“Remember that all models are wrong: the practical question is how wrong do they have to be to not be useful”
- George E.P. Box – 1987
22.9.1 Reason for Air Modeling
I have never seen anything written confirming this, but a former state air permit program manager told me that environmental regulatory agencies typically assess impacts of hourly emissions through air modeling or process weight rates (Section 1.10.3). The purpose of the air modeling is to demonstrate compliance with the National Ambient Air Quality Standards (NAAQS) or state ambient air quality standards[1]. The Environmental Protection Agency (EPA) and state regulatory agencies have established maximum acceptable concentrations of pollutants in the ambient air (i.e., outside the fence-line). When an environmental regulatory agency requires air modeling, you must prove that the facility emissions or construction project emissions will not result in concentrations of pollutants above the National Ambient Air Quality Standards (NAAQS). The air modeling puts a different constraint on your operations: Air construction and operating permits typically constrain the quantity of annual emissions in tons per year, but the air modeling results may constrain the hourly emissions.
22.9.2 Conducting Air Modeling
The specific air model calculations are complex, but the air model does the hard work for you. The parameters used in the calculations are the crucial factors which include the following:
- The state regulatory agencies generally require you to use the highest hourly actual emission rates from each emission unit 24 hours a day, 365 days a year no matter how unrealistic this might be.
- If you use lower emission rates, the regulatory agencies will likely put the lower emission rate as a limit in your air permit.
- If you cannot feasibly run two emission units simultaneously, you typically use the emission unit with the higher emissions, but you might need to run both to show which is worse.
- The state regulatory agencies use these hourly emission rates even extrapolating this to establish an annual emission rate.
- Any limitations you take on these emissions will be reflected on your permit.
- If you set up the air model so you only operate during certain hours of the day, seasons of the year, etc., this will likely end up being a requirement in your air permit.
If air modeling is required, you will likely need to complete it before you can receive your air construction permit and begin construction. To avoid potential issues, you may want to conduct preliminary air modeling to determine if your project might cause an exceedance of National Ambient Air Quality Standards (NAAQS), which can provide you an opportunity to modify your project prior to submitting the air model results to an environmental regulatory agency.
If you think you might have to model, there are some general methods to lessen the impact:
- Try to keep the emission units far from the property line and/or try to have them closer to the prevailing wind side (i.e., so the wind generally blows emissions into the middle of our property)
- If you have stacks, try to have them exhaust vertically without caps to allow the momentum of the exhaust to elevate the emissions where they will dissipate before reaching ground or near-ground level.
- Avoid or limit buildings or structures downwind of emission sources since the emissions tend to accumulate at the downwind base.
22.9.3 Addressing Air Model Results
The results of the air model do not always make sense. This is partly due to the use of unrealistic emission factors, but also due to the complexity of the air model, which uses hourly varying wind conditions and takes into account the wind blowing around buildings and other structures. The results can also be impacted if you have to include background concentrations of pollutants and emissions from nearby sources.
The key to understanding the results is to run a culpability analysis which identifies the contribution of each emission unit to the off-site concentrations. If you are not passing the air model, you can use the culpability analysis to determine which emission units are contributing the most to the off-site exceedance and may need to have lower emissions (e.g., addition of a control device, limits on hours) or modifications to the emission point (e.g., raising up the stack, removing caps on the stacks, etc.). As I mentioned before, this may not make sense in the actual world (e.g., actual emissions may not appear to line up with the modeled emissions).
22.9.4 References
[1] “Air Topics” Environmental Protection Agency website (http://epa.gov/air/criteria.html) accessed August 7, 2022.
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