It’s no secret Colorado struggles with air quality. You’ve probably seen the “brown cloud,” the smog that rises from the Denver metro area in the summer. You’ve probably also seen the notices along I-25 urging motorists to combine trips and avoid outdoor exercise on our hottest summer days. The North Front Range’s air problems are real—but not all of them are visible to the naked eye, which is where ozone comes in.

Last summer, an ozone monitor at Chatfield State Park measured the highest eight-hour average of ozone levels that Colorado saw all summer: 87 parts per billion. That day, August 6, along with July 23, was the worst in the region for ozone levels—and a clear symptom of the state’s long-standing battle with air pollution. According to officials from the Colorado Department of Public Health and Environment (CDPHE), even though Colorado’s ozone levels are the best they’ve been in decades, as federal standards have become more strict, the state has had a hard time keeping up.

Since 2012, Colorado’s North Front Range—or the Denver-Boulder-Greeley-Fort Collins-Loveland nonattainment area, as defined by the EPA—has failed to meet pre-Obama ozone standards. While the population has grown and oil and gas operations have proliferated up north, ozone monitors throughout the nonattainment area have repeatedly measured ozone levels exceeding the 2008 EPA standard of 75 parts per billion averaged over eight hours (the 2015 standard is 70 ppb).

Because of this consistent failure to attain the standard, the EPA announced in December that it has reclassified Colorado’s nonattainment area from “moderate” to “serious”—a decision that requires the state to more strictly permit polluting entities (such as oil and gas operations) and create a state implementation plan outlining the strategy to achieve lower ozone levels.

You may be wondering: Why does this matter? Why is it a problem? And what the heck is ozone, anyway?  We spoke with experts to find out.

The Making of Ozone

Bear with us, this gets a little wonky: Unlike other pollutants, ozone doesn’t have a taste or smell. When it fills the air, it doesn’t form smog. It doesn’t block mountain views. And it isn’t directly emitted by polluting sources. Instead, the invisible gas is formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs)—a.k.a. precursor pollutants—are released into the atmosphere and react with each other with the help of sunlight.

All three components—NOx, VOCs, and sunlight—must be present for the reaction to happen. The more ingredients, the more ozone. When too many precursor pollutants are present in the right circumstances, ozone builds up concentration in the atmosphere. This leaves us breathing the highly reactive gas, which can cause or exacerbate respiratory problems and over long exposures cause lung disease among other conditions.

Where Do Colorado’s Precursor Pollutants Come From?

According to a 2017 study by the National Center for Atmospheric Research (NCAR), the major contributors to the North Front Range’s ozone pollution were emissions from oil and gas operations, as well as traffic. Both sources emit both kinds of pollutants, but motorized vehicles tend to emit more nitrogen oxides while oil and gas operations emit more VOCs. The amount of pollutants varied by location. 

“We saw a bit of a gradient, where the northern part of the Front Range has a little bit more oil and gas influence and the southern part has a little bit more urban influence,” says Frank Flocke, a scientist at NCAR and one of the principal investigators on the project.

On days when ozone levels exceed the health standard, motorized vehicles and oil and gas operations combine to produce about 60 to 80 percent of local ozone.

There isn’t enough recent data to know exactly how the contributions from traffic and oil and gas have changed since 2014, but Flocke and NCAR colleague Gabriele Pfister say it probably hasn’t changed much—even with the greater use of clean cars and improvements to oil and gas facilities—thanks to continual population growth and the increased number of oil and gas operations.

“There’s a lot of variability in the system,” Flocke says.

Is Ozone Really All Our Fault?

On top of a regular supply of precursor pollutants, the North Front Range has a host of natural factors working against it.

First, sunny days. Sunshine is a key ingredient and accelerates the reactions that form ozone. So, even though our tourism economy loves to boast about all that sunshine we get here in Colorado, it gives precursor pollutants ample opportunity to produce the harmful gas.

Wind patterns also play a role. According to Pfister, daytime winds often push Front Range pollution into the mountains and, after the sun goes down, the airflow reverses, bringing the pollution back to the Front Range. If circumstances are right, even more pollution piles on top of it.

“When you have multi-day pollution episodes, some of that can actually be recirculated, recycled pollution from the day before,” Pfister says, “So you basically keep the stuff in the area.”

Background pollution—atmospheric pollution that travels to Colorado from other states or countries—is also a factor. However, according to Flocke and Pfister, the Front Range’s background ozone levels (50–55 ppb in the summer) aren’t much higher than other places in the northern hemisphere (40–50 ppb). “If we only had background ozone, we would probably never exceed the [EPA] standard, or very, very rarely,” Flocke says.

So, What Can Colorado Do About Ozone Pollution?

“There’s no real silver bullet left that could be fired here,” says John Putnam, director of environmental programs for CDPHE.

Over the years, regulations have been passed to improve or decrease emissions from vehicles, power plants, and other common sources of precursor pollutants. Garry Kaufman, director of the Air Pollution Control Division at CDPHE, points out that coal is no longer burned for electricity in the non-attainment area and cars are cleaner than they’ve ever been, although older cars with worse emissions are still in use.

“We’ve done a lot of the things that are the easiest to do,” he says in terms of regulations. “You can’t just say, ‘OK, let’s stop driving or let’s cut driving by 50 percent.’ Those aren’t strategies that really can be employed.”

At this point, CDPHE officials say, improving emissions from cars is a waiting game: As old cars die off and are replaced by newer, cleaner models, the emissions from traffic will improve. In the meantime, the oil and gas industry is the easiest remaining target for emissions regulations.

In December, the Air Quality Control Commission adopted a slew of new or more stringent regulations for the oil and gas industry, after a rule-making process that included key stakeholders, local governments, and the public was completed in the fall. The regulations are expected to reduce VOC emissions from oil and gas by about 7,000 tons per year. Other air quality rules are in the works.

These ozone reduction efforts were in motion before the EPA started considering reclassifying the nonattainment area, but the potential for reclassification added urgency to the department’s efforts. CDPHE is now working on the implementation plan, to address the ozone problem and bring the state into compliance with the federal ozone standard.

“It’s not an easy job,” Kaufman says. “The more you reduce [pollutants], the harder it is to find good, cost-effective ways to get additional reductions. But that’s the place that we’re in.”