Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles 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 presents itself as tiny solid or liquid particles called CCNs. When no CCNs are present,
water vapour can be supercooled below 0
C before droplets spontaneously form . In above freezing temperatures the air would have to be supersaturated to around 400% before the droplets could form.
Encyclopedia
Cloud condensation nuclei or
CCNs are small particles 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 presents itself as tiny solid or liquid particles called CCNs. When no CCNs are present,
water vapour can be supercooled below 0 °
C before droplets spontaneously form . In above freezing temperatures the air would have to be supersaturated to around 400% before the droplets could form.
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 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 centimeter. The total mass of CCNs injected into the atmosphere has been estimated at 2x10
12 kg over a year's time. Large concentrations of particulates are also responsible for
haze in areas with lower
humidity. This dry haze also has an effect on climate by either absorbing or reflecting radiation .
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 . Additionally, while some particles 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 , but the details of this are still not well understood but are the subject of much research by many groups worldwide.
Phytoplankton role
Sulfate aerosol act as CCNs. These sulfate aerosols form partly from the
dimethyl sulfide 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 but no conclusive evidence to support this has yet been reported.
See also
References and external links
- R. Charlson, James Lovelock, M. Andreae and S. Warren . Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature, 326, 655-661.
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