IONS IN THE ATMOSPHERE 
By G. R. WAIT 
Carnegie Institution of Washington 
and W. D. PARKINSON 
Johns Hopkins University 
In considering the subject of atmospheric ions, it has 
been possible, because of space limitations, to consider 
only those conditions of the atmosphere which are 
regarded as normal and to include in the discussions, 
which are of necessity brief, only those items which are 
most important. References to investigations have also 
been restricted to those regarded as most pertinent to 
the subject at hand. 
Tt has been known since late in the eighteenth cen- 
tury that an insulated charged body left m the air will 
slowly lose its charge by a process other than leakage 
across the supporting insulators. This process of gaseous 
conduction of electricity was interpreted by J. J. Thom- 
son [42] as due to the presence in the gas of charged 
particles which, by analogy with the conduction in 
electrolytes, were called “Sons.” 
The concentration of ions in the atmosphere is meas- 
ured by an instrument called an zon counter. In principle 
the instrument is simply a charged cylindrical condenser 
through which air is drawn at a known velocity. The 
rate at which the charge on the central electrode changes 
provides a measure of the number of ions in the air 
stream and consequently the ion concentration of the 
air. If the voltage between the electrodes is sufficiently 
high, all ions from the air stream will be collected. 
Normally this voltage difference is selected so as to 
collect all ions of a particular mobility group and as few 
as possible of the less mobile ions. If a measure of the 
concentration of a lower-mobility group is desired, 
interference from ions of higher mobility may be avoided 
by first passing the air through another cylindrical 
condenser operated with sufficient voltage to remove the 
more mobile ions but only a small proportion of those 
to be measured. The theory and operating details of 
ion counters are discussed in special articles and texts 
on the subject [6, pp. 252-258; 10, 28, 41]. 
Tonic Mobilities 
An electron is released in the formation of a pair of 
small ions, but remains free for only a short time. The 
fact that no high-mobility group of negative ions is 
found shows that very few free electrons are present in 
the lower atmosphere. Small ions of the atmosphere 
have an average life of the order of a minute, so they 
are thoroughly aged during most of their lives, and are 
subject to the influences of many atmospheric constitu- 
ents existing in minute quantities. According to the 
picture recently given by Overhauser [34], an atmos- 
pheric small ion can be imagined as a charged molecule 
120 
which is continually associating with, and dissociating 
from, one or more other molecules. The time a certain 
type of molecule remains associated with an ion depends 
on its electrical properties. 
The average velocity with which an ion drifts through 
a gas under the influence of an electric field is propor- 
tional to the strength of the field. The ratio of the 
velocity to the field strength is called the mobility of the 
ion. The unit of measurement (referred to hereafter as 
centimeters) 1s centimeters per second per volt per 
centimeter. 
Numerous mobility measurements on gaseous ions 
have been made in the laboratory [22]. The value of the 
mobility is found to depend upon both the ion and the 
gas through which it moves. It also is affected by the 
presence of very slight traces of water vapor and other 
impurities. The ion can apparently become attached to, 
or lose its charge to, an impurity molecule. Some molec- 
ular impurities are known to associate readily with a 
positive ion, some with a negative ion, and some with 
either. In- most laboratory measurements, an effort 
is made to remove all traces of water vapor or other 
impurities so the values may be representative of the 
gas. For freshly formed positive and negative ions in 
air, Hrikson [5] obtained a value for the mobility of 
1.87 cm. In a few hundredths of a second the positive- 
ion mobility had decreased to 1.36 em, due, he believed, 
to the ion becoming attached to a neutral air molecule. 
Bradbury [3] obtained what he considered a more 
representative value for each sign after taking extreme 
precautions to remove practically all traces of impur- 
ities. Values obtained by him were 2.21 and 1.60 cm 
as the negative and positive ion mobilities, respectively. 
This requirement for such a high degree of purity for 
the air in mobility determinations leads one to question 
the extent to which laboratory-determimed values can 
be accepted as the values of the small-ion mobilities in 
atmospheric electric work. 
Mobility determinations of the small ions im the 
atmosphere have been made by various methods. Prob- 
ably the method most frequently used is the ratio of air 
conductivity to small-ion content. This will not result 
in high precision unless proper precautions are talxen to 
eliminate various errors which can easily enter and 
unless a correction is made for the lower-mobility ions 
caught by the ion counter. 
The published values [14] of mobilities of small ions 
in the atmosphere, from measurements made many 
years ago, show a great deal of scatter. There are reasons 
