U.S. EPA Finalizes Revisions to the Secondary NAAQS

On December 10, 2024, the United States Environmental Protection Agency (U.S. EPA) issued its finalized revisions to the secondary National Ambient Air Quality Standards (NAAQS). The relatively minor proposed changes, first proposed in April 2024, only affect the secondary standards for sulfur dioxide (SO₂), retaining the secondary standards for nitrogen dioxide (NO₂) and particulate matter (PM). Primary NAAQS are designed to protect public health, while secondary NAAQS protect public welfare, protecting damage to crops, vegetation, and animals, as well as decreased visibility.

The finalized changes:

In the original proposal, U.S. EPA recommended replacing the previous secondary NAAQS for SO₂, which was set at 500 parts per billion (ppb) or 1,310 micrograms per cubic meter (mg/m³) averaged over a three-hour daily maximum, with a new secondary standard based on an annual average in the range of 10 to 15 ppb (26-39 mg/m³) averaged over three years. In the final rule, U.S. EPA chose the low end of the range, 10 ppb, or 26 mg/m³.

Additionally, while U.S. EPA had proposed to retain the current secondary NAAQS for NO₂ and PM, they requested comment on potentially lowering the secondary annual NO₂ NAAQS from 53 ppb down to a range of 35 to 40 ppb, and the secondary annual PM NAAQS from 15 to 12 mg/m³. Ultimately however, U.S. EPA chose to retain those secondary NAAQS as they are.

Also, as was indicated in the proposed revisions, U.S. EPA concluded that no new emissions reductions would be necessary under the revised annual SO₂ secondary NAAQS, after an air quality analysis performed by U.S. EPA using monitored ambient SO₂ data from 2017 to 2023 concluded that those monitors in compliance with the primary one-hour SO₂ NAAQS would already comply with the new secondary annual NAAQS, and that for those monitors showing an exceedance in the one-hour primary NAAQS, the emission reductions necessary to bring the monitor into compliance with the primary NAAQS would also ensure compliance with the new secondary NAAQS.

Finally, while U.S. EPA is not making changes to the New Source Review (NSR) permitting program, it issued a Prevention of Significant Deterioration (PSD) memorandum describing an alternative approach to directly modeling against the new secondary NAAQS, that involves preparing a technical analysis that will provide a justification for relying on the demonstration of compliance with the one-hour primary SO₂ NAAQS as sufficient to comply with the new annual secondary NAAQS. As a result, the burden from an air dispersion modeling perspective is actually lessened by the revision of the secondary NAAQS: rather than having to model both one-hour and three-hour SO₂, an applicant would now model just one-hour SO₂ and reference the memorandum to justify that compliance with the secondary NAAQS was satisfied.

The revised standard will take effect 30 days after publication in the federal register, which has not happened yet as of this writing. It should be noted that the finalized rule falls within the look back period of the Congressional Review Act (CRA), so the new administration will have the ability to review and potentially stay and reverse or revise the rule.

What are the impacts of the final rule?

As was discussed in our blog on the proposed rule in April, the finalized rule is a disappointment to environmentalists who sought to use the reconsideration of secondary NAAQS to circumvent the timing of the review of the corresponding primary NAAQS for these pollutants and install more restrictive standards. Some groups were pushing for secondary NAAQS that were more stringent than the primary NAAQS, something that has never occurred and would potentially complicate implementation of the revised NAAQS.

And, although the final rule selected the lower end of the range it sought comment on for the revision to the secondary SO₂ NAAQS, the final rule is less stringent than what was recommended by U.S. EPA staff, and considerably weaker than what was suggested by the Clean Air Scientific Advisory Committee (CASAC). Not only had CASAC suggested significantly lower levels, but also potentially a new approach where the limits would be based on deposition in waterbodies, which again would have complicated compliance demonstrations as no previous NAAQS has ever taken that form.

What do these proposed revisions mean?

Ultimately the final rule, if it survives review under the CRA, will have a limited impact on industry and require little or no action to show compliance with the revised secondary NAAQS.

If you have concerns about the potential implications of the revisions, feel free to contact your ALL4 Project Manager or Rich Hamel. We’ll continue to monitor NAAQS-related developments and report as news develops.