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U.S. EPA Proposes Guidance on Fine Particulate and Ozone Significant Impact Levels

Posted: August 7th, 2016

Author: All4 Staff 

UPDATE 9/7/16: The values in the tables within this article have been updated per EPA’s corrections to the draft guidance.

On August 1, 2016 U.S. EPA made available for public comment draft guidance and supporting documents recommending Significant Impact Levels (SILs) for ozone (O3) and fine particulate (PM2.5) to be used in the Prevention of Significant Deterioration (PSD) permitting program. Public comments are being accepted until September 30, 2016 at which time U.S. EPA will most likely move forward with official rulemaking. Therefore, the timeline for finalizing the O3 and PM2.5 SILs is somewhat extended and unknown at this time. However, the proposed SIL values could find their way into state policy or guidance documents in the meantime. U.S. EPA is proposing the following SILs for NAAQS and PSD increment:


What are SILs Utilized for?

SILs represent pollutant concentration levels that U.S. EPA has determine represent an insignificant level with respect to a National Ambient Air Quality Standard (NAAQS) or PSD increment. The SILs are pollutant specific and critical to the air quality modeling process for two basic reasons.

First, comparison of air quality modeled concentrations with the SILs is the initial step in the air quality modeling process. As such, a facility can limit its PSD air quality modeling obligation to a single analysis if it is possible to demonstrate project related concentration levels are below the SIL. Second, SILs are also utilized as part of the PSD air quality modeling process to determine if project related concentrations could potential cause or contribute to a violation of the National Ambient Air Quality Standards (NAAQS) or PSD increment. In this instance, the use of the SILs can be critical to a facility when project related concentrations are added to modeled concentrations from other local sources and a violation of the NAAQS or PSD increment is identified. Demonstrating that modeled project related concentrations are below the SIL represents the difference between successfully permitting the project and having to reduce emissions or add additional emissions controls.

NAAQS SILs are utilized to determine if a cumulative NAAQS air quality modeling demonstration is required and PSD increment SILs are utilized to determine if a cumulative PSD increment air quality modeling demonstration is required. If project related impacts exceed a NAAQS SIL then the applicant must undergo a cumulative NAAQS air quality modeling analysis which includes evaluating facility wide impacts, local sources, and representative background concentrations. If a project triggers a PSD SIL then project emissions and potentially facility emissions and local source emissions may need to be evaluated. PSD increment evaluations are currently split up into two classes. Class I areas are areas of special national or regional natural, scenic, recreational, or historic value such as national parks, wilderness areas, or fish and wildlife areas for which PSD regulations provide special protection and Class II areas are all other areas.
Why Is U.S. EPA Proposing O3 and PM2.5 SILs?

In July 2010, Sierra Club petitioned the U.S. EPA to designate an air dispersion model to be used to determine if PSD projects cause or contribute to violations of the O3 or PM2.5 NAAQS. This petition was granted in January 2012 by U.S. EPA and U.S. EPA subsequently proposed revisions to 40 CFR Part 51 Appendix W Guideline on Air Quality Models (Appendix W) to address O3 and PM2.5 air quality modeling in July of 2015. The proposed Appendix W revisions include criteria and steps for choosing analytical techniques or air dispersion models to estimate O3 concentrations from precursor oxides of nitrogen (NOX) and volatile organic compound (VOC) emissions and PM2.5 concentrations from direct and secondary-formed PM2.5. The August 1, 2016 proposed SILs are intended to support the Appendix W revisions. U.S. EPA had intended to finalize Appendix W revisions in July 2016; however, due to a change in the Office of Management and Budget’s (OMB) significance determination based on a congressional inquiry during U.S. EPA’s 2017 budget hearing, the finalized revisions are not expected until sometime after October 2016.

PM2.5 SILs Were Established Years Ago, Right?

Yes, PM2.5 SILs were established in 2010 in 40 CFR Part 51.166 and 52.21 of the PSD regulations. However, in January 2013, the U.S. Court of Appeals for the District of Columbia Circuit (DC Circuit) remanded the PM2.5 SILs as strict indicators of insignificant concentration levels, in other words, the SILs might not always be low enough relative to existing air quality levels for PM2.5. The court decision established a requirement to justify that air quality modeled concentrations below the SILs would not cause or contribute to a violation of the NAAQS considering existing air quality in the project location. The remanded and proposed SIL values are summarized below:

You will notice that the bolded SILs represent concentration values that are proposed to be lowered as part of the proposed SIL guidance. U.S. EPA has concluded that air quality modeled concentrations below 0.2 ug/m3 and 1.2 µg/m3 for the annual and 24-hour PM2.5 SILs, respectively, can be considered an insignificant concentration level and should be considered to not cause or contribute to a violation of the NAAQS. Therefore, no further justification is required if the proposed PM2.5 SILs are used. However, U.S. EPA is proposing that permitting authorities have the case-by-case discretion of utilizing SIL values between 0.2 µg/m3 and 0.3 µg/m3 for the annual PM2.5 NAAQS and between 1.2 µg/m3and 1.3 µg/m3 for the 24-hour NAAQS if it can be demonstrated that there is sufficient “headroom” between ambient PM2.5 concentration levels and the NAAQS to ensure impacts less than the SIL will not cause or contribute to a violation of the NAAQS. This requirement to justify the use of a SIL is no different than is currently required because of the DC Circuit Court remand.

