Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles (typically 0.00002 mm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Without CCNs, water vapour can be strongly supercooled before droplets spontaneously form (this is the basis of the cloud chamber for detecting subatomic particles).
The number and type of CCNs can affect the radiative properties of clouds and hence have an influence on climate change but this is still not well understood.
Note that describing CCNs as "particles" is perhaps slightly misleading. Sulphate aerosol (SO4 droplets) act as CCNs too.
