OZONE RISK FROM WILDFIRES

Small particulates sent up by wildfires are a well-documented health hazard. But recent research finds that ground-level ozone is now on the rise, too.

In July, as smoke drifted down from Canadian wildfires, New York City officials initiated an emergency response to what was expected to be historically poor air quality.

Outreach workers fanned across the city. Police officers handed out masks. For a period on July 16, New York City had the third-worst air quality in the world.

Then the skies began to clear, and the response was concluded. But in a way, the emergency wasn’t really over. A series of studies have found that even after the orange-gray smoke has disappeared, wildfires worsen air pollution in subtle and unexpected ways.

Researchers have quantified the risks over time, and over wide geographic areas, posed by small particulates called PM2.5, a constituent element of wildfire smoke. But scientists are now pointing to another, invisible hazard on the rise: ground-level ozone.

Ozone gas occurs naturally, and in the upper atmosphere it is protective, shielding the planet from solar radiation. But at ground level, ozone is toxic to the lining of human airways and has been linked to increased rates of asthma in children as well as heart disease and lung problems in adults.

“There’s very little doubt that ozone exposure can cause premature mortality,” said Dr. John Balmes, a physician and epidemiologist at the University of California, Berkeley, who has studied ozone.

Ground-level ozone is monitored at about 1,200 locations. But these measurements cover only about 2 percent of the country, and most of the sensors are concentrated in cities.

Jun Wang, an atmospheric chemist at the University of Iowa, set out with his colleagues to make a better map. It was an ambitious undertaking.

Dr. Wang worked with a team that used machine learning, a kind of artificial intelligence, to crunch enormous amounts of data from satellites, surface measurements and atmospheric models to calculate ozone levels down to the square kilometer, across the entire country.

The results, published in June in Science, were sobering.

Like PM2.5 levels, ozone levels steadily declined through the early 2000s. But in 2015, they started climbing again, a shift that had “gone undetected by the sparse monitoring network,” the authors wrote.

No one had noticed that fires had been steadily erasing progress made on ozone for over a decade. “The trend actually reversed solely due to wildfire,” said Meng Zhou, a researcher at NASA and an author of the study. “We did not expect that the impact would be that big.”

- Author: Clayton Dalton, The New York Times

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