Where There’s Smoke (and Rain), There’s Nutrients

By Olivia Barney 

In a world that’s getting continually hotter, wildfires are an increasing worry. They burn fiercely, with smoke and ash particulates presenting a variety of human health concerns. But how do these particulates affect surrounding ecosystems? Do they promote growth or upset the balance of things? These are questions of interest to NHMU’s Curator of Urban Ecology and Professor of City & Metropolitan Planning, Alexandra Ponette-González, Ph.D. Working alongside a team of researchers from the University of Utah (Heather A. Holmes, Ph.D.; Bryn Spielvogel, Ph.D.), Utah State University (Janice Brahney, Ph.D.), and the Cary Institute of Ecosystem Studies (Kathleen C. Weathers, Ph.D.), Ponette-González and collaborators got to work in search of answers. 

In particular, the team was interested in looking at smoke-affected wet deposition, what rains out of the sky and is deposited to ecosystems when it’s also smoky outside. They wanted to see what quantities of nutrients from smoke and ash were deposited in rainfall and how that changed from region to region across the U.S. and in different years. 

Studying Smoke-Affected Rain

Using satellite-derived smoke observations and national network rainfall data, the
researchers quantified the number of days that collection sites experienced (a) smoke,
(b) rainfall and (c) smoke and rain together. Ponette-Gonzáles et. al. Global Change Biology (2026)

For their research, the team chose to focus on data from three years (2014, 2020, and 2022). Since they were interested in looking specifically at smoke-affected rain, they first had to identify when smoke and rain occurred at the same time (calling those instances smoke-rain days) — essentially, they started narrowing down data by asking two key questions. Was there smoke in the air? Did it rain the same day? 

Those questions were answered with the help of two key databases. The NOAA Hazard Mapping System (HMS) provides information about daily smoke presence across North America. Meanwhile, the U.S. National Atmospheric Deposition Program (NADP) has been tracking and testing rainwater from more than 250 sites across the United States and Canada. 

By overlaying these datasets, the team identified the days in which both smoke and rain occurred in the same area. These smoke-rain days made up an average of 1-5% of the year — a substantial number considering those days were found to provide as much as 30% of the yearly nitrogen and phosphorus that’s delivered to ecosystems through rain. 

Particles from rainwater samples collected on filters. Fito Torres / NHMU

Rainwater collected by the NADP is tested for calcium, magnesium, sodium, potassium, sulfate, nitrate, chloride, ammonium, and orthophosphate, nutrients of interest to the team. 

Their results showed that these nutrients were deposited in greater quantities on smoke-rain days when compared to non-smoke-rain days, though the significance of these deposits varied by geographic region. 

“Our results demonstrate that smoke-affected wet deposition can deliver biologically important nutrients to ecosystems, which may be leading to fire-fertilizing of terrestrial and aquatic ecosystems.” 

Questions Left to Explore

Though these nutrients are important across ecosystems, the influx of nutrients may not always have a positive effect. In some cases, nutrients delivered in rain provide a resource the ecosystem has been lacking — like adding fertilizer to slow-growing plants. In other cases, rain might deliver more nutrients than the ecosystem needs. “Too much of a good thing” can cause algae to grow in lakes and streams, for example. 

Up-close view of particles from rainwater samples collected on filters. Fito Torres / NHMU

“In forests, nitrogen deposition originating from wildland smoke can alter tree survival and growth rates… In aquatic ecosystems, including streams and lakes, phosphorus deposited in smoke particles has the potential to stimulate algal production, and potentially, harmful cyanobacterial blooms.” 

So, while all nine nutrients can have beneficial impacts on the ecosystems they’ve been deposited into, in excess they can also cause harm. The actual effects will vary between systems, and it’s an area that still needs to be studied. 

Another area of future study might trace smoke back to its origin. 

“During combustion, different vegetation types emit particulates that vary in their chemical composition, affecting the chemical signature of smoke-affected rainwater and deposition downwind.” 

Understanding the type of “fuel” that burns during fire matters, for example. After all, what goes up must come down. Learning more about exactly what went up (in smoke), would help researchers predict which nutrients are likely to come back down. 

Ponette-González, Brahney, Holmes, Spielvogel, and Weathers plan to continue their research in the coming years, focusing on some of these unanswered questions. They’re also eager to explore dry deposits, which occur on days where there is smoke but no rain, as these days are far more common. 

To learn more about their research, check out their latest publication in Global Change Biology.