U.S. EPA Proposes Revisions to the Secondary NAAQS
Posted: April 17th, 2024
Authors: Rich H.
On April 11, 2024 the United States Environmental Protection Agency (U.S. EPA) issued its proposed revisions to the secondary National Ambient Air Quality Standards (NAAQS). The relatively minor proposed changes, published in the Federal Register on April 15, 2024, only affect the secondary standards for sulfur dioxide (SO2), retaining the secondary standards for nitrogen Dioxide (NO2) and particulate matter (PM). Primary NAAQS protect public health, while secondary NAAQS protect public welfare, protecting damage to crops, vegetation, and animals, as well as decreased visibility.
What is in the proposal?
In the proposal, U.S. EPA is recommending the replacement of the current secondary NAAQS for SO2, which is set at 500 parts per billion (ppb) averaged over a 3-hour daily maximum, with a new secondary standard based on an annual average in the range of 10-15 ppb [26-39 micrograms per cubic meter (mg/m3)] averaged over 3 years. Further, U.S. EPA in the proposal suggests that compliance with the current primary 1-hour SO2 NAAQS, set at 75 ppb (196 mg/m3) should also satisfy the proposed annual secondary standard. Therefore, no additional emissions reductions would be required to address the secondary NAAQS beyond those already required to meet the 1-hour SO2 primary NAAQS. U.S. EPA is also not proposing any revisions to the national monitoring network, believing that the current network, which still typically records annual concentrations of SO2 as was required when the primary annual SO2 NAAQS, which was revoked in 2010, was in effect.
Additionally, U.S. EPA has proposed to retain the current secondary annual NAAQS for NO2 at 53 ppb while taking comments on revising the level to a range of 35-40 ppb averaged over 3 years, and to retain the existing secondary annual PM2.5 NAAQS at 15 mg/m3 while taking comments on revising the standard to 12 mg/m3.
The agency will take comments on the proposal for 60 days, meaning the comment period ends on June 14, 2024. In the Fact Sheet for the proposal, U.S. EPA outlines alternate values and potential forms of the standard on which they request comment. U.S. EPA is required by consent decree to finalize the rule by December 10, 2024
What are the impacts of the proposed rule?
The proposed rule as written would appear to disappoint 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.
The proposal is also 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.
U.S. EPA’s staff recommendations were:
- SO2: 5-15 ppb annual average
- NO2: 35-40 ppb (down from the current 53 ppb), averaged over 3 years
- PM5: 12 mg/m3
Meanwhile, CASAC’s recommendations if U.S. EPA were to continue with ambient air limits and not standards based on deposition were:
- SO2: Annual in the range of 10-15 ppb
- NO2: 10-20 ppb
- PM5: 6-10 mg/m3 and 24-hour in the range of 20-35 mg/m3.
What Do these Proposed Revisions Mean?
Ultimately the proposed rule, if finalized as proposed, will have limited impact on industry and require no action. However, there are a number of alternatives that U.S. EPA is requesting comment on that could result in more restrictive standards should the proposed rule be revised based on those comments.
If you have concerns about the potential implications of the proposed revisions or need help developing comments on the proposal, feel free to contact your ALL4 Project Manager or Rich Hamel. We’ll continue to monitor developments in this and report as news develops. You can also sign up for one of our complementary PM2.5 workshops to learn more about the latest NAAQS happenings and how they may affect your facility.
