124 
methods; then the equation of positive large-ion balance 
becomes 
dN,/dt = Q + momNo — nonoNy 
= yNiNs re EN. (7) 
There will be a similar equation for the negative large 
ions. In this case the value of Q will not necessarily be 
equal for the positive and negative large ions, for the 
situation is not as simple as for small ions where the 
formation is principally due to a process wherein an ion 
pair is produced. 
Kennedy [19] and Nolan [30] found values of y of 
the order of 10~°. Hogg [12] reported a value about ten 
times this, but failed to take into account coalition of 
neutral nuclei and diffusion or falling out of larger 
nuclei as a means of dimunition of nuclei. Wait and 
Torreson [51] found that the value of y varies with the 
age of the ion. In a later study, Wait [48] found that for 
singly charged ions the value of y could be expressed as 
a function of the mobility & of the ion. A provisional 
value of this relationship is given by the equation 
y = 1.25 X 10-8 k%, 
which appears to hold for all mobilities between that of 
the large and that of the small ion. The measured value 
of y for k = 3.2 X 10+ was 3.1 X 10-9. In the free 
atmosphere away from sources of large ions the terms 
Q, yNiNe, and ¢N; are usually so small they can be 
neglected, and (7) reduces to 
dN,/dt = momNo — mn, 
and for large-ion equilibrium conditions, 
momNo = nvnNi, 
which is equation (3). Equation (4) represents the 
analogous equation in the case of the negative large 
ions. 
Ionic Concentrations in the Lower Atmosphere and 
Their Variations 
Both large- and small-ion concentrations vary con- 
siderably from place to place and from time to time at a 
given place. Over land, the two usually vary systemati- 
cally but in opposite directions, both during the day 
and throughout the year. 
The large-ion content (in ions per ec) of the air over 
the oceans averages only a few hundred of each sign. 
More extensive measurements have been made of the 
condensation-nuclei content of the air over the oceans 
from which estimates may be made of the large-ion 
content. Landsberg [20], making use of all available 
measurements at the time, estimated that the average 
nuclei content of the air over the oceans was 940. Hess 
[11] more recently found an average over the Atlantic, 
between America and Europe, of 527 on one leg of a 
cruise and 659 on another leg. On Cruise VII of the 
Carnegie [44], the average nuclei concentration was 
1370 over the Atlantic and 2350 over the Pacific. From 
all of these results one would estimate between 100 and 
200 large ions of each sign for the air over the Atlantic 
and two to three times this number over the Pacific. 
ATMOSPHERIC ELECTRICITY 
The large-ion content over the oceans, as estimated on 
the basis of the number of small ions found during 
Cruise VII, is around 180 of each sign [44], which is in 
reasonably good agreement with the estimates above. 
No diurnal variation was found in the condensation- 
nuclei content of the air over the oceans during Cruise 
VII, from which one might surmise that the large-ion 
content would likewise be constant through the day. 
Over land, Landsberg [20] estimated that the average 
condensation-nuclei content of the air in country dis- 
tricts was around 10,000 per ec, in towns around 30,000, 
and in cities around 150,000. The corresponding large- 
ion content might accordingly be estimated at between 
1000 and 2000 for a country area, between 5000 and 
6000 for a town, and between 20,000 and 30,000 for a 
city. These must be regarded as rough estimates only. 
Most observations on the condensation and large-ion 
content of the air over land have been taken in the 
vicinity of human habitations. Both the annual and 
diurnal variations as well as the absolute values are 
greatly affected by human activities. When observed 
sufficiently far from industrial activities of man, the 
large-ion content, like the condensation-nuclei content 
of the air, shows a maximum during the winter when 
heating of homes is greatest, and a minimum in the 
summer. For a similar reason, the condensation-nuclei 
content of the air generally shows a maximum during’ 
the middle of the day [15, 20, 26, 46, 56]. The diurnal 
variation in large-ion concentration obtained by Yunker 
[56], on the other hand, was more or less opposite to 
this. The curve for large ions found at Washington [49} 
is likewise quite opposite and similar to Yunker’s curve, 
both being plotted on local time. Torreson [43] first 
pointed out the discrepancy between the condensation- 
nuclei curves and the large-ion curves at Washington, 
and Wright [54] suggested an explanation based upon 
a variation in size of the condensation nuclei with 
relative humidity. Sherman [40] could find no evidence | 
of a diurnal variation in the ratio of uncharged to 
charged nuclei in the air, as required by Wright’s 
hypothesis. This apparent paradox has not yet been 
explained, but it raises the question whether condensa- 
tion-nuclei counts tend to include relatively large parti- 
cles while the large-ion counts include smaller particles. 
Additional experiments will be required to find an 
answer to the question as to why the two curves are of 
such different character and to ascertain if a similar 
difference is to be found at other places. 
Over the oceans, even though the rate of small-ion 
production is less than that over land (only 10 to 15 
per cent as great), the small-ion concentration in the 
two areas usually differs but little. This is accounted 
for by a smaller number of large ions over the oceans 
and consequently a slower destruction rate of the small 
ions. Over the oceans, the average small-ion content of 
the air was about 500 and 400, respectively, for the 
positive and negative ions during Cruise VII of the 
Carnegie [44]. Over land the concentration of each sign 
varies from around 100 over polluted areas to about 
1000 for areas unpolluted with industrial smokes and 
gases. 
