Air quality modeling guidance for permits

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 (April 2024).
• Permitting Atypical Event Policy Program Memorandum (April 2024).
• Additional Information on Modeling Determinations (February 2024).
• Permitting Section Addendum to the Modeling Guidance for Incorporation of Environmental Justice Requirements (July 15, 2023).
• Modeling Requirements Determination Form APCD-114 (December 14, 2023).
• 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.
  • Please see 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. Please see this document for 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.
• Permit Modeling Unit (PMU) Determinations.
  • Traditional Construction Permits
    • If an applicant is unsure if modeling will be required to conduct a required National Ambient Air Quality Standards (NAAQS) analysis, the APCD-114 Form (updated December 14, 2023) should be filled out to obtain a decision prior to submitting a permit application or a Modeling Analysis to the Air Pollution Control Division. Form Instructions are provided in the Modeling Requirements Determination Form APCD-114 Instructions. Additional clarification can be found in the Permitting Section Addendum to the Modeling Guideline.
    • Once a determination is made, the applicant will be provided documentation of the PMU Determination by the division. The APCD-114 Form and PMU Determination must be included in the permit application and will be part of the source or modification's permit application record. 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 completed APCD-114 Form and PMU Determination, which demonstrates NAAQS compliance, will be included as part of the source or modification’s permit application record and no further analysis will be required. 
  • New General Permit Registrations: GP01, GP02, GP05, GP06, GP07, GP08, GP09, GP10 and GP11
    • In order to evaluate if modeling will be required to conduct a required NAAQS analysis pursuant to the Colorado Minor NSR Source Modeling Guideline for Air Quality Permits (May 2023), the APCD-114 Form (updated December 14, 2023) should be filled out to obtain a decision prior to submitting a general permit registration or a Modeling Analysis to the division. The instructions are provided in the Modeling Requirements Determination Form APCD-114 Instructions. Additional clarification can be found in the Permitting Section Addendum to the Modeling Guideline.
    • Once a determination is made, the applicant will be provided documentation of the PMU Determination by the division. The APCD-114 Form and PMU Determination must be included in the general permit registration and will be part of the source’s permit application record. If a determination of “Modeling Not Required” is made, the completed APCD-114 Form and PMU Determination will be included as part of the source or modification’s permit application record and no further analysis will be required. If a determination of "Modeling Required" is made, 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, prior to submitting a general permit registration. The Modeling Review Report or determination of “Modeling Not Required” must be submitted with the General Permit registration for the registration to be 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.
• Title V permitting information can be found 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 Unit Impact Multiplier tables are a tool that applicants will be able to use in place of a model analysis to show compliance with the National Ambient Air Quality Standards, where applicable. The final tables will be provided with instructions, a calculation form and technical guidance.

The division is evaluating the use of the proposed Unit Impact Multiplier tables. Instructions on incorporating the tool within the permitting process will be released in fall 2024. 

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 cost 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

• Please 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)
  • 1990 "New Source Review Workshop Manual"
• 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.
• New: Meteorological Data Selector Tool (July 2023)
• Background concentration form (December, 2022)
• 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 Web sites 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
  • National Park Service Air
  • USDA Forest Service

Online Training

• OS411 - Computational Atmospheric Sciences