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On June 2, 2010, the EPA established a revised primary SO2 National Ambient Air Quality Standards (NAAQS) at 75 parts per billion (ppb), which is met at a monitoring site when the 3-year average of the 99th percentile of daily maximum 1-hour concentrations does not exceed 75 ppb (75 FR 35520). On August 5, 2013, the EPA published a notice announcing designations of 29 areas in 16 states as nonattainment for the 2010 primary SO2 standard ("Round 1"), based on certified ambient air quality monitoring data (78 FR 47191). However, at that time, the EPA was not yet prepared to issue designations for the remaining areas of the country. Subsequently, three lawsuits were filed against the EPA in different U.S. District Courts alleging that the agency had failed to perform a nondiscretionary duty under the Clean Air Act (CAA) by not designating all portions of the country by the June 3, 2013, deadline. Under a consent decree entered by the Court on March 2, 2015, in one of those cases, the EPA is required to complete the remaining area designations according to a specific schedule with the following deadlines: July 2, 2016 ("Round 2"); December 31, 2017 ("Round 3"), and December 31, 2020 ("Round 4"). On June 30, 2016, and November 29, 2016, the EPA signed a Federal Register notice completing the designations for the Round 2 areas subject to the July 2, 2016, court-ordered deadline, designating 7 areas nonattainment, 17 areas unclassifiable and 41 areas unclassifiable/attainment (81 FR 45039 and 81 FR 89870). The schedule for completing the remaining designations overlaps with submission of SO2 air quality information by States to the EPA under the SO2 Data Requirements Rule (DRR) (80 FR 51052). This information may be used by the EPA in evaluating areas' air quality in relation to the 2010 SO2 primary NAAQS, including designations and redesignations (80 FR 51052). On December 21, 2017, and March 28, 2018, the EPA signed a Federal Register notice completing the designations for the Round 3 areas subject to the December 31, 2017, court-ordered deadline, designating 6 areas nonattainment, 22 areas unclassifiable, and the remaining areas of the United States that were not, pursuant to the DRR, operating a new EPA-approved monitoring network as attainment/unclassifiable (83 FR 1098 and 83 FR 14597). This is the fourth and final action for the 2010 SO2 NAAQS designations. By the December 31, 2020, court-ordered deadline, the EPA is required to designate all remaining areas of the United States, specifically those areas that, pursuant to the DRR, began operating a new EPA-approved monitoring network.