A view of the Forbidden City is seen on a hazy day in Beijing, Thursday, July 31, 2008. China... 
(AP Photo/Ng Han Guan)
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.
