.png)
Markus Ammann and Fengxia Bao have elucidated how brown carbon produced by wildfires accelerates the conversion of nitrate to nitrogen oxides in the atmosphere.
Photo Credit: © Paul Scherrer Institute PSI/Markus Fischer
Scientific Frontline: Extended "At a Glance" Summary: Atmospheric Nitrogen Oxide Formation from Wildfire Brown Carbon
The Core Concept: Researchers have demonstrated that brown carbon produced by wildfires accelerates the conversion of atmospheric nitrate into harmful nitrogen oxides. By absorbing soft ultraviolet light, these organic substances act as a catalyst that expedites nitrogen oxide production at a rate tenfold faster than previously understood mechanisms.
Key Distinction/Mechanism: While scientists knew that direct photolysis via hard ultraviolet light could convert nitrate back into nitrogen oxide, this known process was too slow to account for observed atmospheric concentrations. The newly identified indirect pathway reveals that brown carbon—derived from the incomplete combustion of biomass—absorbs soft ultraviolet light, enters an excited state, and donates an electron to nitrate, rapidly catalyzing the production of nitrogen oxides.
Origin/History: The findings were published in the journal Science Advances on November 11, 2026, by Markus Ammann and Fengxia Bao at the Paul Scherrer Institute's Center for Energy and Environmental Sciences. The laboratory experiments successfully resolved a longstanding aerosol chemistry hypothesis regarding the unaccounted surplus of nitrogen oxides found far from wildfire sources.
Major Frameworks/Components:
- Aerosol Chemistry and Photolysis: The study of how light initiates chemical reactions within suspended atmospheric particles, specifically the conversion of nitrate back to nitrogen oxide.
- Brown Carbon: Light-absorbing organic particulate matter resulting from the incomplete combustion of wood and biomass, responsible for the grayish-white color of wildfire smoke.
- Nitrogen Oxide (\(\mathrm{NO}_x\)) Cycling: The continuous atmospheric loop where nitrogen oxides oxidize into nitric acid, dissolve as nitrate in aerosol particles, and theoretically deplete, contrasted by the newly proven rapid reversal mechanism.
Branches of Science: Atmospheric Science, Atmospheric Chemistry, Aerosol Physics, Photochemistry, and Climatology.
Future Application: These findings will be integrated into global climate models and air quality tracking systems to significantly improve the accuracy of predictions concerning the long-range environmental and public health impacts of wildfires. Future research utilizing advanced simulation chambers aims to replicate the entire burned material spectrum to map all remaining nitrogen oxide formation pathways.
Why It Matters: Nitrogen oxides are primary drivers of respiratory illnesses, ground-level ozone formation, and the greenhouse effect. Understanding the precise chemical mechanisms that sustain high concentrations of these pollutants across continents enables more effective environmental policies, improved climate change forecasting, and better public health interventions in an era of increasing wildfire activity.
.png) |
| The wood stove at PSI that the researchers use to simulate a wildfire. Photo Credit: © Paul Scherrer Institute PSI/Markus Fischer |
In laboratory experiments, researchers at the Paul Scherrer Institute (PSI) have discovered how brown carbon from wildfires promotes the formation of harmful nitrogen oxides in the atmosphere. This could improve predictions of the effects wildfires have on air quality and climate. The results have now been published in the journal Science Advances.
During a wildfire event, smoke plumes often rise several kilometers and disperse across continents and oceans. Pollutants from wildfires in Canada can still be detected, for example, at the Jungfraujoch in the Swiss Alps. These pollutants, which include particulate matter, carbon dioxide, and nitrogen oxides, contribute directly to respiratory illnesses and accelerate the greenhouse effect. Now, researchers led by Markus Ammann at the PSI Center for Energy and Environmental Sciences have investigated the formation of these pollutants in more detail, and they found an explanation for why elevated nitrogen oxide concentrations are still present even at great distances from wildfires.
The path of nitrogen oxides
Nitrogen oxides (\(\mathrm{NO}_x\)), compounds consisting of one nitrogen atom and one or two oxygen atoms, are produced during combustion processes. Wildfires and lightning are natural sources. However, due to the burning of fossil fuels, current nitrogen oxide levels in urban areas are many times higher than in preindustrial times.
