Things You Should Know About Ions
by C Shaw and G Tamura
Ion Measurement and Ion
Generation
Measurement of ion density or ion concentration - the number
of ions per unit volume of air - is made with an ion counter
whose basic component is usually a cylindrical condenser arranged
so that the air to be counted is passed between two concentric
cylinders at a certain velocity. The ion counter does not count
the number of ions directly, but collects them and measures
the ion current to the condenser. From this the number of ions
is calculated by assuming that each carries a unit charge.
An ion counter can be designed not only to measure small or
large ions but also to measure positive or negative ions simply
by changing the polarity of the potential difference.
Machines for the generation of negative ions have been designed
on several principles, including photoelectric effect whereby
a metal, usually arranged as a screen surrounding an ultraviolet
lamp, ejects electrons when acted upon by the lamp's radiation.
The electrons attach themselves to particles in the air, which
is blown over a screen by a fan. Some designers of negative
ion generators have used the high-voltage corona discharge
effect, involving the ionization of air surrounding an electrical
conductor.
Because certain processes in the generation of negative ions
may also produce products such as ozone, a gas toxic above
a certain concentration, ion generators must be carefully designed
and constructed for safe operation.
Ion Concentration in the Atmosphere
Ion concentration in the atmosphere differs widely from place
to place; the number of small ions, for example, varies over
a range of about 200 to 2200 per cm³ depending on geographical
location. There is normally a slight excess of positive over
negative ions. In the relatively unpolluted air of the country
and in urban areas where there is much vegetation, small ions
dominate, whereas in heavily populated and industrialized areas
large ions greatly exceed small ones. At high altitudes, because
of strong ultraviolet and cosmic radiation, the concentration
of small ions in the air is much greater than at sea level.
Ion concentration exhibits regular daily and annual variations;
for example, in the daily cycle increased concentration of
small ions usually occurs shortly after mid-day, and in the
yearly variation, in the summer months. Changes in meteorological
conditions have significant effects on the ion concentration
of the air. In general, a drop in temperature and humidity
is preceded or accompanied by a sharp rise in ion content.
On cloudy days, ion concentration is usually less than that
on clear days, while foggy weather further reduces it. The
most striking weather effect is heavy rain, especially if it
is accompanied by thunder; in such conditions the number of
small ions, both negative and positive, increases greatly,
as shown in Figure 1, which indicates the effect of various
meteorological conditions. Figure 1 also demonstrates the seasonal
effect on the concentration of small ions.
Ion Concentration in Buildings
Ion concentration in unoccupied rooms with natural ventilation
is not very different from that outdoors. When a room is occupied,
however, the small ion concentration decreases sharply, the
extent depending on the number of occupants and the density
of pollutants such as tobacco smoke. Any decrease in small
ion concentration is always accompanied by an increase in large
ion concentration. This is brought about, in part, because
small ions combine with the condensation nuclei given out with
each breath, and with smoke particles, to form large ions.
The decrease in the small ion content of an occupied room and
the corresponding increase in the concentration of large ions
takes place very rapidly, frequently within an hour, after
which fairly constant levels of ion concentration are established.
When the room is vacated, a slow recovery occurs over several
hours, bringing the air back to its original ion concentration.
Certain components of air-conditioning systems also cause variation
in the ion concentration in buildings, as shown by tests of
an air-conditioned room whose ion content was not affected
by the operation of an air-conditioning system without ductwork.
When the conditioned air was forced through a long metal duct,
however, there was a measurable small increase in the number
of positive small ions. When an electric heater was turned
on, the concentration of positive small ions in the test room
quadrupled, until there were about three times as many positive
small ions as negative small ions. If an air washer was turned
on instead of the electric heater, the number of negative ions
rapidly increased to a level five times as great as the previous
concentration.
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