Section 107(d)(1) of the Clean Air Act (CAA) provides up to one year after EPA’s adoption of a new or revised National Ambient Air Quality Standard (NAAQS) for states to recommend initial designation of areas as “attainment”, “nonattainment” or “unclassifiable”. 

In December 2024, EPA finalized a revised secondary SO₂ NAAQS to improve protection of public welfare, including visibility, ecosystems, and vegetation. This action replaced the previous 3-hour secondary standard with a new annual secondary standard set at 10 ppb, averaged over three years. In accordance with Section 107(d)(1) of the CAA, Connecticut evaluated current SO₂ air quality and determined that the state meets the revised secondary standard. Connecticut submitted a recommendation of “attainment” for the 2024 secondary SO₂ NAAQS and is currently awaiting EPA’s final designation.

Further information on the development and history of the SO₂ NAAQS can be found at Timeline of Sulfur Dioxide NAAQS | US EPA.

EPA's Letter to Connecticut Regarding the SO2 Data Requirements Rule. August 15, 2018

EPA's letter regarding final designations for the 2010 1-Hour SO2 Standard and the technical support document.

Final Infrastructure SIP Revision for the 2010 SO₂ NAAQS

Connecticut's Revised Recommendation Regarding Designations for the 1-Hour SO2 Air Quality Standard (PDF)

SO2 Designation Technical Support Document (PDF)

EPA's reply to Connecticut's SO₂ Designation Recommendation (PDF)

Connecticut’s Recommendation to EPA for an Initial Designation of “Unclassifiable” for the 1-Hour SO₂ NAAQS (PDF)