This memorandum provides information on the schedule and process for the area designations for the 2010 primary sulfur dioxide (SO₂) national ambient air quality standard (NAAQS) that EPA must complete by a court-ordered deadline of December 31, 2020 (“Round 4”). This document supplements, where necessary, our prior designations guidance on area designations for the 2010 primary SO₂ NAAQS to better reflect the Round 4 designations process.

• Memorandum: Area Designations for the 2010 Primary Sulfur Dioxide National Ambient Air Quality Standard – Round 4 (PDF)(11 pp, 780 K, 9/5/2019)