The purpose of this memorandum is to provide information on the schedule and process for designating areas for the 2010 primary sulfur dioxide (SO₂) national ambient air quality standard (NAAQS). In addition, it identifies factors for determining the boundaries for SO2 areas designated nonattainment, attainment and unclassifiable. We recommend that states and tribes consider and address these factors when identifying boundaries for any updated area designation recommendations they choose to submit for the 2010 SO₂ NAAQS.

This guidance updates and replaces the previous designation guidance for the 2010 S02 NAAQS, which was issued on March 24, 2011.

For more information on the rule please go to 2010 Sulfur Dioxide (SO2) National Ambient Air Quality Standards (NAAQS) Implementation Actions.

• Updated Guidance for Area Designations for the 2010 Primary Sulfur Dioxide (SO2) National Ambient Air Quality Standards (NAAQS) (PDF)(11 pp, 719 K, March 20, 2015)