Secondary National Ambient Air Quality Standards (NAAQS) for Nitrogen Dioxide (NO2) and Sulfur Dioxide (SO2) and Particulate Matter (PM)
On this page:
Rule Summary
On December 10, 2024, the Environmental Protection Agency revised an air quality standard to better protect public welfare against adverse effects caused by criteria air pollutants – including ecological effects such as damage to aquatic and terrestrial ecosystems. This action revises the secondary National Ambient Air Quality Standard (NAAQS) for oxides of sulfur (SOX) and retains the existing secondary standards for oxides of nitrogen (N oxides) and particulate matter (PM), aligning standards with the best-available science and ensuring air pollution concentrations remain at historically low levels. The revised standard will take effect 30 days after EPA’s decision is published in the Federal Register.
Rule History
12/10/2024 - Final Decision to Revise the Secondary SO2 NAAQS and Retain Secondary NO2 and PM NAAQS (published 12/27/2024) (pdf) (1.47 MB)
04/15/2024 - Announcement of Public Hearing on Proposed Decision (pdf) (207 KB)
EPA held a public hearing to take comment on this proposed decision. Learn more about the public hearing.
04/03/2024 – Proposed Decision to Revise the Secondary SO2 NAAQS and Retain Secondary NO2 and PM NAAQS (pdf) (1.49 MB)
04/03/2012 – Final Decision to Retain the Secondary NAAQS for NO2 and SO2 (pdf) (4.01 MB)
08/01/2011 – Proposed Decision to Retain the Secondary NAAQS for NO2 and SO2 (pdf) (2.14 MB)
Additional Resources
Final Decision
- Fact Sheet: Final Decision: Review of the Secondary National Ambient Air Quality Standards for Oxides of Sulfur, Oxides of Nitrogen and Particulate Matter (pdf)
- Memorandum to the Docket: Air Quality Analysis Using Sulfur Dioxide (SO2) Air Quality Data, Updated (pdf)
- Alternative Demonstration Approach for the 2024 Secondary Sulfur Dioxide National Ambient Air Quality Standard under the Prevention of Significant Deterioration Program (pdf)
- Initial Area Designations for the 2024 Revised Secondary Annual Sulfur Dioxide National Ambient Air Quality Standard (Revised January 16, 2025)
Proposed Decision
- Fact Sheet: Proposed Decision to Revise the Secondary NAAQS for SO2 and Retain the Secondary NAAQS for NO2 and PM (pdf)
- Memorandum to the Docket: Air Quality Analysis Using Sulfur Dioxide (SO2) Air Quality Data (pdf)
- Technical Analyses to Support Alternative Demonstration Approach for Proposed Secondary SO2 NAAQS under NSR/PSD Program (pdf)
- Ambient SO2 Monitoring Network Review and Background (pdf)
- Scientific and Technical Documents Supporting the Secondary NAAQS for SO2 and NO2
Related Rules
Primary National Ambient Air Quality Standard for Sulfur Dioxide
Primary National Ambient Air Quality Standards for Nitrogen Dioxide
Primary National Ambient Air Quality Standards for Particulate Matter