How Will the Proposed PM2.5 SILs Affect My Facility?

This will largely be dictated by how State permitting authorities implement the case-by-case use of the range of recommended SILs. Especially for the annual PM2.5 NAAQS, it has been a strategic approach to avoid the requirement to conduct PM2.5 air quality modeling as part of the PSD permitting process either through controlling or reducing PM2.5 emissions in an effort to stay below the PSD significant emissions rate (SER). If project emissions are not below the PM2.5 SER, the next strategic approach is to remain below the PM2.5 SILs. This approach is necessary to avoid conducting cumulative NAAQS and PSD increment air quality modeling analyses. Demonstrating that a facility would not cause or contribute to a violation of the annual PM2.5 NAAQS has proven difficult with the current regulatory approach which includes many conservative components. These components include an air dispersion model designed to bias modeled concentrations high (AERMOD), relatively high background PM2.5 concentrations, a conservative approach for applying these background PM2.5 concentrations in the air quality modeling analysis, the need to factor in secondary PM2.5 concentrations, and uncertainty with PM2.5 emissions estimates for some sources.

Therefore, under the proposed regulatory framework lower SILs reduce the flexibility for a facility to avoid cumulative air quality modeling and may increase the cost to comply with air quality standards. Reduced SILs could mean capital cost to add PM2.5 controls or to make stack improvements to enhance the dispersion of PM2.5 emissions (Note: stack improvements must be considered within the framework of good engineering practice – GEP regulations). Evaluating these alternatives with air dispersion models will mean extended timelines and increased costs for developing a PSD application. In the most extreme situation, a project might be derailed because the concentration level for being insignificant (i.e., the SIL for not causing or contributing to an air quality violation) is just too low relative to the existing PM2.5 concentration levels.

Ozone SIL – What the Heck?

As mentioned above the proposed revisions to Appendix W are anticipated to include new requirements to evaluate concentrations of O3 resulting from precursor emissions (NOX and VOC) to demonstrate that a project will not cause or contribute to a violation of the O3 NAAQS. Currently assessing project related O3 concentrations are addressed as part of PSD applications on a case-by-case basis in consultation with the permitting authority. The case-by-case evaluations have varied wildly from state to state, from completely qualitative assessments to quantitative analyses utilizing a photochemical grid model such as the Comprehensive Air Quality Model with Extensions (CAMx) model. Part of the proposed Appendix W rulemaking includes a requirement to establish O3 SILs as the August 1, 2016 proposed guidance is undertaking. Additionally, U.S. EPA intends to develop rulemaking that will establish an intermediate step between the qualitative and quantitative approaches for projects with O3 precursor emissions (NOX and VOC) greater than the SERs and therefore needing to evaluate O3 concentrations with respect to the proposed SILs. This yet to be developed rulemaking will likely propose to establish Model Emissions Rates for Precursors (MERPs).  A MERP would represent a level of emissions of precursors that is not expected to contribute significantly to concentrations of O3. The MERPs are expected to be greater than SERs and be a more appropriate screening threshold for evaluating their impacts as precursor pollutants to O3 formation.

O3 is not directly emitted by sources but rather created by chemical reactions of directly emitted NOX and VOC in the presence of sunlight. The chemical transformation of O3 can vary widely across the country due to different, temperature and sunlight regimes, distribution of anthropogenic and biogenic formed VOC, and topographic settings that lead to unique O3 formation to name a few. Therefore, establishing a uniform SIL and/or MERP that is representative for the entire United States would result in conservative assessments for some areas and less conservative assessments for other parts of the country. The proposed 1.0 ppb O3 SIL represents a conservative value and is only 1.4% of the new 2015 O3 NAAQS (70 ppb). Current qualitative ratio approaches for determining that project related O3 concentrations are below the propose SIL will be difficult for most sources to use. However, next generation air quality models such as CAMx, which accurately predict O3 impacts, are becoming much more accessible for consultants to utilize as federal and state permitting authorities develop a significant portion of the necessary air quality modeling inputs for CAMx. However, as with the lowering of PM2.5 SILs, the evaluation of O3 SILs will also lead to more time and cost for PSD applications.

What Can I Do Now and How Can ALL4 Help?

Public comments are being accepted until September 30, 2016 on the proposed PM2.5 and O3 SILs. Consider providing comments as the final SILs will affect your air permitting projects. ALL4 can assist by evaluating the methods utilized to develop the SILs and with the addition of our new photochemical grid modeling expert, Dr. Bhat, ALL4 can conduct CAMx modeling to evaluate the appropriateness of the proposed O3 SIL or to justify more representative O3 SIL values for your geographic area. In addition Dr. Bhat and Dan Dix will be providing a complimentary webinar on August 11th at 12pm EDT on the proposed O3 and PM2.5 SILs. In the mean time for more information on the proposed SILs and air quality modeling please contact Dan Dix or Abhishek Bhat.

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