China's quest to clear up the capital's polluted skies in time for the Games' opening ceremony on Friday has been marked by a huge effort in what scientists are calling the single largest attempt ever made to improve air quality.
Scores of heavily polluting factories have been shut down and some two million vehicles were pulled off roads across Beijing and a huge swath of northern China - an area roughly the size of Alaska. During the weekend the hazy skies finally gave way to swirling blue.
Beijing's massive experiment with controlling pollution is offering international researchers a unique opportunity to study the effects on an urban laboratory.
"It has never been done before," said Veerabhadran Ramanathan, an atmospheric scientist from the University of California. "I doubt it will be repeated. This is it. We've got a golden opportunity to fast-forward our research."
Ramanathan is one of dozens of scientists from around the world who are gathering in the region to conduct experiments investigating areas such as how pollution travels across continents and testing the impact of dirty air on cardiovascular functions.
Ramanathan said he first learned about China's plans to restrict cars and shut down factories last year from reading a newspaper article. "I immediately jumped off my seat. I thought, this is what I've been waiting for. I said, 'Thank God for the Olympics.' For me, this is 10 times better than winning the lottery."
With China's polluted air ending up over neighbouring countries and continents the data being collected may have larger ramifications beyond these Games. If China's efforts can be shown to have had a major impact other countries could consider taking similar action.
Since the factory closures and traffic restrictions began on July 20 Beijing's air pollution levels have fluctuated, although the general trend is downwards. What is increasingly clear is how much of a role weather conditions play in cutting down pollution. "If mother nature co-operates I expect there would be an impact. But it all depends on the wind directions," Veerabhadran said.
In the past two weeks four days failed to meet the national air quality standard, with pollution levels classified as unhealthy for sensitive groups. On those days Beijing suffered sweltering temperatures and a thick, grayish haze that reduced skyscrapers to ghostly outlines.
But strong winds and rainfall in the last week helped scatter much of the smog, giving Beijing residents a rare spate of sunshine and blue sky over the weekend. The air pollution index showed a decrease in pollutants, dropping to a level considered healthy by the World Health Organisation.
From a researcher's point of view China's attempts to ensure blue skies for the Olympics are of huge scientific interest, said Staci Simonich, an associate professor of chemistry and toxicology at Oregon State University. "It's a giant science experiment on air pollution. As far as I know it's the biggest case where a city that had air quality problems took strong measures to improve air quality. They've taken it very seriously. It's exciting from a science standpoint."
Although Los Angeles in 1984 and Atlanta in 1996 both took measures to improve their air quality when they hosted the Olympics neither city went as far as Beijing in its steps. "They didn't have to go to quite the extremes that Beijing has. Beijing has had to come a long way further than LA or Atlanta," she said.
"It's not just about China. It's about megacities across the world. What's learned here can perhaps be applied to other cities," Simonich said.
Other scientists have chosen to focus on the health impact of the dirty air, an area of huge concern to many athletes. Several countries have taken the step of providing athletes with air masks.
Qinghua Sun from Ohio State University is collecting data on the mechanics of how air pollution affects human health, especially cardiovascular diseases."Hopefully with our data China can see the need to take a dramatic policy strategy to continue the good policies they conducted during the Games," he said. AP