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Part 231 New Source Review for New and Modified Facilities

Part 231 implements the Clean Air Act's New Source Review program. It provides for the review of the air pollution impacts of new major stationary sources and modifications to existing major stationary sources in air pollution attainment and non-attainment areas of New York State (NYS).

New Source Review (NSR)

NSR is a permitting process that requires industry to undergo a pre-construction review to determine the appropriate air pollution controls. NSR consists of two components: attainment NSR (a.k.a. prevention of significant deterioration (PSD)) and non-attainment NSR.

Facilities Subject to Part 231

New major stationary sources and existing major sources which make modifications are required to determine their emissions of regulated air pollutants and, in the case of an existing air pollution source, compare those emissions post-modification to baseline emissions to determine the significance of the emission increase. Sources that make minor modifications may have reporting and recordkeeping requirements. Sources that make major modifications, in addition to reporting and recordkeeping requirements, will need to obtain a new or modified air pollution permit and apply pollution control equipment, and may need to obtain emission offsets. Requirements vary depending on whether the facility is subject to the "attainment" or "non-attainment" provisions of Part 231.

Attainment Contaminants

The attainment contaminants in NYS are sulfur dioxide, particulate matter, particulate matter less than 10 microns (PM10) (outside New York County), particulate matter less than 2.5 microns, carbon monoxide, oxides of nitrogen (NOx) (NYS is in attainment for NOx; however, as an ozone precursor, there is a dual review for attainment and non-attainment), lead, and any other NSR regulated contaminant.

Non-Attainment Contaminants

Ozone and PM10 are currently designated as non-attainment contaminants in NYS. Ozone is regulated by its precursors; volatile organic compounds and NOx. Section 200.1 defines the specific areas of NYS that are designated as non-attainment for these contaminants.

Applicable Requirements

A proposed major project that becomes subject to Part 231 must apply air pollution controls based on a control technology analysis. Attainment contaminants subject to Part 231 must apply best available control technologies (BACT). Non-attainment contaminants subject to Part 231 must apply lowest achievable emission rates (LAER) and obtain emission offsets.


BACT is a top-down analysis used to determine the best control technology available for controlling an attainment contaminant. This analysis contains both an economic (dollars per ton of contaminant removed) and a technical aspect (i.e. can the control physically be installed).


LAER is a control technology analysis that requires the most stringent controls be used to reduce non-attainment contaminants. Unlike BACT, a LAER analysis does not have an economic aspect. For a new facility to become subject to the requirements, the emissions from the proposed facility must equal or exceed the applicability thresholds in Subpart 231-13. For an existing facility to become subject to the requirements, two conditions must be met: a modification must be proposed, and the emissions from the proposed modification must equal or exceed the applicable thresholds in Subpart 231-13, thus resulting in an NSR major modification.


Netting is the process used by an existing major facility to avoid being subject to the BACT or LAER requirements of Part 231 for a proposed project. The netting process is a calculation involving the proposed project emissions increase in addition to past emission increases and decreases that occurred during the contemporaneous period (as defined in Subpart 231-4) associated with the proposed project.

Emission Reduction Credits (ERCs)

Emission offsets are emission reductions (which have been approved as ERCs) that are required to be obtained by a proposed new major facility or by an existing major facility that undertakes an NSR major modification. Emission offsets are required by facilities located in a non-attainment area or an attainment area of the state within the ozone transport region. A proposed project for a non-attainment contaminant must offset their emissions by obtaining ERCs at an offset ratio specified in Subpart 231-13. The offset ratio varies depending on the contaminant and non-attainment status of the area of the state that the project is occurring.


A modification is a physical change in or change in the method of operation of a major stationary source that would result in a significant net emissions increase of any regulated NSR pollutant. A modification does not include routine maintenance, repair and replacement (RMRR) and other specific activities (as defined in Subpart 231-4). RMRR determinations are made on a case-by-case basis considering four factors, namely the nature/extent, purpose, frequency, and cost of the work. The proposed project is no longer considered a modification if it is determined to be RMRR.

RMRR Activities

There are no predetermined activities that are considered RMRR activities. However, there are some activities that may qualify as RMRR activities. For an electric generating facility these activities include, but are not limited to:

  • individual tube repair/replacement;
  • inspection, repair and replacement of refractory in the slag necks and troughs;
  • inspection and repair of ductwork and expansion joints;
  • water blasting, inspection, stud replacement and new refractory (cyclone fired boilers);
  • inspection and repair of boiler casing, doors and inspection ports;
  • inspection and repair of gas path deflection baffles and flow distributors;
  • inspection and repair of all dampers (air and gas);
  • removal, disassembling, inspection and repair of ignitors;
  • cleaning, inspection and repair of external steam header vestibules;
  • chemical cleaning of water-side tubing to remove internal deposits, inspections for leak detection (on pipes, tubes, and/or valves).

Non-RMRR Activities

There are no predetermined activities that are considered to extend the life of a facility (non-RMRR activities). Some activities that are likely to extend the life of a facility (using an electric generating facility as an example) may include, but are not limited to:

  • the replacement of a boiler's super heater;
  • the replacement of a fuel feed system;
  • the replacement of a burner or burners;
  • repairs that allow an emission source to regain lost operating capacity;
  • replacement of components that increase the capacity of the emission source beyond its original design.

Construction Timeline

The existence of a valid permit shall not be construed as authorizing construction if construction is not commenced within 18 months after the date of permit issuance, if construction is discontinued for a period of 18 months or more, or if construction is not completed within a reasonable time as determined by DEC. DEC may grant one extension, for a period not to exceed 18 months, upon a satisfactory showing that an extension is justified. A permit shall become subject to revocation or modification if construction is not commenced and completed within the time frames described above.

Construction of a proposed project cannot commence prior to the issuance of a permit. The timeframe for DEC's review of a permit application and the issuance of a permit is governed by the provisions of Part 621.

  • Contact for this Page
    Division of Air Resources
    Part 231 Implementation Guidance
    625 Broadway
    Albany, NY 12233-3254
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