On this page: Recent updates | General permitting questions | Colorado modeling guidance for air quality permits | Supplemental modeling guidance, maps, forms, and information | Unit Impact Multiplier tables | External Model Review Program | Major NSR source (PSD) modeling resources | Meteorological & geophysical data for air quality permit modeling | Federal modeling support | Online training
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 Dec. 1, 2024. After that date, permittees must submit APCD Form 114 using the new electronic submission process.
Recent updates
- 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.
- Permitting Clarification on PM2.5 Modeling Thresholds Memorandum (August 2024).
- Permitting Atypical Event Policy Program Memorandum (August 2024).
- Additional Information on Modeling Determinations (February 2024).
- Permitting Section Addendum to the Modeling Guidance for Incorporation of Environmental Justice Requirements (August 2024).
- Meteorological Data Selector Tool (July 2023).
- Guidelines for Disproportionately Impacted Community Source-Specific Monitoring Requirements (July 2024).
- For more information, visit the disproportionately impacted community permitting rule web page.
- See the Permitting Section Addendum to the Modeling Guidance for Incorporation of Environmental Justice Requirements (July 15, 2023) to determine the applicability of this requirement.
General Permitting Questions
- For general permitting questions, contact the Stationary Sources Program. Contact the Permit Modeling Unit at cdphe_apcd_modelingdeterminations@state.co.us or (720) 235-8143 for modeling questions and determinations.
- Modeling Procedure Guidance.
- Guidance on how to request modeling files and general permit modeling questions.
- Completeness Checklist for Modeling Submittals.
- FAQs on Air Quality Modeling Data and Techniques.
Colorado Modeling Guidance for Air Quality Permits
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.
Supplemental modeling guidance, maps, forms, and information
- Construction and Oil & Gas permitting information can be found on the APENs and air permits web page.
- Learn more about Title V permitting on the Title V operating permits web page.
- Air Pollution Emission Notice (APEN) and application forms.
- Air Quality Control Commission regulations.
- Colorado EnviroScreen: Environmental Justice.
- Federal Class I Areas (interactive map).
- Nonattainment/Maintenance Area Boundaries (interactive map).
- OnBase Public Access Search Tool.
- OnBase APCD Map Portal and OnBase Records Search Tool.
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:
- UIM instructions.
- Facility information form.
- UIM tables by Colorado region.
- Contractor technical guidance.
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
- Contact the division at cdphe_apcd_modeling_files@state.co.us if you have a PSD project.
- Checklist for Major NSR PSD Model Protocol (June 17, 1997) (PDF).
- Class II areas with the protection of Class I increments for sulfur dioxide (Figure 6 from Colorado Modeling Guideline May 2022) (image).
- Colorado Modeling Guideline for Air Quality Permits (May 2022).
- Please reference for Major PSD Source Modeling Requirements.
- Colorado PSD baseline areas and minor source baseline dates (PDF).
- Database of "scenic and/or important views" (March 11, 2005) (PDF).
- Federal Class I Areas (Figure 7 from Colorado Modeling Guideline May 2022) (image)
- New Source Review (NSR).
- Prevention of Significant Deterioration (PSD) Increment Tracking (February 26, 2001; updated 12/28/05).
Meteorological & Geophysical Data for Air Quality Permit Modeling
- Please follow the detailed instructions found in the Modeling Procedure Guidance to request ambient background concentrations, nearby source inventories and meteorological data. The division Permit Modeling Unit has data from current and historic meteorological towers in Colorado that are suitable for use in regulatory models. Contact the division at cdphe_apcd_modeling_files@state.co.us for all data requests. Division approval should be obtained on a case-by-case basis before datasets are used for regulatory (e.g., air permit) modeling.
- Meteorological Data Selector Tool (July 2023).
- Background concentration form (August 2024).
- Meteorological Determinations and Application Dispersion of Models.
- Nonattainment Area Map.
- Queried Sources of Air Pollution.
- Web based map tool for stationary sources of air pollution based on origin coordinates, radius, and pollutant. Map tool allows users to navigate to a point of interest, and then click to set the coordinates.
Federal Modeling Support
- U.S. EPA.
- U.S. EPA State Guidance.
- Support Center for Regulatory Atmospheric Modeling (SCRAM).
- EPA guidance, models, user guides, and related information.
- The primary EPA modeling guideline is Appendix W to 40 CFR Part 51.
- Technical Air Pollution Resources.
- A collection of related websites containing information about many areas of air pollution science, technology, regulation, measurement, and prevention.
- OAQPS Air Quality Modeling.
- Topics and areas associated with air quality modeling.
- Federal Land Managers.