Cloud condensation nuclei



  Cloud condensation nuclei or CCNs (also known as cloud seeds) are small particles (typically 0.0002 mm, or 1/100 th the size of a cloud droplet [1]) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a cloud chamber for detecting subatomic particles). In above freezing temperatures the air would have to be supersaturated to around 400% before the droplets could form. The concept of cloud condensation nuclei has led to the idea of cloud seeding, that tries to encourage rainfall by seeding the air with condensation nuclei appropriately

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 nucleus (albedo).

There are many different types of atmospheric particulates that can act as CCN. The particles may be composed of dust or clay, hygroscopic properties of these different constituents are very different. Sulfate and sea salt, for instance, readily absorb water whereas soot, organic carbon and mineral particles do not. This is made even more complicated by the fact that many of the chemical species may be mixed within the particles (in particular the sulfate and organic carbon). Additionally, while some particles (such as soot and minerals) do not make very good CCN, they do act as very good ice nuclei in colder parts of the atmosphere.

The number and type of CCNs can affect the lifetimes and radiative properties of clouds as well as the amount and hence have an influence on climate change [2] [3], but the details of this are still not well understood but are the subject of much research by many groups worldwide.

 

Phytoplankton role

Main article: CLAW hypothesis

Sulfate aerosol (SO42- and methanesulfonic acid droplets) act as CCNs. 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. The idea that an increase in global temperature would also increase phytoplankton activity and therefore CCN numbers was seen as a possible natural phenomenon that would counteract climate change. This is known as the CLAW hypothesis [4] (named after the authors' initials of a 1987 Nature paper) but no conclusive evidence to support this has yet been reported.

A counter-hypothesis is advanced in The Revenge of Gaia, the book by James Lovelock. Warming oceans are likely to become stratified, with most ocean nutrients trapped in the cold bottom layers while most of the light needed for albedo associated with low clouds. As of 2007 this hypothesis remains speculative.

See also

  • Bergeron process

References and external links

  • R. Charlson, James Lovelock, M. Andreae and S. Warren (1987). Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature, 326, 655-661.
  • Apollo.lsc.vsc.edu
  • www.agu.org
  • www.grida.no
  • Condensation Nucleus National Science Digital Library - Cloud Condensation Nucleus
  • DMS and Climate
  • Dimethyl Sulfide - Summary
  • AGU Association between CCN and Phytoplankton
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cloud_condensation_nuclei". A list of authors is available in Wikipedia.