NUCLEI OF ATMOSPHERIC CONDENSATION* 
By CHRISTIAN JUNGE 
Meteorological Institute for Northwestern Germany 
The Condensation Effect of the Nuclei 
According to Wall [88], the condensation effect of a 
nucleus can be compared to that of a droplet of pure 
water on which, because of the surface curvature, con- 
densation will take place only when there is a definite 
amount of supersaturation (Thomson’s equation). The 
following distinctions between different types of nuclei 
can then be made: 
1. Particles msoluble in water and wnwettable. These 
may serve, as tests have shown [20], as nuclei of con- 
densation. They require (when spherical) a larger 
amount of supersaturation than do droplets of pure 
water. Their importance for the natural aerosol is slight. 
2. Particles insoluble in water, but wettable. Accord- 
ing to the existing humidity and to their degree of 
wettability, these particles are surrounded by one or 
more molecular layers [40] of adsorbed water. They 
begin to condense at, or somewhat below, the value 
given by Thomson’s equation. If these particles are of 
irregular or flaky structure, condensation begins before 
saturation, either in cavities (as capillary condensation) 
or in porous nuclear substances (as absorption of water). 
3. Droplets of solutions. These form an important 
group of nuclei. Owing to the dissolved substance, the 
degree of supersaturation necessary for condensation 
falls to as low a value as 14 of that indicated by Thom- 
son’s formula [25]. 
Between particles of types 2 and 3 there are many 
intermediate types, because, as a result of coagulation, 
many nuclei will contain both soluble and insoluble 
matter (mixed nuclei). Figure 1 shows the relationship 
between the radius of the nuclei and the amount of 
supersaturation necessary for condensation to occur. 
It is evident that, in the presence of condensation 
nuclei, the range of supersaturation necessary for the 
formation of clouds extends from 0 to about 20 per 
cent. 
The processes of condensation on nuclei at tempera- 
tures below OC, which have recently been the object of 
thorough research, will be mentioned only briefly here, 
because they represent a transition into the field of 
cloud physics [27]. Weickmann [41] found that the 
nuclei act fundamentally as condensation nuclei for the 
liquid phase even at temperatures below freezing, that 
is, only at saturation with respect to water does con- 
densation take place in the form of droplets, some of 
which may subsequently freeze. Sublimation nuclei, in 
the sense of Bergeron-Findeisen, which may form ice 
particles already at supersaturation with respect to ice, 
seem to exist only in negligible quantities, if at all. 
According to Lafargue [22], the droplets initially 
formed freeze at about —41C in the size range between 
* Translated from the original German. 
1 uw and 20 u, rather independently of the presence of 
dissolved substances. Heverly [13] found that this spon- 
taneous freezing point rises to —16C for drops of 0.4 
mm diameter and then remains constant for drops up 
to about one mm diameter, likewise largely independ- 
ently of the source of the water. However, according to 
Weickmann, the presence of certain solid particles, so- 
called freezing nuclei, appears to modify these processes 
by raising the freezing point. This modification depends 
on the size and nature of the freezing nuclei. 
Size, Number of Nuclei, and Methods of Measurement 
If we disregard the small ions (radius r Y 10~ em), 
which are not of interest here, the size range of con- 
densation nuclei extends approximately from r = 4 X 
10~ to 10 em (Fig. 1). Although all types of particles 
may be active as condensation nuclei in a nuclei counter 
(see next paragraph and [20]), only those nuclei which 
require the lowest degree of supersaturation (7.e., haze 
droplets and large nuclei droplets) are active in the’ 
actual atmosphere. It is within this group of particles 
—recently re-examined by Dessens [8, 9] and Woodcock 
and Gifford [43]—that we find the real meteorological 
condensation nuclei. By what method, now, can we 
measure these particles which range in size over three 
orders of magnitude? 
All nuclei, whether hygroscopic or even unwettable, 
with radii ranging from about 4 X 10-7 to 2 X 10% 
cm, are counted in nuclei counters, so called after Aitken 
(3, 23]. The lower limit of this range is determined by 
the ratio of expansion; however, it appears that the 
values of supersaturation computed from this ratio are 
substantially too high [17]. The upper limit—which is 
highly uncertain—can be established with a certain 
degree of probability by assuming that larger nuclei 
droplets grow so rapidly in the saturated air of the 
counting chamber that they are probably precipitated 
before the actual measurement (Fig. 3). This seems to 
be corroborated by the photographs of condensation 
nuclei taken with an electron microscope, as described 
by Linke [26]. For these photographs, the nuclei were 
precipitated on an object screen by condensation in the 
nuclei counter and show an upper limit at 7 = 10-* cm, 
which corresponds to droplets of solution of r } 2 X 
10 em (Fig. 1). However, the aerosol, which was taken 
from a room, undoubtedly contaimed larger particles. 
It is possible that these limits are subject to variations 
when different types of counters, or even different 
instruments of the same type are used; this would 
explain the discrepancies recently found by Israél and 
Krestan [17] when they made comparative measure- 
ments with different nuclei counters. 
While the nuclei counter gives only the number of 
nuclei, the method developed mainly by Israél [18] 
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