An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »

Sulfur Dioxide Designations

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.

You may need a PDF reader to view some of the files on this page. See EPA’s About PDF page to learn more.