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The U.S. Environmental Protection Agency (EPA) Issues Draft Guidance for 1-Hour Sulfur Dioxide National Ambient Air Quality Standards (NAAQS) State Implementation Plan (SIP) Submissions


On September 22, 2011, EPA released draft guidance for states implementing the new 1-hour sulfur dioxide (SO2) NAAQS and for preparing related SIP submissions.  The draft guidance addresses EPA’s proposed approach for states to use to demonstrate whether areas are in attainment with the 1-hour SO2 standard, as well as the content of SIPs for attainment, unclassifiable, and nonattainment areas.  EPA promulgated the new 1-hour SO2 NAAQS on June 2, 2010 to address short-term exposure to elevated SO2 ambient air concentrations.  The 1-hour SO2 standard is set at 75 parts per billion (ppb) (3-year average of the 99th percentile daily maximum 1-hour concentration) and is substantially more stringent than the prior standards.

States were required to submit initial attainment/nonattainment designations for the 1-hour SO2 NAAQS by June 3, 2011.  EPA expected that the initial designation recommendations would be based upon SO2 monitoring data from existing ambient air monitoring stations.  Kentucky submitted its recommendations for SO2 nonattainment areas on June 2, 2011 and noted that ambient air monitoring data at SO2 monitors in Kentucky were below the 75 ppb standard in all locations except Jefferson County.  Kentucky therefore recommended that Jefferson County be designated as nonattainment and the rest of the areas in Kentucky be designated as attainment/unclassifiable.  EPA is to make final area designations based upon the state submittals by June 2012.

However, in order to obtain SIP approval, EPA has proposed in the rule and guidance a hybrid modeling and monitoring approach for states to demonstrate continued or ultimate compliance with the 1-hour SO2 NAAQS.  In this regard, EPA will approve SIPs and designate areas as attainment only if an area has both monitoring data and refined modeling results showing no violations of the standard.  Areas that include SO2 ambient air monitors showing no violations, but that lack refined ambient air modeling to further demonstrate no violations of the standard by major stationary sources of SO2 emissions, would be designated as unclassifiable.  Any area that has monitoring data or refined dispersion modeling results showing a violation of the NAAQS would be designated as nonattainment.  EPA notes that it does not expect to use this hybrid designational approach for other NAAQS pollutants.

While EPA did not expect states to have refined modeling completed at the time of the June 2011 initial area designation recommendations, EPA expects refined modeling by states to be included to support SIPs to be submitted by June 2013 to provide for implementation and maintenance of the 1-hour NAAQS in areas initially designated as unclassifiable.  For example, because Kentucky did not submit refined modeling with its designation recommendations, all areas of the state, except Jefferson County, will likely be designated as unclassifiable by EPA in June 2012.  SIPs for unclassifiable areas would, under EPA’s draft guidance, have to include refined modeling and any monitoring that demonstrates attainment and maintenance of the new 1-hour SO2 standard as expeditiously as practicable.

With respect to the refined modeling, EPA expects states to evaluate whether significant sources of SO2 emissions, either individually or in combination with other nearby sources, would cause or contribute to a violation of the 1-hour SO2 standard.  The September 22 draft guidance includes modeling recommendations and guidance for making these demonstrations.  EPA expects that states would focus the attainment demonstration modeling on sources of SO2 emissions over 100 tons per year (tpy), as well as any other sources that may be expected to cause or contribute to a violation.  EPA notes that within the United States over 1,900 stationary sources are estimated to have actual emissions above 100 tpy.  The draft guidance provides for the modeling to be conducted based upon the maximum allowable emissions or federally enforceable emissions limits for the sources, taking into account control strategies that may be employed due to nationally enforceable requirements such as upcoming National Emission Standards for Hazardous Air Pollutants (NESHAP) or reductions required by the Cross State Air Pollution Control Rule.  It is anticipated that after the modeling guidance is finalized, DAQ will conduct modeling to evaluate the sources in Kentucky that have actual emissions of SO2 above 100 tons per year (approximately 50 sources).

As part of the June 2013 SIP submittals for areas initially designated as unclassifiable or attainment, the SIPs would have to include enforceable emission limitations on major sources, based upon the results of the refined modeling, that would demonstrate compliance with the 1-hour SO2 standard, along with timetables for compliance and appropriate testing/reporting to assure compliance.  EPA expects any required SO2 controls will be in place to achieve the standard by August 2017, which is the same compliance/attainment deadline that would be required for areas designated as nonattainment.  The EPA guidance indicates that SIP submittals would not be approved as sufficient to support designation of an area as attainment unless they contain the refined air quality dispersion modeling.  Based upon EPA’s statements in the draft guidance, controls such as SO2 scrubbers may be required on significant sources of SO2 emissions, especially those with short stack heights or that are located in complex terrain, in order to demonstrate compliance with the 1-hour SO2 standard.

EPA has indicated that it also plans to promulgate a regulation to address the 1-hour SO2 NAAQS implementation elements.  The elements to be addressed include:  the requirement that attainment designations be based upon both air quality modeling and monitoring station data; the modeling requirements to demonstrate compliance; the minimum scope of analysis required to demonstrate attainment and maintenance of the 1-hour SO2 standard; and the time period for sources to reduce SO2 emissions to comply with the SIPs.  Given the June 2013 deadline for submittal of SIPs for attainment and unclassifiable areas, it appears that states will have very little time to prepare or refine SIPs after promulgation of any such final rule.

Pursuant to an extension published in the October 28, 2011 Federal Register, the public comment deadline regarding the draft guidance is December 2, 2011.  DAQ reports that it intends to submit written comments on the guidance.

To view a complete PDF of the Third Quarter 2011 issue of the Air Quality Letter, click HERE.


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