The Truth About Ozone

Written By Waverly McDaniel

Almost daily we hear in the media or in legislative committee rooms that “oil and gas is the largest contributor to ozone in Colorado.” It’s usually followed by an outraged call for industry “to be held accountable.”

But where is the data supporting this claim coming from? Is it even true?

Let’s spend a few minutes unpacking the claim. The answer may surprise you.

Before diving in too much, it’s important to understand what ozone is and where it comes from. The U.S. Environmental Protection Agency (EPA) defines ozone as “a highly reactive gas composed of three oxygen atoms. It is both a natural and a man-made product that occurs in the Earth’s upper atmosphere, where it blocks UV radiation, and at ground level.” [1] The EPA also provides further insight into how ozone is formed, stating, “Ground-level ozone – what we breathe – is formed primarily from photochemical reactions between two major classes of air pollutants, volatile organic compounds (VOC) and nitrogen oxides (NOx).”

Oil and gas don’t emit ozone. Industry -- along with cars, trucks, power plants, lawn mowers, leaf blowers, water heaters, and even pine trees – emits VOCs and NOx that may form ozone. How much ozone forms, and when and where, depends on the weather. Colorado state agencies use scientific models to predict ozone levels, and monitors to measure what is actually in the air. The models show that oil and gas cause only a small share of Colorado’s ozone -- less than vehicles, much less than emissions that blow into our state, and much less than what occurs naturally.
It is simply inaccurate to say oil and gas is the largest contributor to ozone pollution.

First, there is a difference between total sources of ozone pollution in the non-attainment area (NAA) and man-made sources.

Total ozone pollution includes ozone from natural sources, man-made, and ozone that travels here from out of state. Government in Colorado can only try to control man-made emissions inside our state, but interstate transport and natural sources are real and can’t be ignored. Disregarding them skews the data and makes local man-made sources seem more impactful to total ozone than they actually are. To further analyze the data, it is helpful to understand what each of the different sources means.

Natural – Includes contributions from wildfires, vegetation, lightning, etc. [2] That fresh pine forest smell in the mountains? Those are VOCs and they cause ozone.
Man-Made – Ozone caused by emissions from human activity. Industrial sources and everyday actions like spray-on sunscreen are in this category.
Transported – This includes atmospheric pollution, both natural and man-made, that travels to Colorado from other states or countries (as far as Asia). [3]

To better understand ozone pollution in Colorado, we need to look at local “source apportionment” studies. Colorado agencies work with academics and expert contractors to evaluate the sources of ozone in the NAA. We’ll focus on the results for the National Energy Renewable Laboratory (NREL) monitoring site in Golden, where the highest ozone levels usually occur. In April 2021, Ramboll Engineering provided local air quality agencies with a report modeling the sources of ozone pollution in Colorado through 2023 based on data from two measurement tools at NREL.

The results are powerful.

  • On days with the highest ozone reading at NREL, all local man-made ozone only account for about 20 ppb out of 71 ppb, i.e., not even 30%. [4]

    • Of this 20 ppb, only 12% (2.6 ppb) of man-made ozone is attributable to oil and gas. Again, that’s 2.6 ppb out of 71 ppb.5

    • This means oil and gas only account for 3.6% of ozone at NREL when considering all sources.


The Fort Collins West monitoring site has historically seen elevated ozone, but lower than at NREL. Fort Collins West is closer to oil and gas wells than NREL. Even there, the total oil and gas contribution only equates to about 7% of the total average ozone.

There is nowhere in the NAA that oil and gas accounts for even 10% of total ozone, let alone the 40-45% publicly claimed recently.

So, you ask, how could this gross overestimation be possible?

Media reports appear to be confusing emissions with actual ozone. Remember that VOCs and NOx are ‘precursors’ to ozone pollution – they might form ozone, but when, where, and how much ozone depends on where the emissions occur, the winds, sunlight, and chemistry. Colorado’s State Implementation Plan for ozone shows that oil and gas produce 57.6 tons per day of NOx out of 148.7 tons per day – in other words, 38.7% of local man-made NOx, [5] but this happens away from the areas with the highest ozone concentrations. The State Implementation Plan and satellite data also show the industry has cut its methane and VOC emissions by 50% - 70% since 2013. [6]

The fact that oil and gas produces almost 40% of NOx from human activity in the NAA [7] does not mean that oil and gas contribute 40% of the ozone in the NAA.

We know this because of the data from NREL mentioned earlier. Only 3.6% of the total ozone at NREL came from oil and gas sources. So even though NREL had high ozone levels, the main source was not oil and gas. Similarly, at Fort Collins West, we saw an increase to 7% in ozone sources attributable to oil and gas, but that still means 93% of total ozone pollution came from other sources – which is in an area much closer to oil and gas production, and with lower overall ozone concentrations.


What does all this mean?


Oil and gas produce, on average, between 3 – 7 % of total ozone in the NAA. Not 40%. Not 45%. Not the majority. Not even close.

This is a complicated topic, but conversations around ozone need to be steeped in facts.
We encourage politicians, legislators, and reporters to provide information to the Colorado public that is truthful and verifiable. We are facing significant new legislation (HB23-1294) that is being hailed as an altruistic attempt to lower ozone pollution across the state—a solution to a problem that doesn’t exist.

Even worse, this bill, and many more like it, aims to do away with an industry that is vital to the Colorado economy, our way of life in the 21st century, and to our country’s national security.

As an industry, we should strive to continue to reduce our emissions. As a society, we need to fight for common sense legislation that’s based on facts, not scare tactics.

In the Media:


Denver Post – 

Oil, gas industry faces new crackdown on emissions as Colorado tries to clean air


Governor Polis – 

Letter to COGCC & CDPHE

  • “For 2023 oil and gas is projected to be the largest contributor of ozone precursor emissions in the non-attainment area on a ton per day basis, and it is estimated that oil and gas extraction activities are responsible for almost half of total ozone in the Denver metro area.”


Colorado Public Radio – 

Colorado air regulators vastly underestimated ozone pollution from some oil and gas operations due to a data error

  • “To explain Colorado’s consistent smog problem, regulators and scientists often point to two main sources of local air pollution: traffic and oil and gas.”

  • “The revised calculations predicted nitrogen oxide emissions from drilling and hydraulic fracturing in 2023 were likely nearly double the state’s original estimates. As a result, those two activities alone appeared likely to account for more ozone-causing emissions than all cars and trucks along the Front Range. In total, the industry is expected to account for 45 percent of the region’s nitrogen oxide emissions in 2023.”


Denver Post – 

Colorado Democrats propose tougher oil and gas permit rules to curb air pollution – risking showdown with Polis

The Colorado Sun – 

Polis orders Colorado regulators to set new rules for oil and gas industry to sharply cut ozone by 2030

Waverly McDaniel
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COGA and API Respond to AQCC Rulemaking Revisions to Ozone State Implementation Plan