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Air Pollution from Sulfur Dioxide

General information on sulfur dioxide and associated air quality standards, Texas' compliance status, and latest air quality planning activities.

What is sulfur dioxide?

Sulfur dioxide (SO2) is one of a group of highly reactive gases called sulfur oxides (SOX). Exposure to SO2 can affect the respiratory system, especially for people with asthma. Studies show connections between short-term exposure and increased visits to emergency departments and hospital admissions for respiratory illnesses, particularly in populations at risk (including children, the elderly, and asthmatics). SOX can react with other compounds in the atmosphere to form small particles that cause or worsen respiratory disease or aggravate existing heart disease.

The largest source of SO2 emissions is fossil fuel combustion at power plants and other industrial facilities. SO2 emissions also come from extraction of metal from ore and burning high-sulfur fuels in locomotives, large ships, and non-road equipment.

National Ambient Air Quality Standards (NAAQS) for SO2

History of the SO2 NAAQS

The United States Environmental Protection Agency (EPA) issued annual and 24-hour primary (health-based) SO2 standards as well as three-hour and annual secondary SO2 standards in 1971. As a result of the EPA's review of the SO2 NAAQS in 1973, the secondary annual SO2 standard was revoked and the secondary three-hour standard was retained. Following a subsequent review of the NAAQS in 1996, the EPA retained both the annual and 24-hour primary SO2 standards.

Effective August 23, 2010, the EPA strengthened the SO2 NAAQS, adding a primary one-hour standard of 75 parts per billion (ppb) (75 FR 35520).Exit the TCEQ A one-hour standard was determined to better protect the public from exposure to high short-term SO2 concentrations, especially in communities located close to coal-fired power plants, industrial boilers, petroleum refineries, metal processing plants, and diesel exhausts.

In setting the 2010 primary one-hour SO2 standard, the EPA revoked the annual and 24-hour primary standards for areas not previously designated nonattainment, to become effective one year after designations for the 2010 standard are finalized. For areas of the country previously designated nonattainment for SO2 under the 1971 standards, the annual and 24-hour standards remain in effect until state implementation plans to attain or maintain the 2010 standard are approved by the EPA.

Texas Compliance with the SO2 NAAQS

Texas currently has no SO2 nonattainment areas. No areas of Texas have yet been designated under the 2010 one-hour SO2 air quality standard.

In April 2012, Texas recommended to the EPA that Dallas, Ellis, El Paso, Galveston, Gregg, Harris, Jefferson, Kaufman, McLennan, and Nueces Counties be designated attainment for the 2010 SO2 standard; and all remaining counties be designated unclassifiable. Texas' recommendation was based on 2011 design values for the state's existing SO2 regulatory monitors.

On August 5, 2013, the EPA published final nonattainment area designations for areas with 2009 through 2011 monitoring data indicating violations of the one-hour SO2 standard (78 FR 47191) Exit the TCEQ. No areas of Texas were designated.

Texas revised its recommendation for designations on September 18, 2015 (see the Latest Air Quality Planning Activities section for additional information).

Implementation of the 2010 Primary SO2 NAAQS

A consent decree to resolve EPA’s failure to timely promulgate designations was ordered by the U.S. District Court on March 3, 2015. The court-ordered settlement establishes deadlines for the EPA to complete designations. Areas with newly monitored violations or large emissions sources not announced for retirement as of March 2015, with 2012 SO2 emissions greater than 16,000 tons per year (tpy) or greater than 2,600 tpy with average emission rate greater than 0.45 pounds per million British thermal units (lbs/MMBtu), are to be designated by July 2, 2016. 

In a letter dated March 20, 2015, the EPA identified 12 electric power plants in Texas with emissions meeting the court-ordered consent decree criteria for designation by July 2016: Big Brown, Coleto Creek, Harrington, Limestone, Monticello, Twin Oaks, San Miguel, Sandow, Sandy Creek, W.A. Parish, Martin Lake, and Tolk. No monitors with violations of the 2010 primary SO2 NAAQS were identified in Texas. The EPA’s letter provided Texas with an opportunity to revise previously recommended designations, and to submit supporting data and any additional information for EPA consideration, by September 18, 2015. (see the Latest Air Quality Planning Activities section for Texas' revised recommendation).

The EPA’s Data Requirements Rule (DRR) for the 2010 primary one-hour SO2 NAAQS, finalized on August 10, 2015 and published on August 21, 2015 (80 FR 51052)Exit the TCEQ, provides three options for states to characterize and assess SO2 air quality near sources that emit greater than 2,000 tpy: (a) modeling, (b) monitoring, or (c) enforceable emissions limits (below 2,000 tpy). The rule requires states to identify and submit a list to EPA of all applicable emissions sources by January 15, 2016. By July 1, 2016, Texas must identify the characterization approach planned for each identified source. For any source to be evaluated with modeling, states must submit a modeling protocol by July 1, 2016, a modeling analysis by January 13, 2017, and annual reports thereafter, to the EPA. For sources to be monitored, the SO2 monitors must be operating by January 1, 2017. Any enforceable emissions limits agreed to must be adopted and effective by January 13, 2017.

For more information, please visit the EPA's Sulfur Dioxide Implementation Web page. Exit the TCEQ

Latest Air Quality Planning Activities

Last updated: 10/14/2015

Revised Designation Recommendations for the 2010 Primary SO2 NAAQS

On September 18, 2015, Texas submitted revised designation recommendations and supporting information to the EPA to address 12 facilities in Texas with SO2 emissions meeting the court-ordered consent decree criteria for designation by July 2, 2016. Texas recommends attainment designations for Dallas, El Paso, Ellis, Galveston, Gregg, Harris, Jefferson, Kaufman, McLennan, Navarro, and Nueces Counties. These 11 counties have certified air monitoring data showing compliance with the 2010 SO2 NAAQS. Texas recommends unclassifiable/attainment designations for the remainder of the state (243 counties).

Infrastructure and Transport SIP Revision for the 2010 SO2 NAAQS

On April 23, 2013, the commission adopted the Federal Clean Air Act (FCAA), §110(a)(1) and (2) Infrastructure and Transport SIP Revision for the 2010 SO2 NAAQS (Project No. 2012-022-SIP-NR).

The SIP revision identifies basic program elements enabling Texas to meet infrastructure requirements for the 2010 SO2 NAAQS, as stipulated in the FCAA. The revision specifically addresses FCAA, §110(a)(2)(D)(i)(I) transport requirements with a technical demonstration showing Texas does not contribute significantly to nonattainment nor interfere with maintenance of the SO2 NAAQS in any other state.

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