Scott Paul at the Washington Note:
On the very worst day, I couldn't see clearly from one side of Tienanmen Square to the other. That was a Friday, which is important because, according to locals, that is the day of the week on which the clouds are seeded to clear out the pollution. I was told on a clear Friday afternoon that there would be heavy rain at precisely 7:30 and sure enough, the rains came.
According to various reports, China aims to ‘schedule’ rain in order to (1) ensure good weather for the Olympics’ opening ceremonies and (2) wash away as much atmospheric pollution as possible. James Fallows has photos of Beijing’s smog here.)
Boingboing noted this about two years ago, and WiReD’s Science Blog about a year later.
For more info, see sources below; a short summary follows.