Colorado Front Range Ozone: Prepare Now for Extreme Nonattainment

Colorado’s Denver Metro/North Front Range (DMNFR) ozone nonattainment area appears headed for another reclassification, this time from “severe” to “extreme” under the 2008 8-hour ozone National Ambient Air Quality Standard (NAAQS). Ozone monitoring data from 2024 through 2026 will determine whether the area is reclassified, so the outcome is not yet final. However, 2026 year-to-date monitoring data indicate that attainment is very unlikely, and facilities should treat an extreme reclassification as a planning assumption rather than a remote possibility.

The stakes for industrial facilities are significant. An extreme classification would lower the major source threshold for volatile organic compounds (VOC) and nitrogen oxides (NO_X) from 25 to 10 tons per year (tpy), bringing many of today’s minor sources into Title V permitting and making virtually any VOC or NO_X emissions increase at a major source subject to Nonattainment New Source Review (NNSR). Evaluating emissions and future needs now will set facilities up for a smoother transition once the new thresholds take effect.

Background on Ozone Nonattainment in Colorado

The DMNFR nonattainment area includes Adams, Arapahoe, Boulder, Broomfield, Denver, Douglas, and Jefferson counties, along with portions of Larimer and Weld counties (all of Weld County is included for the 2015 standard, while only a portion is included for the 2008 standard). Two 8-hour ozone standards are currently in effect in the area:

• 2008 Standard (75 parts per billion [ppb]): The DMNFR area was originally classified as marginal and has since been reclassified (“bumped up”) to moderate, then serious, and, in November 2022, severe. The severe attainment date is July 2027.
• 2015 Standard (70 ppb): The area was classified as marginal in August 2018 and is currently classified as serious, with an attainment date of August 2027.

Ground-level ozone is typically not emitted directly; rather, it forms when VOC and NO_X react in the presence of sunlight. For this reason, ozone nonattainment requirements focus on emissions of VOC and NO_X.

Likelihood of Reclassification

To attain the 2008 standard by July 2027, every monitor in the area must have a 2024–2026 design value (the three-year average of each year’s fourth-highest daily maximum 8-hour ozone concentration) of 75 ppb or less. The 2024 ozone season was a difficult one, with fourth-highest values of 85 to 88 ppb at several key monitors. Although 2025 values were considerably lower (71 to 73 ppb at the same key monitors), year-to-date data for 2026 are showing numerous days with monitored concentrations above the standard of 75 ppb, making attainment unlikely unless a significant number of data points are excluded (e.g., through EPA-approved exceptional event demonstrations for wildfire smoke).

The 2026 data still must be certified, and EPA will make the formal determination after the attainment date, so the reclassification is not final. Even so, facilities should plan as if it will occur.

When Would Reclassification Take Effect?

Under the Clean Air Act, EPA must determine within six months after an area’s attainment date whether the area attained the standard, and an area that fails to attain is reclassified to the next higher classification. In practice, EPA has historically taken longer, and prior DMNFR reclassifications under the 2008 standard have taken effect roughly 12 to 18 months after the applicable attainment date.

If EPA follows a similar timeline, an extreme reclassification could take effect sometime between mid-2028 and early 2029.

What an Extreme Classification Means for Industrial Sources

Table 1 summarizes how the VOC and NO_X major source thresholds, significant emission rates (SER), and emission offset ratios change with each nonattainment classification.

Table 1. VOC and NO_X Thresholds by Nonattainment Classification

As shown in the table, following reclassification, facilities in the 2008 ozone nonattainment area with a potential to emit (PTE) of 10 tpy or more of VOC or NO_X would become major sources for purposes of both the NNSR and Title V programs.

Implications of this lower threshold include:

• Nonattainment New Source Review. Because the SER for extreme areas is zero, NNSR would apply to any net increase in VOC or NO_X emissions at an existing major source, and projects would require a project emissions accounting analysis to determine whether a net emissions increase occurs. New facilities with a VOC or NO_X PTE of 10 tpy or more would also be major sources subject to NNSR. NNSR permitting is complex and time-consuming. It requires Lowest Achievable Emission Rate (LAER) controls, which do not take cost of control into consideration, and requires that emission increases be “offset” at the ratios shown in the table above (1.5:1 for extreme nonattainment areas). Note that at this time, there are no Emission Reduction Credits (ERCs) available for purchase based on Colorado’s Emissions Credit Tracking Certificates database.
• Title V. The Title V major source threshold for VOC and NO_X would also drop to 10 tpy. Thresholds for sulfur dioxide, carbon monoxide, and particulate matter are not affected by the ozone reclassification and therefore remain at 100 tpy. Newly major sources typically must submit a Title V permit application within one year of reclassification. Title V brings ongoing obligations, including semi-annual monitoring and deviation reporting, annual compliance certifications, Compliance Assurance Monitoring (CAM) plans where applicable, five-year permit renewals, and annual compliance inspections.
• Reasonably Available Control Technology (RACT). RACT is required for existing major sources, and therefore could be newly applicable to sources at facilities with VOC or NO_X PTE between 10 and 25 tpy. New or modified equipment at minor sources may also be subject to RACT at lower emission thresholds, and the cost threshold used to determine economic infeasibility may increase. Whether APEN and construction permitting thresholds will also change remains to be seen.
• Annual Emissions Reporting. The major source threshold also determines whether a facility is a “true minor” or “synthetic minor” source, which affects applicability of Colorado’s annual toxic air contaminant (TAC) and criteria pollutant reporting under Regulation Number 3. Facilities that become major sources, or rely on permit limits to stay below the 10 tpy threshold and are therefore synthetic minor sources, would be required to do the annual reporting.

Actions Facilities Should Consider Now

Although reclassification is not yet final, decisions made over the next 12 to 24 months will shape how each facility is treated once the new thresholds take effect. Facilities in the DMNFR nonattainment area should consider the following steps:

• Calculate facility-wide PTE for each facility in the nonattainment area. Be sure to include APEN-exempt and insignificant activities such as boilers and heaters, engines, and tanks at their PTE, as these sources can easily push a facility over 10 tpy. For sources that do not have federally enforceable (e.g., permitted) limits, PTE is generally based on uncontrolled emissions at 8,760 hours per year (or 500 hours per year for emergency generators).
• Reduce PTE now if you want to avoid major source status. If facility PTE is 10 tpy or more of VOC or NO_X, consider permitting actions to reduce emissions before reclassification. Options may include:
  • Federally enforceable limits on throughput or hours of operation
  • Add-on controls
  • Electrification to remove fuel-fired heaters or engines
  • Site-specific emission factors to replace conservative default factors
• Evaluate generating Emission Reduction Credits (ERCs). If you are reducing emissions, consider whether those reductions can be banked as ERCs. With offsets required at 1.5:1 under an extreme classification, creditable reductions may carry significant value.
• Start permitting planned projects now. If you need to increase throughput at a facility that is already above 10 tpy, or plan to build a new facility with emissions above 10 tpy, begin permitting now so that the permit is in hand before the reclassification effective date and NNSR can be avoided. Build in schedule margin for Division review times.
• Prepare for Title V if major source status is unavoidable. Facilities that will remain at or above 10 tpy should begin identifying applicable requirements, evaluating CAM applicability, and budgeting for ongoing Title V compliance.

Trinity will continue to track 2026 ozone monitoring data, EPA’s attainment determination, and related Air Quality Control Commission rulemakings. Be on the lookout for future Trinity eNews articles and training events as additional details become available!

For additional information or assistance evaluating your facility’s emissions and permitting strategy ahead of reclassification, contact Trinity’s Colorado office at 720.638.7647.