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Regulatory Updates

PM2.5 Annual NAAQS Revised

May 7, 2024

On Feb. 7, 2024, the United States Environmental Protection Agency (U.S. EPA) issued a final rule based on its reconsideration of its 2020 decision to retain the primary and secondary National Ambient Air Quality Standards (NAAQS) for particulate matter (PM) without revision. The reconsideration included three main actions:

  • Revised the primary PM2.5 Annual NAAQS down to 9.0 μg/m3 from 12.0 μg/m3;
  • Revised the ambient monitoring requirements, including data collection, updated quality assurance (QA) calculations (to account for lower concentration measurements), probe and monitoring path siting criteria and use of automated continuous Federal Equivalent Methods (FEMs); and
  • Revised the Air Quality Index (AQI) breakpoints where the “Good” AQI limit (0-50) has been revised to 9.0 μg/m3 from 12.0 μg/m3 to match the new PM2.5 annual NAAQS.

The final rule was published in the Federal Register on March 6, 2024, with an effective date of May 6, 2024.

Small particulate matter (PM) continues to be under scrutiny from regulatory agencies and the public because of its link to negative health and welfare effects. In addition to regulatory requirements, industrial sites also address PM emissions issues through increasing environmental justice (EJ) concerns. The EPA concluded that the available scientific information supports strengthening the primary annual PM2.5 standard to ensure it adequately protects public health and noted that recent studies suggest adverse health effects from exposure to PM2.5 are occurring at concentrations allowed by the previous standard of 12.0 μg/m3 (set in 2012). Additional studies demonstrate improvements in public health, including reductions in mortality, following reductions in PM2.5 in areas with air quality below 12.0 μg/m3.

What’s Next?

Stationary Source Permitting

In addition to PM2.5, major source sites located in attainment areas subject to Prevention of Significant Deterioration (PSD) requirements will have to include the precursor pollutants NOX and SO2 in the new source review analysis while major source sites in non-attainment areas (non-attainment new source review) will have to address the precursor pollutants NOX, SO2, VOC, and ammonia.  States define the significant emissions rate for ammonia.

No Exemptions
If a PSD permit is not issued for a new major source or major modification when a new non-attainment designation is made, the site would then be required to address the nonattainment new source review (NNSR) preconstruction permit requirements.

Area Designations

The EPA is expected to make area designations in two to three years after the promulgation of the new standard. The general steps in this process are outlined below.

  • Within 1 year after promulgation, States make the designation recommendation – attainment, nonattainment or unclassifiable. Under the Clean Air Act, the response from the State cannot be required sooner than 120 days after promulgation.
  • Within 2 years after promulgation, but as soon as practicable, EPA makes designations. The States can comment on EPA’s designations.

The EPA intends to issue a designations memorandum to provide information regarding the designation process including addressing exceptional events, such as wildfires.


State Implementation Plans (SIPs)

After the EPA promulgates a new or revised NAAQS, States are required to make a new SIP submission within 3 years after the effective date of a new or revised primary standard to establish that they meet the necessary structural requirements for the new or revised NAAQS or make changes to do so. These are known as “Infrastructure SIP” submissions. Within 18 months from the effective date of a nonattainment area designation, States must submit a “Nonattainment Plan SIP” submission to provide for attainment and maintenance of the revised standard.


TRC is uniquely qualified to assist our clients

TRC’s Air Quality Practice is well-positioned and uniquely qualified to address our client’s PM2.5 issues. Through our experienced group of professionals, we provide technically sound solutions and creative approaches to address PM2.5 regulatory requirements and public concerns. We provide resources for testing, ambient and meteorological monitoring, and permitting, compliance and modeling.


Stack Testing
  • Provide both traditional (Method 5/202 and 201A/202) and new approaches like Other Test Method (OTM) 37 to measure condensable particulate matter.
Ambient Air Monitoring
  • Experienced in both Federal Reference Methods (FRMs) and Federal Equivalent Methods (FEMs) to ensure precise, reliable and quality data through network management and calibration to ensure accurate, consistent and defensible measurements. 
  • FRMs and FEMs are superior to low-cost sensors for many reasons including data quality, reliability, and regulatory compliance. They are built to meet strict regulatory standards and undergo rigorous calibration and quality control processes to ensure accurate measurement of PM concentrations.
  • While low-cost sensors are valuable for providing spatial coverage in monitoring networks or for community-based initiatives, they are generally not suitable for applications where accurate and reliable measurements are essential such as regulatory compliance monitoring or scientific research.
  • Our monitoring team provides extensive experience in plan development; setup, operation, calibration, maintenance and troubleshooting; inspections and validation; and internal and external audits.
Permitting, Compliance, and Modeling
  • Evaluate PM2.5 emission factors and determine if your industry’s factors are too conservative.
  • Identify appropriate emission factors and control technologies to lower emission rates.
  • Develop compliance strategies.
  • For modeling, assess impact of existing ambient background concentrations, understand current site operations to inform future projects/impacts, implement novel techniques and Modeled Emission Rates for Precursors (MERPS).

Looking for effective solutions to your problems?

Turn to the experts at TRC.

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