In the lower atmosphere, nitrogen oxides are responsible for the formation of ozone, a greenhouse gas and irritant. Furthermore, nitrogen oxides oxidize to nitric acid (\(\mathrm{HNO}_3\)) in the atmosphere. This dissolves as nitrate in tiny aerosol particles, which act as nuclei for cloud droplets. Rain carries the nitrate back to earth, where it is deposited in the soil. Theoretically, this continuous conversion should deplete atmospheric nitrogen oxide concentrations to levels significantly lower than observed measurements.
There is also a known reverse pathway in the atmosphere: triggered by sunlight, nitrate can be converted back into nitrogen oxide. This process is called photolysis. "However, this process is far too slow to account for the observed amounts of nitrogen oxide," Ammann says. Consequently, aerosol chemists hypothesized the existence of an alternative, accelerated photolysis pathway mediated by other compounds released during wildfires.
Simulated wildfire in a wood-burning stove
Wildfires differ from other sources of nitrogen oxide in that they also produce a large amount of particulate matter from organic compounds, including some soot. The cause of this is incomplete combustion of wood and other biomass. This organic particulate matter is responsible for the grayish-white color of the smoke. Some of the organic compounds absorb light in the visible spectrum. These are collectively referred to as brown carbon.
To investigate the influence of brown carbon on photolysis, researchers at PSI conducted a laboratory experiment. In a wood-burning stove, they first simulated a wildfire by reducing the air supply to produce more smoke, and thus more brown carbon. This was collected using filters, dissolved in water, and placed in a temperature-stabilized glass tube.
The researchers then exposed nitrate to the light of various UV lamps that simulated sunlight under different conditions, once without the added brown carbon and once with it. In each case, they measured the concentration of nitrogen oxides produced.
Nitrogen oxides were always produced with light containing a particularly high-energy ultraviolet component, that is, hard UV light. The photolysis of nitrate under hard UV light is known. However, the amount of nitrogen oxide was significantly higher in the samples that also contained brown carbon.
"When we then used lamps without a hard UV component, we could only detect nitrogen oxides in the presence of brown carbon," says Fengxia Bao, a researcher in Markus Ammann’s group and first author of the study.
Since direct photolysis with the remaining soft UV light is not possible, the researchers concluded that the path to nitrogen oxide in this case is indirect: the organic substances in the brown carbon absorb the soft UV light. This causes them to enter an excited state and donate an electron to the nitrate. That’s how the nitrate is converted into nitrogen oxide. In this way, nitrogen oxide production is accelerated tenfold compared to the known process with hard UV light.
Catalytic Impact
"This is a clear, powerful result," Ammann says. "We were able to demonstrate experimentally what up to now had only been vaguely suspected. Our study will enable better predictions about the effects of wildfires on air quality and climate."
Further research will be needed to clarify additional pathways for the formation of nitrogen oxides, Ammann continues: "Our result can explain a larger proportion of the nitrogen oxides present in the atmosphere, but not yet all of them."
In the future, he would like to make his research, which was funded by the ETH Board’s ReCLEAN project, even more realistic. For example, the simulation chamber at PSI could use all of the burned material, not just the components filtered out and dissolved in water.
Research material: ReCLEAN (Reactive nitrogen at the CLimate, Energy, Agriculture, water, and health Nexus)
Funding: F.B. acknowledges the ETH Domain Joint Initiative Reactive nitrogen at the CLimate, Energy, Agriculture, water, and health Nexus (ReCLEAN) and the Swiss National Science Foundation (grant no. 231265). C.L.H. acknowledges the NOAA Climate Program Office (NA16OAR4310112). M.A. and L.I. acknowledge the Swiss National Science Foundation (grant no. 188662). The atmospheric simulation chamber at Paul Scherrer Institute is a facility of the ACTRIS ERIC and receives funding from the Swiss State Secretariat for Education, Research and Innovation (SERI grant).
Published in journal: Science Advances
Title: Brown carbon in biomass burning aerosol enhances HONO production
Authors: Fengxia Bao, Lucia Iezzi, Wenjie Wang, Arianna Tronconi, Yanfang Chen, Therese S. Carter, Colette L. Heald, Kaspar Rudolf Dällenbach, David M. Bell, and Markus Ammann
Source/Credit: Paul Scherrer Institute | Oliver Morsch
Edited by: Scientific Frontline
Reference Number: as111226_01
