Michigan's Ozone Status Update: Southeast Achieves Attainment, West Faces New Serious Nonattainment Challenges

Environmental ConsultingEnvironmental Consulting
06/11/2025
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Michigan’s ozone attainment landscape has undergone a significant transformation in recent years. While southeast Michigan has successfully navigated the redesignation process to reach attainment under the 2015 ozone National Ambient Air Quality Standard (NAAQS), West Michigan now faces new regulatory challenges after being reclassified to “serious” nonattainment as of January 16, 2025. Understanding the regulatory implications, timelines, and compliance expectations is critical for affected industries moving forward.

The Role of Design Values and Monitoring in Ozone Attainment

The U.S. EPA revised the primary and secondary ozone NAAQS on October 26, 2015. Initial designations for nonattainment under this standard became effective August 3, 2018. In Michigan, the affected areas included Berrien, Livingston, Macomb, Monroe, Oakland, St. Clair, Washtenaw, and Wayne Counties, as well as portions of Allegan and Muskegon Counties.

Ozone attainment classification is based on a three-year average of the fourth-highest daily maximum 8-hour ozone concentrations, known as the “design value.” This design value determines whether an area meets or exceeds the NAAQS and drives the classification of nonattainment severity: marginal, moderate, serious, severe (severe-15 & severe-17), and extreme.

Southeast Michigan’s Road to Redesignation

Southeast Michigan was originally designated as marginal nonattainment but failed to attain the standard by the initial August 3, 2021 deadline. As a result, the area was bumped up to moderate nonattainment, effective March 1, 2023.
However, based on monitoring data from 2019 through 2021, the region demonstrated attainment of the ozone NAAQS. Michigan submitted a formal redesignation request to the U.S. EPA on January 3, 2022. This request was briefly delayed due to an exceedance observed on July 4, 2021, which required further investigation.
The exceedance was ultimately attributed to Canadian wildfire smoke during June 2020, and Michigan submitted an Exceptional Events Demonstration to support excluding that data point. The U.S. EPA accepted this justification and issued a Clean Data Determination on May 19, 2023, finalizing the redesignation of southeast Michigan to attainment.
Importantly, the redesignation also included Michigan’s maintenance plan, which outlines how the area will continue to meet the standard for the next 20 years. This includes contingency measures that can be implemented if future monitoring indicates noncompliance.

West Michigan’s Reclassification to Serious Nonattainment

In contrast, West Michigan has seen a worsening ozone classification. Originally classified as marginal nonattainment, the area failed to meet the ozone NAAQS by the August 3, 2024 deadline based on 2021-2023 monitoring data. As a result, the U.S. EPA finalized the reclassification to serious nonattainment, effective January 16, 2025.
The reclassification applies to three West Michigan areas and triggers several new regulatory obligations under the Clean Air Act. Facilities in these areas must now prepare for:
  • VOC and NOX Reasonably Available Control Technology (RACT) requirements for major sources (defined as >50 tons/year under serious classification) and Control Technique Guidelines (CTG) sources.
  • Reasonable Further Progress (RFP): A 9% VOC/NOX emissions reduction is required between 2024 and 2026, building upon a baseline of a 15% reduction achieved from 2017-2023.
  • Attainment Demonstration: West Michigan must now submit a formal plan demonstrating how it will achieve attainment by the serious nonattainment deadline.
  • Contingency Measures: Additional emissions reductions that must be available for implementation if the area fails to meet attainment milestones or RFP targets.
  • Transportation Control Demonstration: West Michigan must show that projected motor vehicle emissions are consistent with the area’s attainment strategy and will not prevent progress toward meeting ozone standards. This includes modeling transportation-related VOC and NOX emissions and aligning regional transportation plans with air quality goals.
  • Increase in the NSR offset ratio from 1.15:1 (moderate) to 1.2:1 (serious).
The effective date of serious nonattainment classification sets the clock ticking. The statutory attainment deadline for serious areas is August 3, 2027.

Looking Ahead: What Facilities Should Expect

Facilities located in West Michigan’s serious nonattainment areas should begin preparing for increased permitting requirements and control expectations. This includes reviewing whether they meet the revised major source thresholds and evaluating compliance with applicable RACT and CTG standards.
For Southeast Michigan facilities, the attainment designation offers some relief, but the requirements of the maintenance plan remain binding. Facilities must continue to track emissions and remain compliant with existing permit and monitoring obligations.

Ozone Attainment Timeline Recap

October 26, 2015: EPA revises ozone NAAQS
August 3, 2018: Nonattainment designations become effective.
November 7, 2022: West Michigan bumped to moderate nonattainment.
March 1, 2023: Southeast Michigan bumped to moderate nonattainment.
May 19, 2023: Southeast Michigan redesignated to attainment.
August 3, 2024: Deadline to attain 2015 ozone standard.
January 16, 2025: West Michigan reclassified as serious nonattainment.
August 3, 2027: Attainment deadline for serious nonattainment areas.

Conclusion

If your facility is located in a newly classified serious nonattainment area, or if you are unsure how the redesignation impacts your obligations, reach out to Trinity’s Ann Arbor office at 734.474.7709. Our experts are ready to support your compliance strategy through these evolving regulatory conditions.

I joined Trinity Consultants because I wanted to take my experience as an engineering student and apply it to a job that was people-oriented and allowed me to explore a wide range of industries. In my time at Trinity, I’ve had the opportunity to both work on a variety of projects and develop my own areas of expertise. As someone who was interested in air dispersion modeling early on, I’ve had the opportunity to grow my experience in that subject area without sacrificing opportunities to try new projects and work with great people. As a Senior Consultant, I now support clients in a variety of industries including data centers, surface coating, Portland cement, lime manufacturing, oil and gas, and more. My project work covers a broad range as well, including air dispersion modeling, routine compliance support, new construction permitting, and stack testing support.

Sam Najmolhoda
Senior Consultant

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