On December 17, 2024, the United States Environmental Protection Agency (USEPA) redesignated the existing Moderate ozone nonattainment areas in Southeast Wisconsin as Serious ozone nonattainment areas, with an effective date of January 16, 2025. The affected areas consist of the entirety of Milwaukee and Ozaukee counties, along with portions of Washington, Racine, Waukesha, Kenosha, and Sheboygan counties. This article will discuss the background that determines nonattainment, the path towards attainment, and the permitting impacts on facilities located inside of the nonattainment area.
Nonattainment Overview
The Clean Air Act (CAA) Amendments of 1990 set forth the national ambient air quality standards (NAAQS), codified in 40 CFR Part 50, covering six criteria pollutants, carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), particulate matter (PM2.5 and PM10), and sulfur dioxide (SO2). The standards are separated into primary and secondary standards, primary are designed to protect human health, including protecting the health of “sensitive” populations, such as asthmatics, children, and the elderly, with secondary designed to protect public welfare, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings. Not every pollutant has a primary and secondary standard, and some, like ozone, have a combined primary and secondary standard.
Additionally, the USEPA will review the existing standards and periodically revise standards, for example, ozone was updated in 1997, 2008, and most recently in 2015. The 2015 ozone standard was promulgated as a primary and secondary standard of 0.070 ppm (70 ppb) with an 8-hour averaging time, specifically based on the annual fourth-highest daily maximum 8-hour concentration, averaged over 3 years. As part of a newly promulgated NAAQS, states are required to submit attainment designations to the EPA based on local ambient air sampling. Since the initial promulgation of the 2015 standards, the Wisconsin Department of Natural Resources (WDNR) and the USEPA had made minor changes to nonattainment status from the shorelines of nine counties to the Milwaukee metropolitan area as it exists today. Once the nonattainment designations are in place, the USEPA requires states to prepare and submit a State Implementation Plan (SIP) that outlines the states compliance with the new NAAQS as well as plans to bring each noncompliance area into compliance.
Permitting Impacts
Nonattainment construction permitting in Wisconsin is governed by NR 408 and follows the USEPA guidelines for Nonattainment New Source Review (NNSR) of new major sources and existing sources undergoing major modifications. Importantly, the major source and major modification thresholds are redefined in Serious nonattainment, see Table 1.
Table 1 – Lower Major Source Thresholds for Nonattainment Areas
| Ozone Nonattainment Designation | Major Source Threshold | Major Modification Threshold |
| Marginal | 100 tpy VOC or NOX | 40 tpy VOC or NOX |
| Moderate | 100 tpy VOC or NOX | 40 tpy VOC or NOX |
| Serious | 50 tpy VOC or NOX | 25 tpy VOC or NOX (with aggregation) |
| Severe | 25 tpy VOC or NOX | 25 tpy VOC or NOX (with aggregation) |
| Extreme | 10 tpy VOC or NOX | Any increase in VOC or NOX |
The decrease in the major source threshold will also affect Wisconsin facilities by requiring any minor or synthetic minor sources with the potential to emit (PTE) greater than 50 tpy for either NOX or VOC to submit an application to either become a major source and subject to Title V permitting, or to submit an application to limit the PTE to below 50 tpy for NOX or VOC. The WNDR has stated that any facility requiring a Title V permit, or permitted emissions limitations are required to submit a permit application no later than January 16, 2026. This decrease will also affect the Registration Operation Permits, as ROPA and ROPC permits are limited to up to 25% of the major source threshold and ROPB are limited to up to 50% of the major source threshold. The WDNR has stated that any facility with 12-month rolling actual emissions in excess of the new Registration Operation Permits thresholds are required to submit an application to comply with the new thresholds immediately.
In addition to permitting changes with the bump from Marginal to Serious nonattainment, new major sources or major modifications to existing facilities will be required to implement emissions offsets for the total amount of ozone precursors at a ratio of 1.2 tons of offset to every 1 ton of permitted emissions, as well as the implementation of the Lowest Achievable Emission Rate (LAER). LAER is similar to Best Available Control Technology (BACT), with the exception that LAER cannot take into account economic or cost considerations and are non-negotiable case-by-case determinations.
Conclusion
The reclassification from Moderate to Serious nonattainment for ozone will require existing facilities in the impacted areas to determine which permitting option fits the facility, and to meet the deadline for permit applications. Affected facilities will need to understand the future permitting requirements and determine compliance with any future requirements. Timely action on securing resources, corporate/shareholder input, and compliance with DNR requirements will be key to maintaining compliance with the new nonattainment category. If you have any questions about the process, reach out to Steve Tasch or Harrison Rae from our Milwaukee office.
