Sulfur dioxide (SO2) is a colorless gas with a noticeable odor. The department monitors sulfur dioxide levels in the air at locations across the state. SO2 is one of the Environmental Protection Agency's criteria air pollutants – commonly found air pollutants that can harm people and the environment at high enough levels. SO2 also contributes to the formation of secondary fine particle pollution, which is a subset of particulate matter, another criteria pollutant. 

SO2 Standards

In June 2010, the EPA established a new 1-hour primary sulfur dioxide standard of 75 parts-per-billion. The previous sulfur dioxide primary standards set in 1971 included a 24-hour standard at 140 parts-per-billion and annual standard at 30 parts-per-billion. There is also a secondary sulfur dioxide standard based on a 3-hour average set at 500 parts per billion. When the EPA revised the sulfur dioxide standard in 2010, they revoked the two previous primary standards, replacing them with the new 1-hour primary standard and they retained the existing secondary standard. 

2010 1-hr SO2 Primary Standard: 75 ppb

When the EPA revises a National Ambient Air Quality Standard, states are allowed to submit boundary designation recommendations to the EPA for consideration when establishing the final boundary designations. For most criteria pollutants, states are given one year to submit their recommendations and the EPA finalizes boundary designations within two years.

The EPA has chosen a different approach to establish boundary designations under the 2010 sulfur dioxide standard. The EPA has 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.

Infrastructure SIP

Infrastructure SIP Actions Table

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
July 1, 2013

Infrastructure Elements for the 2010 SO2 Standard

 

Sept. 24, 2018

March 22, 2018

Approval of Element 110(a) (2)(D)(i) prong 4: 
[83 FR  48242]

Approval of all elements except 110(a)(2)(D)(i) prongs 1, 2 and 4:
[83 FR 12496]

Initial Round - Existing Monitors

Initial Round: Boundary Recommendations Actions Table

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
April 26, 2013

Revised Initial Round 2010 SO2 Boundary Recommendations

Aug. 5, 2013

Final Boundary Designations for Initial Round 
[78 FR 47191]
Effective: 10/04/2013

July 19, 2011 Original Initial Round 2010 SO2 Boundary Recommendations Feb. 6, 2013

EPA 120-Day Letter for Initial Round SO2 Boundary Designations

Attachment to EPA 120-Day Letter

Jefferson County (Partial) Actions Table

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
April 9, 2021 Jefferson County SO2 Maintenance Plan Supplement
  June 29, 2021

Proposed Approval

[86 FR 34177]

Dec. 21, 2017

Jefferson County 2010 SO2 Redesignation Request and Maintenance Plan

Feb. 2, 2016 Jefferson County 2010 SO2 Clean Data Determination Request Sept. 13, 2017

Approved - Clean Data Determination
[82 FR 28605]

May 29, 2015

Jefferson County 2010 SO2 Nonattainment Area Plan
(All elements withdrawn from EPA consideration except the base year emissions inventory - see entries below)

Feb. 13, 2019 Approved - Based Year Inventory for Jefferson County SO2 Nonattainment Area
[84 FR 3703]
March 28, 2018 Withdrawal Request - Jefferson County 2010 SO2 Nonattainment Area Plan N/A N/A
March 30, 2018 Withdrawal Request - 2010 SO2 Rule N/A N/A
May 4, 2018 Withdrawal Request - Clarification Letter N/A N/A

Jackson County (Partial) Actions Table

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
Sept. 3, 2021 Jackson County SO2 Maintenance Plan Supplement Pending Pending
Feb. 7, 2021 Jackson County SO2 Maintenance Plan Pending Pending
Feb. 7, 2021 Jackson County SO2 Redesignation Request Pending Pending
May 4, 2018

Jackson County 2010 SO2 Clean Data Request

July 9, 2020

Approved - Clean Data Determination
[85 FR 41193]

Oct. 9, 2015

Jackson County SO2 Nonattainment Area Plan (All elements withdrawn from EPA consideration except the base year emissions inventory - see entries below)

Feb. 13, 2019 Approved - Base Year Inventory for Jackson County SO2 Nonattainment Area
[84 FR 3703]
June 11, 2018 Jackson County Withdrawal Request N/A N/A
March 30, 2018 SO2 Rule Withdrawal Request N/A N/A
Round Two - Consent Decree Round

