Observations over a continuous 3-month period in Beijing showed 26 new particle formation (NPF) events, generally coinciding with periods with relatively clean air when the wind direction was from the less industrialized north. Large particles in the atmosphere suppress the gaseous supersaturation that is required for NPF. No events were observed when the daily mean PM_2.5 concentration exceeded 43 µg m⁻³. These results provide useful insight into the formation of haze events in megacities.