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EPA Issues Draft Guidance on State Implementation of New 1-Hour SO2 NAAQS


By Larry Kane, Attorney, Bingham Greenebaum Doll LLP

The U.S. Environmental Protection Agency (EPA) announced on September 21, 2011 that draft guidance for State Implementation Plan (SIP) submissions for the 1-hour SO2 NAAQS is available for public review and comment. See The new draft guidance (“Draft Guidance”) would supplement earlier EPA guidance for implementation of the 1-hour primary SO2 NAAQS adopted by the agency on June 2, 2010 (the 1-hour standard is 75 parts per billion, based on the three-year average of the 99th percentile of 1-hour daily maximum concentrations). From its preface, it may be inferred that the Draft Guidance provides a preview of EPA’s thinking on a projected rulemaking to codify key concepts for implementation of the short-term SO2 NAAQS.

Although the Draft Guidance addresses the development of both maintenance SIPs under Section 110(a) of the Clean Air Act (CAA) and SIPs for restoring nonattainment areas to attainment with the new SO2 standard, the main emphasis of the guidance is on maintenance SIPs. This is because EPA anticipates that most areas of the country will initially be designated as “unclassifiable.” That unclassifiable designations are expected to predominate derives from the approach EPA has identified for designations under the short-term SO2 standard which requires dispersion modeling to determine compliance status of more significant SO2 sources and such modeling is not anticipated to be available in time for the June 2012 deadline for initial designations.

Area Designations for the Short-Term SO2 Standard

EPA has previously indicated its intended approach to area designations, which may be summarized as:

  • areas with 2008-2010 monitoring data or refined dispersion modeling results showing a violation of the NAAQS will be designated as nonattainment;
  • areas that have both monitoring data and refined modeling results showing no violations will be designated as attainment;
  • all other areas, including those with SO2 monitors showing no violations but without refined modeling showing no violations, will be designated as unclassifiable. Areas with no SO2 monitors will be designated as unclassifiable as well.

The approach outlined by EPA reveals its belief that refined dispersion modeling will be needed, in addition to monitoring results, to determine compliance with the short-term SO2 standard for most significant sources. EPA refers to this as a “hybrid” modeling and monitoring implementation approach and intends to codify it by rulemaking.

Nonattainment Areas. States with areas designated nonattainment in June 2012 must submit SIPs to EPA within 18 months after the effective date of such designations (i.e. by February 2014) that define the actions they will require to bring such nonattainment areas into attainment with the short-term SO2 standards. Any nonattainment areas must attain the standard within five years from the effective date of designation (expected to be August 2017).

Attainment/Unclassifiable Areas. States with areas designated “attainment” or “unclassifiable” must submit SIPs to EPA by June 2013 – i.e., within three years following the promulgation of the new short-term SO2 NAAQS. These SIPs must satisfy Section 110(a) of the CAA which requires such SIPs to ensure that the state will implement, maintain, and enforce the new SO2 standard. They must also fulfill the requirement of Section 110(b) to provide necessary programmatic elements (“infrastructure”) to assure that attainment will be achieved and maintained. Thus, such SIPs are sometimes referred to as maintenance SIPs and/or “infrastructure” SIPs. Notably, EPA indicates its belief in the Draft Guidance (at p. 10) that it is generally reasonable for attainment with the SO2 standard to be achievable in unclassified areas within the same five-year timeframe as mandated for nonattainment areas.

Development of Maintenance/Infrastructure SIPs for Attainment/Unclassifiable Areas

EPA indicates that the Section 110(a)(1) requirements for unclassifiable areas will generally be accomplished by: (i) evaluating whether significant sources of SO2 emissions, individually or in combination with other proximate sources, will cause or contribute to violations of the short-term SO2 standard; and (ii) imposing adequate new control measures in the SIP with timetables for compliance to assure the standard is attained and maintained. (Guidance, at 8.) For areas that receive an attainment designation, EPA expects that the 110(a) requirements will be satisfied by SIP provisions ensuring that any applicable SO2 emission limits are permanent and enforceable and addressing the infrastructure requirements of Section 110(a)(2). (Guidance at 11.)

