8/20/2019 Cloud Condensation Nuclei

1/2

Cloud condensation nuclei

From Wikipedia, the free encyclopedia

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 vapour to a liquid. In the atmosphere, this surface presentsitself as tiny solid or liquid particles called CCNs. When no CCNs are present,

water vapour can be supercooled below 0 °C (32 °F) before droplets

spontaneously form (this is the basis of the 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 (aerosol) is on the order of 0.0001 mm or 0.1 micrometer or greater in diameter. The number of cloud

condensation nuclei in the air can be measured and ranges between around 100 to 1000 per cubic centimetre. The total mass

of CCNs injected into the atmosphere has been estimated at 2x1012 kg over a year's time. Large concentrations of particulatesare also responsible for haze in areas with lower humidity. This dry haze also has an effect on climate by either absorbing or

reflecting radiation (see albedo).

There are many different types of atmospheric particulates that can act as CCN. The particles may be composed of dust or

clay, soot or black carbon from grassland or forest fires, sea salt from ocean wave spray, soot from factory smokestacks or

internal combustion engines, sulfate from volcanic activity, phytoplankton or the oxidation of sulfur dioxide and secondary

organic matter formed by the oxidation of VOCs. The ability of these different types of particles to form cloud droplets varies

according to their size and also their exact composition, as the 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. One such experiment is CLOUD, a facility to explore the relationship between CCNs and Galatic

cosmic rays.

Phytoplankton role

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

Aerosol pollution over Northern India andBangladesh - NASA

Contents

n 1 Size, abundance, and compositionn 2 Phytoplankton rolen 3 See alson 4 Referencesn 5 External links

Phytoplankton bloom in the North Sea and

the Skagerrak - NASA

Make a donation  to Wikipedia and give the gift of knowledge!You can support Wi kipedia  by making a tax-deductible donation.

Page 1 of 2Cloud condensation nuclei - Wikipedia, the free encyclopedia

10/9/2008file://E:\vsm\Cloud_condensation_nuclei.htm

8/20/2019 Cloud Condensation Nuclei

2/2

 photosynthesis in the warm top layer. Under this scenario, deprived of nutrients, marine phytoplankton would decline, as

would sulfate cloud condensation nuclei, and the high albedo associated with low clouds. As of 2007 this hypothesis remains

speculative.

See also

n Bergeron processn Ice nucleus

References

n Charlson, Robert J.; Lovelock, James; Andreae, Meinrat O.; Warren, Stephen G. (1987). "Oceanic phytoplankton,atmospheric sulphur, cloud albedo and climate". Nature 326 (6114): 655–661. doi:10.1038/326655a0.

External links

n www.agu.org n www.grida.no n Condensation Nucleus National Science Digital Library - Cloud Condensation Nucleusn DMS and Climate

n AGU Association between CCN and Phytoplankton

Retrieved from "http://en.wikipedia.org/wiki/Cloud_condensation_nuclei"Categories: Clouds | Particulates

n This page was last modified on 14 August 2008, at 18:28.n All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.)

Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a U.S. registered 501(c)(3) tax-deductiblenonprofit charity.

Page 2 of 2Cloud condensation nuclei - Wikipedia, the free encyclopedia

10/9/2008file://E:\vsm\Cloud_condensation_nuclei.htm

http://www.agu.org/http://www.grida.no/http://en.wikipedia.org/wiki/Cloud_condensation_nucleihttp://en.wikipedia.org/wiki/Cloud_condensation_nucleihttp://www.grida.no/http://www.agu.org/