WASHINGTON (Reuters):

When officials in Beijing kept about 800,000 cars off the road for three days last year, it cut the amount of nitrogen oxide air pollution almost instantly by about 40 per cent, scientists reported on Monday.

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.

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."