The Clean Air Act regulates sulfur dioxide as a criteria pollutant. The U.S. Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards for sulfur dioxide (SO2). These standards include both standards to protect public health and secondary standards to protect the environment. The annual and 24-hour standards were set in 1971, along with secondary annual and 3-hour standards.
EPA reviews pollutant standards every five years. Sometimes, a review results in a tightened standard. The secondary annual standard was revoked in 1973, but the existing 3-hours secondary standard of 500 parts per billion (ppb) was retained. When EPA revised the sulfur dioxide standard in 2010, they revoked the two previous standards, replacing them with a new 1-hour standard of 75 ppb. The form of the 1-hour standard is the 3-year average of the 99th percentile of the yearly distribution of 1-hour daily maximum SO2 concentrations.
When EPA revised the 2010 sulfur dioxide standard, they also chose a different approach to establishing boundary designations. EPA decided to use a hybrid monitoring-modeling approach for sulfur dioxide, with a series of variable designation dates based on the designation approach selected. The initial round for boundary designations was based on available ambient air quality monitoring data at the time EPA established the standard. The second round was targeted at areas surrounding facilities that had the largest annual SO2 emissions. The third round was targeted at areas surrounding facilities with emissions more than 2,000 tons per year that elected to characterize air quality using refined dispersion modeling analyses. The fourth and final round is targeted at areas surrounding facilities with emissions more than 2,000 tons per year that elected to characterize air quality using newly deployed ambient SO2 monitors in the areas of maximum expected SO2 concentrations surrounding such sources.
The SO2 standard below includes the state implementation plan elements developed for the standard and the relevant documents associated with the boundary designations for each round. If you are looking for a document that is not available on our website, please submit a Open Records/ Sunshine Law Request.
- Infrastructure SIP
Infrastructure SIP Actions MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document July 1, 2013 Section 110 Infrastructure Requirements for the 2010 Sulfur Dioxide National Ambient Air Quality Standard Sept. 24, 2018 Air Plan Approval; Missouri; Regional Haze Plan and Prong 4 (Visibility) for the 2012 PM2.5, 2010 NO2, 2010 SO2, and 2008 Ozone NAAQS [83 FR 48242] March 22, 2018 Approval of Implementation Plans; State of Missouri; Elements of the Infrastructure State Implementation Plan Requirements for the 2008 Ozone, 2010 Nitrogen Dioxide, 2010 Sulfur Dioxide, and 2012 Fine Particulate Matter National Ambient Air Quality Standards (NAAQS) [83 FR 12496] - Initial Round - Existing Monitors
Initial Round: Boundary Recommendations Actions MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document April 26, 2013 Revised Boundary Recommendation of Nonattainment and Unclassifiable Areas Per the 2010 1-Hour Sulfur Dioxide National Ambient Air Quality Standard Aug. 5, 2013 Air Quality Designations for the 2010 Sulfur Dioxide (SO2) National Ambient Air Quality Standard [78 FR 47191]
Effective: Oct. 4, 2013July 19, 2011 2010 Sulfur Dioxide National Ambient Air Quality Standard Summary of Recommendation of Nonattainment and Unclassifiable Boundaries in Missouri Feb. 6, 2013 EPA Response to Recommendations on 2010 National Ambient Air Quality Standard for Sulfur Dioxide - 120-day letter for the initial round designations
- Round Two - Consent Decree
EPA designated no nonattainment areas in Missouri during this round.
Round Two - Boundary Recommendations Actions MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document Sept. 2, 2020 Redesignation Request for the Labadie Unclassifiable Area Under the 2010 Sulfur Dioxide Standard Aug. 6, 2020 EPA Response to Redesignation Request for Portions of Franklin and St. Charles Counties Under the 2010 Sulfur Dioxide Standard April 19, 2016 MoDNR Response to EPA Proposed February 2016 Sulfur Dioxide Area Designation July 12, 2016 Air Quality Designations for the 2010 Sulfur Dioxide (SO2) National Ambient Air Quality Standard-Round 2 [81 FR 45039]
Effective: 9/12/2016Sept. 24, 2015 2010 1-Hour Sulfur Dioxide Standard Area Boundary Recommendations, July 2016 Designation Feb. 16, 2016 EPA Response to Recommendations on Air Quality Designations for 2010 National Ambient Air Quality Standard for Sulfur Dioxide - 120-day letter for round two designations
- Round Three - Modeling
In August 2015, EPA finalized the data requirements rule, which details options for evaluating areas in rounds three and four of the 2010 SO2 standard boundary designations. Round Three is for sources characterized by refined dispersion modeling of actual emissions. EPA designated no nonattainment areas in Missouri during this round.
- Round Four - Monitoring
Round Four is for sources characterized by newly installed SO2 monitoring networks located in the areas of expected maximum concentration near high-emitting SO2 sources. The EPA data requirements rule required Round Four monitors to begin operating by Jan. 1, 2017. State boundary recommendations were due by May 1, 2020, and EPA final boundary designations was due by Dec. 31, 2020. Three areas of Missouri have ambient SO2 monitoring networks that were installed according to EPA’s data requirements rule. The Labadie, Missouri source had already been designated unclassifiable under the Round Two Designations, which may be considered for redesignation based on monitored SO2 concentrations surrounding the facility.
Round Four - Monitoring Actions MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document Oct. 16, 2020 Missouri Response to EPA 120-Day Letter on Round Four Designations
March 26, 2021 Air Quality Designations for the 2010 Sulfur Dioxide (SO2) National Ambient Air Quality Standard-Round 4 [86 FR 16055]
Effective: April 20, 2021April 30, 2020 Proposed Area Boundary Recommendations for the 2010 Sulfur Dioxide Standard: December 2020 Designations Aug. 13, 2020 Round Four - Monitoring: Nonattainment Area Plans
EPA finalized designations for Round Four with an effective date of April 30, 2021. EPA designated a portion of New Madrid County as nonattainment in this round. This required the department to develop a nonattainment area plan for the area within 18 months.
New Madrid County Nonattainment Area Actions
MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document May 3, 2023 New Madrid County Nonattainment Area Plan 2010 Sulfur Dioxide Standard
Pending Pending Nov. 3, 2021 New Madrid Nonattainment Area Plan for the 2010 Sulfur Dioxide Standard Presentation, Nov. 3, 2021 - Annual Ongoing Data Requirements Report
For sources designated attainment according to the EPA SO2data requirements rule based on modeling of actual emissions (generally Round Three sources), the federal rule requires the department to submit annual ongoing emissions evaluations from these sources to ensure emissions have not increased to a level that may cause a violation of the 2010 SO2 standard.
- 2023 Annual Ongoing Data Requirements Report for Sulfur Dioxide
- 2022 Annual Ongoing Data Requirements Report for Sulfur Dioxide
- 2021 Annual Ongoing Data Requirements Report for Sulfur Dioxide
- 2020 Annual Ongoing Data Requirements Report for Sulfur Dioxide
- 2019 Annual Ongoing Data Requirements Report for Sulfur Dioxide
- 2018 Annual Ongoing Data Requirements Report for Sulfur Dioxide
- 2017 Annual Ongoing Data Requirements Report for Sulfur Dioxide
- Other SO2 SIPS