“We found order in the chaos,” says Rawad Saleh, a postdoctoral researcher at Carnegie Mellon University and lead author of the study. “Now we have this framework that unifies the seemingly discordant results reported by various research groups: the warming effect of brown carbon depends on the conditions under which it is produced.”
These findings, published in the journal Nature Geoscience, could help improve global climate predictions.
The team focused on the major sources of brown carbon: wildfires.
Smoke from wildfires accounts for one-third of the Earth’s “black” carbon—the familiar black soot that is second only to carbon dioxide in warming the planet. While black carbon is relatively simple—consisting solely of carbon—brown carbon contains a complex soup of organic material, making it difficult to identify, characterize, and model.
“You might call brown carbon frustrated black carbon,” says Neil Donahue, a professor of chemical engineering and a principal investigator of the study. “You get brown carbon when the wood isn’t fully cooked all the way.”
Brown carbon warms the atmosphere similarly to black carbon, but the prevalence and warming efficiency of brown carbon in wildfire smoke has been a large question mark.
To study brown carbon from wildfires, the researchers simulated a series of wood burning events, setting fire to a range of tree, grass, and brush types representative of vegetation in areas known for wildfires.
The team then measured the smoke’s ability to absorb light—and thus to induce warming—with a collection of state-of-the-art air monitoring equipment.
The study found that the amount of brown carbon produced in a fire had little dependence on what was being burned. Instead, brown carbon emissions correlated with the amount of black carbon in the smoke, which is dictated by the burning conditions, such as flaming and smoldering phases, and is relatively easy to measure.
“That’s a huge simplification in terms of estimating the emissions of brown carbon and predicting the effects on climate,” says Allen Robinson, head of the Department of Mechanical Engineering and a principal investigator of the study. “If every different tree produced different amounts of brown carbon, that just makes it really, really complicated.”
The Department of Energy Atmospheric Science Research Program and the National Science Foundation Atmospheric Chemistry Program supported the research.
Source: Carnegie Mellon University