Sulfur Dioxide (SO2)

Contact: Air Quality Related Issues: Bob Irvine, 517-284-6749
Agency: Environmental Quality

The United States Environmental Protection Agency (USEPA) revised the primary National Ambient Air Quality Standard (NAAQS) for sulfur dioxide (SO2) on June 2, 2010. The new short-term standard is based on the 3-year average of the 99th percentile of the yearly distribution of 1-hour daily maximum concentrations. This level was set at 75 parts per billion (ppb). The designations for the new 1-hour SO2 standard were performed in three rounds.


Round One

The designations for Round One covered areas which, based on certified ambient air quality monitoring data for the years 2009-2011, showed violations of the 1-hour SO2 standard. That standard was not being met at the Michigan Department of Environmental Quality (MDEQ) monitoring station located at Southwestern High School in Detroit. Consequently, in July 2013, the USEPA formally designated a portion of southern Wayne County as "nonattainment" with the SO2 standard. This formal designation required the MDEQ to develop an air pollution abatement State Implementation Plan (SIP).

On May 31, 2016, the MDEQ submitted its SO2 SIP strategy for southern Wayne County to the USEPA for final approval. This SIP is the strategy for bringing the area into compliance with the health-based NAAQS for SO2.

Our strategy requires substantial SO2 reductions from two DTE coal-fired power plants and a U.S. Steel facility. In addition, Carmeuse Lime will be required to increase the height of their smokestack to lessen ground level impacts. DTE and Carmeuse Lime committed to controlling their SO2 emissions by modifying their existing air use permits. We developed State Rule 430 to make sure reductions from U.S. Steel are permanent and enforceable.

The May 31, 2016, SIP submittal contained the draft version of Rule 430. The Rule since has become final with an effective date of June 14, 2016. An addendum to the strategy, containing final Rule 430 and related updates, was submitted to the USEPA on June 30, 2016.

Concurrent with the development of this abatement SIP, our air monitoring station at Southwestern High School has come into compliance with the 75 ppb NAAQS. While air quality in the area has obviously improved, computer modeling of strategies within our plan is needed to demonstrate compliance with the NAAQS for those areas within the nonattainment area where we do not have air monitoring stations.

SO2 SIP Documents for Wayne County Nonattainment Area


Round Two

The designations in Round Two covered stationary sources that emitted more than 16,000 tons of SO2 in 2012 or emitted more than 2,600 tons of SO2, has a 2012 emission rate of at least 0.45 pounds (lbs) SO2 per million BTU (mmbtu), and that had not been announced (as of March 2, 2015) for retirement. USEPA identified eight coal-fired power plants which meet this criteria. The MDEQ proceeded with a designation analysis for these facilities and this information was submitted to the USEPA.

On July 1, 2016, the USEPA confirmed that six counties containing large, coal-fired power plants are in attainment of the federal NAAQS for SO2 based on MDEQ’s computer modeling. The counties are Bay, Eaton, Ingham, Marquette, Monroe, and Ottawa.

On July 1, 2016, the USEPA also confirmed that the air in southeastern St. Clair County exceeds the NAAQS for SO2. The finding, based on MDEQ’s computer modeling and recommendation, designates the southeastern portion of the county in nonattainment with the Clean Air Act’s NAAQS for SO2. To address this public health concern, the MDEQ must develop a SIP by March 12, 2018 to lower SO2 air pollution in the county. The plan must require SO2 reductions in the area sufficient to attain the NAAQS within five years, and sooner if possible.

The MDEQ’s modeling shows the emissions from two coal-fired power plants, DTE Belle River and DTE St. Clair, are causing SO2 levels that exceed the new 1-hour standard. The MDEQ is working with DTE to develop a SIP that will reduce SO2 emissions to protect public health and bring the area’s air into compliance with the federal standard. DTE recently announced that the St. Clair power plant will cease operating in 2020-2023.

The MDEQ will keep the public informed as a SIP is developed, and will make a draft SIP available for public review and comment.

SO2 Designation Documents and SIP Documents for St. Clair County


Round Three

The Round Three designations affect stationary sources subject to the USEPA Data Requirements Rule (DRR). Under this rule, designations are required for areas having sources that emit more than 2,000 tons per year of SO2, and were not addressed in previous rounds. Two facilities were identified as falling within the emissions levels referenced in the DRR. The MDEQ designation analysis for these two facilities showed impacts are meeting the NAAQS for SO2 based on computer modeling. The MDEQ submitted its designation recommendations and supporting documents of this analysis to the USEPA in January and February, 2017. The USEPA must designate these areas by December 31, 2017.

SO2 Designation Documents


DEQ Documents & Other Resources