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Air quality modeling guidance for permits

Important update: As of October 2024, permit applicants are encouraged to use the division’s new electronic submission process to submit Modeling Requirements Determination Form APCD-114.

As a courtesy, the division will continue to accept the Modeling Requirements Determination Form APCD-114 (August 2024) as a PDF through Jan. 6, 2025. After that date, permittees must submit APCD Form 114 using the new electronic submission process.

Recent updates

General permitting and modeling questions

Colorado modeling guidance for air quality permits

Colorado Minor NSR Source Modeling Guideline for Air Quality Permits - Draft open for public review through Dec. 8, 2024.

Minor update: Colorado Minor NSR Source Modeling Guideline for Air Quality Permits May 2023 (minor revision, Appendix C February 2024).

  • Additional minor update to Table 1: Modeling Thresholds, made April 2024.

 

Dispersion Modeling Unit (DMU) Determinations.

  • If an applicant is unsure if modeling will be required to conduct a required National Ambient Air Quality Standards (NAAQS) analysis, use the electronic submission process to complete the APCD 114 Form to obtain a decision prior to submitting a permit application or a Modeling Analysis. Instructions for filling out the new electronic form can be found in the guidebook. Additional clarification can be found in the Permitting Section Addendum to the Modeling Guideline. Please see below for specific instructions based on the type of permit being submitted.
  • If you are submitting an APEN for a construction permit or general permit with the electronic submission process, follow the below instructions on how to include the modeling determination as part of the APEN. The following APEN categories are currently available in the electronic submission platform. Others will be added over time.
    • Crematory Operations.
    • Surface coating operations.
    • Crushers/screens.
    • Fuel service stations.
  • Once the Modeling Determination Form is submitted via the electronic submission process, the applicant will be issued a Modeling Determination Number. While submitting the APEN, for any of the above listed categories, include the modeling determination number when asked for in the APEN. If the applicant believes that a modeling determination is not required for the project, mark that in the APEN instead of providing a modeling determination number. Attaching a copy of the completed Modeling Determination is not necessary, as the permit engineer will be able to find the Modeling Determination with the number provided.
  • The applicant will receive an email once the Modeling Determination has been reviewed and approved by the Permit Modeling Unit. This email will include a link to download the completed Modeling Determination and will inform the applicant if modeling is required for the project. If a determination of "Modeling Required" is made, the applicant must submit a Modeling Analysis demonstrating compliance with the NAAQS with the permit application. If a determination of “Modeling Not Required” is made, no further analysis will be required.
  • For all other types of applications that may require a modeling determination:
    • Once the Modeling Determination Form is submitted via the electronic submission process and the Modeling Determination has been reviewed and approved by the Permit Modeling Unit, the applicant will receive an email. The email will include a link to download the completed Modeling Determination and inform the applicant if modeling is required for the project. If a determination of "Modeling Required" is made, the applicant must submit a Modeling Analysis demonstrating compliance with the NAAQS with the permit application. If a determination of “Modeling Not Required” is made, the determination should be downloaded by the applicant and included as an attachment to the permit application. The completed APCD-114 Form and Dispersion Modeling Unit Determination, which demonstrates NAAQS compliance, will be included as part of the source or modification’s permit application record.
    • For applications requesting a General Permit where a determination of "Modeling Required" is made for a source registering to a General Permit, the applicant must submit a Modeling Analysis demonstrating compliance with the NAAQS to the division and obtain a Modeling Review Report, which is written approval of the Modeling Analysis, before submitting a general permit registration. The Modeling Review Report or determination of “Modeling Not Required” must be submitted with the General Permit application for the application to be deemed complete.

Unit Impact Multiplier tables

The new air modeling process and Unit Impact Multiplier (UIM) tables are finalized and available for use. The new process will allow specific air permit applicants to show compliance with the National Ambient Air Quality Standards (NAAQS), which are federal health-based standards for pollution in outdoor air. Permit applicants can use the UIM tables for air dispersion modeling requirements. The available resources include: 

External Model Review Program

The Permit Modeling Unit oversees the option for external third-party review services.
If there is a statutory backlog, an external independent contractor may be available upon request to review the air modeling associated with a permit application for a source not subject to Part C of the Clean Air Act (Prevention of Significant Deterioration of Air Quality). Colorado Senate Bill 22-1193, revised statutes 25-1.5-101-25-1.5-113, created the option to use external contractors if a modeling review backlog occurs within the air division.

An external modeling review process could be faster and would impose additional costs to the applicant. To learn more about the External Model Review Program, email cdphe_apcd_modeling_files@state.co.us.

Major NSR Source (PSD) modeling resources

Meteorological and geophysical data for air quality permit modeling

Federal modeling support