Area Designations for the 2010 Primary Sulfur Dioxide National Ambient Air Quality Standard - Round 4 (September 2019)
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.