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Theory of Air Ion

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1. Basics of air ions (1)


1-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".


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1-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.


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1-C. Mobility

(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)
SI unit of mobility: (m/s) / (V/m) = m2/Vs
In most cases, mobility is determined by the ratio between the amount of electronic charge the ion possesses and the mass of the ion.

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.


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1-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).


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2. Basics of air ions (2)


2-A. How are air ions created?

Air ions in the atmosphere are generated in various ways. While new ions are generated some cease to exist in order to maintain a certain balance.

(1) UV rays

Gaseous molecules are intensively ionized when UV rays pass through the molecules.
(Research conducted by Leonard, Thompson, Brenry)

This intensive ionization phenomenon can be found in the upper layers of the atmosphere. It has been said that molecules are ionized about 10 times more intensively at an altitude of 10km than at the ground surface. These highly concentrated ions are diffused to the ground surface at low speeds. Negative and positive ions combine together and they are electrically cancelled out on their way back to the ground surface. This results in lower ion densities at or near the earth's surface.

(2) Lenard's effect (Waterfall effect)

When a drop of water changes its shape (e.g. splits into smaller droplets), the droplets and the surrounding air are charged positively and negatively respectively. This is the Lenard's effect.

Two electronic layers always exist on the surface of a drop of water. The inner/outer layer is charged negatively/positively respectively. As soon as a newly formed water droplet has contact with the air positive ions in the air are absorbed into the droplets outer layer. As a result the surrounding air is charged negatively and thus negative air ions are generated.

(3) Sunlight

Ions are generated by the photoelectric effect (when a light of a certain wavelength hits a metallic surface, the surface emits electrons). Electrons collide with molecules in the air to ionize the molecules.

It has been said that the photoelectric effect is less likely to cause ion generation because few substances, which can cause the photoelectric effect, exist at or near the surface of the earth.

(4) Radioactive substance

The majority of ions found at or near the surface of the earth are generated by alpha, beta or gamma rays.
(A) Alpha ray
This ray is positively charged. A helium atom becomes a positively charged tiny particle after losing two electrons. When the particle passes through the air it collides with gaseous molecules and ionizes them intensively.

(B) Beta ray
This is an electron and it is charged negatively and it passes through the air at very high speeds. It does not ionize other molecules so intensively as the alpha ray.

(C) Gamma ray
This is a very short wavelength electromagnetic wave. Its transmittance is higher than that of X rays while it ionizes molecules as intensively as the X ray.

(5) Radium emanation

The radioactive series (e.g. Uranium, Radium, Actinium, Thorium) exist extensively in the earth. "Emanation" is a gaseous form of decayed substances found at the surface of the ground. It decays more rapidly in the atmosphere while emitting alpha, beta and gamma rays, and it ionizes the air.

Negative ions are generated when atmospheric pressure falls because air is drawn out of the earth together with Emanation due to the pressure changes.

(6) Ion balance in the atmosphere

In most cases, positive and negative ions are generated in pairs. While few ways generate positive ions only, the Lenard's effect, beta ray emission and so on generate negative ions only. In theory, more negative ions should exist in the air. However, negative ions decrease by recombination as the air gets polluted.


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2-B. Actual air ions floating in the air

Here are some examples of air ions found at or near the surface of the earth.

(1) Negative air ions

Electrons are released from molecules in the air when radiant or cosmic rays in the atmosphere collide with the molecules. These electrons are absorbed by molecules (e.g. oxygen or carbon dioxide) in the air to form negative ions. Actually, they combine with water molecules to exist stably.

Here are some examples of small negative ions.
Oxide molecule ion: O2- + (H2O)n
Carbon dioxide ion: CO3- + (H2O)n
Nitric acid ion: NO3- + (H2O)n
"-" of O2-/CO3-/NO3- means a neutral molecule in the air has received an electron.

"n" of (H2O)n shows the number of water molecules and it changes depending on humidity.

A large negative ion is formed when a small ion is absorbed by a larger particle.

(2) Generation of positive ion

The Oxonium ion is the most well known positive air ion. It is formed when hydrogen ions combine with water molecules.
Oxonium ion: H3O+ + (H20)n


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2-C. How do air ions decrease?

Air ions decrease by:

(1) Electronic neutralization

Positive and negative air ions combine together and they are electrically cancelled out.

(2) Diffusion

Air ions tend to spread out to have uniform density. (i.e. higher to lower density) Generally, diffusion speeds of small negative ions are higher than positive ions. This means negative ions vanish more rapidly (i.e. shorter life span).

