To the northwest of Beijing is sparsely populated Inner Mongolia.
Thus, wind from there cleans things out. Here’s what happens on average to PM 2.5 levels with wind from different directions in Beijing:
When wind comes from the south or east, PM 2.5 is above average. When wind comes from the north (on the far left on the graph), PM 2.5 is usually a lot lower.
Special thanks to Calvin Quek (Twitter @clearroads), who did the original analysis and made the graphs here.