Cloud condensation nuclei

Water vapor will not condense very readily unless there are small particles onto which it can form droplets. These particles were first identified by John Aitken (see the sidebar) and they are known as Aitken nuclei or cloud condensation nuclei. The air contains large quantities of them, especially over land.

Cloud condensation nuclei are of a particular size and the air contains particles of many other sizes. Some are solid and some liquid. The bigger ones are visible to the naked eye. You can see them floating in sunbeams. Most are too small to see, however.

Atmospheric aerosols

John Aitken and cloud condensation nuclei

John Aitken (1839-1919), a Scottish physicist and engineer, was born and died at Falkirk, Scotland. His health was poor, and he was never fit enough to hold an official position. Instead, he worked from a laboratory at his home, where he built his own apparatus. He was a member of the Royal Society of Edinburgh, and many of his discoveries were first described in articles in its journals.

Aitken became interested in dust and discovered that water vapor will not condense to form cloud droplets unless particles of a suitable size, between 0.005,um and 0.1 ,um, are present. These particles are now called cloud condensation nuclei. The smallest of them, less than 0.4 ,um across, are called Aitken nuclei. He determined to measure the amount of dust present in the air, and to this end he designed and built a dust counter, now known as the Aitken nuclei counter.

This comprises a chamber with a pump, containing a graduated disk and sodden filter paper. The filter paper ensures that the air inside the chamber is always close to saturation. A sample of air is drawn into the chamber, and then the pump withdraws air, causing the air inside to expand rapidly. The air cools as it expands, and as its temperature falls below the dew point temperature, water vapor condenses. Some of the resulting droplets fall onto the graduated disk, where they can be counted with the help of a small microscope. Each droplet is assumed to contain one cloud condensation nucleus, so Aitken was able to estimate the number of particles in a unit volume of air. He found that there are usually about 820-980 nuclei in each cubic inch of air (5-6 million per liter) over land and about 164 in-3 (1 million I-1) over the ocean.

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