The Draft Guidance further identifies the following essential elements for a maintenance SIP to satisfy Section 110(a):

  • Attainment Emissions Inventory

EPA indicates that states need to develop an accurate “attainment inventory” that identifies the emissions in the air quality area that are sufficient to attain the short-term SO2 standard. It is said that sources with SO2 emissions at or above 100 tpy should be included, as well as smaller sources that, in combination with other sources, potentially cause or contribute to a violation of the standard.

  • Maintenance Demonstration

Consistent with its preference for modeling of SO2 emissions in relation to the short-term standard, the Draft Guidance indicates EPA’s belief that air quality monitoring is not sufficient, in general, as the sole basis of a demonstration of attainment and maintenance of the standard. (Guidance at 13.) Thus, EPA reemphasizes that refined dispersion modeling should be conducted with respect to sources, whether individually or collectively, that are considered to have the potential to cause or contribute to violations of the standard. As Appendix A to the Draft Guidance, EPA has provided a 32-page update to modeling guidance regarding demonstration of compliance with the short-term SO2 standard.

As a concession to the burdens on states of performing dispersion modeling on possibly several hundred SO2 sources, EPA identifies a general threshold of 100 tpy of potential emissions of SO2 above which sources must be modeled. (Guidance at 16-17.) This threshold would be subject to the judgment of a state agency whether there are groupings of smaller sources that may have a potential to cause or significantly contribute to violations of the short-term standard. EPA is requesting input on what guidance or criteria to use to determine when air quality impact of smaller sources should be modeled.

  • Control Strategy

In this element, EPA states its expectation that the improvement in air quality from a maintenance SIP will be based on SO2 emission reductions resulting from control measures that are permanent and enforceable. (Guidance at 18). Considerable emphasis is made on the point that several recently adopted or proposed regulations of national scope will or are expected to produce considerable reductions in SO2 emissions over the next several years, such as the Cross State Air Pollution Rule, the Industrial Boiler MACT, and the Mercury and Air Toxics Standards Rule. However, since in some cases these rules do not expressly set limits on SO2 emissions, states may want to consider adopting SO2 emission standards that are based upon the control standards set by these rules. A warning is made that source-specific factors always should be considered with respect to the impact of these regulatory controls.

  • Contingency Plan

Interestingly, EPA indicates that contingency measures for maintenance SIPs for the short-term SO2 standard may differ from those for other criteria pollutants. (Guidance at 23.) This belief is due to two factors: (i) EPA considers the expected use of dispersion modeling to produce more certain results in projecting attainment than tools available for other pollutants; and (ii) SO2 controls are well-understood and less prone to uncertainties that may attend attainment projections for other pollutants. Thus, EPA appears to believe that a contingency plan in this context may be focused on a comprehensive program to identify and enforce sources in violation of the short-term standard.

  • Verification of Continued Attainment

EPA states its expectation that this element can, in most cases, be satisfied with an effective program to track (i) compliance with applicable emission limits and (ii) changes in background concentrations.

In addition, the Draft Guidance offers a summary of EPA’s thoughts on a requisite showing to justify reclassification of an unclassifiable area to an attainment area. The proposed criteria generally tend to follow those elements described above for a Section 110(a) SIP for an unclassifiable area and, EPA acknowledges, are not clearly specified by the CAA. It would be perhaps helpful for EPA to address such redesignation criteria by rule.

EPA also discusses SIP requirements for nonattainment areas but, since these are described to be the same as for a nonattainment area for any other criteria pollutant, they will not be discussed further here.

While the Draft Guidance is more directly aimed at state air quality agencies, SO2 sources will ultimately be impacted by the final guidance and any rulemakings spawned by the Guidance. Thus, sources may want to scrutinize the Draft Guidance for any issues of concern.


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