Cloud Condensation Nuclei (CCN) - Condensed water vapor that is so small that it can only be seen through a microscope. CCN are actually the center of the droplet. Many of these nuclei are tiny salt particles, sulfate or nitrate aerosol, or small particles present in smoke. See dimethyl sulfide. Most droplets measure from 1/2,500 to 1/250 inch (0.01 to 0.1 millimeter) in diameter.
[Journal of Applied Meteorology. v 32, 666, 1993.]
[Journal of Aerosol Science. v 23 supp. page S865, 1992.] 
Atmospheric Chemistry Glossary