended for technical experts who run dispersion models and interpret their results.
Nitrogen Dioxide Modelling Guidance (PDF, 1MB)  Updated: August 2022	Must be used alongside the B.C. Air Quality Dispersion Modelling Guideline when modelling nitrogen dioxide (NO2) concentrations from emissions of nitrogen oxides (NOx).
Dispersion Modelling Plan Template (DOCX, 53KB)  Updated: August 2022	Qualified Professionals undertaking a dispersion modelling assessment should submit a Dispersion Modelling Plan to the B.C. Ministry of Environment and Climate Change Strategy. A Dispersion Modelling Plan can be filled in electronically and emailed or printed and filled in manually, then faxed or mailed to the ministry.
Lower Fraser Valley Photochemical Modelling Guideline (PDF, 1.5MB) Published: August 2022	Provides recommendations on photochemical modelling techniques for estimating impacts of project emissions on secondary pollutants (such as, ground-level ozone and secondary particulate matter). This document is intended for Qualified Professionals who have expertise in conducting photochemical models and interpreting their results.
Weather Research and Forecasting Model (WRF) interactive mapping tool	An interactive mapping application to access WRF data for modelling application in B.C. For more information, visit Weather Research and Forecasting Model Data (WRF).
CALNO2 Post-processor for Nitrogen Dioxide Modelling	A post-processor for the CALPUFF system to calculate nitrogen dioxide (NO2) concentrations based on modelled nitrogen oxides (NOx) using either the Ambient Ratio Method (ARM) or the Ozone Limiting Method (OLM). Visit CALPUFF Modelling System to download this tool and its user guide.