'Order in the Chaos' Found by CMU Researchers Examining Brown Carbon

Aug 11, 2014

A team of researchers, including some from Carnegie Mellon University, have figured out a hard-to-understand pollutant called brown carbon.

A lot of attention is paid in the media to pollutants that contribute to climate change, especially to greenhouse gas emissions from power plants and other sources. But, some sources are lesser-understood and don’t come from areas that can be regulated — namely brown carbon, which comes from smoke from wildfires.

“There are a lot of tiny particles and droplets floating in the air, in the atmosphere, which we call aerosols,” said Rawad Saleh, a postdoctoral researcher at Carnegie Mellon University and lead author of the study. “Depending on where these aerosol particles come from, they are made of different stuff.”

Saleh said if you have an aerosol particle coming from a forest fire, it contains black carbon, or what people commonly call soot. Black carbon is known to cause warming of the climate because it absorbs sunlight and warms the atmosphere. He said there’s another “flavor” inside of the particles made of harmful organic compounds – and that is brown carbon.

“Brown meaning that they can also absorb sunlight, but in a different manner than black carbon,” Saleh said. “Until our study, there was no way to predict the brownness of an aerosol. You can measure it, but you can’t predict it. What we did is that we developed a method that we can predict the brownness of these organics and thus the warming of the atmosphere. So if you tell me what your fire is, I’ll tell you how brown your aerosols are.”

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.

“What this means going forward, in terms of science, is implementing our findings into climate models,” Saleh said, “and we are in the process of doing that right now in our research group, and I believe that a lot of research groups around the world will pick up the results and enter them into their climate models.”

This, he said, could help improve global climate predictions. The findings were published in the in journal Nature Geoscience.