The report was published Friday in the journal Environmental Science and Technology, and funded by the National Science Foundation in the U.S. and the National Science Foundation in China.
The Chinese government's plans to control air pollution for the Olympics gave international researchers a unique opportunity to observe a large-scale experiment. Scientists from Oregon State University and Peking University looked at Beijing's worst air pollutant - tiny dust particles known as particulate matter - over an eight-week period before, during and after the games.
When Beijing won the bid for the Olympics in 2001, China poured some $20 billion into "greening" the city, including doubling the number of subway lines, retrofitting factories with cleaner technology and building urban parks.
Beijing officials also imposed drastic cleanup measures just before the games in mid-July, including pulling half the city's 3.3 million vehicles off the roads, halting most construction and closing dozens of factories.
The study found that particulate air pollution decreased by about one-third during the two-week Olympic period compared with other periods.
But further investigation suggested that the weather, such as rainfall and strong winds from the north and northwest, played a much larger factor in clearing the air.
Meteorological conditions accounted for 40 percent of the variation in concentrations of coarser particulate matter, or PM 10, while pollution control measures accounted for only 16 percent, the study said.
"It was a giant experiment and a noble effort. But in the end, the extra added measures didn't help reduce PM concentration as much as had been expected," said Staci Simonich, an associate professor of chemistry and toxicology at Oregon State University who worked on the study.
At the same time, the findings don't invalidate the government's efforts, said Zhu Tong, professor at Peking University's College of Environmental Sciences and Engineering, and a co-researcher on the project.
"We learned a lot about how air pollution forms in a mega-city like Beijing, and how much pollution comes from which sources," Zhu said.
The findings also showed that the weather ushered some air pollution in to Beijing from industrial regions south of the capital that were not subject to pollution curbs, including Hebei, Shandong, and Shanxi. Those results indicated the difficulties in trying to control pollution from a city level when air masses tend to move regionally.
The level of particulate pollution that athletes faced in Beijing was twice as bad as in Athens, three times worse than Atlanta and 3.5 times higher than that of Sydney.
Levels of PM 10, the coarser particulate matter, exceeded levels that the WHO considered safe about 81 percent of the time, while concentrations of the smaller particulate pollution PM 2.5, which can cause more serious health consequences, exceeded WHO guidelines 100 percent of the time.
However, there has been no evidence so far of any health problems linked to the short-term exposure of athletes or spectators during the Olympics, researchers noted.
Pollution expert Fang Ming, now retired from Hong Kong University of Science and Technology, said the findings don't break new ground in terms of understanding how air pollution works.
"Having said that, it is useful to know the effectiveness of the huge 'green Olympic' effort to clean up the air in Beijing," he said in an e-mailed response.
Overall, the Olympic pollution control efforts were worthwhile because "it demonstrated to the Chinese government that they need to pay more attention to the environment and it is good for the country. It also says that this is doable and the people have to be a part of the effort," he said.