EPA designated no nonattainment areas in Missouri during this round

Round Two - Boundary Recommendations Actions Table

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
Sept. 24, 2015

2nd Round 2010 Sulfur Dioxide Boundary Recommendations

July 12, 2016

Final Boundary Designations for Second Round (Consent Decree)
[81 FR 45039]
Effective: 9/12/2016

April  19, 2016

Response to EPA 120-Day Letter

 

Feb. 16, 2016

EPA 120-Day Letter for SO2 Round 2 Designations

Attachment to EPA 120-Day Letter for SO2 Round 2 Designations

Sept. 2, 2020 Redesignation Request for the Unclassifiable Area Surrounding the Labadie Energy Center Aug. 6, 2020

EPA 120-Day Letter on Redesignation for the Unclassifiable Area Surrounding the Labadie Energy Center

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 3 - Boundary Recommendations Actions Table

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
Dec. 8, 2016

3rd Round 2010 Sulfur Dioxide Boundary Recommendations

Jan. 9, 2018

Final Boundary Designations for Third Round (Modeled Sources)
[83 FR 1098]
 

Oct. 23, 2017

Response to EPA 120-Day Letter for Round Three SO2 Boundary Designations

Aug. 22, 2017 EPA 120-Day Letter for Round Three SO2 Boundary Designations
May 3, 2018 Clarification Letter - Sibley Generating Station N/A N/A
July 2016 Original Modeling Protocol for Round Three DRR sources N/A NA
June 28, 2016 Methods for Characterizing SO2 Air Quality for Round Three and Four DRR Sources N/A N/A
Jan. 15, 2016 List of Sources Subject to the 2010 SO2 DRR N/A N/A
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 for Round Four are due by May 1, 2020, and EPA must establish final boundary designations by Dec. 31, 2020.

Three areas of the state have ambient SO2 monitoring networks that were installed pursuant to EPA’s data requirements rule and were operating by Jan. 1, 2017. One of these sources, in Labadie, Missouri had already been designated unclassifiable under the Round Two Designations (Consent Decree), which may be considered for redesignation based on monitored SO2 concentrations surrounding the facility.

Round Four - Monitoring Actions Table

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
    March 26, 2021

EPA finalizes round-4 designations for the 2010 SO2 standard [86 FR 16055]

Oct. 16, 2020

Missouri Response to EPA 120-Day Letter on Round Four Designations

Aug. 13, 2020

EPA 120-Day Letter on Round Four Designations

EPA Technical Support Document for 120-Day Letter

April 30, 2020 SO2 Round Four Boundary Recommendations May 26, 2021

Final Boundary Designations for Fourth Round (Monitored Sources)

[86 FR 16055]

Effective Date: April 20, 2021

Round Four - Monitoring: Nonattainment area plans

EPA finalized designations for Round 4 with an effective date of April 30, 2021. EPA designated a portion of New Madrid County as nonattainment in this round. This requires the department to develop a nonattainment area plan for the area within 18 months.

New Madrid County Nonattainment Area Actions Table

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
Nov. 3, 2021 Stakeholder Meeting Presentation on the New Madrid Nonattainment Area Plan for the 2010 Sulfur Dioxide Standard, Nov. 3, 2021    

October 2022

The department is currently developing the nonattainment area plan for the New Madrid County SO2 nonattainment area.    
Annual Ongoing Data Requirements Report

For sources designated attainment pursuant to the EPA SO2 data requirements rule based on modeling of actual emissions (generally Round Three sources), the federal rule requires the department to submit annual ongoing evaluations of emissions from these sources to ensure emissions have not increased to a level that may cause a violation of the 2010 SO2 standard.

Annual Ongoing Data Requirements Report Actions Table

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
June 23, 2021 2021 Annual Ongoing DRR Report N/A N/A
June 26, 2020

2020 Annual Ongoing DRR Report

N/A

N/A

June 24, 2019 2019 Annual Ongoing DRR Report N/A N/A
June 29, 2018 2018 Annual Ongoing DRR Report N/A N/A
June 29, 2017 2017 Annual Ongoing DRR Report N/A N/A

Related Links

Weekly Summary of Monitoring SO2 Values

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