Wildfires Contribute More to Atmospheric Warming, New Study Shows

Jul 15, 2013

Scanning electron microscope images revealing soot (bottom left) and tarball particles (top left, bottom right) collected from 2011 Las Conchas fire.
Scanning electron microscope images revealing soot (bottom left) and tarball particles (top left, bottom right) collected from 2011 Las Conchas fire.
Credit LANL (China, S, Mazzoleni, C, Gorkowski, K, Aiken, AC, Dubey, MK; Nature Communications, 2013)

As the country recovers from the worst wildland firefighting accident in years, there’s more attention on fire crews and the homes they’re trying to protect. But an often invisible result of wildfire can have a big effect on human health and climate... even after the flames die down. Science correspondent Ellis Robinson takes a look at the effects of wildfire smoke on air quality. And that means understanding something called a “tarball.”

If you get too close to fire, your senses scream one, very important thing at you: That fire is hot. Really hot. But from a different perspective, it's not that hot at all. In fact, it's almost downright cold.

Wildfires simply don’t burn hot enough to completely break down wood. If they did, the only products from fire would be carbon dioxide and water. Instead, along with CO2 and water, you get stuff called partial combustion products... like soot, fine particles, and thousands of organic chemicals. What this mess of smoke does in the atmosphere has scientists working hard to understand it.

"There is a host of organics, there's a whole stew of organic chemistry that we don't know what the chemical composition of smoke is."

That's Manvendra Dubey, a senior scientist at Los Alamos National Lab. He recently made a discovery about smoke that was published last week in the journal Nature. In 2011, the Las Conchas fire burned right outside of Dubey's lab, giving his team a unique opportunity to examine smoke close-up. They measured thousands of wildfire particles and found two major types: soot particles and what he calls “tarballs.”

“We find that tarballs are ten times more abundant than soot.”

Think of tarballs as microscopic particles of organic goo. Not unlike tiny bits of tar floating around in the air. Now, soot has long been known to absorb sunlight, which is easy to understand since it’s dark, which contributes to atmospheric warming. But tarballs used to be considered “white,” in that they only scattered sunlight, which cools the atmosphere. Dubey says this old picture is wrong.

“We think that these molecules tend to absorb in short wavelengths, they are a key element of brown carbon.”

And because they found so many of these tarballs in wildfire smoke, if they absorb even a little bit of sunlight, tarballs could warm the climate as significantly as soot. This would be a big change from how climate modeling of fire is most often done.

Once it gets into the sky, smoke... actually isn't even smoke at all. Does that sound confusing? It should, until you consider a process called secondary chemistry. As smoke reacts with sunlight and other stuff in the air, it begins to change as it gets blown away by the winds. This process creates new molecules out of smoke that are not themselves emitted from wildfire. And one important example is something you’ve probably heard of: ozone. Atmospheric scientist Gabi Pfister:

“In order to form ozone, what you need is nitrogen oxides, you need volatile organic compounds, and you need sunlight.”

Since wildfires spew out these ingredients in abundance, they can be big contributors to ozone formation, which is bad for our lungs and bad for crops. Interestingly though, Pfister says that ozone from forest fires is felt more strongly far away from the fire than right up close.

“The highest ozone impacts can be very, very far downwind. A few miles to a hundred of miles downwind of a fire. And then you don’t see it anymore. The smoke you see, the ozone you don’t see, and that makes it tricky.”

And trickiness is the name of the game for understanding how wildfires impact air quality. Joost de Guow, a senior scientist at NOAA in Boulder, says many unanswered questions remain, especially when it comes to the chemistry of smoke in the atmosphere.

“There have been studies that show production of fine particles and ozone, but there have also been studies that shown no production of fine particles and ozone. That’s something we have to learn to understand if we want to predict what the air quality implications of a wildfire in Utah are for the air in the Front Range.”

In other words, fires can produce ozone and fine particles, which are bad for air quality, but predicting how much continues to perplex scientists. Kind of startling, considering that fire is one of our oldest inventions. But with discoveries like Dubey’s tarballs, we get a little closer.

In science, for Aspen Public Radio, I’m Ellis Robinson