Air Dispersion Modeling: New Source Review (NSR)

Attainment areas are regions that meet the National Ambient Air Quality Standards (NAAQS). In these areas, air dispersion modeling is a key component of Prevention Significant Deterioration (PSD) permitting under the New Source Review (NSR) program. The objective of PSD air dispersion modeling is to demonstrate that emissions from new major sources or major modifications will not significantly degrade existing air quality.

Under PSD, air dispersion modeling is used to:

• • Predict ambient concentrations of regulated pollutants
  • Demonstrate compliance with NAAQS, which is based on predicted ambient concentrations, plus representative background concentrations
  • Evaluate PSD increment consumption
  • Support Best Available Control Technology (BACT) determinations

EPA-approved models such as AERMOD are typically used with site-specific emissions data, meteorology, terrain, and receptor grids to assess project impacts and support permitting decisions.

In nonattainment areas, where one or more NAAQS are not met, air dispersion modeling plays a critical role in supporting Non-Attainment New Source Review (NNSR) permitting. Dispersion modeling is utilized to evaluate how emissions from new sources of major modifications may affect existing air quality conditions and to support regulatory determinations.

Under NNSR, air dispersion modeling is used to:

• • Predict ground-level concentrations of criteria pollutants at nearby receptors
  • Evaluate whether a project would cause or contribute significantly to air quality impacts
  • Support permitting determinations tied to Lowest Achievable Emission Rate (LAER) controls
  • Inform emission offset strategies by quantifying project impacts

Unlike attainment modeling, NNSR Modeling:

• • Is applied in areas where one or more National Ambient Air Quality Standards (NAAQS) are not met
  • Focuses on evaluating whether emissions from new sources or major modifications would cause or significantly contribute to existing air quality violations or interfere with progress toward attainment
  • Does not include PSD increment consumption analyses, as nonattainment areas do not have an allowable air quality increment to preserve
  • Is commonly paired with emissions-based demonstrations and emission offset requirements
  • Relies on conservative modeling assumptions to support regulatory review and public participation

To learn more about NNSR regulations and air permitting requirements, visit our NNSR page here.