2024 News & Events

During the summer of 2023, Canada experienced its most intense wildfire season on record. More than 40 million acres burned, an area the size of Georgia, injecting an enormous amount of smoke into the atmosphere, where photochemical reactions generated ozone pollution. In late spring, which is early for the wildfire season, wave after wave of smoke billowing from burning forests in western Canada poured across the sky and into the upper U.S. Midwest.

A new analysis by NOAA's Chemical Sciences Laboratory (CSL) found that ozone pollution readings in the Upper Midwest caused by these massive fires were the worst in decades.

"We were watching the Environmental Protection Agency's air quality indices, and all across the Upper Midwest they were showing ozone exceedances," said Owen Cooper, a research physical scientist at CSL. "On June 3, 2023, every ozone monitor in Iowa, about a dozen of them, exceeded the ozone standard of 70 parts-per-billion. It was unbelievable. Then it happened again on June 20th."

Wildfires are not new to the U.S. and Canadian west. Most western forests rely on wildfire as a natural disturbance that renews forest ecosystems and provides habitat diversity for native species. What has changed in the past 30 years is the size and intensity of the fires and the amount of emissions they generate. According to data reported by the U.S. National Interagency Fire Center and the Canadian National Fire Database, the average combined area burned annually across the United States and Canada from 2015-2024 increased 70% from the 1990s.

The 2023 fire season was the warmest and driest for Canadian forests since at least 1980.

Wildfires produce large quantities of carbon monoxide, carbon dioxide, fine particulates (PM_2.5) and a variety of other gaseous compounds, including reactive nitrogen oxides and volatile organic compounds &nash; which are the photochemical precursors of ozone. These compounds quickly react in sunlight to form ozone in the fire plume and can be transported far downwind with the smoke.

Ozone and PM_2.5 both impair lung and cardiovascular function. Recent studies suggest that exposure to wildfire smoke containing both elevated ozone and PM_2.5 can have more severe health impacts than exposure to either pollutant alone.

A major western fire can generate a smoke plume that degrades air quality all the way to the Atlantic Ocean.

Analyzing the maximum daily 8-hour average surface ozone and 24-hour average surface PM_2.5 measurements archived by the EPA, researchers found that the western Canadian fires produced the highest regional-scale surface ozone levels ever recorded across the northern tier of the U.S. during the period from May to August. Over a period of 45 days, some ozone monitors recorded more than 20 periods when ozone exceeded the national standard.

"Looking at the ozone and PM_2.5 exceedances in the decades prior to 2023, we can see that the 2023 wildfires were truly extraordinary," said co-author Kai-Lan Chang, a CIRES research scientist working at CSL.

When exceptional events like major wildfires that can't reasonably be controlled cause exceedances of federal clean air standards, regulations include a process for excluding those periods from counting toward a violation.

The research team, which also included scientists from NOAA's Global Monitoring Laboratory, NASA's Jet Propulsion Laboratory, the University of Colorado Boulder, and the U.S. Geological Survey, also analyzed estimates of carbon monoxide and peroxy acyl nitrates, a class of air pollutants that play a key role in the formation of ozone. The estimates were produced by the NASA TROPESS project using observations taken by the Cross-Track Infrared Sounder instrument, currently flying on the Suomi National Polar-orbiting Partnership, a joint NOAA-NASA mission, as well as the NOAA-20 and NOAA-21 polar-orbiting satellites.

One of the startling findings was that even when the smoke was days old and more than 1,000 miles downwind from the fires, it still contained the ingredients necessary for continued ozone production.

"The satellite data further confirm the surface measurements by showing the enormous extent and impact of these fires both in North America and the rest of the world," added co-author Kevin Bowman, the TROPESS project scientist at the Jet Propulsion Laboratory. "This underscores the fact that these fires can affect us all."

In 2023, the eastern half of the country experienced numerous days of heavy smoke from fires in both western Canada and Quebec, even as the U.S. experienced its smallest area burned within its borders since 1998.

Climate model projections indicate that temperatures similar to those experienced in 2023 are likely to be typical by the 2050s, even under a moderate climate mitigation scenario. "As warmer temperatures and drought conditions increase the likelihood of intense wildfire seasons, these large ozone pollution events could become more common in the future," Cooper said.