Researchers Try to Track ‘Sootprints’ to Smog Sources

The plain-looking package addressed to scientists at the Massachusetts Institute of Technology arrives every few months. Inside, there is a vial containing a sample for MIT’s powerful electron microscope.

A deadly virus? A bioengineered wonder drug?

Actually, it’s a smidgeon of soot.

The researchers are using a new technique to compile a library of black-and-white “sootprints” showing the molecular structure of various types of pollution from everything from woodburning stoves and diesel trucks to smokestacks and jet engines.

Ultimately, the scientists hope to use the sootprints to track pollution to its source. That, in turn, could help policymakers design more effective environmental regulations and make it easier to catch polluters.

“I want to be able to use ‘sootprints’ the way we use fingerprints,” said John Vander Sande, a professor of materials science. “I’d like to be able to tell you this was a specific engine manufacturer using a specific lubricant burning a specific fuel--going uphill.”

The “sootprints” themselves even look like fingerprints: white circles sliced with distinctive patterns of short, black lines.

After a year, researchers have archived about 15 “sootprints” from a single diesel engine. Vander Sande and his partner, chemical engineering professor Adel Sarofim, plan to expand the library to include soot from other sources, such as jet engines, factories and power plants.

Knowing which soot comes from where would be extremely useful to lawmakers and regulators, said Kathleen Nauss, director for scientific review and evaluation at the Health Effects Institute in Cambridge.

“Where do you put most of your regulating money? On power plants? On 18-wheelers? It’s important to know,” Nauss said.

The American Lung Assn. has called soot, or particulate matter, a deadly threat. People who work long hours in rail yards and other soot-filled places have been shown to have a slightly elevated cancer risk, Nauss said.

But identifying the source of the bad air is often a process of elimination.

Take the case of Denver and its notorious Brown Cloud. Studies over the last 20 years have blamed the winter smog on everything from coal-fired power plants to cars and fireplaces. In the late 1980s, regulators forced power plants to switch from coal to natural gas. The Brown Cloud persisted.

“Turned out it didn’t make any difference. So, OK, we know that wasn’t it,” said William Pierson, a research professor at the Desert Research Institute in Reno, Nev.

Existing techniques for identifying soot rely on analyzing chemical makeup and tracking trace metals. Sarofim and Vander Sande also look at the soot’s physical structure--the spacing of certain carbon molecules.

The MIT work is especially interesting because it begins with diesel soot, which is notoriously hard to track back to its source with the existing techniques, said Glen Cass, a Caltech professor.

“None of the methods I have can make that distinction,” Cass said.

The process begins with a $2-million electron microscope the size of a hot tub. The microscope image is digitized, then run through a computer program to generate a new picture based on the spacing of carbon atoms. Finally, the image is reconstructed once more to make the “sootprint.”

A spectroscopic analysis of trace elements provides a cross-check that further distinguishes one sample of soot from another.

Sarofim and Vander Sande received a $100,000 federal grant for their work.

The greatest challenge may be the dizzying number of sources contributing to air pollution. There are 70 major types of emissions in Los Angeles alone, Cass said.

That means only a comprehensive, regional approach--not a crackdown on a single source--can thin the urban haze.

“I have never seen an urban air pollution problem in which there was a silver bullet. All I have seen is analyses that have produced a false silver bullet,” Cass said.