2023 News & Events

Were wildfires responsible for Denver's record ozone season of 2021?

20 June 2023
adapted from the story by NOAA Communications

Denver skyline comparison
The Denver skyline in 2021 on July 8 and again on August 7, the smokiest day of summer. Photo comparison: Colorado Department of Public Health and Environment

The summer of 2021 was a smoky one for Denver and northeastern Colorado.

Smoky haze from wildfires in Arizona, California, and the Pacific Northwest shrouded the Front Range mountains and cast a gray pallor over the sky on a near-daily basis. The typical pattern of monsoon-driven summer thunderstorms that normally flush out stagnant air in July and August largely failed to materialize, allowing smog cooked under the summer sun in 90-degree heat to pool along the base of the foothills to the west.

All that added up to a record number of days when ground-level ozone, which has long bedeviled the "Queen City of the Plains," exceeded the 2015 National Ambient Air Quality Standard (NAAQS) of 70 parts per billion averaged over 8 hours set by the US Environmental Protection Agency. In 2022, the U.S. Environmental Protection Agency proposed to downgrade the northern Front Range from a "serious" to "severe" violator of the 2008 federal ozone standard.

So how bad was it?

"The smoke was unpleasant to be sure," said Andrew Langford, a research chemist with NOAA's Chemical Sciences Laboratory, and lead author of a paper published today in the Journal of Geophysical Research: Atmospheres. "Particulates, which you can see, only exceeded the air quality standard a few times that summer. Ozone exceeded the standard on more than half the days. But you can't see ozone, so this was a much more insidious problem."

In 2021, exceedances were recorded at one or more of the 16 Northern Front Range monitors operated by the Colorado Department of Public Health and Environment (CDPHE) on 66 of the 122 days from June 1 to September 30. In July, the Rocky Flats monitor operated by the state health department just west of Denver, exceeded the 8-hour standard on 22 of the 31 days.

"We estimate that without the extra ozone in the smoke, the Rocky Flats Monitor would have exceeded the standard on only 10 of those days," Langford said.

Wildfires generate large quantities of carbon monoxide, carbon dioxide and fine particulates (PM2.5), and a variety of other gaseous compounds, including reactive nitrogen oxides and volatile organic compounds (VOCs) – which are the photochemical precursors of ozone. These compounds quickly react to form ozone in the fire plume and can be transported far downwind with the smoke.

Denver-Boulder area surface ozone in 2021
The top graph shows the 66 days during the summer when ozone levels exceeded the 2015 national standard; the bottom graph shows four days with high PM2.5 levels. The horizontal dotted lines represent National Ambient Air Quality Standards (NAAQS). Figure: Langford et al., Journal of Geophysical Research, 2023

Ozone and PM2.5 both impair lung and cardiovascular function. Recent studies suggest that exposure to wildfire smoke containing both ozone and PM2.5 can have more severe health impacts than exposure to either pollutant alone.

With a team made up of colleagues from the Chemical Sciences Laboratory, along with researchers from the NOAA's Global Systems Laboratory and the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder, Langford analyzed surface observations and upper air measurements taken in 2021 with a sensitive Light Detection and Ranging instrument, or LIDAR, to detect the smoke aloft and estimate how much ozone was in the wildfire smoke. The ozone lidar at CSL is one of only six in the entire U.S., and the only one that can measure ozone all the way from the surface to 8 km (about 5 miles).

Analysis found that maximum daily 8-hour average ozone concentrations were 6 to 8 ppb higher than in 2019, 2020, or 2022. Lidar measurements showed the extra ozone imported with the wildfire smoke to be as large as 12 ppb on some days.

Denver and mountain range relief map
The state of Colorado and the Denver Metro/North Front Range non-attainment area, and the location of ozone and PM2.5 air quality monitors used in the analysis. The Front Range Urban Corridor extends from Pueblo, Colorado in the south to Cheyenne, Wyoming in the north and includes the Wattenberg Gas Field of the Denver-Julesburg Basin. The red crosses mark the locations of the red crosses mark the locations of four large wildfires during the June - September measurement period. Map: A. Langford, NOAA

That might not seem like a lot, but it represents a serious challenge for the Intermountain West, where many of the large metro areas regularly flirt with NAAQS violations even without wildfire smoke. For example, on July 12, smoke caused increased ozone concentrations in both rural and urban areas, Langford said, but it only caused exceedances in Denver, Salt Lake City, Utah, Boise, Idaho, and the Cheyenne-Laramie area of southern Wyoming – metro areas that already had elevated ozone from local pollution.

These cities are not alone. Some 200 US counties are in non-attainment with the EPA's current ozone standard, and the agency is currently deciding whether to tighten it even further.

"You can think of transported ozone in wildfire smoke kind of like the tide," Langford said. "Both the little ships and the big ships in the harbor will rise as the tide comes in, but now the big ships won't be able to pass under the bridge."

Langford said the likelihood of more smoke in more places highlights the need for effective public health communication. "People can see smoke but not ozone," he said. "There may be days when it doesn't look too smoky, but there's a lot of ozone."

He added that the prospect of more smoke arriving on the wind should also encourage air quality managers to keep the focus on cleaning up air in local areas. "It will be more and more important to reduce the sources of ozone that we have some control over," Langford said.

Langford, A.O., C.J. Senff, R.J.A. II, K.C. Aikin, R. Ahmadov, W.M. Angevine, S. Baidar, W.A. Brewer, S.S. Brown, E.P. James, B.J. McCarty, S.P. Sandberg, and M.L. Zucker, Were wildfires responsible for the unusually high surface ozone in Colorado during 2021?, Journal of Geophysical Research, doi:10.1029/2022JD037700, 2023.


Ground-level ozone (O3) was unusually high in northern Colorado in the summer of 2021 with maximum daily 8-hr average (MDA8) concentrations 6 to 8 parts-per-billion by volume (ppbv) higher than in 2019, 2020, or 2022. One or more of the monitors on the Colorado Front Range exceeded the 2015 U.S. National Ambient Air Quality Standard (NAAQS) of 70 ppbv on 66 of the 122 days from 1 June to 30 September, and this record number of exceedances coincided with the near daily presence of dispersed smoke haze from wildfires in Arizona, California, and the Pacific Northwest. In this paper, we use regulatory and non-regulatory surface O3 and PM2.5 measurements in conjunction with ground-based lidar observations to estimate how much O3 was associated with the wildfire smoke. Analyses of the surface measurements suggest that pyrogenic O3 transported to northern Colorado with the smoke increased the surface concentrations in northern Colorado by an average of 8 ppbv in July, 3 ppbv in August, and 2 ppbv in September. Analysis of the lidar measurements showed these contributions to be as large as 12 ppbv on some days. Production of O3 from reactions of pyrogenic VOCs and locally emitted NOx appears to have been minimal (<3 ppbv) in the Boulder area, but may have been much larger in the suburbs southwest of downtown Denver.