EPA has delayed the issuance of attainment designations under the National Ambient Air Quality Standards (NAAQS) for sulfur dioxide (SO2). Initial area designations under the June 2010 1-hour primary SO2 NAAQS had been due on June 3, 2013, but EPA did not meet this deadline, citing limitations in the current monitoring data. Currently monitored areas found to violate SO2 NAAQS are still scheduled to receive area designations in 2013, but remaining areas may not receive designations until 2020. State agencies, industry groups, and environmentalists have argued that failing to issue all area designations by June 3 violates the statutory deadline under the Clean Air Act. Environmental groups filed a notice of intent to sue on this basis on June 4, 2013.

The current timeline for SO2 NAAQS implementation proposed by EPA includes:

  • Mid-2013: Issuance of Technical Assistance Documents (TADs) for Modeling and Monitoring
  • Late 2013: Proposed “data requirements” rule (setting out what SO2 sources must be evaluated for attainment purposes) made available for public comment
  • Late 2014: Final “data requirements” rule
  • January 2017: New monitors operational and modeling data submitted for unmonitored areas
  • December 2017: Final designations issued for modeled areas
  • May 2020: Monitoring data and designation recommendations submitted for monitored areas
  • December 2020: Final designations issued for remaining areas, including newly monitored areas

An announcement by EPA in February 2013 signaled a shift in the Agency’s approach to SO2 NAAQS implementation. The preamble to the 2010 SO2 NAAQS asserted that twothirds of existing monitors are not located to characterize maximum concentration sourceoriented impacts. Initial EPA draft guidance suggested remedying these deficiencies by relying on modeling, rather than monitoring. Stakeholders expressed concern that the conservative assumptions employed in modeling would lead to false designations of nonattainment. Response from stakeholders led EPA to adopt a more flexible approach. State agencies will now be given latitude to demonstrate attainment using monitoring as well as modeling, as long as the monitoring is targeted, source-oriented, and otherwise adequate. The adoption of this “dual-pathway” approach has led EPA to issue preliminary technical guidance on both monitoring and modeling provided in the TADs.

On May 21, 2013, EPA released draft TADs for monitoring and modeling. The TADs aim to provide suggestions as to how state, local, and tribal agencies might design monitoring and modeling regimes in preparation for the new data requirements rule. When finalized, they will not constitute final agency action or impose binding and enforceable requirements or obligations on any person or state agency. The Draft Modeling TAD reflects stakeholder concerns raised in response to previous modeling guidance. The draft supersedes guidance issued in March 2011, and includes increased use of actual data. Most significantly, the TAD calls for the use of 3 years of actual emissions data in modeling rather than maximum allowable emissions. It also suggests the use of 3 years of meteorological data rather than one to five years and use of actual stack height instead of Good Engineering Practices stack height.

In the Draft Monitoring TAD, EPA provides three approaches for identifying source-oriented SO2 monitoring sites: (1) using existing emissions, monitoring, and modeling data; (2) conducting new modeling to identify candidate sites; and (3) using exploratory monitoring to identify or evaluate possible placements. EPA expects that once monitors are sited they will be operated equivalent to other monitors in the State and Local Air Monitoring Stations network, per federal guidelines. The comment period for the Draft TADs remains open through July 22, 2013.

EPA has also begun the five-year review process for SO2 NAAQS and is scheduled to promulgate an updated standard in November 2017. We will provide additional updates on the SO2 NAAQS implementation process as more information becomes available.