AQD | 2024 Revised Secondary (Welfare-Based) NAAQs for SO2

Air Quality Division

2024 Revised Secondary (Welfare-Based) National Ambient Air Quality Standard for SO2

Revised On: Dec. 8th, 2025 - 03:18 pm

On Dec. 10, 2024, the U.S. Environmental Protection Agency (EPA) strengthened the annual secondary (welfare-based) National Ambient Air Quality Standard (NAAQS) for sulfur dioxide (SO₂), to be more protective of public welfare based on the latest available scientific evidence. Unlike primary NAAQS, which prioritize public health, secondary standards provide for protection of public welfare such as protection against decreased visibility and damage to animals, aquatic life, crops, vegetation, and buildings.

Some sources of SO₂ pollution include:

Fossil fuel fired power plants and industrial facilities;
Industrial processes such as extracting metal from ore;
Natural sources such as volcanoes; and
Locomotives, ships and other vehicles and heavy equipment that burn fuel with a high sulfur content.

In this action, EPA revised the annual secondary (welfare-based) standard for SO₂ from 0.5 parts per million (ppm) as a 3-hour average, not to be exceeded more than once in a year, to an annual standard with a level of 10 parts per billion (ppb), averaged over 3 years.

Read the full text of EPA’s final rule and learn more about the revised SO₂ NAAQS | View >

State of Arizona, 2024 Secondary SO2 Boundary Recommendation

Letter from the Governor to the EPA | View Letter >
Appendix A: Final TSD | View Appendix A >
Appendix B: Final Public Process Documentation | View Appendix B >

ADEQ’s Final Proposed Boundary Recommendation Package

ADEQ submitted the final proposed SO₂ boundary recommendation package, including the final proposed boundary recommendation, response to comments, and Technical Support Document (TSD), to the Governor’s Office on Nov. 10, 2025.

ADEQ’s Final SO₂ Proposed Boundary Recommendation Package:

ADEQ’s letter to the Governor’s Office | View Letter >
Appendix A: SO₂ 2024 NAAQS Boundary Recommendation TSD | View Appendix A >
Appendix B: Public Process Documentation and Responses to Comments Received During the Public Process | View Appendix B >

Proposed Boundary Recommendation Public Comment Period & Hearing

Pursuant to A.R.S. § 49-405, ADEQ held a public comment period from July 10, 2025, to Aug. 11, 2025, for its proposed SO₂ 2024 NAAQS Boundary Recommendations Draft Report and Technical Support Document. A public hearing was held on Aug. 11, 2025, at 10:30 a.m. The draft recommendations and TSD proposed areas for attainment and unclassifiable designations throughout the state | View Public Notice >

ADEQ evaluated comments received during the public comment period and drafted a formal response to comments. In accordance with A.R.S. § 49-405, the final proposed boundary recommendation, response to comments, and TSD was provided to the Governor’s Office. Pursuant to A.R.S. § 49-405(C)(4)(b), ADEQ has posted the final proposed boundary recommendation, response to comments, and TSD within five days after ADEQ’s submission to the Governor.

ADEQ’s Draft Proposed SO₂ Boundary Recommendation | View >

Boundary Designations (Attainment, Nonattainment, and Unclassifiable Areas)

When EPA releases a new or revised NAAQS, the Governor of each state is required to submit recommended boundary designations to EPA, identifying all areas of the state as:

Nonattainment (areas that do not meet the NAAQS, or areas that contribute to a nearby area not meeting the NAAQS),
Attainment (areas that meet the NAAQS), or
Unclassifiable (areas that cannot be classified as attainment or nonattainment based on available information).

The Arizona Governor’s recommendations for the 2024 revised annual secondary SO₂ NAAQS are due to EPA by Dec. 10, 2025.

Pursuant to Arizona Revised Statutes (A.R.S.) § 49-405, ADEQ is the lead agency assisting the Arizona Governor in making the boundary recommendations. When developing the initial boundary recommendations, ADEQ will consider EPA’s five factors: air quality monitoring data, emissions and emissions-related data, meteorology, geography/topography, and jurisdictional boundaries.

EPA will make the final decision regarding how areas are classified. If EPA decides that it is necessary to modify a state's recommendation, then EPA will notify the state at least 120 days prior to finalizing the area designations to allow the state an opportunity to comment on the potential modification. EPA has until Dec. 10, 2026, to finalize the initial area designations, unless there is insufficient information. In such circumstances, EPA may take up to 1 additional year to make the designation decisions (i.e., no later than 3 years after promulgation of the standard).

The timeline for developing the boundary recommendations is summarized in the following graphic for the 2024 revised annual SO₂ secondary standard.

ADEQ welcomes interested parties to sign up to receive future updates and details on the boundary designations by subscribing to ADEQ’s Secondary SO₂ Boundary Redesignation list. When selecting subscription topics, select Air Quality, proceed to the next page(s), select Other Air Quality Topics and then select Secondary SO₂ Boundary Redesignation where listed | Subscribe >

Permitting and Other Impacts on Stationary Sources

Stationary sources of SO₂ emissions may be subject to additional control requirements as a result of the revised standard.

Existing Sources

Existing sources will not need to amend their current permits as a result of the new standard, unless they undertake a modification.
However, if EPA designates nonattainment areas, and the nonattainment State Implementation Plan (SIP) development process is underway, air quality planning agencies will have to consider whether additional controls on existing stationary sources are necessary and should be included in the SIP.

New Sources and Modifications

A new source is a new industrial plant, such as a new factory or mine.
A modification is a physical or operational change to an existing source, including installation of new equipment, which results in an increase in emissions.
Anyone applying for a permit for a new source or modification may have to demonstrate through modeling that emissions of SO₂ from the source or modification will not cause or contribute to a violation of the revised secondary SO₂ NAAQS.
- This requirement will apply to any source subject to the Prevention of Significant Deterioration (PSD) permit program or to Minor New Source Review (NSR).
- A source that has obtained a final permit under either of these programs before the effective date of the revised NAAQS (Dec. 31, 2024) will be grandfathered out of this requirement.
- The NAAQS revision does not change which sources are subject to PSD or Minor NSR; rather, it requires sources and modifications already subject to those programs to model for compliance with the revised SO₂ NAAQS.
If an area is designated nonattainment for the revised SO₂ NAAQS, applicants for a permit or permit revision to construct a new source or modification in the area may be subject to nonattainment NSR. Among other requirements, an applicant subject to nonattainment NSR must obtain emission reductions from other sources in the area to completely offset the increase in emissions from the new source or modification.
As detailed in the Technical Support Document linked in ADEQ’s Final Proposed Boundary Recommendation Package, ADEQ is not recommending that any area in Arizona be designated as nonattainment.