The difference was apparent from space, observed by a NASA satellite, said Michael McElroy, a Harvard University professor of environmental studies and co-author of the report in Geophysical Research Letters.

Nitrogen oxides are key ingredients in ozone, which can exacerbate lung ailments such as asthma and emphysema.

Limiting the number of motor vehicles on the streets of the Chinese capital reduced nitrogen oxide levels dramatically during a Beijing summit in November, the scientists found.

Measured using satellite-based equipment, the change occurred almost as soon as the number of cars operating in Beijing dropped from the usual 2.8 million to about 2 million, he said, and nitrogen oxides rose back to normal levels shortly after the restrictions were lifted.

"I was frankly surprised that we were able to see it and that it was such a clean-cut experiment," McElroy said by telephone. "I thought it would be much fuzzier."

The experiment took place during a meeting between Chinese and African officials. Such vehicle bans are expected to be repeated this summer and then again next year in preparation for the Beijing Olympic Games, McElroy said.

"One of the big issues in the Olympics, a very serious problem, is the possibility that ozone levels during the Olympics will be very high," he said.

McElroy said Chinese officials would probably work hard to make sure Olympic visitors to Beijing will find a clean, modern city: "They don't want people arriving and coughing."

This might happen by banning all private cars from the city during the games, using buses and other means to ferry people about, he said. It is also possible that much of the Beijing-based industry, including chemical plants, will be shut down for the duration, according to McElroy.

There was some skepticism about whether the vehicle ban was responsible for the decline in ozone-forming oxides, since it coincided with a period of northwest winds that cleared the Beijing air, McElroy said.

Data collected by the NASA Aura spacecraft confirmed the correlation between the ban and the drop in nitrogen oxide.

Because ozone pollution is often regional, rather than local, McElroy said, emissions controls may be needed in the surrounding areas, as much as 620 miles (1,000 km) from Beijing.