Alternative Demonstration Approach for the 1971 Secondary 3-Hour Sulfur Dioxide National Ambient Air Quality Standard under the Prevention of Significant Deterioration Program
On December 10, 2024, the U.S. Environmental Protection Agency (EPA) revised the secondary sulfur dioxide (SO2) National Ambient Air Quality Standard (NAAQS) to an annual average of 10 parts per billion (ppb), averaged over three consecutive years. The 2024 rulemaking added a new annual secondary NAAQS in 40 CFR Part 50 without altering the 1971 secondary 3-hour SO2 NAAQS, which remains in effect. Under the Prevention of Significant Deterioration (PSD) program, any permit issued on or after the effective date of such SO2 NAAQS for construction of or at a stationary source that increases SO2 emissions in significant amounts needs to be supported by a demonstration that the increased emissions from the proposed major stationary source or major modification will not cause or contribute to violation of those standards. To help facilitate implementation of the 2024 secondary annual SO2 NAAQS under the PSD program, the EPA developed a streamlined, alternative PSD demonstration approach, Alternative Demonstration Approach for the 2024 Secondary Sulfur Dioxide National Ambient Air Quality Standard under the Prevention of Significant Deterioration Program. Through the technical analysis conducted for the alternative PSD demonstration approach, the EPA determined that a demonstration that increased SO2 emissions will not cause or contribute to a violation of the 2010 primary 1-hour SO2 NAAQS can suffice to demonstrate that increased SO2 emissions will also not cause or contribute to a violation of the 2024 secondary annual SO2 NAAQS. Thus, permit applicants and reviewing authorities have relied on the demonstration for the 2010 primary 1-hour SO2 NAAQS to also satisfy the demonstration requirement for the 2024 secondary annual SO2 NAAQS, resulting in no additional burden on permit applicants.
Recognizing the alternative PSD demonstration approach for the 2024 secondary annual SO2 NAAQS and that the 1971 secondary 3-hour SO2 NAAQS remains in effect, air permitting agencies have requested the EPA support a similar alternative PSD demonstration approach to demonstrate compliance with the 1971 secondary 3-hour SO2 NAAQS to further streamline and reduce burden on permit applicants. To evaluate this request, the EPA conducted the attached technical analysis. Based on this technical analysis, the EPA has determined that a demonstration that increased SO2 emissions will not cause or contribute to a violation of the 2010 primary 1-hour SO2 NAAQS can suffice to demonstrate that increased SO2 emissions will also not cause or contribute to a violation of the 1971 secondary 3-hour SO2 NAAQS. Thus, permit applicants and reviewing authorities may rely on the demonstration for the 2010 primary 1-hour SO2 NAAQS to also satisfy the demonstration requirement for the 1971 secondary 3-hour SO2 NAAQS. This additional alternative PSD demonstration alleviates the need for permit applicants to conduct a separate PSD demonstration for the 1971 secondary 3-hour SO2 NAAQS. Permit applicants may demonstrate compliance with the 2010 primary 1-hour SO2 NAAQS to satisfy the demonstration requirement of all three current SO2 NAAQS (1971 secondary 3-hour SO2 NAAQS, 2010 primary 1-hour SO2 NAAQS, and 2024 secondary annual SO2 NAAQS).
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