Counties Affected:

Parts of Pinal and Gila County | View eMap >

Pollutant of Concern:

Sulfur Dioxide (SO₂) (2010 National Ambient Air Quality Standard (NAAQS), 75 parts per billion (ppb))

Nonattainment Area Designation Date:

Aug. 5, 2013 — Designation as nonattainment for the 2010 One-Hour SO₂ NAAQS  | View 78 Federal Register 47191 > 

Summary: 

The concentration of SO₂ in the air in Hayden has been above federal standards for several years. The Hayden area was designated as nonattainment for the 2010 one-hour SO₂ NAAQS in 2013. This triggered a requirement to develop a State Implementation Plan (SIP) for the area to reduce emissions and bring the area into attainment of the NAAQS. ADEQ submitted a nonattainment SIP to the U.S. Environmental Protection Agency (EPA) in March 2017. Subsequently, ADEQ submitted the associated final rules, Arizona Administrative Code (A.A.C.) R18-2-B1302, establishing control requirements for SO₂ emissions from the copper smelter located in the Hayden, AZ Nonattainment Area to the EPA in April 2017. 

In 2018, numerous improvements to the Hayden smelter were completed by ASARCO as part of a $180 million converter retrofit project that has substantially reduced SO₂ emissions. Some of the controls implemented at the copper smelter include:

• ventilation hooding over furnaces;
• wet gas cleaning system improvements;
• high surface area lime injection system for secondary hood baghouse and furnace ventilation baghouse; and
• stringent SO₂ emission limits established to meet federal standards.

On Nov. 5, 2020, EPA finalized limited approval/limited disapproval of ADEQ’s SO₂ control measures for the Hayden smelter that were codified in A.A.C. R18-2-B1302 | View 85 Federal Register 70483 >

On Nov. 10, 2020, EPA finalized partial approval/partial disapproval of Arizona’s 2017 SIP for attaining the 2010 one-hour SO₂ NAAQS for the Hayden SO₂ Nonattainment Area | View 85 Federal Register 71547 >

On Jan. 31, 2022, EPA issued a final rule finding, among other things, that the Hayden SO₂ Nonattainment Area had failed to attain the 2010 one-hour SO₂ NAAQS by the applicable attainment date of Oct. 4, 2018. This action required ADEQ to submit a SIP revision to EPA that would provide for expeditious attainment of the 2010 one-hour SO₂ NAAQS no later than Jan. 31, 2027 | View 87 Federal Register 4805 >

On Oct. 3, 2023, ADEQ submitted a SIP revision to EPA to show attainment of the 1971 and 2010 SO₂ NAAQS. As part of this SIP revision, the Hayden Smelter will implement three projects to reduce fugitive SO₂ emissions:

• replacement of the existing preheater at the double contact acid plant with a redesigned unit;
• completion of an uptake improvement project to install and vent a partial enclosure around the INCO flash furnace uptake;
• completion of a fuming ladle capture project to construct a hood and retaining walls to collect emissions from fuming ladles, which sometimes occur after a ladle is poured; and
• installation of an anode furnace secondary hood system to allow for improved capture of SO₂ emissions during operations.

This SIP revision also included a revised good engineering practice (GEP) demonstration study, as required by Clean Air Act (CAA) § 123 and EPA’s regulations. In 2022, ADEQ re-assessed the GEP determination in the 2017 SIP revision. This re-assessment confirmed that the original GEP stack height determination was consistent with the 1985 regulations and that the 1,000-foot stack remains an appropriate GEP determination. Therefore, ASARCO’s main stack was modeled at its true height of 1,000 feet. 

The control strategies adopted in the 2023 and subsequent 2025 submittals are meant to subsume the control strategies in ADEQ’s 2002 SIP revision for the  revoked 1971 24-hour and annual primary SO₂ NAAQS, as the 2010 NAAQS is more protective of public health and the environment.

On May 1, 2024, ADEQ proposed a rulemaking to amend Arizona Administrative Code (A.A.C) Title 18, Chapter 2, Article 13 to incorporate the SO₂ control measures that had been included as conditions in the permit for the Hayden Smelter into A.A.C. R18-2-B1302. Codifying these Title V permit terms ensures emission limits and operational controls relied upon in the SIP are federally enforceable. On Dec. 9, 2025, ADEQ submitted a Notice of Final Rulemaking to the Secretary of State, amending existing rules and adopting new rules to incorporate the Hayden smelter’s permit conditions into A.A.C. R18-2-B1302. The rules became effective on Feb. 7, 2026. 

On Dec. 10, 2025, ADEQ submitted the Revised Hayden SO₂ SIP with the amended rules in A.A.C. R18-2-B1302 to the EPA.

On March 4, 2026, EPA proposed approval of the Hayden SO₂ plan as meeting the CAA requirements, and is currently pending with a final action | View 91 Federal Register 10531 >

Submitted SIPs for the Hayden SO₂ Nonattainment Area: 

• 2025 SIP Revision: Rules Incorporating Hayden Smelter Permit Conditions to Supplement Arizona’s 2023 SO₂ SIP | View 2025 SIP Supplement >
• 2023 Hayden SO₂ SIP for the 1971 and 2010 SO₂ NAAQS | View 2023 SIP >
• 2017 Hayden SO₂ SIP Supplement*
• 2017 Hayden SO₂ SIP for the 2010 NAAQS*

*Documents available at the ADEQ Records Center | Submit Records Request >

Rulemaking

Learn more about the completed state level rulemaking, which incorporated controls for the Hayden SO₂ Nonattainment Area into the Arizona Administrative Code  | View Final Rules >

Resources

To learn more about SO2 and other Criteria Pollutants, visit our Criteria Pollutant SIPs page | View Criteria Pollutant SIPs >

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