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After review of the public comments the USEPA received on its proposal to revoke the 2009 New Source Review (NSR) Aggregation Action, the USEPA has now decided to not revoke this action. This 2009 action clarified implementation of the NSR permitting program under the Clean Air Act with respect to treating related physical or operational changes as a single "modification" for the purpose of determining NSR applicability at a stationary source. The action was originally published by the USEPA under the name "Prevention of Significant Deterioration (PSD) and Nonattainment New Source Review (NSR): Aggregation and Project Netting." The proposal to revoke the ruling came on April 15, 2010 with a subsequent public comment period. This decision to not revoke the action is effective on November 15, 2018.

There is a two-step process for determining NSR applicability for projects at stationary sources. "Step 1" takes into account emission increases from the project itself, also known as "project emissions accounting". "Step 2" considers emission increases at the source as a whole, taking into account emission increases and emission decreases attributable to other projects undertaken at the source within a specific time frame, also known as "contemporaneous netting." In order to be subject to NSR major requirements, the project must result in both a significant emissions increase in "Step 1" and a significant net emissions increase in "Step 2." Under this current action taken by the EPA, stationary sources are allowed to participate in "project aggregation"; this allows the sources to treat nominally-separate projects occurring at a source as a single project for NSR applicability purposes where it is unreasonable not to consider them a single project. This project aggregation of "substantially related" activities can occur at Step 1, thus allowing the source to assess whether the project is major before reaching Step 2, where one must consider all contemporaneous increases and decreases across the source.

Unfortunately, there is no pre-determined list of activities that should be aggregated for a given industry. Rather, determining what constitutes a "project" is done on a case-by-case basis by the USEPA. The USEPA has sought to clarify its policy on project aggregation through interpretations contained in USEPA letters and memoranda, with the most commonly cited being a 1993 memorandum providing guidance to a 3M facility in Minnesota. The USEPA has stated regarding what constitutes a single project, "it is not reasonable to imagine perfect clarity could ever be achieved." However, they do believe that in most cases a similar standard to the 2009 NSR Aggregation Action is already being implemented by state and local air agencies.

A link to this final action in the Federal Register can be found here.