The research, published June 2 in Geophysical Research Letters, found a threefold increase in the amount of nitrogen dioxide (NO2) above background levels downwind of the 2021 Dixie Fire, the second largest wildfire in recorded California history. The team's analysis showed that the fire had released large amounts of NO2, not only by directly burning vegetation but also by heating and drying forest soil, causing a rise in microbial production of nitrogen oxide gas (NO), which reacts in the atmosphere to form NO2.
"It has long been known that vegetation burning emits nitrogen dioxide into the atmosphere," said corresponding author Jenny Fisher, an assistant professor in the department of atmospheric and environmental sciences at the University at Albany, State University of New York. "However, our results show unexpectedly high nitrogen dioxide concentrations in the forest air after the fire has been extinguished, originating not from smoldering, but from microbial processes in soils."
The researchers used satellite observations of NO2 from the TROPOspheric Monitoring Instrument (TROPOMI) spectrometer and surface observations of carbon monoxide (CO) and other pollutants from the California Air Resources Board's network. They found the maximum post-fire NO2 concentration was 2.5 times larger than typical values observed over the remote Sierra Nevada region, and estimated that the emission of soil NO had increased by a factor of about three.
They also simulated the transport of smoke and the evolution of nitrogen oxides using computer models, to estimate the amount of post-fire NO2 enhancement above the background levels.
Wildfire-induced soil emissions were found to be particularly persistent, with NO2 enhancements lasting up to a month after the fire. According to the researchers, post-fire soil emissions should be better accounted for in regional air quality modeling, as they could make a significant contribution to air pollution events in remote areas during and following wildfires and potentially interact with other pollutants from long-range transport to affect the composition and chemistry of the troposphere, or the lowest level of Earth's atmosphere.
