Air pollutant: Sulfur dioxide (SO2)
Sulfur dioxide (SO2) is a gas primarily emitted from fossil fuel combustion at power plants and other industrial facilities, as well as fuel combustion in mobile sources such as locomotives, ships, and other equipment. Over 100,000 tons of SO2 were emitted in 2009 in Minnesota. Current scientific evidence links SO2 exposure with adverse impacts on the respiratory system. In recent reviews of the standard, EPA has determined that even short term exposure to high levels of SO2 can have a detrimental effect on breathing function, particular for those that suffer from asthma. SO2 also reacts with other chemicals in the air to form tiny sulfate particles, contributing to levels of PM 2.5.
SO2 also reacts with other chemicals in the air to form acids, which fall to the earth as acid rain. Acid rain damages forests and crops, changes the makeup of soil, and makes lakes and streams acidic and unsuitable for fish.
In the 1990 Clean Air Act Amendments, EPA implemented the Title IV cap-and-trade program to reduce emissions of SO2 and nitrogen oxides (NOX) from power plants. This has resulted in major decreases in SO2 across the United States.
For more information on SO2 in the air, visit EPA’s sulfur dioxide page.
Sources of SO2 emissions
Data for electric utilities and point sources are from the 2009 Minnesota Emissions Inventory. All other data are from the 2005 Minnesota Emissions Inventory.
The Minnesota Criteria Pollutant Emissions Inventory includes emissions from four principal source categories:
- Point sources: Large, stationary sources with relatively high emissions, such as electric power plants and refineries.
- Nonpoint sources: Smaller stationary sources such as dry cleaners, gasoline service stations and residential wood burning. May also include diffuse stationary sources such as wildfires and agricultural tilling.
- On-road vehicles: Vehicles operated on highways, streets and roads.
- Non-road sources: Off-road vehicles and portable equipment powered by internal combustion engines. Includes lawn and garden equipment, recreational equipment, construction equipment, aircraft and locomotives.
Point sources are estimated annually, while the other categories are estimated every three years.
SO2 monitoring in Minnesota
MPCA monitors SO2 in the Twin Cities to help show compliance with the National Ambient Air Quality Standards (NAAQS) and the data is also reported as part of the Air Quality Index. Currently, monitoring data indicates that Minnesota meets the NAAQS.
For more information on SO2 monitoring, see the Annual Air Monitoring Network Plan.
2010 SO2 monitoring sites in Minnesota
Regulation of SO2 under the NAAQS
A new one-hour NAAQS for SO2 went into effect on August 23, 2010. Previously, EPA had standards for levels of SO2 over 24-hour and annual averaging periods. However, based on new evidence showing adverse health impacts even after shorter periods of exposure, EPA chose to set a standard with a shorter averaging time, and to eventually revoke the prior NAAQS.
The new standard is a one-hour standard of 75 ppb (197 µg/m3), calculated as the three-year average of the 99th percentile of the annual distribution of daily maximum values.
Designation and implementation process
When a new standard is promulgated, States must determine if they attain the standard. The state usually does this through reviewing monitoring data. The state must then make a recommendation to EPA on whether all or part of the State should be designated as meeting the standard (attainment), not meeting the standard (nonattainment) or if insufficient data exists to make a recommendation (unclassifiable).
EPA guidance for designating areas for this new standard is available on the EPA website.
On May 23, 2011, the MPCA submitted a designation recommendation to EPA. The MPCA recommended that all counties in Minnesota that do not contain any sources with a potential to emit over 100 tons per year of SO2 be designated in attainment with the standard. The MPCA recommended an unclassifiable designation for all other counties. EPA will review this recommendation, and is scheduled to promulgate initial area designations by June 3, 2012.
States with areas that are designated as nonattainment with the new standard will need to submit a State Implementation Plan (SIP) in February 2014 that shows attainment by July 2017. Minnesota does not have any monitored violations of the standard, so this will not apply to Minnesota
States with areas that are designated as in attainment or unclassifiable with the new standard need to submit a 110(a) or “infrastructure” SIP by June 2013. In general, states use the 110(a) SIP to demonstrate that they have appropriate authorities and infrastructure (rules, monitoring, permitting and compliance/enforcement programs) to ensure that the state will maintain the NAAQS.
EPA initially planned to require more robust 110(a) SIPs that demonstrate attainment and maintenance of the new SO2 standard. Draft EPA implementation guidance, released in September 2011, stated that the 110(a) SIP would have to include:
- SO2 emission reductions from compliance with national and regional regulations
- A demonstration, using refined modeling, that all sources that could cause or contribute to a modeled or monitored violation of the standard will be sufficiently controlled to ensure attainment
- Any source emitting over 100 tpy of SO2, along with any source included in Minnesota’s existing SO2 maintenance SIP, will probably be considered as one that could cause or contribute to a violation
- Enforceable emission limitations, timetables for compliance, and testing/reporting to assure compliance with emission limitations, if these limitations are needed to ensure attainment
- Facilities that model over the standard at current allowable emissions will need to reduce allowable emissions in order to demonstrate compliance
In April 2012, EPA announced a change in the implementation guidance for the SO2 standard. EPA stated that the 110(a) SIP would not need to include refined modeling. Instead, EPA would work with states and stakeholders to develop an approach for implementation. More information is available on the EPA's SO2 implementation website.
The initial hybrid implementation approach required significant modeling, as EPA indicated that any source that could cause or contribute to a violation of the new standard should be modeled.
In summer 2011, the MPCA requested parameter information from facilities that 1) emit over 100 tpy of SO2, based on recent years of data from MPCA's emission inventory; 2) have a potential to emit over 100 tpy; 3) have a screening model or previous detailed modeling showing that the facility could exceed the new standard; or 4) were a culpable source in Minnesota's prior nonattainment of the existing SO2 NAAQS and therefore has an Administrative Order or permit with Title I SIP conditions to control SO2 emissions.
The MPCA used this information to conduct baseline modeling for implementing the new SO2 standard, based on a facility's current allowable emissions. A summary of the modeled results is below. This modeling was conducted using very conservative assumptions, and allowable (not actual) emissions from each facility. It is not fully refined modeling and should not be considered definitive evidence of compliance status.
Notification of modeled results was sent to the facilities in March 2012. Facilities that showed a modeled exceedance must provide to the MPCA by September 28, 2012, a demonstration of modeled compliance and information on any necessary emission limits to resolve the exceedance.This information request was placed on hold on April 12, 2012, pending EPA's final implementation guidance for the standard.
Information below includes modeling guidance, the modeling parameters workbook, and a template for the previously proposed response to the MPCA.
- SO2 1-hr Modeling Information Guidance
- SO2 Modeling Parameters Workbook
- SO2 Exceedance Notification Response Template
Modeled sources and results
Maps of modeled sources
Statewide map of modeled sources of SO2
Metro map of modeled sources of SO2
For more information
- State Implementation Plan: Catherine Neuschler, 651-757-2607
- Permits: Dick Cordes, 651-757-2291
- Modeling: Ruth Roberson, 651-757-2672
- Emissions: Nathaniel Edel 651-757-2332
- Monitoring: Cassie McMahon 651-757-2564