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Tracking Sootprints

October 1, 1997

Was a coal miner’s lung cancer triggered by coal dust or cigarette smoke? Which is more to blame for a city’s poor air quality: diesel buses in the streets or gas-fired turbines at a nearby electric power plant? And how should government target air-pollution-control efforts to best protect human health? The answers to all these questions, John Vander Sande believes, lie in the structure of individual particles of soot.

Scientists concerned about air pollution have long studied concentrations of particulate-or soot-in the air emitted from smokestacks and tailpipes. Some researchers have also looked at the chemical composition of those emissions. But a team of researchers led by Vander Sande, a professor of materials science and engineering at MIT, is the first to mine the wealth of information found in each tiny chunk of impure carbon.

What makes this forensic feat technically possible, Vander Sande explains, is his team’s discovery that each soot particle has a unique internal structure-a complex, partially ordered lattice of carbon atoms he dubs a “sootprint.” Particles from different sources may also contain traces of different metals or clump together in different shapes-for instance, those discharged by a diesel engine look significantly different from those spewed from a smokestack. In fact, Vander Sande says his group has been able to discern differences in the sootprints that are produced by a diesel engine when it is idling versus when it is revving.

Soot Sources

Vander Sande estimates that about 20 major sources produce soot, including oil-or coal-fired power plants, industrial gas turbines, diesel engines, cooking stoves, and wood-burning processes. While soot particles from the same type of source may vary somewhat, the team’s goal is “to differentiate among these broad categories of airborne particulate accurately enough so that we can reliably assign [soot specimens] to specific sources.”

Vander Sande says a particle’s sootprint reflects its so-called thermal history: the temperature at which it burned, the amount of oxidation at the time it was formed, and the type of fuel it derives from. “Ultimately,” Vander Sande says, “we hope to be able to use our growing understanding of the variations in these soot signatures to learn more about the combustion process that forms them.”

“So far, however, we don’t understand enough about the relationship between structure and history to begin to have predictive power,” says Adel Sarofim, a renowned expert on combustion. A former member of Vander Sande’s research team, Sarofim is now a research professor of chemical engineering at the University of Utah. To identify a soot-print, then, scientists must compare it with the sootprints of particles whose source has been identified-an empirical approach that is based on “brute force and ignorance,” as Vander Sande puts it.

Toward that end, Vander Sande’s research group has begun to amass a library of soot particles. The project began in 1993, with a grant from the U.S. Bureau of Mines, and has subsequently received funding from the Environmental Protection Agency. Vander Sande and his colleagues have painstakingly analyzed the structure and composition of soot specimens from five different types of combustion, with four to six variants in each category. Their goal is to analyze at least 10 variants of all 20 major types of soot. The resulting images will provide a database that will allow scientists to match unidentified soot particles with the most likely source.

A Battery of Tests

Members of Vander Sande’s team employ a battery of tests to obtain a full picture of a given soot particle. First, they analyze the specimen under a high-resolution electron microscope, which reveals a mosaic of grains similar to the pattern of bumps on the skin of an orange. Computer enhancement techniques then transform the image into a maze-like series of lines that actually resembles a fingerprint. Using pattern recognition technologies similar to those employed by law enforcement officials to match fingerprints, scientists can d e t e r m i n e how closely these different kinds of soot structures resemble one another.

A technique called energy-dispersive x-ray analysis provides additional clues to the soot’s origins. Researchers use a special device to shoot electrons at the particulate samples and image the x-ray spectra that result. Because different types of atoms, such as iron or zinc, generate distinct patterns of x-rays, the technique helps scientists identify the many trace metals and other infinitesimal-and potentially toxic-elements imbedded in the carbon structure.

Quickly, Vander Sande ticks off three more tests in his laboratory’s arsenal. One, electron energy loss spectroscopy, allows researchers to graph a curve showing the rate at which electrons lose energy as they encounter a soot sample. The test yields slightly different curves for different types of soot. Lab team members also conduct precise size measurements because the particles’ sizes vary according to the type of combustion. And the team images groups of soot particles as well as individual particles to observe the way they aggregate into clumps. As Vander Sande explains, particles generated by wood burning tend to form “necklaces” while those from a coal-fired plant are likely to resemble “a grape cluster.”

This forensic tool kit gives scientists “enough sensible, meaningful metrics not only to differentiate among kinds of particles but to pinpoint their source with a great degree of confidence,” according to Vander Sande.

Right now, the major obstacle to progress is the labor involved in imaging each individual particle. Tests must be done essentially by hand.

Because the tests are so painstaking and costly, Vander Sande points out, automating the imaging techniques is essential if sootprinting is to become widely available. If the process can be made less cumbersome, Vander Sande hopes to create a hybrid instrument that state or regional environmental agencies could use to test and analyze soot particles on behalf of private parties or state or federal officials.

The opportunity to link air emissions to different types of sources has caught the attention of many environmentalists. Steven Dujack, a spokesperson for the Environmental Law Institute, based in Washington, D.C., suggests that people suffering from pollution-related health problems could use sootprints to support class-action suits against polluters whose emissions exceed legal limits. Neighbors might use the new evidence to force tighter emission standards for airports or power plants. “More legal cases might be brought,” Vander Sande concurs, “but frivolous lawsuits will also be more easily dismissed.”

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