I had written previously about the environmental conditions during the Olympics in Beijing. Real Climate points to a study which compared NO2 levels in the atmosphere in China before and during the Olympics. China had announced to regulate pollution more strict during the Olympics than before or after.
Real Climate reports:
The team at GSFC have released preliminary images (here and here) from the NO2 analysis showing the before and during the pollution controls. In both images, Beijing shows up as a huge hotspot of pollution, but relatively, the levels during the Olympics were significantly smaller:
August 2008 levels were therefore about 50% less than a similar period the year before. Meanwhile values at other hotspots in China were steady or got even worse. So there was a significant effect, but the scale of the task was indeed Olympian.