Air Dispersion Modeling: Non-Attainment Permitting

What Is Nonattainment Permitting Air Dispersion Modeling?

Nonattainment permitting air dispersion modeling is required for new or modified major stationary sources located in areas that do not meet the National Ambient Air Quality Standards (NAAQS).

When an area is classified as nonattainment, permitting authorities must evaluate whether emissions from a proposed project would worsen air quality or interfere with future attainment. Air dispersion modeling is used to predict ambient pollutant concentrations and support permitting determinations under Nonattainment New Source Review (NNSR).

Additional regulatory context is provided on our Nonattainment Area Standards page.

Types of Impact Analysis for Modeling

Air dispersion modeling for nonattainment permitting generally involves one or both of the following analyses, depending on project impacts and regulatory expectations.

1. 1. Source Impact Modeling (Project Sources Only) – Source Impact modeling evaluates emissions from the proposed new or modified project to determine its direct, localized air quality impact.
      1. Purpose: To determine whether the proposed project triggers the need for a more detailed cumulative air quality analysis
      2. Scope: Includes only emissions from new or modified units
      3. Threshold: Analysis may conclude if modeled concentrations are below applicable Significant Impact Levels (SILs) – Any pollutants that are non-attainment must be below their SILs
      4. Results: Used to determine whether the project causes a significant air quality impact that warrants further analysis
   2. Cumulative Impact Modeling – Cumulative impact modeling evaluates the combined impact of the proposed project and surrounding emission sources to assess compliance with air quality standards. This analysis would only be for pollutants that are in attainment for the facility’s area. Any pollutants that are non-attainment must be below the SILs.
      1. Purpose: To determine whether the combined emissions from the project and nearby sources would violate NAAQS
      2. Scope: Includes emissions from the proposed project, nearby industrial sources, and background concentrations (modeled and/or monitored)
      3. Threshold: Assesses total predicted concentrations against applicable regulatory standards
      4. Result: Determines whether the project would cause or contribute to an air quality standards violation
   3. Key Differences
      1. Source impact modeling is a screening step; cumulative impact modeling is a comprehensive regulatory assessment
      2. Source impact modeling evaluates only project emissions; cumulative modeling evaluates combined regional or localized impacts
      3. Cumulative impact modeling is generally required only if source impact modeling exceeds applicable SILs, subject to state-specific requirements
      4. Cumulative impact modeling is not used for non-attainment pollutants, since the background concentration alone violates the NAAQS

Note: Significant Impact Levels (SILs) are EPA-established screening thresholds for criteria pollutants used in air dispersion modeling to determine whether a proposed project requires cumulative air quality analysis under NSR and nonattainment permitting.

Requirements for Modeling Under Nonattainment Permitting

Air dispersion modeling is a required component of nonattainment permitting and must: 

• • Follow EPA’s Guideline on Air Quality Models
  • Demonstrate that emissions increases will not cause or contribute to a violation of NAAQS
  • Support determinations related to Lowest Achievable Emission Rate (LAER)
  • Provide regulator-ready documentation suitable for review and compliance certification

State Regulations for Nonattainment Dispersion Modeling

All states with EPA-delegated air permitting authorities require air dispersion modeling for nonattainment permitting. These requirements are federally mandated under the Clean Air Act and implemented through each state’s State Implementation Plan (SIP).

While the EPA framework is consistent nationwide, state-specific requirements may vary with respect to:

• • Model selection and configuration
  • Treatment of background concentrations
  • Receptor placement and modeling domain
  • Expectations for cumulative impact analysis

Air dispersion modeling may be required under multiple permit types, including Nonattainment NSR, PSD, and Title V, depending on project characteristics. SEC helps navigate these state-level differences efficiently and provide support tailored to your facility.

Staying Ahead of Nonattainment Permitting Modeling

Nonattainment designations and regulatory requirements can change over time, increasing permitting risk for future projects. Early engagement and proactive air dispersion modeling can help facilities:

• • Identify potential permitting constraints before project submittal
  • Reduce agency review timelines
  • Support long-term compliance planning under evolving SIP requirements
  • Protect attainment areas from adverse air quality impacts

SEC delivers expert, regulator-ready modeling that supports permit approvals and long-term operational success and flexibility.