A. Discovery of air ions
(1) Coulomb's experiment
"An insulated charged body exposed to atmospheric air gradually loses its charge." (Coulomb 1785)
However, at the time, it was not clear why it loses the charge.
(2) Theory of air ion conductivity
"Positively or negatively charged tiny particles can be found not only in electrolytic solutions but also in the air.
Electricity flows through the air by these particles." (Discovery and Research of Becquerel and X rays)
This theory was applied to explain "Electronic conductivity in the air" by Ester, Geitel and Wilson, and resulted in better understanding of Coulomb's experiment.
These tiny particles are called "Air Ions".
Faraday named these particles "Ions".
B. What is an air ion?
(1) What is an air ion?
An air ion is a positively or negatively charged tiny particle in the air. (e.g. oxygen molecule)
A negative ion possesses the same amount of electric charge as an electron.
(2) How is an air ion created?
Ions charged positively are positive ions and those charged negatively are negative ions.
Generally, free-floating electrons are released into the air and they collide/stick with/to gaseous molecules (e.g. oxygen molecule) to form negative ions or negative ion molecules.
(1) About mobility
An air ion moves according to the electric field because of its electronic properties.
The velocity is proportional to the electronic field strength. The velocity per 1 (V/m) is defined as the mobility.
SI unit of velocity: (m/s)
In most cases, mobility is determined by the ratio between the amount of electronic charge the ion possesses and the mass of the ion.
SI unit of mobility: (m/s) / (V/m) = m2/Vs
Ions which have high/low mobility values are large/small ions respectively.
(2) Importance of mobility
Effects on the human body are closely connected with mobility of air ions.
Small ions are more efficiently absorbed by the human body than the same amount of large ions because they have larger mobility values and diffusion coefficients.
D. Classification of ions
Ions are generally categorized into the following groups based on mobility values and dimensions.
(1) Free-floating electron
An free-floating electron exists by itself and weights only about 1/1800 of the hydrogen atom.
Its mobility values are as large as beta rays generated by cathode rays or radiant substances.
It is generally found at high altitudes where the air is rarefied, or in highly purified nitrogen, helium and argon.
(2) Ionized atom
An atom, which has lost an electron, is a positively ionized atom.
An electronically neutral atom, which has obtained an electron, is a negatively ionized atom.
Both types of ions along with electrons exist only in the upper layers of the atmosphere.
(3) Small ion
Most ions found in the atmosphere belong to this group (also known as Lightweight or Normal ion).
As soon as an electron or ionized atom shows up in the atmosphere,
it attracts gaseous molecules and combines with them to form a small ion molecule while positioning itself in the center.
A small ion molecule consists of 2 to 30 molecules.
Generally, positive ions weigh more than negatively charged ions, and mobility values are larger than 0.4-0.8 (cm2/Vs).
(4) Large ion
A large ion (also known as Heavy ion) is a negative or positive small ion (molecule) absorbed by dust, mist or another tiny particle.
While having the same structure as small ions, it can weight 1,000 times more. Mobility values range from 0.0005 to 0.01 (cm2/Vs).
Many exist in polluted air.
(5) Middle ion
This group of ions was discovered by Pollock and exists only in low humidity conditions, and does not exist near the earth's surface.
Mobility values range from 0.01 to 0.1 (cm2/Vs).
Author: Masaharu Nemoto
(Universal Plan Co., Ltd.)