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PSD / NSR Modification Calculator

Screen process modifications against PSD significance levels per 40 CFR 51.166

Free PSD/NSR modification screening calculator. Compare baseline emissions against projected post-modification emissions for NOx, SO2, VOC, CO, PM10, PM2.5, and Lead. Checks each pollutant against federal PSD significance levels to determine whether a process change may trigger Prevention of Significant Deterioration or New Source Review.

Pro Tip: Baseline actual emissions should represent the average of the two highest consecutive years within the last five years. Do not use permitted limits or PTE as your baseline - regulators want actual throughput-based emission records. If your records are incomplete, stack test data multiplied by actual production rates is the next best option.

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PSD / NSR Modification Calculator
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How It Works

  1. Enter Baseline Emissions

    Input current actual emissions for each pollutant in tons per year. Use the average of the two highest consecutive years from the past five years of operating records.

  2. Enter Projected Emissions

    Input expected emissions after the modification. Include effects of capacity increases, fuel changes, new controls, and any operational changes.

  3. Set Facility Context

    Select whether the facility is currently a major or minor source, and the area attainment status. Nonattainment areas have different significance thresholds for NOx and VOC.

  4. Review PSD Screening

    The calculator compares the emissions increase for each pollutant against federal PSD significance levels. Any pollutant showing a potential PSD trigger warrants a formal applicability review.

Built For

  • Plant engineers evaluating whether a capacity expansion triggers PSD review
  • Consultants screening process modifications for air permitting requirements
  • Environmental managers planning capital projects with emission implications
  • Operations staff assessing fuel switching or equipment replacement impacts

Assumptions

  • Emission increases are calculated as simple projected minus baseline values.
  • PSD significance levels are federal values from 40 CFR 51.166(b)(23).
  • Netting analysis is not performed; only the gross increase is evaluated.
  • State-specific significance levels may differ from federal values.

Limitations

  • Does not perform netting analysis with contemporaneous emission changes.
  • Does not evaluate NNSR thresholds for specific nonattainment classifications.
  • Does not assess de minimis exemptions or routine maintenance exclusions.
  • State-specific rules may impose different thresholds or calculation methods.

References

  • 40 CFR 51.166 - Prevention of Significant Deterioration of Air Quality
  • 40 CFR 51.166(b)(23) - Significant Net Emissions Increase Definitions
  • 40 CFR Part 52 - Approval and Promulgation of Implementation Plans
  • EPA New Source Review Workshop Manual (Draft October 1990)

Frequently Asked Questions

PSD significance levels are the emission increase thresholds that trigger Prevention of Significant Deterioration review under 40 CFR 51.166. If a modification at a major source causes a net emissions increase above these levels, the source must undergo PSD review, which requires BACT analysis, air quality modeling, and public notice. Key thresholds: NOx 40 tpy, SO2 40 tpy, VOC 40 tpy, CO 100 tpy, PM10 15 tpy, PM2.5 10 tpy.
Baseline actual emissions equal the average annual emissions from the two consecutive calendar years with the highest emissions during the five-year period immediately preceding the modification. For electric generating units, the lookback period extends to ten years. Use actual fuel records, production data, or CEMS data to calculate this value.
PSD (Prevention of Significant Deterioration) applies to major sources in areas that meet national ambient air quality standards (attainment areas). NSR (New Source Review) is the broader program that includes both PSD for attainment areas and NNSR (Nonattainment New Source Review) for areas exceeding standards. NNSR has stricter requirements including emission offsets and LAER instead of BACT.
Yes. Netting allows you to credit emission reductions from other contemporaneous changes at the same facility against the proposed increase. The netting period is generally five years. However, only federally enforceable reductions count, and the reductions must be surplus, quantifiable, permanent, and real. A successful netting analysis can avoid PSD review.
Disclaimer: This calculator provides preliminary screening estimates. Formal PSD/NSR applicability determinations require detailed engineering analysis, consideration of contemporaneous changes (netting), and coordination with your state or EPA regional office. This tool does not constitute legal or regulatory advice. Consult an air permitting specialist for formal determinations.

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