The Nature of Air Ions

by C Shaw and G Tamura

Several types of electrically charged submicroscopic particles - air ions - are found in the atmosphere. They may be atoms, molecules, a group of molecules, or particles such as dust or liquid droplets that have become electrically charged. If an atom loses an electron it forms a positive ion; if it gains an electron, and thus manifests a negative charge, it becomes a negative ion. Ions are continuously produced in nature formed by the bombardment of air molecules by alpha and beta particles, by the absorption of radiation such as X-rays, gamma rays, cosmic rays and short-wave ultraviolet radiation, by the shearing of water droplets, and by drifting snow and dust. Such sources of energy dislodge electrons to form positive ions, and the free electrons are subsequently captured by other air molecules to form negative ions.

Although ions are continuously being formed, they are also neutralized by combination with ions of opposite polarity, so that their concentration in the atmosphere is fairly constant. Ionization due to cosmic rays and radioactive matter in soil is approximately constant with time at a particular location, but that due to radioactive material in the air varies because of air turbulence and because the rate of exhalation of radioactive gases from the soil is affected by such factors as temperature, wind, and ground covering.

Ions are classified not only by their electrical charge (polarity) but also by size as small, intermediate and large. Small ions, consisting of only a few molecules, are between 0.001 and 0.003 micron in diameter. Large ions are at least ten times the size of a small one and may be as large as 0.1 micron in diameter. A small ion is highly mobile compared with a large one, its velocity in an electrical field being about 5000 times greater than that of a large ion.

There is an additional physical difference between large and small ions. If a small one is neutralized, the molecules that have been held together by the charge move apart, leaving no trace of the original collection; if a large ion loses its charge, on the other hand, it continues to exist as an uncharged particle. The two classes also differ in length of life. The average time interval between formation and destruction of a small ion in clean air is about 4 to 5 min, in polluted air generally less than a minute. Large ions have a life of about 15 to 20 min in clean air and about an hour in polluted air.