For decades, humans have been reducing emissions of nitrogen oxides—potent pollutants released by power plants and vehicles—thanks to increasingly stringent environmental regulations. Now, climate change may be undoing that.

“In another 10 or 20 years, vast tracts of California will have burned, and those emissions are going to affect the chemistry of our atmosphere immensely.”

In a new study published in Environmental Research Letters, researchers found rapidly rising nitric oxide and nitrogen dioxide, collectively known as NOx, emissions from forests and soils in California. The extra nitrogen could be enough to offset decreases in human-caused sources of NOx in the state, according to researchers, and may contribute to poorer air quality across the West.

Hotter, drier conditions could lead to further increases in NOx emissions from soils and especially wildfires, which release vast quantities of the gases, said study coauthor Ian Faloona, an atmospheric scientist at the University of California, Davis.

“In another 10 or 20 years, vast tracts of California will have burned, and those emissions are going to affect the chemistry of our atmosphere immensely,” Faloona said.

Getting to Know NOx

Nitrogen oxides are created during combustion reactions and lightning strikes and when microbes in the soil metabolize nitrogen. They don’t last long—typically, just a day or less—but they react with molecules in the atmosphere to form ozone and particulate matter, both of which can pose health risks.

“Understanding what produces ozone…and what we can do to reduce ozone concentrations is really important, and NOx is one of those really important factors,” said Jeffrey Geddes, a scientist at Boston University who studies air quality monitoring. Geddes wasn’t involved in the new study.

Ground stations in urban areas keep a close eye on NOx levels, but emissions in rural areas, where monitoring is sparse, aren’t as well studied. To see what’s happening outside of cities, Faloona and his colleagues turned to data from NASA’s Aura satellite, which has been monitoring atmospheric gases since 2004. They looked at emissions between 2009 and 2020, breaking down the land cover below into five categories: urban, forestland, cropland, scrubland, and barren land.

Faloona said the group was initially interested in NOx emissions from the soil, including in agricultural regions where farmers use nitrate-containing fertilizers. To estimate these emissions, the researchers drew from previous work that modeled how soil emissions responded to temperature and paired that with real-world nitrogen observations from Aura.

During the 11-year period, soil emissions not only rose with increased temperatures but rose at a much higher rate than previous studies indicated they should—almost 7 times more NOx leached from soils at 30°C than at 20°C. That could mean hotter temperatures caused by climate change could cause significant increases in NOx emissions from soils across the state.

Those results may not hold everywhere, Geddes cautioned, because nitrogen emissions can vary widely from place to place. Soil emissions also plateau above around 30°C. But with the limited current understanding of NOx emissions in general, this kind of data is valuable.

“It’s really important that we continue to build out our understanding of how NOx is emitted from soils across a broad range of ecosystems,” he said.


The most alarming data on NOx emissions didn’t come from soils, though. The researchers found that in forests, NOx levels grew an average of 4% per year for more than a decade, spiking in 2020. The dramatic jump was too big to be explained by soil microbes alone, Faloona said. Instead, much of the NOx was coming from wildfires.

“We have to get our wildfire emissions under control.”

Climate change is making wildfire more likely across the United States and elsewhere by creating hotter and drier conditions. The new findings indicate that NOx emissions from both wildfires and soils could increase in coming years, not only in California but around the world.

“Looking at the data and looking at the wildfire information, I became quite alarmed,” Faloona said. “We have to get our wildfire emissions under control.”

Although sources of atmospheric NOx can vary greatly in different environments, Geddes said it’s becoming clear that we should be looking to rural areas, and not cities, to understand nitrogen emissions, especially as climate change creates conditions ripe for future increases. These sources are “becoming more important to really understanding NOx variability,” Geddes explained.

The research “adds to our knowledge about why it becomes increasingly important to think about how we can mitigate wildfires and how we can adapt, given climate change, to try to reduce those risks,” said Carlos Restrepo, an urban planning researcher at New York University who wasn’t involved in the research.

This study and others like it are beginning to put numbers to a previously ill-defined source of air pollution, helping to make a case for action to policymakers. That could be especially important in the case of wildfire management, an area in which regulators and environmental agencies are struggling to catch up with rapidly changing conditions brought on by climate change.

—Nathaniel Scharping (@nathanielscharp), Science Writer

Citation: Scharping, N. (2023), Natural nitrogen emissions are rising in California, Eos, 104, Published on 13 October 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

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