Credit: JIM MUMAW

For much of August, Beijing switched off many of the engines that churn out its infamous pollution — and atmospheric scientists are waiting downwind to find out what happens.

In an effort to make the air healthier for athletes competing in the Summer Olympics — and clean up China's image for the international press — Chinese officials ordered what they called a 'great shutdown'. For the duration of the games, which ran from August 3 to 24, half of Beijing's several million cars were barred from the road, and its factories and power plants were expected to cut activity by up to 30 per cent.

In effect, the Olympics organizers performed a tantalizing experiment, reducing regional emissions of soot and other aerosol pollutants on a scale that only climate modellers could dream of before now. On August 9, scientists from the Scripps Institution of Oceanography at the University of California, San Diego, began launching small unmanned planes (pictured) that will monitor the results of this experiment through the end of September.

In charge of the project is atmospheric scientist Veerabhadran Ramanathan, who last year reported with co-authors that aerosols in 'brown clouds' make a large contribution to regional warming, among other effects (Nature 448, 575–578; 2007). Taking off from the South Korean island of Cheju, about 1,165 kilometres southeast of the Chinese capital, instruments on the airborne labs will gauge how the Olympian edicts have altered the quantity and size of particles drifting from Beijing. Specific sensors on the Cheju fleet will measure how those particles in turn affect meteorological conditions such as temperature, humidity and the sunlight that filters through the haze, and will tease out the impacts of different pollutants.

Unlike long-lived greenhouse gases, soot sifts out of the atmosphere in only a few weeks, so the outcome of Beijing's temporary industrial shutdown could be striking.