AERMOD - Friendly Air Quality Modelling

My favourite air quality model to use in Alberta is AERMOD.  Why?  This Gaussian Plume model is cost effective to use, well understood, up to date, scientifically defensible and generally accepted throughout the world.  As an environmental consultant, I use this model for assessing near-range air pollution impacts 20 km or less from the air emissions sources under scrutiny.

Example of dispersion output drawing where black curves are isopleths of equal concentration (similar to elevation contours joining points with equal elevations.)

This is where all the action occurs in most Alberta industrial facilities. How so?  Except for the major cities and big complexes such as the oil sands, sites in Alberta are generally quite isolated, with the most important sources concentrated into a small footprint.

A large plus for this model is its relative ease of use; a model run can be executed on an office desktop or laptop PC and it usually takes less than a few hours for the run to finish.  The model methodology and results can be shown to our regulatory agencies such as Alberta Environment and Sustainable Resource Development, the Alberta Utilities Commission or the Alberta Energy Regulator for straightforward communication of predicted air quality impacts.  

Meteorology and Input Data used in AERMOD

The set of weather conditions reported for each hour is assumed to remain constant from the time of plume release to time of ground impact. We refer to this property as "steady-state", and this assumption is normally valid over short distances. The AERMOD environmental software works best when:

• chemical transformations after release are handled in a simple fashion if needed at all,
• terrain rises and falls gently with increasing distance from the sources,
• the weather at any time is identical across the modelling domain and
• winds are rarely calm.

This describes most of Alberta with the exception of the mountainous regions. Basic meteorological and topographical data (required for modelling) are readily available for most areas of the globe and Fifth-Generation Penn State/NCAR Mesoscale Model (MM5) data is preferred for our assessments, as outlined in the Alberta Air Quality Model Guideline.

This model also tends to overpredict concentrations arising from relatively short stacks, thus offering a degree-of-safety in assessments that are used for regulatory purposes.

AERMOD integrates the six distinct stability classes into a continuum, incorporating a planetary boundary-layer similarity theory to produce turbulence and dispersion coefficients used for internal calculations.  When atmospheric conditions become unstable, non-Gaussian dispersion coefficients take over and produce more accurately predicted concentrations in the vicinity of the stacks than would be indicated otherwise during periods with strong vertical mixing.  

What about more complicated facilities?  

The use of Plume Rise Model Enhancements (PRIME) with output from the Building Profile Input Program (BPIP) further increases close range accuracy and is especially designed for plant sites with buildings located among the stacks.  

This model was developed in the 1990's and has replaced the long-standing model of choice, Industrial Source Complex (ISCST3), in the US Environmental Protection Agency (EPA) and other regulatory environments over the last ten years or so.  My assessments can provide calculations for many sources, buildings and receptors simultaneously, enough to efficiently provide a thorough study of the area.

You will probably find an AERMOD modelling assessment to be the most effective and efficient solution to your regulatory air dispersion modelling needs.  Please call Barry J. Lough at Calvin Consulting Group Ltd. at 403-547-7557 for help with any questions you might have.

Thank you.