Guiding a small squadron of robotic aircraft and fielding a network of ground stations, scientists from the United States, China, and South Korea this summer are putting vast plumes of Asian air pollution under some of the most exhaustive scrutiny ever.
The three-month campaign, dubbed CAPMEX, represents the latest in a series of ambitious international field experiments during the past decade aimed at closing a gap in atmospheric science: the need to understand how human-produced soot and tiny particles dubbed aerosols affect climate.The air-pollution issue took on added visibility with the 2008 Summer Olympics in Beijing, which ended Sunday. Air quality was a key concern for many athletes, particularly those taking part in strenuous outdoor events.
But beyond public-health concerns, pollution in the form of aerosols and soot – byproducts from burning coal and oil as well as from burning vegetation to clear farmland or as fuel for cooking – are increasingly seen as important yet complex players in Earth’s climate.
One sign of that complexity appeared earlier this month, when scientists from the US and Israel showed how smoke from fires set to clear farmland in the Amazon in the dry season could either prevent clouds from forming, or accelerate cloud formation. The outcome depended on the amount of aerosols, how cloudy the skies were to begin with, and cloud height. Ironically, either outcome can reduce rainfall: The aerosols can warm and dry out the air, or they can spawn so many cloud droplets that they exhaust available moisture before droplets can grow heavy enough to fall as rain. The work, led by the Weizmann Institute’s Ilan Koren, appeared in the journal Science.
That raises an urgent question as to what effect reducing human-made aerosols and soot in the atmosphere will have on climate.
“I’m concerned about what’s going to happen over the next 10 to 20 years as we clean up the atmosphere,” says Veerabhadran Ramanathan, an atmospheric scientist at the Scripps Institution of Oceanography in La Jolla, Calif., and one of CAPMEX’s two lead scientists. “As the air clears, are we going to see a major acceleration in warming?” The answer would have a direct bearing on how deeply and quickly countries must cut their greenhouse-gas emissions in order to stave off what the UN Framework Convention on Climate Change has termed “dangerous human interference” on climate.
In addition to addressing this concern, CAPMEX is providing an opportunity to track the results of China’s Olympic-related pollution-control efforts. Dr. Ramanathan notes that those strenuous efforts will last for another two weeks, through the International Paralympic Games, giving his team time to catch a during-and-after view before research flights end Sept. 30.
Over the past year, at least, Beijing’s efforts bore fruit, notes Staci Simonich, an Oregon State University atmospheric chemist. In collaboration with scientists at Beijing University, she set up a monitoring station there. She notes that particulate levels at this site fell between 20 and 40 percent during that period.
CAPMEX, whose operations are based at an airfield on South Korea’s Cheju Island, also is taking such studies in a new direction. It’s gathering information to see whether pollution plumes – which can dim sunlight reaching the surface directly or indirectly through stimulating the formation of dense, reflective clouds – have an effect on tiny ocean phytoplankton. These bottom-of-the-food chain organisms rely on sunlight for photosynthesis.
Why the focus on Asia? “It’s simple,” says Dan Murphy, a researcher with the National Oceanic and Atmospheric Administration’s Earth Systems Research Laboratory in Boulder, Colo. He was a lead researcher in this past spring’s ARCPAC project to gather data on the effect of aerosols on solar radiation and clouds in the Arctic. “Asia has a large population, its economies are developing so rapidly, and its emissions are large and changing rapidly.” Moreover, the historical record for air pollution there is not as extensive or consistent as similar measurements in the West.
Indeed, one of Ramanathan’s goals is to help set up a baseline against which efforts to improve China’s air quality can be measured.
In May, the US Department of Energy in a separate effort set up a portable atmospheric-radiation monitoring station in Shouxian, 300 miles west of Shanghai. The experiment, in collaboration with Chinese researchers, aims to better understand the effect that soot and aerosols have in the region through their effect on clouds.
Satellite data show that clouds in southern China hold a lot of liquid water, but the clouds aren’t producing the amount of rain such moisture-laden clouds might be expected to drop, says Mark Miller, an atmospheric scientist at Rutgers University in New Brunswick, N.J., and one of the US scientists involved in the effort. “We’re trying to find out why clouds in southern China are operating under a different set of rules,” he says.
Vast plumes of aerosols, soot, and smoke, dubbed atmospheric brown clouds, can have far-reaching effects, geographically. While aerosols appear to have a net cooling effect, black soot particles warm things up. Work by scientists at Scripps suggests that soot’s warming effect may be contributing to an increasingly early snow melt and runoff in the Sierras and other western ranges.
In the Arctic, Asian pollution has been measured as far east as Greenland. And now, scientists are beginning to build a historical record of soot-fall there. Earlier this month, a team led by Joseph McConnell from the Desert Research Institute in Reno reported finding soot dating back to 1788 in ice cores from Greenland. Based on the trends they see, soot – largely from eastern North America – may have been responsible for early 20th century warming in the Arctic. And migrating soot and aerosols often carry chemical pollutants that accumulate where the particles land.
In the southern hemisphere, researchers in Australia have shown through modeling studies how Asian haze over the tropical Indian Ocean appears to redirect monsoon-related winds, driving more rain to northwestern Australia than might otherwise be the case.
[Editor's note: The original version of this article misstated Joseph McConnell's affiliation. He works for the Desert Research Institute in Reno.]