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Sulfur Dioxide NAAQS Implementation

The U.S. Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards (NAAQS) for criteria pollutants, including sulfur dioxide (SO2). On June 2, 2010, EPA revised the SO2 standard, establishing a one-hour primary standard of 75 ppb. As of January 12, 2022, Wisconsin is in attainment of this NAAQS statewide.

The EPA completed area designations for this NAAQS in four rounds. The following sections provide information about each round of designations in Wisconsin, the State Implementation Plan (SIP) revisions DNR developed to describe how areas will be brought into attainment and other relevant documents. More information about EPA's designations for this standard can be found on EPA's Air Quality Designations for Sulfur Dioxide page.

For general information about SO2, visit Air Pollutants and Standards.

Round 1

The designations for Round 1 covered areas that violated the 2010 one-hour SO2 NAAQS based on certified ambient air quality monitoring data for the years 2009 to 2011. The EPA designated one nonattainment area in Wisconsin in Round 1: a portion of Oneida County including the city of Rhinelander and the towns of Crescent, Newbold, Pine Lake and Pelican.

On January 12, 2022, EPA finalized redesignation of the Rhinelander nonattainment area to attainment/maintenance status and approved the state’s maintenance plan for the area. The rule and supporting materials can be found in EPA’s docket for the rule.

Area Designation Documents

State Implementation Plan documents

Round 2

Round 2 designations involved two types of areas: (1) areas that violated the 2010 one-hour SO2 NAAQS based on air monitoring data collected from 2013 to 2015, and (2) areas containing sources that emitted SO2 above specified levels in 2012. The EPA designated Columbia County as unclassifiable/attainment in Round 2.

Area Designation Documents

Round 3

In December 2017, EPA finalized designations for all areas, except areas where states have established new monitors per EPA's Data Requirements Rule. All remaining areas of Wisconsin were designated unclassifiable/attainment in Round 3, except for Outagamie County, which EPA addressed in Round 4.

Area Designation Documents

State Implementation Plan Documents

Round 4

In December 2020, EPA designated all remaining areas of the country in its Round 4 designations, including Outagamie County. While EPA finalized a designation of nonattainment for a portion of Outagamie County based on 2017 to 2019 data, the area attained the NAAQS based on 2018 to 2020 data. Based on this information, in April 2021 EPA withdrew its initial nonattainment designation and finalized a designation of attainment/unclassifiable for this area.

Area designation documents

DNR comments on EPA implementation rules and guidance