Demonstrating Secondary SO2 NAAQS in PSD

On December 10, 2024, Peter Tsirigotis, Director, Office of Air Quality Planning and Standards, released guidance entitled "Alternative Demonstration Approach for the 2024 Secondary Sulfur Dioxide National Ambient Air Quality Standard under the Prevention of Significant Deterioration Program." In the Guidance, the EPA stated that "the EPA has determined that a demonstration that increased SO2 emissions will not cause or contribute to a violation of the primary 1-hour SO2 standard can suffice to demonstrate that SO2 emissions will also not cause or contribute to a violation of the secondary annual SO2 standard." Guidance, at 1.

ANALYSIS

On December 10, 2024, the EPA revised the secondary SO2 NAAQS to an annual average of 10 ppb averaged over three consecutive years. 89 Fed. 105692 (Dec. 27, 2024). Under the D.C. Circuit's decision in Murray Energy, new NAAQS are effective immediately and so sources with permits issued on or after January 27, 2025, would need to demonstrate attainment of the 10 ppb secondary SO2 NAAQS. 936 F.3d 597 (D.C. Cir. 2019). Unfortunately, EPA did not release a "significant impact level" or SIL for the new secondary NAAQS and AERMOD is not yet equipped with a processor to directly calculate an annual value for SO2 in ppb, requiring additional post-processing work. EPA also noted that the existing monitoring network is mostly "source oriented" and not background oriented, so determining an appropriate background concentration may be difficult. To reduce the burden on the regulated community, the EPA undertook an alternative demonstration.

The EPA conducted a two-prong evaluation of the relationship between the secondary annual SO2 standard and the primary 1-hour SO2 standard to determine whether a sufficient basis exists to determine that demonstrating attainment with the 1-hour primary standard will also demonstrate attainment with the new 10 ppb annual secondary standard.

First Prong: Does an increase less than the 1-hour SIL result in a corresponding small increase in the secondary annual average

EPA began its analysis using AERMOD data from the 2018 Risk and Exposure Assessment from the most recent review of the SO2 primary standard. It averaged emissions from multiple facilities in three areas to calculate an annual average emission rate. It then calculated an annual design value based on that emission rate using AERMOD. It then calculated the 3 year average of the highest one-hour concentrations and compared that modeled value to the SIL (3 ppb or 7.86 ug/m3). All concentrations at all receptors were then multiplied by the ratio to determine the impact of an increase in emissions equal to the 1 hour SIL. A new annual design value was then calculated. The calculation showed that for most facilities, the annual design increased by less than 1 percent, for all but two facilities were less than 2 percent, and the maximum impact was 3.5 percent of the 10 ppb annual secondary standard. Based on this analysis, the EPA concluded that "overall, results ... suggest that a small increase in 1-hour SO2 concentration produces a comparably small increase in annual SO2 concentration" and supports a belief that demonstrating attainment of the 1-hour SO2 NAAQS also would demonstrate attainment of the annual secondary SO2 NAAQS. Guidance, at A-7 through A-8

Second Prong: Would the expected increases from the first prong result in an exceedance of the secondary NAAQS in a cumulative impact analysis?

In the second prong, the EPA analyzed the data for both primary and secondary SO2 NAAQS and determined that all monitors demonstrating attainment with the primary NAAQS also demonstrated attainment with the new secondary NAAQS. Accordingly, the EPA concluded that the likelihood of a source demonstrating attainment of the primary NAAQS resulting in a violation of the secondary NAAQS was acceptably small given the relative relationships demonstrated in the first prong.

COMMENTARY

The EPA took several good administrative steps with the release of the new annual secondary SO2 NAAQS standard. First, it released the implementation Guidance discussed in this NSR Law Blog article on the same date as it released the new annual secondary NAAQS. This meant that the guidance on how to demonstrate attainment was available immediately to affected sources and represents a very positive step. The EPA deserves credit for its effort to address implementation at the same time as it releases the new standard.

Second, the EPA undertook a forthright analysis showing that relatively small increases in hourly concentration will result in corresponding small changes in the annual concentration, thus enabling sources to demonstrate attainment until such time as the EPA updates AERMOD to include the new annual secondary calculation methodologies. This minimizes time and expense and uncertainty for state and local permitting authorities and sources alike and also provides an assurance to the public that sources following the guidance will not adversely affect their surrounding communities.

NSR Law Blog is hopeful that the EPA will continue to release implementation aids for the NSR programs at the same time it releases new NAAQS and increments.