The northern Front Range had only 23 days violating EPA caps for toxic ground-level ozone in the 2025 summer season, a steep fall from the record 67 days in 2021, according to a preliminary review in conversations with the Regional Air Quality Council. Only seven of those 2025 high-ozone days were in the worst category above 75 parts per billion.
Though the season isn’t technically over yet, RAQC officials don’t expect late-September weather to result in more violations in 2025, and are pleased with a 120-day ozone season that saw far fewer exceedances than the 41 violation days in 2024.
With Colorado air pollution control officials under strict EPA orders to reduce violations and bring Front Range emissions consistently below 70 parts per billion through new policy controls on driving, oil and gas production or other sources, a respite in 2025 was more than welcome.
Praise the wind, or a frequent series of massive updraft-downdraft cycles, but residents who breathed a little easier this summer can generally thank the weather if not slow-acting past policy changes, RAQC chief Mike Silverstein said.
“We can attribute the lower values to just less stagnant air masses over our region,” said Silverstein, whose agency collects and analyzes data on Front Range air pollution and recommends policy changes to state and local bodies like the Air Quality Control Commission.
“We had hot days, we had plenty of sunshine, but we had a lot of convective activity. If you talk with the meteorologist, they would tell you we had more cloud cover, we had more thunderstorm activity, we had less stagnation, a more turbulent atmosphere,” he said.
Anatomy of a violation
Ozone is formed when a group of chemical precursors from fossil fuel activity, such as volatile organic compounds from oil and gas production and nitrogen oxides from vehicle engines, combine and then bake in the hot Colorado sun on relatively still days. Excess ozone can cause or exacerbate asthma in children, aggravate respiratory and heart conditions in adults and cause other health damage.
In 2008, the EPA set an ozone limit in metro areas of 75 parts per billion, then tightened the cap in 2015 to 70 parts per billion. The RAQC is working on a “blueprint” of recommendations to state officials that would go into a State Implementation Plan, or SIP, required to show progress in eventually meeting the EPA ozone caps. The current compliance period runs through 2027, and each summer the Front Range monitors show official ozone readings above the limits, Colorado risks further EPA sanctions.
Air quality officials are also counting on recent policy decisions to kick in and start contributing to a drop in ozone. One measure RAQC championed that was eventually passed by the state was a summer ban on gasoline powered lawn equipment use by governments and institutional users. Replacing fossil fuel engines with cleaner battery-powered versions could chip away at ozone counts on hot days.
“Everything helps,” Silverstein said. “So we will take credit for all the emission reduction activities that either we champion, or that we do, or that the state does, or all of the other partners in our region, and the natural change in technology to cleaner-emitting devices.”
Also, Silverstein said, RAQC doesn’t reject luck as a factor.
“We’ll take anything,” he said. “We get penalized for bad meteorology. Let’s celebrate the good meteorology.”
Building a better crystal ball
If metro Denver and the northern Front Range are going to get out from under the EPA sanctions that come with being a “severe” ozone violator, air quality officials are going to need a better crystal ball.
For years now, they have conducted computer modeling based on emissions and weather predictions to set new pollution control policies meant to comply with federal ozone caps within a few years. But in the last couple of years, the reality in the air as measured by real-time scientific instruments has recorded Front Range ozone blowing past the forecasts. So Colorado gets downgraded again in the EPA’s watchful eyes, and must come up with even more restrictive commuting and industrial policies in search of compliance.
Research teams were out in force this summer on a multiagency, coordinated plan to bulletproof the models.
From Fort Collins down to southern Denver, researchers set up highly accurate fixed and mobile instruments to record volatile organic compounds, nitrogen oxide and other emissions from burning fossil fuels that contribute to creating ozone. “Precursors” is the official term. The data gathering, called FROZE for Front Range Ozone Study, is a joint effort of RAQC, the Air Pollution Control Division and Colorado State University.
This summer’s measurements, at a lot more places and times than usually recorded, will help policymakers learn which industries or which highways on the map contribute more VOCs or NOx to the baking ozone. They may learn how constricting emissions at certain times of the day in certain places could trim some of the toxic peaks. The scientists will spend 2026 and 2027 analyzing all the data collected in 2025.
“It actually allows us to identify individual sources, and then we can look at our emissions inventory, and we’re counting up everything,” said Tom Moore, planning director for RAQC.