Data gathered by aircraft flying over northern Alaska and the Arctic Ocean in April 2008 hint that many springtime plumes of arctic haze in the region, long thought to be pollutants associated with industrial emissions, may in fact result from forest fires and agricultural burning in Asia.
Distinct layers of dirty air, often dubbed arctic haze, have been regularly observed at high latitudes of the Northern Hemisphere since the 1950s. The plumes are particularly apparent during the spring, when the long polar night is coming to an end, but dissipate somewhat as summer progresses. Their sources have never been well identified, says Charles A. Brock, an atmospheric physicist at NOAA’s Earth System Research Laboratory in Boulder, Colo.
Many scientists have presumed that the plumes of haze include industrial emissions from Europe, Asia and North America that are carried to the high Arctic by weather systems in the winter and early spring, Brock notes. Indeed, many studies indicate that mercury and soot, among other industrial emissions, routinely make their way to remote and otherwise pristine arctic locales. But new analyses, reported by Brock and his colleagues online January 30 in Geophysical Research Letters, suggest that many plumes of arctic haze, including those prominent in the spring, may stem from other sources.
In April 2008, a NOAA aircraft laden with sensors made a half-dozen flights north from Fairbanks, Alaska, to the Arctic Ocean, slurping in samples of dirty air as it flew through arctic haze plumes. Over the six missions, the researchers identified nearly 50 separate plumes of haze, says Brock. Some of the layers were found at altitudes as high as 6.5 kilometers, but others were detected on low-altitude passes over the Arctic Ocean — including some flight segments just 40 meters above the ice-strewn waters.
The dark plumes were full of tiny particles of black carbon averaging about 300 nanometers across — less than one-hundredth the width of a human hair, Brock notes. Those layers also contained concentrations of acetonitrile, benzene and carbon monoxide that were much higher than average — all three compounds can be produced by either industrial sources or fires, whether manmade or natural. Analyses indicate that the plumes contained little if any propane (C3H8) or tetrachloroethene (C2Cl4), substances that would betray an industrial origin. So, Brock and his colleagues speculated, the plumes of arctic haze came from fires.
The researchers then used computer simulations to track the smoky plumes back to their sources. Of the 49 plumes they identified, 39 had originated in forest fires raging near Lake Baikal in southern Siberia. The other 10 plumes came from agricultural burning in Kazakhstan, where fires are often set in the spring to prepare the fields for planting, says Brock. Satellite images confirmed that fires were burning in those regions at the time.
Not one of the plumes detected during the April 2008 flights was tracked back to industrial sources. “That was certainly a surprise,” Brock says. “We were expecting more anthropogenic plumes.” However, he notes, some of the plumes did contain higher-than-expected concentrations of carbon monoxide — a sign that the air was tainted by industrial emissions before it reached the area where the fire added smoke.
The new analyses “are an interesting way of looking at different emissions” that end up in the Arctic, says Rick Shetter, an atmospheric scientist at the University of North Dakota’s National Suborbital Education and Research Center in Grand Forks. The smoky plumes are “altogether quite a big input [of emissions]” to the region, he adds.
The tiny particles of black carbon — essentially, soot — in the plumes absorb infrared radiation, thereby heating the air, says Brock. When these particles fall to Earth’s surface, they darken snow and accelerate its melting. The long-term warming trends that could result from such phenomena could be self-reinforcing, the researchers speculate: If plumes of smoke warm the climate, the forest fire season may become longer, which would produce more smoke, heating the air even more.
Also during April 2008, NASA aircraft were conducting similar air-sampling missions out of Fairbanks, some of which extended across northern Canada to Greenland. During those flights — at least 20 in all — scientists detected plumes of polluted air that could be tracked back to Europe, North America and Asia, says James H. Crawford, an atmospheric scientist at the NASA Langley Research Center in Hampton, Va. Some of the plumes that the NASA aircraft flew through originated in fires. But the largest sources of arctic pollutants — those that taint the vast volumes of air outside those plumes — are industrial emissions generated throughout the Northern Hemisphere. “Anthropogenic emissions are still the dominant fraction” of pollutants in the Arctic, Crawford notes.
