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Sulfur Dioxide (SO2)


Sulfur oxides (SOx) are colorless gases that are the result of burning sulfur. All fuels used by man (oil, coal, natural gas, wood, etc.) contain some sulfur, and during the combustion process, sulfur reacts with oxygen to form SOx. The primary source of sulfur oxides is the burning of these fossil fuels, particularly coal, at industrial facilities. Sulfur dioxide(SO2) is used as an indicator of all SOx concentrations in the ambient air, because it is the most easily measured.

Sulfur dioxide is known to irritate the respiratory system. SO2 is particularly detrimental to individuals who suffer from respiratory diseases such as asthma and chronic bronchitis. SO2 can also combine with particles and moisture in the air creating an even greater health risk. It contributes to the formation of acid rain by transforming into sulfuric acid in the atmosphere. Acid rain can damage lakes and aquatic life, building materials, and plant life.

Sulfur dioxide emissions from combustion at power plants are controlled by “scrubbing” the gas leaving the plant or by removing sulfur from the fuel before it is burned. Individuals can reduce SO2 emissions by limiting the use of electricity, by turning off electric devices not in use, and by making use of more efficient electric devices (lights, refrigerators, motors, etc.) and alternate energy sources.

Emissions of SO2 in Nonattainment Areas by Source

Standard Status

In 1978, the EPA designated Salt Lake and Tooele Counties as a nonattainment area for SO2 based on ambient data collected at air monitoring locations in Magna and Tooele. In 1981, the EPA removed the nonattainment status for all of Tooele County except the eastern portion above 5,600 feet. Later in 1981, the State submitted an implementation plan (SIP) to the EPA that addressed the SO2 violations. The primary control measure was replacement of Kennecott Copper Corporation’s reverberatory smelter with a new and cleaner Noranda smelter. The EPA approved the State’s SIP in 1994, and no violations of the standard have been recorded since the new smelter and the “tall stack” were constructed. In 2005, Utah requested that EPA re-designate the area to attainment.

On June 2, 2010, EPA strengthened the primary National Ambient Air Quality Standard (NAAQS) for SO2 by establishing a new 1-hour standard designed to protect sensitive individuals from high, short-term (5-minutes to 24-hours) exposure. The level is set at 75 parts per billion (ppb). This new 1-hour standard will replace the two existing primary standards, which were:

  • 140 ppb averaged over 24-hours—EPA felt that retaining the existing standard would not add any protection beyond what the new 1-hour standard provides.
  • 30 ppb as an annual average—There is little health evidence to suggest an association between long-term exposure to SO2 and adverse health effects.

EPA’s current action focuses only on the primary SO2 standard. It will address the secondary standard for SO2 as part of a separate review. The current secondary standard to protect public welfare is a 3-hour average set at 500 ppb.

EPA intends to determine which areas are or are not attaining the revised primary standard by June 2012.

  • States with areas designated nonattainment in 2012 would need to submit SIPs to EPA by 2014. These areas will actually need to meet the standards by August 2017.
  • All other areas, states would need to submit “maintenance” or infrastructure SIPs by June 2013.

The final rule also changes the Air Quality Index to consider the revised SO2 standard.

SO2 concentrations are not a function of meteorology to the same extent as CO, PM10 and ozone. Therefore, we do not typically compare SO2 to any meteorological parameter. SO2 concentrations have decreased dramatically over the past eight years due to major SO2 controls at industrial facilities in the area. Most notably, Kennecott’s Flash Smelting Process came on line in June 1995 and made a significant difference in SO2 concentrations in the Wasatch Front region.

Sulfur dioxide tends to be a localized problem and therefore the network locates a monitor near the largest sources of SO2. The maximum values of SO2 have been far below the old NAAQS for the past 25 years and are also below the new NAAQS

Area Designation Process

Nonattainment designation is determined by the EPA when an area or areas within a state persistently exceed the National Ambient Air Quality Standards.

The Clean Air Act allows the States to recommend initial designations for all areas following promulgation of new or revised NAAQS.

Areas should be designated as attaining, not attaining, or as unclassifiable. EPA must finalize the area designations as expeditiously as practicable, but not later than two years following the effective date of the revised NAAQS.


For more information contact Glade Sowards (gladesowards@utah.gov) (801) 536-4020.

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