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.