"Beijing sits in a soupy haze of pollution from nearby factories, coal-fired power plants, and traffic that increases dramatically by the day, making the city one of the most air polluted in the world. China spent billions of dollars trying to control emissions that could hinder athlete's performances on game day. From 20 July to 20 September 2008, the Chinese government temporarily closed factories and regulated the number of cars on the road in Beijing and in nearby areas, all with the hopes of curbing aerosols--fine particles suspended in the atmosphere. China tried a similar traffic strategy in 2006 during a 3-day political summit and achieved 40% to 60% reductions in aerosol concentrations, according to one study. But this study covered only a short period and concentrated on aerosols at ground level, not throughout the larger atmosphere. For the 2008 Olympics, Chinese officials called for reductions of 60% to 70% in automobile emissions and up to 30% in industrial emissions."
"...It turns out that the Chinese only achieved a modest reduction in aerosols. The researchers report in a paper in press in Geophysical Research Letters that pollution-control efforts reduced the overall amount of aerosols in the atmosphere by about 10% to 15%. That small change highlights the importance of factors such as wind direction in determining local pollution, says Cermak. In spite of the reduction in local emissions, winds from the south and southeast sullied Beijing's air by bringing in pollution from distant industrial areas, he says."