Sunlight transforms wildfire smoke into toxic air

Wildfire season isn’t just about charred forests and hazy skies anymore. The smoke from those fires is doing something sneaky – and scientists say we’ve been underestimating just how much it’s disrupting the air around us.

When wildfires burn, they fill the air with smoke that can drift thousands of miles. That smoke contains tiny particles that pollute the air and make it hard to breathe.

But it turns out those particles aren’t just sitting there doing nothing. Under sunlight, they actively change, becoming tiny chemical factories that crank out even more air pollution.

Smoke reactions puzzle scientists

Air pollution experts have known for a while that wildfire smoke worsens air quality. But there was a strange gap in the data.

Scientists kept noticing high levels of peroxides – a group of highly reactive pollutants – during wildfires, even in places where the usual chemical reactions that create them shouldn’t work.

That’s because these reactions are normally blocked in cities by other pollutants like nitric oxide, which comes from car exhaust and other fuel-burning sources. Yet somehow, peroxides were still showing up – in large amounts.

It wasn’t until researchers at King Abdullah University of Science and Technology (KAUST) and the Chinese Academy of Sciences took a closer look that things started to make sense.

Smoke’s hidden chemical power

Smoke from wildfires contains lots of organic material – stuff that comes from burned plants and trees. Some of this material includes colored molecules that react to sunlight. These molecules are known as “photosensitizers.”

When these photosensitizers absorb sunlight, they become energized. That kick of energy sets off a chain reaction inside the smoke particles.

The result is the formation of highly reactive chemicals called peroxy radicals, which quickly turn into peroxides. This all happens within the particles themselves – not in the surrounding air like most traditional pollution reactions.

“This particle-driven pathway is surprisingly efficient – orders of magnitude faster than what classical pathways can supply,” said Professor Chak Chan, co-author of the study and dean of KAUST’s Physical Science and Engineering Division.

Pollution travels far beyond

Peroxides aren’t greenhouse gases, but they do play a big role in how air pollution spreads and how particles form in the atmosphere. They contribute to haze, worsen respiratory health risks, and help create new particles that float in the air. These secondary particles can hang around long after the flames are gone.

This means wildfire smoke is doing double duty – it’s not just releasing harmful particles directly into the air. It’s also generating new pollutants as sunlight interacts with the smoke long after it leaves the fire.

And that’s a problem for places far away from wildfires. Even cities hundreds of miles from a blaze can see elevated peroxide levels and worsening air quality – especially if the models used to predict pollution don’t account for this sneaky chemistry.

“This overlooked chemistry means that current air-quality and climate models are underestimating oxidant production from wildfires,” Chan said. “Updating these models is essential for communities, including here in Saudi Arabia, to better anticipate the health risks and environmental impacts of a warming world.”

Bigger, hotter, deadlier wildfires

Wildfires are not only becoming more common; they’re also getting bigger and more intense. In parts of the western United States, fire sizes have quadrupled since the 1980s. In the Mediterranean, burn areas have more than doubled over the last 20 years.

As more fires burn, more smoke enters the atmosphere. And now, thanks to this study, scientists know that smoke is changing in unexpected ways when exposed to sunlight. It’s producing reactive pollutants that were not being counted before.

That’s bad news for public health, especially for people with asthma or other breathing issues. And it makes the job of forecasting air quality – already a tough task – even more complicated.

This new research is pushing scientists to rethink how wildfire smoke works. Atmospheric models need to be updated to account for this “internal chemistry” that smoke particles perform under sunlight.

Until that happens, we’re likely underestimating just how much pollution wildfires add to our skies.

The full study was published in the journal Science Advances.

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