California Air Resources Board

There is one federal primary standard for sulfur dioxide (SO₂) – a 1-hour standard of 75 parts per billion (ppb), revised in 2010 – and one federal secondary standard – an annual average standard of 10 ppb, revised in 2024.  All areas in California are designated attainment/unclassifiable for SO₂.

Maps of current area designations can be found through the Resources tab on the left.

Sulfur Dioxide Primary Standards

• December 2017 - U.S. EPA completed the third round of sulfur dioxide (SO₂) designations for California. All areas in California were designated as attainment/unclassifiable.
• February 2013 - U.S. EPA published notice in the Federal Register (FR) of proposed nonattainment designations for the 2010 primary federal sulfur dioxide (SO₂) standards.  No California areas were included in the proposal; all areas of the State remained undesignated. 
• August 2012 - U.S. EPA published notice extending the deadline for designating areas with respect to the 2010 primary federal SO₂ standard for up to 1 year.  U.S. EPA had until June 3, 2013, to complete the initial designations (FR Notice August 3, 2012).
• June 2011 - CARB staff submitted area designation recommendations on June 20, 2011.
  • Transmittal Letter
  • Enclosure 1: Staff Report
  • Appendix 1: Five Factor Analyses
  • Appendix 2: 2009 SO₂ 1-Hour Design Values
  • Enclosure 2: Area Boundary Descriptions
• June 2010 - U.S.EPA revised the primary SO₂ standards (FR Notice June 22, 2010; effective August 23, 2010).

Additional information about activities related to SO₂ can be found on U.S. EPA's SO2 home page.

Sulfur Dioxide Secondary Standard

• December 2025 - CARB staff submitted area designation recommendations on December 3, 2025.
  • Transmittal Letter
  • Enclosure 1: Staff Report
  • Appendix 1: Five Factor Analysis
  • Appendix 2: 2024 SO₂ Annual Design Values
  • Appendix 3: Area Boundary Descriptions
• December 2024 - U.S. EPA revised the secondary SO₂ standard (FR Notice December 27, 2024; effective January 27, 2025).
• March 2012 - U.S. EPA took final action to retain the existing SO₂ and NO₂ secondary standards (FR Notice April 3, 2012; effective June 4, 2012).

Additional information about activities related to the secondary standards can be found on U.S. EPA's NO₂/SO₂ secondary standards webpage.