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).
Size, abundance, and composition
A typical raindrop is about 2 mm in diameter, a typical cloud droplet is on the order of 0.02 mm, and a typical cloud condensation nuclei (particle or droplet) is on the order of 0.0002 mm or 0.2 micrometer in diameter. The number of cloud condensation nuclei in the air has been estimated to average from 100 to 1000 per cubic centimeter. The total mass of CCNs injected into the atmosphere has been estimated at 2x1012 kg over a year's time. A large concentration of aerosol CCNs also is responsible for haze even in areas with low humidity. This dry haze also has an effect on climate by either absorbing or reflecting radiation.
The particles may be minute dust or clay particles, soot or black carbon particles from grassland or forest fires, sea salt from ocean wave spray, various pollutants from factory smokestacks or internal combustion engines, and sulfate aerosols from volcanic activity or phytoplankton.
The number and type of CCNs can affect the radiative properties of clouds as well as the amount and hence have an influence on climate change   , but the details of this are still not well understood.
Note that describing CCNs as "particles" is perhaps slightly misleading. Sulfate aerosol (SO42- and methanesulfonic acid droplets) act as CCNs also. These sulfate aerosols form partly from the dimethyl sulfide (DMS) produced by phytoplankton in the open ocean. Large algal blooms in ocean surface waters occur in a wide range of latitudes and no doubt contribute considerable DMS into the atmosphere to act as nuclei.
References and external links
- Condensation Nucleus National Science Digital Library - Cloud Condensation Nucleus
- DMS and Climate
- Dimethyl Sulfide - Summary
- AGU Association between CCN and Phytoplankton