Calvin Quek from Greenpeace analyzed how PM2.5 changes with depending on which way the wind is blowing in Shanghai:
The data is simple. When the wind is coming from the industrialized areas to the west of Shanghai, toxic air pollution is more likely.
When the wind blows in from the ocean, bad air days are much less likely.
In Beijing, the situation is different.
Special thanks to Calvin Quek (Twitter @clearroads), who did the original analysis and made the graphs here.