Dispersion Modeling Audits: The Audit Process

I do not go out to eat that much. The main reason is, eating a meal outside of my home, stresses me out. At home, I know where everything is. I know the format of how the food is going to be dispersed and served. I know where all the tools are. I know where everything goes. Having a level of certainty makes the whole process more relaxing.

Generally, knowing what to expect lessens the stress level.

Having a dispersion modeling analysis go through the review process can be stressful if you do not know what goes into the process, who the players are, or how it is conducted.

What is the Process?

In general, the dispersion modeling review process is pretty straightforward. There are usually just two parties involved: the applicant and the regulator.

The applicant has three items to take care of:

  • Show what was done;
  • Explain what was done; and
  • Demonstrate compliance with the air quality standards based on what was done.

The regulator has three complementary items of their own:

  • Confirm what was done by the applicant;
  • Verify that explanations by applicant match with what the applicant did; and
  • Determine if the demonstration of compliance is sufficient or not.

Having been on both sides of the equation, I am not sure which is more difficult, but one side can always make the other side more challenging if the roles are not strictly adhered to. If the applicant has done a thorough job and lays out all the information in a logical and organized fashion, the work and time of the regulator is substantially reduced.

What Are All The Parts?

The complexity of the dispersion modeling review is due to the many sources of data needed to make the demonstration of compliance. The data can be categorized into three areas:

Data From the Applicant:

  • Emission source data;
  • Emissions data; and
  • Building data.

Data From the Regulatory Agencies

  • Emissions inventory for offsite sources;
  • Ambient monitoring data;
  • Location of Class I areas; and
  • Location of nonattainment areas.

Data from other governmental agencies

  • Terrain data;
  • Land cover data; and
  • Meteorological data

All of these data need to be collected, compiled, analyzed, and manipulated into the appropriate input files for AERMOD, AERMAP, AERMET, AERSURFACE, AERMINUTE, and BPIPRM. It is a lot of different pieces going into a lot of other different pieces. In the end, the applicant has a complete air quality analysis (AQA) to ready to submit.

The AQA Audit

The audit of an AQA can be broken down into four distinct sections.

Data Consistency Check

Much of the same data appears in input file after input file. These data should be identical from file to file. For example, source location and exit parameters appear not only in multiple AERMOD input files, but will also appear in BPIPPRM and AERMAP input files, too.

The representation of the meteorological input will appear in multiple AERMOD input files. All representations should be the identical.

One item that is nearly always overlooked is the consistency of the time period of the meteorological observations (from the NWS) with the time period of the meteorological surface characteristics. If the year of the land cover data used for the surface characteristics was 1992, then the meteorological inputs may not be representative.

Data Accuracy Check

If the site under review is a greenfield site, there may not be much to check for accuracy. For an existing site, stack and building heights can be checked by measuring shadows. It is not easy, but it can be done. If the stack diameters are big enough and the aerial imagery is of high resolution, the diameters can be measured. Remember, you are not going to get an exact measurement, but if a height or diameter is significantly off, it will be noticed.

Receptors can be checked for accuracy, too. They are where they are, but is the elevation for that location correct. If an incorrect horizontal datum or datum transformation was used to convert coordinates, the locations can be off from 30-40 meters to over 200 meters. Depending on the project, that could be a show stopper.

Report Representations

All of the data, files, and other documentation used needs to match what is described in the AQA report. This can be one of the most tedious and time consuming part of the audit. When I mentioned how one side of the audit equation can either make things easier or more difficult for the other side, this is what I was referring to. If the information is presented in an orderly, logical, and readable fashion, the audit goes smoothly, quickly, and thoroughly.

The purpose of the report is to summarize, document, and explain. However, it must contain or refer to everything used in the analysis.

Demonstration Made or Not

Though it is a part of the AQA report, making the case that ALL standards and guidelines will not be violated is a separate and apart from the presentation of all the data and information used in the analysis. Most AQA reports that I have reviewed over the year stated THAT the demonstration was made. As a regulator, I am looking at HOW the demonstration was made. The role of the regulator is to decide whether the HOW is sufficient and then document their decision for the public record.


A dispersion modeling analysis has a lot of moving parts coming from several data sources. Also, multiple parties are involved in the audit process, and the smoothness of the process is contingent on the parties working cooperatively. If not, the audit process can drag on.

Regardless of how it is done, compiling an AQA or auditing an AQA and supporting documentation, it comes down to the quality and the clarity of the data. If the data are not reliable or available, then the whole audit process gets bogged down and opens one or both parties to potential liability due to lack of clarity.

If you found this article informative, there is more helpful and actionable information for you.  Go to http://learn.naviknow.com to see a list of past webinar mini-courses. Every Wednesday (Webinar Wednesday), NaviKnow is offering FREE webinar mini-courses on topics related to air quality dispersion modeling and air quality permitting. If you want to be on our email list, drop me a line at [email protected].

One of the goals of NaviKnow is to create an air quality professional community to share ideas and helpful hints like those covered in this article. So if you found this article helpful, please share with a colleague.


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