U.S. scientists to study Arctic smog

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

Despite its pristine image, the Arctic has a serious smog and soot problem.

Scientists from three federal agencies are now engaged in the most ambitious effort yet to measure airborne pollutants in the Arctic and gauge their effect on the region's climate.

For three weeks this month, and another three weeks this summer, they are marshaling satellites, instrument-laden aircraft, oceanographic ships, and ground stations to study the gases, aerosols, and black-carbon soot that accumulate in the region from human activities and wildfires.

By some accounts, these pollutants – especially soot from inefficiently burned fossil fuels and from burning biomass – could be responsible for a significant portion of the region's recent warming.

That warming has outpaced projections from climate models. And it's led to a dramatic loss of summer sea ice in the Arctic Ocean. If current trends continue, some researchers say, the Arctic Ocean could be ice-free in the summer within the next 10 years – with ripple effects that would touch climate and weather patterns at lower latitudes.

Scientists have long observed that atmospheric circulations have carried pollutants into the skies above the Arctic, turning them into something of a dumpster for the Northern Hemisphere's air pollution. More recently, the vast northern forests in Russia, Canada, and Alaska have seen rising numbers of wildfires, adding their emissions to the mix.

"There is an urgent need to better understand changes going on in Arctic pollution" and its effects on the region's climate, says Harvard University's Daniel Jacob, who specializes in atmospheric chemistry and is chief scientist for ARCTAS, the National Aeronautics and Space Administration's contribution to the effort.

The two-phase study is part of a broader international effort – associated with the International Polar Year – to understand the sources and effects of airborne pollutants reaching the region.

The ambitious project follows on the heels of major field efforts in the late 1990s and last year looking at similar issues in the Indian Ocean and across the northern Pacific Ocean. Taken together they should provide a more detailed global picture of the impact air pollution can have on weather and climate and how those effects change with latitude, according to Veerabhadran Ramanathan, director of the Center for Clouds, Chemistry, and Climate at the Scripps Institution of Oceanography in La Jolla, Calif. He played a leading role in these earlier research projects.

"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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