(3) Absorption

A tiny particle (e.g. dust, steam) lowers ion density and mobility when it absorbs an air ion. Absorption speeds are determined by diffusion speeds, therefore mobility of the ion. Generally, negative ions are more likely to be absorbed because of higher mobility values than positive ions.

(4) Electric fields

An air ion moves according to the electric field. However, it is said that it does not affect the movement much except under extraordinary events. (e.g. when thunder occurs.)


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2-D. Conditions where air ions are created

Air ions in the atmosphere are generated in various ways. While new ions are generated some cease to exist in order to maintain a certain balance.

(1) Humidity

As referred on "Examples of air ions", ionized molecules combine with water molecules (drops of water) in the air. When humidity is extremely high the number of air ions decreases as drops of water merge to form a fewer number of drops. When humidity is low air ions decrease because of a fewer number of drops of water as a result of evaporation.

Thus, high or low humidity is not an ideal condition for air ion generation. It is said that relative humidity of 40 to 60% is best.

(2) Temperature

Some research shows that positive ions increase as temperature rises while other research shows that negative ions are generated when the surface of the earth is heated and water evaporates. It seems ion generation has no particular tendencies regarding temperature variables.

(3) Pressure

Pressure changes have more effect on ion generation than absolute pressure values. When atmospheric pressure drops radioactive substances are drawn out of the ground to form negative ions.

(4) Wind

Negative ions are generated when a cluster of water molecules in the air is dispersed by winds. This is one of the reasons why fresh air makes you feel better when ventilating a room.


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2-E. Ion as the core of mist

(1) Visible lionization tendency

An ion becomes the core to form a small drop of water under saturated steam conditions.

You can see the small drop with the naked eye.
(Wilson's cloud chamber)

(2) Negative ions more likely to be the core

Negative ions are more likely to be the core of mist than positive ions. This is not due to the amount of electrons which an ion possesses but to the two electronic layers of water.


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2-F. Conductivity and electronic field

(1) Conductivity and Electronic field

An electrical current flows from an ion molecules in the air. There are 4 kinds of ions, large-positive, large-negative, small-positive and small-negative. Where the electric field strength is F, the total electrical current which flows from these 4 kinds of ions is it, and all ions have different mobility values (k/K/+/-) and ion counts (n/N/+/-).

[Mobility values]
k+: Positively charged small ion
k-: Negatively charged small ion
K+: Positively charged large ion
K-: Negatively charged large ion

[Ion counts]
n+: Positively charged small ion
n-: Negatively charged small ion
N+: Positively charged large ion
N-: Negatively charged large ion

Each current which flows from each ion:
[Current: Ion]
en+Fk+: small positive ion
en-Fk-: small negative ion
eN+FK+: large positive ion
eN-FK-: large negative ion

The total electrical current is the sum of each current and it is described by:

it = (en+Fk+) + (en-Fk-) + (eN+FK+) + (eN-FK-)

= F(en+k++en-k-+eN+K++eN-K-) ----- (A)
(e=4.77X1010C)

where

λ+ = (en+k++eN+K+) ----- (B)
λ- = (en-k-+eN-K-) ----- (C)
λ = λ+- ----- (D)

(A) can be descried by:

it = F(λ+-) = Fλ ----- (E)

Each term in λ+- is the ratio of the electronic field and the electrical current density, which shows the electrical conductivity.

λ+: Positive polar conductivity
λ-: Negative polar conductivity
λ: Whole conductivity

The SI unit of conductivity is: Sm-1

(2) Conductivity depending on environment

e.g. #1
where the air is well purified and n/N/k/K/+/- are:
n+ = 600
n- = 600
N+ = 2000
N- = 2000
k+ = 1.5(cm2/Vs)
k- = 1.5(cm2/Vs)
K+ = 0.0004(cm2/Vs)
K- = 0.0004(cm2/Vs)
ekn:eKN = 1125:1 (ref. to B/C)
This shows that the electronic conductivity in the purified air is largely determined by small ions.

e.g. #2 where the air is polluted and n/N/k/K/+/- are:
n+ = 100
n- = 100
N+ = 50000
N- = 50000
k+ = 1.5(cm2/Vs)
k- = 1.5(cm2/Vs)
K+ = 0.0004(cm2/Vs)
K- = 0.0004(cm2/Vs)
ekn:eKN = 7.5:1 (ref. to B/C)
This shows that large ions play a partial role to determine the electronic conductivity in the polluted air.


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Author: Masaharu Nemoto
(Universal Plan Co., Ltd.)
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