Water requires a non gaseous surface to make a transition from a vapor to a liquid. Cloud Condensation Nuclei or CCN (also known as cloud seeds) are small particles about which cloud droplets coalesce.
When no CCN is present, water vapor can be supercooled below 00C before droplets spontaneously form. In the above freezing temperatures the air would have to be super saturated to around 400% before the droplets could form.
- The concept of CCN has led to the idea of cloud seeding, that tries to encourage rainfall by seeding the air with condensation nuclei.
- There are many different types of atmospheric particulates that can act as CCN.
- The particles mat by composed of dusty or clay, soot or black carbon from grassland or forest fires, sea salt from ocean waves spray, soot from factory, sulphate from volcanic activity, phytoplankton or the oxidation of SO2 and secondary organic matter formed by oxidation.
- The ability of these particles to form cloud particles varies according to their size and also their exact composition, as the hygroscopic properties of these different constituents are very different.
- Sulphate and sea salt for instance readily absorb water whereas soot, organic carbon and mineral particles do not.
- The number and type of CCN can affect the lifetime and radiative properties of clouds as well as the amount and hence have an influence on climate change.
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.
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