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Air Quality

Air Pollutants and Monitoring - Criteria Pollutants

State Implementation Plan (SIP) for Sulfur Dioxide (SO2)

On May 21, 2013, the EPA published two draft Technical Assistance Documents (TADs) that advise states on the use of modeling and monitoring to determine if an area meets the 2010 1-hour sulfur dioxide National Ambient Air Quality Standard (1-hour SO2 NAAQS). These documents and are available on EPA's website.

These documents do not specify an emission threshold and other specifics in determining sources which should be modeled or monitored to show compliance with the standard. EPA will publish the regulatory action which establishes these threshold requirements (which EPA refers to as the 'data requirements rule') at a later date as a proposal for public comment (expected late 2013 or early 2014).

On July 22, 2013, DHEC submitted a comment letter on the proposed TADs.


On October 3, 2011, EPA published its proposed guidance on its plan for implementing the new 1-hour SO2 standard which relied on a hybrid modeling and monitoring approach to show compliance with the 1-hour SO2 standard.

Under this approach, states with areas designated nonattainment in 2012 would have needed to submit State Implementation Plans (SIPs) to EPA by early 2014 outlining actions that would be taken to meet the standards as expeditiously as possible, but no later than August 2017. Also under this proposed approach all other areas, even those designated unclassifiable, would need to submit to EPA robust "maintenance" or infrastructure SIPs by June 2013, three years following the promulgation of the new SO2 standard. On December 2, 2011, DHEC submitted a comment letter to EPA on the proposed guidance.

However, based on adverse comments to the aforementioned implementation strategy, on April 12, 2012, EPA sent a letter to all states outlining next steps with regard to implementation of the 1-hour SO2 standard.

In the letter, EPA stated that they plan to:

  1. Move forward with the current designations process as soon as possible. Areas that have a violating monitor will be classified "nonattaintment." All other areas will be classified "unclassifiable."
  2. Initiate stakeholder outreach in the near future to ask for input on monitoring, modeling, and implementation issues, particularly for unclassifiable areas.
  3. No longer expect Infrastructure SIP submittals (due June 2013) to contain modeling demonstrations showing attainment of the standard in unclassifiable areas.

EPA sought additional input from states, tribes, and other interested parties to refine its approach for implementing the SO2 standard by holding several stakeholder meetings and accepting written comments.

EPA released a final version of a white paper detailing options for implementation of the 1-hour SO2 standard. The white paper will form the basis for discussion at upcoming meetings with stakeholders to discuss implementation of the SO2 standard. On June 29, 2012, DHEC submitted comments on the final version of the white paper.

On February 6, 2013, EPA issued a letter to SC indicating that it would not designate any areas of the state for the new 1-hour SO2 National Ambient Air Quality Standard (standard). The EPA also announced that it would be working on developing an updated strategy for accurately identifying SO2 levels throughout the country which will focus on modeling and/or monitoring sources which emit SO2 above a certain threshold. Under this anticipated approach, EPA expects states to identify sources by early 2015, and provide modeling demonstrations or establish monitoring networks by late 2015. Those sources which are modeled would be designated 18 months after modeling demonstrations are submitted (December 2017) and attainment SIPs would be due August 2019. Those sources that are monitored would be designated after three years of monitored data is obtained (expected in December 2020) and attainment SIPs would be due August 2022. The EPA has published a document outlining its anticipated approach. The EPA intends to follow rulemaking to establish a final implementation rule. A proposed implementation rule is expected in late 2013, and a final rule in late 2014.

More info about these actions, and future efforts can be found at EPA's website.

For more information please contact the Bureau of Air Quality at (803) 898-4123 or by email.