U.S. scientists to study Arctic smog

Key question: Is air pollution from lower latitudes causing the region's recent warming?

posted April 16, 2008 at 4:12 p.m. EST

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"What they are trying to do is very important," he says. "The chemistry of how you go from emissions to pollution is very different" in different regions of the world. Even when the chemistry is the same, the pollutants' behavior can vary with temperature, humidity, and a region's broader climate patterns. "To put the global puzzle together, we need to understand the different contexts" in which gaseous emissions, aerosols, and black-carbon soot find themselves.

The Arctic illustrates the point. Taken globally, soot and tiny particles called aerosols have a mixed effect on temperatures at the Earth's surface. Soot has a warming effect, because its dark surface absorbs and reemits heat. Aerosols present a more mixed picture. On their own, they can lead to cooling. But they also provide the seeds around which cloud droplets can grow. The size and number of particles affect the thickness of the clouds. That thickness can help determine whether the clouds will cool the surface by blocking sunlight and reflecting it back into space or keep things relatively toasty by letting some light through and trapping heat coming up from the surface.

Globally, the net impact of aerosols is to cool climate, partly offsetting the warming from a build up of human-generated greenhouse-gases, explains Ravi Ravishankara, who heads the chemical sciences division of the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo.

"But," he adds, "the Arctic is a strange place." Aerosols that form Arctic haze appear to warm the region, he says.

Although the project is roughly halfway through its first three weeks in the field, researchers are already noting the region's role as a caldron for emissions flowing up from lower latitudes.

"We've seen European pollution, North American pollution, Russian pollution. We've seen Siberian forest-fire plumes already, in April. We've seen plumes coming all the way up from Indochina," where locals use fire to clear farmland, Dr. Jacob says. This stew is aging in the Arctic, combining to form Arctic haze.

Moreover, he says, the team has been gathering details on the haze's color, which can vary from nearly white to dark gray. The relative abundance of these different tones can play a significant role in tilting aerosols' net effect toward or away from warming.

By understanding how pieces of the Arctic air-pollution puzzle fit together, researchers say they hope to give modelers the information they need to better simulate the changes there.

"The researcher we're doing is not simply about whether warming is under way," says James Crawford, who heads the tropospheric chemistry program at NASA headquarters in Washington. "It has to do with predictability of consequences" from shifts in the delivery of pollution to the Arctic as climate changes and as countries strive to clean up their emissions.

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