Sulfur dioxide

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

Sources of Sulfur Dioxide emissions The Minnesota Criteria Pollutant Emissions Inventory includes emissions from four principal sources:

  • 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

Monitoring sites


SO2 concentrations

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.


Minnesota statewide map of sulfur dioxide modeled sources


Twin Cities area map of sulfur dioxide modeled sources

For more information

  • State Implementation Plan: Amanda Smith, 651-757-2486
  • Permits: Dick Cordes, 651-757-2291
  • Modeling: Ruth Roberson, 651-757-2672
  • Emissions: Nathaniel Edel 651-757-2332
  • Monitoring: Kari Palmer 651-757-2635