An airplane equipped with flares disperses silver iodide particles into clouds to force rain showers.

by Courtesy Bruce A. Boe; Courtesy Daniel Breed/National Center for atmospheric research

An airplane equipped with flares disperses silver iodide particles into clouds to force rain showers.

At this time next year, the world’s finest athletes will converge in Beijing for the 2008 Olympics. They will sprint, leap, and toss their way to gold-that is, if they can get a breath of clean air. China has some of the worst pollution in the world, and bad air kills 400,000 Chinese annually [see “China’s Green Evolution.” In anticipation of the Olympics, Beijing has launched a massive cleanup. Some 200 factories in the area will be relocated. More than 60,000 taxis and buses will be kicked off the road by the end of this year. And now Chinese authorities are looking to the heavens for help.

For athletes, the two biggest pollution threats are fine particulate matter-bits of carbon, sulfates and other microscopic industrial by-products-and ozone. Lungs limit the amount of air they absorb in response to high ozone levels. This can lead to coughing, wheezing and headaches. “We cannot have athletes exerting themselves under these conditions,” says atmospheric scientist David Streets of Argonne National Laboratory.

The summer months are among the rainiest in Beijing, and officials are hoping that rain clouds will sweep in at the right time and wash away the bad air. “Whether pollution is a big problem or not depends on the weather,” Streets says. “Pollutants will build up, but they can be dispersed either by rain or wind.”
Last year, Beijing’s Weather Modification Office announced plans to use a technique called cloud seeding, which attempts to induce precipitation by filling moisture-laden clouds with tiny particles of silver iodide. The particles, dispersed by plane or shot into the air from ground-based rockets, enable water vapor to condense and form raindrops.

Invented in 1946 and used all over the world, including the U.S., cloud seeding remains controversial, in part because it’s so difficult to prove whether it actually works. Dan Breed, a scientist with the National Center for Atmospheric Research, says that even if the Chinese seed a series of rain clouds prior to the Games and precipitation then washes away the pollution, it doesn’t constitute proof. The clouds might have rained anyway, without the particles.

Nor would a foolproof technique eliminate the element of chance, notes Bill Cotton, an atmospheric scientist at Colorado State University. Beijing will still need the right kind of clouds, and in the right place. “You can’t make it rain if the conditions are not set for rain,” echoes Streets.
Compounding the issue is a study Streets published last spring showing that 50 percent of the bad air in Beijing originates in the industrial provinces to the south, so even banning cars and inducing thunderstorms may not be enough to clean the air. “Even if Beijing shut down, you’d still get pollution,” he says. “This is going to be the challenge for the Olympic Games.”

Streets thinks officials will postpone events if the air is bad. But Kenneth A. Rahn, a visiting atmospheric chemist at Beijing’s Tsinghua University, who is conducting a massive study of pollution in China, suggests another strategy, based on the fact that periodic shifts in weather patterns can carry clean, fresh air down from Mongolia in the north, clearing Beijing’s skies. His advice: “Pray to the Mongolian weather gods.”