Tracking the "Secret" Life of Soot

January 24, 2014
Originally published July 27, 2013

For a long time scientists have known that breathing in soot from vehicles and power plants is bad for us. But the soot itself might not be the problem—at least not entirely. Scientists have found that particles live a "secret life" once released into the atmosphere, picking up toxic gases and other hitchhikers before making their way into our lungs.

One of the places where this science is being tested is in Pittsburgh’s Fort Pitt Tunnel. On a recent afternoon Daniel Tkacik, a graduate student at Carnegie Mellon University, stood one floor above I-376, which courses through the tunnel. He peered threw a narrow ventilation slots—yes, big enough for someone to fall through—at cars and trucks racing below. All of a sudden, a "whoosh" went through the slot.

“Oh boy!” Tkacik says. “A semi just drove by and it was like three feet from our heads.”

Tkacik’s is in the tunnel to study car and truck exhaust. He is interested in knowing  not just what’s in the exhaust—but what it will look like it in a few days. He ran the exhaust through a machine that’s like a software program that tells you what you’ll look like in the future.

“We’re taking these particles, and we’re aging them, we’re aging them by four days,” says Tkacik. Particles typically only last two weeks in the atmosphere. “So four days is a pretty significant chunk of its life. It’s like we’re catching it in its mid-20s, maybe?”

Secret Life Unveiled

For a long time, scientists thought the soot coming out of these tailpipes was basically unchanged from the time it was created to the time we breathed it in. But in the last few decades, they’ve found soot, and other small particles change pretty radically while they’re in the air.

Harvey Jeffries, a retired chemist at the University of North Carolina who’s studied these particles, says that for decades scientists have known the smallest of these particles were the worst for us. This is probably because they can slip through our natural defense systems and end up deep in our lungs. So they started studying them, and what they found was unexpected.

“The mass of these things changed over time, and in fact you gained mass,” Jeffries says. There was more. “They began to find it was gooey, it was sticky.  As they examined it more and more they figured out it was a kind of complicated organic soup.”

What was happening? In a word—oxygen. It makes up about 20 percent of our atmosphere. And a lot of times this oxygen forms something called a hydroxyl radical. This is basically a water molecule, that’s missing one hydrogen atom. So it’s really "jonesing" for an extra hydrogen. It turns out there’s a lot of hydrogen atoms in the atmosphere, attached to molecules that are just sort of hanging around. And a lot of the time, these molecules come from stuff we create—exhaust from buses and cars, weed whackers, and those fumes from spray paint cans. With the help of a little sunlight, the hydroxyl radical can go to work on these molecules, says Carnegie Mellon's Neil Donohue, an atmospheric chemist.

The hydroxyl radical, Donohue says, is “just ravenous.”

“It sees all these hydrogens on these very big molecules, and it starts ripping them off here, there, and everywhere.”

And when it does that, it makes a new substance, that’s heavier, and goopier. This new substance has a name—secondary organic aerosol. And it wants to condense, much like how steam from a shower condenses onto a bathroom mirror. Instead of a mirror, it uses particles in the air, like, a piece of soot.

A Surprising Source

This process happens quickly—after a few days, there can be hundreds of thousands of different chemical compounds stuck to a particle, many of them made by humans.

But there’s also one surprising source for goopy stuff.

Rich Kamens is a professor of environmental sciences at the University of North Carolina. At a small lab he held a bottle of clear liquid demonstrating how trees—yes, trees—contribute to particle pollution. The liquid gives off a pine-tree scent, when opened.

Inside the bottle is alpha-Pinene. “This is the material that’s coming out of pine trees in liquid form,” Kamens says.

Kamens puts a drop of the pine tree liquid into a chamber in a small greenhouse. Then he runs a souped up hair dryer, to simulate what would happen to the tree’s chemicals in the sun.

Under the right conditions, the pine tree chemical will stick onto a piece of soot—or even make new particles of its own. And that’s exactly what Kamens sees when he looks at a screen to see how many particles have been created.

“We’re seeing this initial burst of particles in the very very smallest sizes. Now, all of the sizes we’re measuring especially the ones above 100 nanometers are going to penetrate deep into our lungs.”

A Toxic Effect

So are trees big sources of air pollution? Not exactly.

“Before people, the atmosphere was fine,” Kamens says.

But when tree emissions combine with human-made sources, like from burning fossil fuels, particle pollution becomes a problem. The longer these particles stay in the air, the more pollution they pick up. Fine particles are responsible for 15,000 premature deaths in the U.S., according to the EPA. Some of this toxicity may be coming not from the particle itself, but from what it picks up in the atmosphere, Jeffries says.

“The particle is itself is not such a toxin as it is, in our view, as it is a carrier of toxicity. And that toxicity is being created in the atmosphere. The atmosphere itself is a source of toxicity,” he says.

Scientists want to know what’s the best way to keep the atmosphere clean. By doing this, they think we can prevent the particles already in the atmosphere from becoming worse for us by the time we breathe them in. Maybe someday soon, the second life of a soot particle won’t be so secret anymore.