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AQD | National Ambient Air Quality 2024 Revision

Air Quality Division

2024 Revised Primary (Health-Based) National Ambient Air Quality Standard for PM-2.5

Posted On: March 1st, 2024 - 7:00 a.m.

On Feb. 7, 2024, the Environmental Protection Agency (EPA) strengthened the annual primary (health-based) National Ambient Air Quality Standard (NAAQS) for particulate matter with a diameter less than 2.5 micrometers, known as PM2.5 or fine particulate matter, to be more protective of public health based on the latest available scientific evidence. Some sources of PM2.5 pollution include; vehicles, power plants, wood-burning stoves, and wildfires. In this action: 

  • EPA revised the annual primary (health-based) NAAQS for PM2.5 from 12.0 µg/m3 to 9.0 µg/m3.
  • EPA revised the breakpoints in the Air Quality Index (AQI) for reporting when daily air quality is considered good, moderate, unhealthy, very unhealthy, and hazardous based on the concentration of PM2.5 in an area | Learn More >
  • EPA made changes to the PM2.5 monitoring network requirements to enhance the protection of air quality in communities overburdened by air pollution.  
    • Because ADEQ’s air quality monitoring network already meets EPA’s new requirements, no changes to air quality monitoring in Arizona are expected.

Read the full text of EPA’s final rule and learn more about the revised PM2.5 NAAQS | View >

Permitting and Other Impacts on Stationary Sources 

  • Stationary sources of both direct PM2.5 emissions and PM2.5 precursor pollutants may be subject to additional control requirements as a result of the revised standard.
    • Precursor pollutants of PM2.5 for PM2.5 attainment areas are:
      • Nitrogen oxides (NOx), and
      • Sulfur dioxide (SO2).
    • Precursor pollutants of PM2.5 for PM2.5 nonattainment areas are:
      • NOx,
      • SO2,
      • Volatile organic compounds (VOCs), and 
      • Ammonia. 
  • 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, once EPA has designated 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 PM2.5 or PM2.5 precursors from the source or modification will not cause or contribute to a violation of the revised PM2.5 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 (May 6, 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 PM2.5 NAAQS.
    • After an area is designated nonattainment for the revised PM2.5 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.

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 PM2.5 primary NAAQS are due to EPA by Feb. 7, 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 Feb. 7, 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 PM2.5 primary standard. 

image of a timeline

ADEQ plans to hold informational sessions and workshops before formally proposing the draft boundary designations. Interested parties can sign up to receive registration details and updates on the boundary designations by subscribing to ADEQ’s PM-2.5 Boundary Designations list | Subscribe >