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Western wildfires drive increasing overlap of particle pollution and ground-level ozone

Increasingly frequent wildfires in the western U.S. are driving particle pollution to occur with late summer ozone episodes and raising new public health concerns, according to research published in Science Advances on Wednesday.

“The year 2020 actually had the largest extent of simultaneous air pollution co-occurrence,” said Dmitri Kalashnikov, a doctoral student at Washington State University Vancouver.

“I had an inkling that we are seeing an increase in co-occurrence, because we hear stories about wildfires increasing every summer, but I was surprised just how strong the trend increased over the last 20 years,” Kalashnikov said. He analyzed a 1 degree x 1 degree grid of the western U.S. overlaying fine particulate matter and ozone data collected by the U.S. Environmental Protection Agency and Canada’s National Air Pollution Surveillance Program from 2000 to 2020.

The grid was generated by co-author Jordan Schnell, a researcher at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder.

Fine particulate matter is an inhalable form of particle pollution, one-thirtieth the size of a strand of human hair. Wildfires produce about half the fine particulate matter in the air. Ground-level ozone is generated through a chemical reaction of air pollutants like car exhaust in sunlight.

In the past, the presence of fine particulate matter peaked in the winter, while ozone pollution commonly occurred in the late summer heat.

This analysis found the presence of fine particulate matter more than doubled across the western U.S., with some areas experiencing an 80% increase. Rather than in the winter, particle pollution now peaks from July to September alongside ozone episodes. The air pollutants converge an average two weeks longer during late summer compared to data collected in 1979.

These co-pollutants lingered longer in recent years, spreading over wide geographic areas. Between the 2017, 2018 and 2020 fire seasons, a cumulative 600 million people were exposed to simultaneous particle pollution and ground-level ozone.

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The paper estimates 46 million people alone experienced simultaneous particle pollution and ozone on Aug. 21, 2020, during a record-breaking wildfire season.

Researchers point to an increase in high-pressure weather patterns, or atmospheric ridging, as driving the co-occurrence, since high-pressure ridging holds hot stagnant air in place and allows the buildup of air pollution. At the same time, this produces more ozone and the hot, dry conditions ripe for wildfire.

“It's this perfect storm,” Kalashnikov said. “Atmospheric ridging patterns are increasing so we're seeing more hot weather, and as a result we're seeing more wildfire smoke, mixing with more ozone, creating more of these co-occurring air pollution episodes which are becoming larger and more frequent and more persistent.”

The impact of these weather patterns is worsened by the topography of the western U.S., which is full of valleys like the Los Angeles Basin and the Willamette Valley where stagnant air easily settles and air pollution accumulates.

“Understanding how regional factors influence air pollutant characteristics and contribute to their changing risks is critical for assessing their public health impacts and anticipating future trends associated with climate variability and change,” the researchers wrote.

Fine particulate matter can cause cardiovascular and respiratory illnesses in people, potentially leading to death. Ozone affects humans as well as plants and the broader environment. Research indicates the combined air pollutants are particularly harmful to pregnant women and children.

Kalashnikov said he hopes this research encourages people to treat air pollution like any other severe weather condition and avoid outdoor activities and work during poor air quality peaks.

“Given that we are seeing climate change and we are expecting to continue seeing these trends, I think it's very important to increase awareness on the dangers of air pollution," Kalashnikov said. "If there's wildfire smoke, change your plans: don't go running, don't go hiking."

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Dmitri Kalashnikov

Satellite image of wildfire smoke from numerous large fires burning across California spreading across the western U.S. on Aug. 21, 2020, which contributed to 46 million people being simultaneously exposed to high concentrations of multiple harmful air pollutants. Image was generated using the NASA Fire Information for Resource Management System online data explorer.

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