EPA Issues Final Guidance on SILs for PM2.5 and O3

EPA Issues Final Guidance on SILs for PM2.5 and O3

MAY 01, 2018

It’s been a long road, with several bumps and detours, however on April 17, 2018 the EPA published final guidance on Significant Impact Levels (SILs) for fine particulate matter (PM2.5) and ozone (O3), two pollutants that continue to present technical challenges for regulated sources attempting to demonstrate compliance with their respective ambient air quality standards (AAQSs) and allowable increases of ambient concentrations (PSD increments). PM2.5 and O3 can be produced through complex chemical reactions in the atmosphere from other emissions (so-called “precursor emissions”), so complying with SILs for these two pollutants is even more critical than other pollutants to avoid extensive, complex, time-consuming, and costly additional modeling analyses. Although the final SILs are considered by many to be too low, they will provide many emission sources a pathway out of additional complex modeling compliance demonstrations. 

EPA promulgated PM2.5 SILs in October 2010 but subsequently, during litigation, asked the court to vacate the SILs due to inconsistencies in the final rule. The court granted their request in 2013. Since then, EPA has developed, and offered for informal comment, several documents that support the revised final guidance:

The guidance and support documents establish the basis for the optional use by permitting agencies to allow applicants for an air permit to assess whether a proposed emissions increase may result in insignificant impacts of PM2.5 and O3 and bypass a burdensome path to a final permit issuance. If the modeled impacts from the proposed project are greater than the SILs, the applicant can choose either to reduce the emissions of a specific pollutant or go through a more rigorous compliance demonstration modeling analysis. 

The SILs guidance issued by EPA for PM2.5 and O3 differ from SILs issued for other pollutants in the way they were derived, e.g., it is based on a statistical analysis of the variability of observed air quality data nationally, not on a calculated fraction of the NAAQS (typically 4%) and in that they are provided as “non-binding guidance.” EPA explains that it intends to derive information on the application of these SILs, and if these prove to be “…suitable in all circumstances to show that an increase in air quality concentrations below the values does not cause or contribute to a violation of the NAAQS or PSD increments,” it may assess and refine these values and if appropriate, it may eventually codify these via a binding rulemaking. The guidance provides permitting authority the discretion to apply these SILs on a case-by-case basis and use the documentation provided to support their use or develop their own SIL values, provided they are properly supported in the permitting record.

The statistical evaluation of monitoring data EPA performed indicated that appropriate SIL values should be 1 part per million (ppm) for O3 and 1.5 micrograms per cubic meter (µg/m3) for the 24-hour average and 0.3 µg/m3 for the annual average PM2.5 SILs, respectively. However, EPA explains in the new guidance document that the PM2.5 SIL values derived from the statistical evaluation cannot be used, as they are higher than those values that EPA determined to potentially cause or contribute to a violation, and was codified in the 2010 rulemaking and listed in 40 CFR 51.165(b)(2). Therefore, the final SIL values recommended for NAAQS compliance demonstrations in the final guidance, Table 1, applicable for all areas are as follows:

When considering the potential impact of the proposed increase in emissions on allowable PM2.5 increments, there are no ozone increments currently, and the EPA recommends applying the same SILs for Class II areas and lower values for protected Class I areas, such as national parks and wilderness areas. The recommended values provided in Table 2 of the guidance document are as follows:

The recommended SILs can be used by permitting authorities to exempt permit applicants from a multisource compliance demonstration if the results of the modeling analyses for just the emissions increases due to a proposed project are less than the respective SIL. If the project-only analysis indicates a significant impact, the SIL values can also be used in cases where violations of the standard or exceedances of an increment are predicted, to assess whether the project’s contribution to the predicted violations is significant. If the project’s contribution to those violations is, in all such cases, below the SILs, then the project is assumed to not cause or contribute to those violations and the permitting agency can proceed with the issuance of the construction permit.

EPA has been very diligent in preparing this final guidance and supporting documents that provide permitting agencies the support they need to determine potential impact significance from a proposed project. The permitting agencies have a choice to use these recommended values, develop their own, or request additional information from an applicant to support a case-specific determination. 

It is important for an air permit applicant that will have precursor emissions, i.e., primary PM2.5, sulfur oxides, nitrogen oxides, and volatile organic compound emissions, that may result in secondary formation of PM2.5 and O3 to assess their potential emissions impacts and work closely with the permitting agency to prepare a compliance demonstration that is supported by the appropriate data and modeling analyses. Woodard & Curran’s permitting and modeling experts can provide the support needed to take full advantage of the final guidance SILs to ease the air quality compliance demonstration burden, expedite the project schedule, and interact with the permitting agency to successfully permit projects.



Meteorologist, Project Manager
Air Services

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Kelley Begin, PE; Matt Jones, CCM; Michael Newman, PE
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