Mountain States

Here’s the Real Story Behind Ozone in Utah

This week, the Salt Lake Tribune published an in-depth story on ozone pollution in Utah – an issue that affects much of the western United States.

While the story attempts to fault many local industrial sources for most of the ozone pollution – including the oil and natural gas sector – the facts simply don’t back up that claim.

The reality is that the majority of Utah’s ozone comes from foreign countries (mostly industrial pollution from China) and from naturally occurring sources.

It’s why the Utah Division of Air Quality (DAQ) submitted a 179(b) International Transport Demonstration to the U.S. Environmental Protection Agency in May. If granted, this will acknowledge that the emissions sources that are beyond the agency’s control impede the state’s ability to ever be taken off the ozone nonattainment status list.

The 179(b) demonstration would also give state regulators the time and flexibility to develop real solutions to address air quality issues outside the required EPA-mandated State Implementation Plan process. For Utah, continuing to be classified in nonattainment comes with severe economic consequences while doing little to reduce pollution. Further, it disregards the significant efforts underway in the state to reduce emissions DAQ can control.

Here’s the real story behind ozone in Utah:

China is the Primary Human Contributor

Contrary to the Tribune article, Utah actually has very little control of its overall ozone levels, primarily because it’s so heavily impacted from pollution from China. In its demonstration to EPA, DAQ explains how pollution floats across the Pacific and increases ozone levels in the state:

“A semi-permanent low-pressure system off the coast of China lofts pollutant-laden air to the mid and upper free troposphere. Fast winds within that region of the atmosphere then move this air and associated pollutants eastward toward the U.S. Pacific coast.

“This occurs within days to weeks with ozone persisting at these altitudes because of the relative lack of chemical sinks and low temperatures in this part of the atmosphere. Semi-permanent high-pressure system over the U.S. Pacific Coast then brings down the upper tropospheric air back to the surface over the western U.S.”

While the Tribune relies on a separate DAQ analysis that describes limited influence from foreign sources, the article doesn’t provide the full context. That DAQ analysis wasn’t built to estimate the effects of long-range ozone transport from Asia, so did not incorporate the approximately 10 parts per billion of foreign ozone that lingers over Utah on a daily basis.

A more complete picture requires photochemical modelling, such as the analysis by Ramboll, a consulting company whose model showed that “but for” the international emissions the Wasatch Front would meet the ozone standard. DAQ is currently in the process of completing their own photochemical model, both to estimate the impact of international emissions and to determine the most effective control strategies to reduce ozone. The Ramboll analysis was also built with the EPA’s own modeling, including peer-reviewed scientific literature. DAQ included the Ramboll model results in the 179(b) demonstration the state has filed.

Similarly, a 2020 DAQ blog also noted:

“Intercontinental transport of ozone and nitrogen oxides (NOx) from Asia contributes up to 20 percent of the West’s total ozone concentrations and it has been shown to be growing by 0.5–1 parts per billion (ppb) per year.”

A study from NASA’s Jet Propulsion Laboratory said that the United States’ human contributions to ozone had dropped but that the west was still dealing with pollution because of China:

“A new study finds that the western United States reduced its production of ozone-forming pollutants by a whopping 21 percent between 2005 and 2010, but ozone in the atmosphere above the region did not drop as expected in response. The reason: a combination of naturally occurring atmospheric processes and pollutants crossing the Pacific Ocean from China.”

As NPR reported:

“The U.S. is producing less air pollution, but smog levels are still rising in the western U.S. because of pollutants released in Asian countries that then drift over the Pacific Ocean.”

Natural Sources and Local Topography Are Also Significant Factors

Pollution from China isn’t the only factor that’s outside of Utah’s control. In fact, as the chart below from DAQ’s demonstration shows, naturally occurring ozone makes up the majority of ozone-creating pollutants in Utah.

DAQ also explains how the natural topography and elevation in Utah and across the west combined with Chinese pollution leads to higher ozone levels:

“This vertical transport of air from aloft is also enhanced by complex topography by creating winds that enhance O3 mixing down mountain slopes, leading to high altitude locations throughout the western US experiencing greater impacts from intercontinental transport of O3 as compared to lower-elevation locations. This intercontinental transport persists throughout the summer season in Utah, leading to enhancements of local ozone concentrations.”

The NASA study also noted the “natural atmospheric processes” that contribute to ozone, explaining:

“The large contribution from the stratosphere is part of a natural up-and-down cycle of upper-atmosphere winds. We know pretty well what will happen to the stratospheric contribution in the next few decades; it will continue to go up and down every two years or so.”

Neighboring Colorado faces many of the same ozone issues as Utah because of its similar topography and elevation – a fact one of the state’s top environmental regulators admitted last year:

“It’s one of those things where on one hand I see some type of evidence on health effects that obviously set the standards.

“And yet from a control perspective, just based on what blows into the state and natural background in this environment, I’m not sure how we do that. Practically, what are the consequences for doing that?”

Utah Contributes Very Little to Its Own Ozone

It’s clear that pollution from China, natural sources, and local topography are the biggest factors for ozone in Utah. But how much does the state itself actually contribute?

The answer is very little, according to a white paper from EPA.

When EPA assessed four Utah counties that make up a significant share of the state’s population and had ozone levels above 70 ppb (the EPA’s threshold), it found that natural and international sources contributed to at least half of local ozone levels.

It also found that human sources from elsewhere within the United States contributed approximately another 25 percent, while human sources within Utah contributed no more than 20 percent.

Oil and Natural Gas Are Far From the Only Domestic Human Contributors

Despite efforts to pin the blame on Utah’s oil and natural gas industry for higher levels of ozone, the reality is that a wide range of human sources contribute to the problem.

As the Utah Department of Environmental Quality notes, 90 percent of ozone is known as “good ozone” that sits about 10-31 miles above the Earth’s surface and protects the planet from the sun’s ultraviolet rays. The remaining 10 percent, or “bad ozone” sits just above the Earth’s surface and is formed “through the photochemical reaction of nitrogen oxides (NOx) and volatile organic compounds (VOCs).”

DEQ explains how humans contribute to “bad ozone” and it’s clear that nearly all facets of modern-day life are contributors, not just the oil and natural gas industry:

“Nitrogen oxides and volatile organic compounds are emitted by cars and trucks, industrial facilities, refineries, power plants, household products and cleaning supplies, and paints and solvents.”

Utah’s Air Quality is Getting Better

In a letter to EPA that was sent along with the demonstration, DAQ notes that Utah’s air quality has consistently improved in recent years:

“Over the last 15 years, the Wasatch Front airshed has achieved nearly a 40% reduction in volatile organic compounds (VOC) and NOx emissions – the precursor emission that lead to both PM2.5 and ozone pollution.”

It means that Utah, like most of the rest of the United States is making major progress on air quality thanks to innovation and technological advancements in various industries.


There is clear evidence that sources outside of Utah’s borders and naturally occurring sources are the primary drivers of the state’s ozone issues, despite the Tribune’s efforts to pin the blame on the oil and natural gas industry.

Moreover, the Tribune alleges that the state’s “demonstration was taking valuable time and resources away from working on a plan to meet federal ozone standards.”

But the truth is that the model DAQ is building for the 179(b) demonstration would also be needed for EPA-mandated SIP. Even if used for the 179(b) demonstration, the model is critical in determining what controls will be needed to reduce ozone levels.

This all shows that ozone issues in Utah, and across the west, are more complex than the Tribune portrayed and despite progress the state has made, it will continue to struggle to meet EPA thresholds if foreign and natural sources aren’t taken into account.

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