Ontario's air pollution regulation, 419/05, uses an American computer model to
estimate how much air pollution from a stationary source reaches a
point of impingement, usually the property boundary. This model, AERMOD, is used to determine whether
organizations are complying with air emission limits in the
regulation, and in their Environmental Compliance Approvals.
Like all computer models, AERMOD is imperfect, and it has been
designed to be very conservative, that is to overestimate the
emissions of individual facilities. In December, the US
Environmental Protection Agency released an update to the model so
that it more accurately reflects weather conditions as night turns
to day and vice versa. The result is a dramatic reduction in
the predicted air impacts of many facilities: 40 to 50% or
more. This may have a significant impact on what organizations have
to spend to reduce their emissions. Many Ontario businesses are
facing significant new air limits under
