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 prediction of the effects wildfires have on air quality and climate. The results have now been published in the journal Science Advances.
Wildfires bring to mind a blazing sea of flames. But what happens to the smoke? Anyone observing a wildfire from a safe distance notices mainly the enormous clouds that tower above the fire. These rise several kilometres and then embark on journeys that can extend 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 – have diverse and well-documented effects on air quality and climate. Among other things, they lead to respiratory illnesses and accelerate climate change through 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 (NOx) – 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 pre-industrial times.
In the lower atmosphere, nitrogen oxides are responsible for the formation of ozone, a greenhouse gas and irritant. Furthermore, nitrogen oxides oxidise to nitric acid (HNO3) 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.
This continuous conversion of nitrogen oxides to nitrate should ultimately result in a much lower amount of nitrogen oxides in the atmosphere than is actually present.
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.
One hypothesis in aerosol chemistry was, therefore, that in addition to this known, slow photolysis, there must be another, faster photolysis process – and that other substances released during wildfires could play a role in this.
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 greyish-white colour 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, then dissolved in water, and finally placed in a temperature-stabilised 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.
A powerful effect
“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.