{22 
length, by breaking drops, and by drifting snow and 
dust. Under particular circumstances one or more of the 
above-mentioned processes may add appreciably to the 
ion population of the air. None of these, however, are 
normally important as ionizers of the atmosphere. The 
chief ionizer in the lower stratosphere and the tropo- 
sphere, except in the lower atmosphere over land, is the 
cosmic rays. In the lowest kilometer or so over land, 
ionization of the air is due chiefly to radiations from 
radioactive matter in the earth and in the air. 
The average rate of ionization of the atmosphere q, 
over land and near the earth’s surface, has been esti- 
mated by Hess [10, pp. 167-171] at about 10 pairs per 
ec per sec. This value is based on the average amount of 
radioactive matter in the earth and in the air and is 
summarized in Table II. According to this estimate, 
approximately half of the total ionization is due to 
radioactive matter in the air, one third is due to radio- 
active matter in the soil, and one sixth due to cosmic 
rays. The ionization due to cosmic rays and radioactive 
matter in the soil is probably more or less constant with 
time at a given station. That due to radioactive matter 
in the air, however, is subject to variations, since the 
TasxiE II. Ionization or THE AIR NEAR THE HaRTH’S 
Surrace Over Lanp In Pur Cunt or Toran 
Tonizing ray 
Tonizer 5 Total 
> B *y Caspic 
Thorium) ai fee Pte 10}! 
Radium \-. : 
an naetrna ne soil — 1 32 30 
Cosmie rays 16 16 
PRO Gale aa ees AAI 48 3 33 16 | 100 
quantity of radioactive matter in the air varies with 
time. The amount of radioactive matter in the air 
depends upon two factors: (1) the rate at which it is 
dissipated in the atmosphere, and (2) the rate of exhala- 
tion from the soil. 
The rate of exhalation of radioactive gas from the soil 
is subject to considerable variation, being affected by 
such factors as temperature of the soil, wind force, 
dryness of soil, and covering on the ground [4, 57, 58). 
The rate of dissipation in the atmosphere depends upon 
several factors, but particularly upon air turbulence. 
Zeilinger found a diurnal variation in the rate of exhala- 
tion from the soil, a maximum occurring in the morning 
hours and a minimum in the evening. A diurnal varia- 
tion curve with similar characteristics has also been 
found for the radon content of the atmosphere [57]. 
A systematic diurnal variation in the rate of ioniza- 
tion of air near the earth over land would be expected, 
with a maximum occurring in the morning and a mini- 
mum in the evening. Continuous observations on the 
rate of ionization have been reported at three stations, 
Canberra, Australia [13], Washington, D. C. [48], and 
Huancayo, Peru [35], in which diurnal variation curves 
were obtained. A curve for each of the three stations is 
ATMOSPHERIC ELECTRICITY 
shown in Fig. 1. The curves are of the type expected 
when each is plotted on local time. Slightly different 
methods were employed in measuring the ionization. 
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1933 a= | ite oR 
HUANCAYO, PERU 
JULY 1947 +— 
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1) 
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WASHINGTON, D.C. 
1935 
{o) 
2 4 6 8 10 12 14 16 18 20 22 24 
HOURS (LOCAL MEAN TIME) « 
IONIZATION (ION PAIRS/CC/SEC) 
[o) 
Fic. 1.—Daily variation in ionization of the lower atmosphere. 
Hogg [13] measured the ionization of the air after it was 
drawn into a thick-walled sealed chamber. Wait [48] 
and Parkinson [35] measured the ionization in a thin- 
walled ionization chamber (the stopping power of the 
wall for alpha particles was equivalent to 1.5 cm of 
air). The values plotted for Huancayo represent the 
rate of ionization inside the chamber and must be | 
multiplied by about 1.6 to correspond to the ionization 
of the atmosphere. 
Small-Ion Balance 
The processes of ion formation just described are 
balanced by the processes of small-ion destruction. One 
such process is the recombination between small ions of 
opposite signs. Ions of other mobility groups will also 
be active in neutralizing small ions. This is particularly 
true of the large ions. A third process is one not of 
neutralization but of the conversion of a small ion into 
a large ion by coalition of a small ion and a neutral 
condensation nucleus. The equation of ion balance for 
positive small ions, taking into account the processes 
mentioned so far, is 
(1) 
There is an analogous equation for the negative small 
ions. The symbols have the following meanings: q is the 
rate of small-ion formation (expressed in lon-pairs per 
ec per sec); a is the recombination coeflicient for small 
ions; m; and n, are the concentrations of positive and 
negative small ions, respectively (in ions per cc); Ne 
is the concentration of negative large ions; and No is 
the concentration of neutral condensation nuclei. The 
constants 72 and mo are known as combination coef- 
ficients. 
In general the small-ion concentration is accounted 
for by such an equation. In some places it is necessary 
to take into account the effect of intermediate ions, 
but their effect is always small compared to that of the 
large ions and uncharged condensation nuclei. Over 
land, the term due to recombination is generally small 
compared to the other terms, and equation (1) can be 
written: 
g = anne + nwNoni + moN om. 
g = m(m2N2 + moNo). (2) 
