Black Carbon Belchers?

Tiny particles known as black carbon can pack a heavy punch when it comes to climate change, by trapping heat in the atmosphere and by alighting atop, and melting, Arctic ice. With an eye toward controlling these emissions, researchers have tracked black carbon production from fossil fuel combustion in gasoline-burning cars and diesel-burning trucks. Once thought to be minor players, gasoline-burning engines could put out twice as much black carbon as was previously measured, according to new field methods (Environ. Sci. Technol., DOI: 10.1021/es2033845).

Black carbon particles, which come from many combustion processes, have become a focus this month of an international agreement to control climate impacts from short-lived but powerful actors such as methane, hydrofluorocarbons, and black carbon (C&EN, Feb. 20, page 8). The new findings could lead to controls on gas-burning vehicles, long considered to produce less black carbon than diesel-burning trucks and cars.

A team of Canadian government scientists stumbled upon the hidden black carbon emissions after deploying a new laser-based method to monitor soot and other tiny particles in the chaos of traffic. In 2010, John Liggio and his colleagues from Environment Canada and the National Research Council Canada deployed the new device along highways around Toronto.

The instrument’s pulsed laser beam heats black carbon particles up to thousands of degrees Celsius, making them light up like “red-hot glowing charcoal,” says project leader Jeffrey Brook. A photometer measures the brightness of the particles to estimate their mass in real time. The team compared their device to a commonly used single-particle analyzer, known as SP2, which they set out at the same time.

The researchers followed 30 heavy-duty, diesel-burning vehicles and collected samples of their exhaust. They found that the trucks belched out levels of black carbon similar to those documented by prior studies.

While the scientists were chasing trucks, stationary monitors measured particles moving downwind of the same multi-lane highways. Video monitoring helped the team track the number and types of vehicles passing by. After measuring the mass of black carbon as it fluctuated over 17 days, the scientists compared those data with estimates of total black carbon based on expected black carbon emissions of diesel- and gasoline-burning vehicles. But their estimates always fell short of the actual black carbon measurements.

Gasoline-burning vehicles would have to emit more to take up the slack, the researchers concluded. They calculated that such vehicles must emit about 75 mg of black carbon per kilogram of fuel, about twice the amount that other scientists have reported and that the SP2 measured. They surmised that, unlike the new device, other methods miss tiny black carbon particles as small as 7 nm.

Researchers should be aware of the limitations of SP2, an instrument that is growing in popularity despite its lower sensitivity, says Dan Lack of the National Oceanic and Atmospheric Administration. However, other monitoring techniques such as absorption filters can track smaller particles, he says. For now, the discovery of a hidden well of black carbon holds true only for the team’s Toronto sites, he points out.

Mark Jacobson of Stanford University comments that if the results hold true elsewhere, they “imply an even greater urgency for taking measures to reduce particle emissions from vehicles.”