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.
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