When wildfire smoke and rain collide, it may fall as fertilizer, U. study finds

SALT LAKE CITY — Federal meteorologists were baffled five years ago when a cold front didn't push wildfire smoke out of Utah like they had suspected, instead creating a smoky type of rain that fell over most of Utah's northern half.

Air quality levels were considered unhealthy at times, especially along the Wasatch Front and northern Utah, as the storm pushed through. But what exactly is in the rain when it collides with wildfire smoke?

These types of events can cause "large bursts" of nitrogen, phosphorus and potassium to tumble to the ground with the rain, potentially fertilizing land and water ecosystems, according to a new study led by the University of Utah.

"When rain hits an ecosystem, the nutrients are bioavailable — organisms, whether in lake or on land, can immediately access them because they're dissolved in water," said Alexandra Ponette-González, an urban ecologist at the University of Utah and the Natural History Museum of Utah, the study's lead author, in a statement.

The study, published in Global Change Biology on Tuesday, is based on a review of 2014, 2020 and 2022 data from 250 climate sites scattered across the 16 regions. This included data from the National Atmospheric Deposition Program, which was established to collect data on the chemical composition of rainwater.

A team of researchers from the University of Utah, Utah State University and the Cary Institute of Ecosystem Studies in New York used satellite observations of smoke along with the national network rainfall data to determine the frequency of smoke and rain events and what type of nutrients were left behind after these events.

As Ponette-González puts it, what goes up must come down.

In this case, fires burn through landscapes, which send particles and gases into the atmosphere that can fall to the ground when they mix in with rain clouds. Nutrients from the smoke end up falling on plants and other organisms through this process of wet deposition.

"You can see the remnants of material that are left behind when the water evaporates," she explained.

While the rain-smoke events were mostly sporadic, researchers found that they delivered a surprising amount of nitrogen, phosphorus and potassium when they formed. For example, rain-smoke days accounted for 20% to 30% of annual rain-deposited nitrogen, phosphorus and potassium, despite accounting for just 5% of days.

The team found more types of rain-smoke events in 2022 than in 2014, which could be tied to growing "frequency and magnitude of extreme wildfire events" across the globe, the authors wrote.

They also found shifting regional hot spots over the surveyed years, with the Northern Rockies, the Southwest and the Ohio Valley all having their moments. The Upper Midwest, however, was the "persistent hot spot" for these types of weather events.

Yet, researchers found only "modest increases in smoke-affected wet deposition" within the region, which could be caused by different materials introduced by fires or the distance the smoke traveled.

The study largely shows that wildfire smoke can be a key source of material deposited into ecosystems, but it's unclear exactly what the impacts are, its authors say. They don't know if it creates benefits, aiding growth or productivity, especially in areas with limited nutrients, or challenges, like harmful algal blooms.

It may differ based on the atmospheric deposition and elements of the ecosystem receiving it, said Janice Brahney, a USU watershed scientist and one of the study's coauthors, in a statement.

"We can't characterize atmospheric deposition as simply 'good' or 'bad,'" she added. "With more wildfire and more smoky days, it is increasingly important to understand how much material is being transported in smoke and deposited across the landscape, and what that means for ecosystems."

The findings are a starting point for further research, the team added.

That includes studying more about how various elements of fires produce the chemistry of depositions that end up falling to the ground in rain, and whether different types of fires can affect nutrient types in them.

"This study puts a number on how much wildfire can contribute to nutrient delivery and raises important questions for what that means for ecosystem health," Ponette-González said.

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