There are seemingly constant changes with air dispersion modeling and in advance of the release of the new modeling guidance from the Minnesota Pollution Control Agency (MPCA) this spring, the MPCA held their annual winter modeling seminar earlier this January, inviting stakeholders to discuss recent updates to modeling practices with the modeling and permitting staff at the agency. This article will discuss recent changes related to air dispersion modeling.
Appendix W Final Rule
One major change in 2025 is the new final rule for the Guideline on Air Quality Models, Appendix W to 40 CFR Part 51 published on November 20, 2024. The revised Appendix W provides the EPA-preferred models and other recommended techniques for use in conducting air dispersion modeling demonstrations. The effective date of the Appendix W revision was March 21, 2025.
The major revisions in Appendix W include:
- Addition of a new Tier 3 detailed screening technique for NO2 (Generic Reaction Set Method, GRSM),
- Incorporation of COARE algorithms into AERMET to model marine boundary layer environments for offshore sources,
- Addition of RLINE as a near-roadway mobile emissions source, and
- Refining recommendations on how to determine the appropriate background concentration for cumulative impact modeling.
GRSM differs from the other Tier 3 options by adding reaction rate based on solar radiation and travel time from an emission source to a receptor. Trinity has done case studies of GRSM’s performance and concluded that GRSM resulted in lower NO2 concentrations when compared other Tier 3 methods.
RLINE is designed to be a more efficient way to model mobile source emissions, however, EPA has indicated that RLINE could also be appropriate to model industrial haul roads. RLINE has some advantages in modeling road sources, when compared to area and volume sources, as it uses an algorithm to integrate the contribution of point sources along a line to determine concentration. Trinity has done case studies of RLINE and determined that RLINE resulted in lower concentrations compared to volume, area, and line sources.
Appendix W now recommends developing a representative background concentration to include the impacts of nearby sources and other sources. Previously, Appendix W recommended the use of a “concentration gradient” to define nearby sources that should be included in a modeling demonstration. However, the EPA removed this term due to the lack of definition of the term and its inconsistent usage. Now, the recommend methodology is to first determine a background concentration that would be representative a facility’s location and nearby sources and then explicitly modeling a few sources. It’s likely this will result in fewer nearby sources needing to be included in a modeling analysis.
For more detailed information on the updates to Appendix W, the EPA gave a webinar presentation and
slides are available on their website. The MPCA recommends consulting with the agency before using these new methods in a modeling demonstration and has been noncommittal on their stance of these items.
NAAQS Updates
On December 10, 2024, the EPA revised the secondary National Ambient Air Quality Standards (NAAQS) for sulfur oxides (SOX) and retained the existing standards for nitrogen oxides (NOX) and particulate matter (PM). The form of the new SO2 secondary NAAQS changed from a 3-hour standard to an annual standard, which is now 10 parts per billion (ppb), averaged over three consecutive years. At the same time, the EPA released a supporting memo on an “alternative demonstration” to show compliance with the new standard, which implies that compliance with the 1-hour NAAQS is a reasonable surrogate for compliance with the annual NAAQS. The standard became effective on January 27th, 2025. The MPCA emphasized that this alternative demonstration is not currently MPCA’s policy, but the agency will evaluate the memo when an SO2 modeling demonstration using the methodology is submitted and may allow it to be utilized to streamline an SO2 analysis.
Separately, in 2024, the EPA also revised the primary annual PM2.5 NAAQS from 12 μg/m3 to 9 μg/m3 and the recommended significant impact level (SIL) from 0.3 μg/m3 to 0.13 μg /m3. While the new NAAQS is in effect, the MPCA clarified that the previous SIL of 0.3 μg/m3 will be listed in the upcoming modeling guidance that is to be issued this spring. Given EPA only provides recommended values for the SIL, the MPCA does have the authority to retain the previous SIL.
New Air Dispersion Modeling as Part of Permitting Practice
The MPCA also had further discussions about the new permitting practice that was implemented in March of 2025. This practice establishes certain project emission increase thresholds that trigger modeling and covers all pollutants with ambient air quality standards except carbon monoxide (CO) and ozone. The agency provided the regulatory basis for the new permitting practice at the modeling seminar and why the new practice will still be protective of the NAAQS even if not all projects will need to show compliance with ambient air quality standards via air dispersion modeling.
In the new practice, projects that qualify for a minor or lesser amendment or have project hourly emission increases below the new thresholds do not, in general, need to go through an air dispersion modeling exercise. However, this practice does not supersede existing remodeling requirements in a facility’s permit. The agency has said that it will consider removing these remodeling requirements on a case-by-case basis, but that it would require a major amendment to do so. Additionally, the MPCA retains the right to require air dispersion modeling for any project on a case-by-case basis.
In the event that project hourly emissions increases are above the listed threshold, an air dispersion modeling analysis is required. Alternatively, the applicant can submit an applicability determination request to justify why air modeling should not be required. The air dispersion modeling can be completed via AERSCREEN or a more refined modeling analysis utilizing AERMOD.
Table 1. Pollutant Thresholds
| Pollutant | Threshold (lb/hr) |
| PM10 | 3.42 |
| PM2.5 | 2.28 |
| NOX | 9.13 |
| SO2 | 9.13 |
| Lead | 0.11 |
Source: CH-19 Permit Action Emissions Screening Form
This practice was effective as of March 3, 2025 and there are new forms available (CH-19, CH-20) that will be used to determine whether a project requires an air dispersion modeling demonstration.
If you have any questions regarding air dispersion modeling in Minnesota, please contact in Trinity’s Minneapolis office.
