184 
ever, has seldom been found. Tables I and II may serve 
as a guide. 
The large nuclei (those with radii greater than about 
1 to 2 X 10~ em) are above the resolution limits of the 
TasLE I. Horizontal DistRIBUTION oF AITKEN-NUCLEI [23] 
Number of Nuclei per cc 
Location Abso- 
leeale obser- Mean Mean ineolat 1 
ities | ¥8, | Aversee) mast) mint | maximum | mini 
Large city. .| 28 | 2500)147 ,000|379 000/49, 100/4 000,000) 3,500 
Town....... 15 | 4700) 34,300/114,000) 5,900; 400,000) 620 
Open coun- 
try.......| 25 | 3500} 9,500) 66,500} 1,050) 336,000 180 
Ocean......| 21 600 940) 4,680 840 39,800 2 
optical microscope. In this category only solid particles 
have been measured so far, in the well-known dust 
counters of Owens and Zeiss. It was only recently that 
Tasie Il. Mean Vertican DisTRIBUTION OF AITKEN-NUCLEL 
(Data FROM 28 BAaLLoon ASCENSIONS [23]) 
Altitude (km)..... 
0-0.5) 0.5-1) 1-2 4-5 
11,000 
2-3 
780 
3-4 
340 
Nuclei per ce. .... 2,500 170 
22,300 
Dessens, and Woodcock and Gifford succeeded in ob- 
serving the haze droplets, thus closmg a gap in our 
methods of observation. Dessens [9] moved fine spider 
threads through the air, thus capturing the haze drop- 
lets quantitatively. It is uncertain whether or not solid 
particles can be collected in this way. Woodcock and 
Gifford [43] deposited haze droplets on small glass plates 
and showed that over the ocean practically all these 
droplets consist of NaCl-solutions. Dust and haze-drop- 
let counters enable us to determine both the total 
number of suspensions and their size distribution (Fig. 
il). 
The dust concentrations show—particularly in in- 
dustrial areas—a close correlation with the Aitken- 
nuclei concentrations [44], which is not surprising since 
both types of particle often have the same source. The 
concentration of the dust particles and haze droplets 
over land amounts to about 10-200 cm=%, that of the 
haze droplets over the ocean to about 1-10 em- [43]. 
From the foregoing discussion we can conclude that 
it is not yet possible to measure the nuclei spectrum 
in its entirety; so far there is only the substitute of 
counting nuclei, ions, dust, and haze droplets simul- 
taneously. 
Form and Physical State of Nuclei 
Photographs of Aitken-nuclei taken on the stage of 
an electron microscope show irregular clusters of mole- 
cules interspersed with a few hexagonal crystal outlines 
[26] that can be attributed to hexagonal or cubic crys- 
tals. Soot, for example, consists of loosely connected 
platelets; cigarette smoke, of small crystals [41]. It is 
likely, however, that these are essentially the boiled- 
down residues of nuclei droplets. 
CLOUD PHYSICS 
Previous examinations of samples collected with dust 
counters have revealed a similar picture. These samples 
showed, in addition to various crystalline shapes (e.g., 
cubic forms) and some organic substances, a preponder- 
ance of irregular particles. However, according to recent 
investigations by the author, these dust particles, when 
deposited on suitable surfaces, consist to a considerable 
extent of droplets. This is to be expected from Dessens’ 
observations. The proportion of droplets varies greatly 
and increases with humidity. Many of these droplets 
contain both soluble and insoluble substances. It is 
conceivable that the Aitken-nuclei have a similar com- 
position, since the mixed nuclei among them constitute 
an even larger portion owing to the greater coagulation, 
and all mtermediate types between pure water droplets 
and solid particles covered with a hygroscopic film 
probably exist. 
Important clues to the physical state of nuclei may 
be deduced from the study of their growth with in- 
creasing humidity. Figure 2 shows a few typical results 
100 
90 
80 
(op) 
(oe) 
RELATIVE HUMIDITY % 
oO 
(oe) 
7 6 5 4 VISIBILITY CODE NO, 
I 2 3 4 5 OPAGITY 
A4—+A (ACCORDING TO [I6]) 
(ACCORDING TO [I9] ) 
O——oO (AGGORDING TO [48]) 
Fic. 2.—Effect of the growth of nuclei with increasing rela- 
tive humidity on the opacity (circles), on the visibility 
total number of nuclei 
1 th in@ (P= = 
(sausizes) sand on the rable total number of large ions 
(triangles). 
of observations of various quantities related to natural 
aerosols. The dependence of visibility on relative hu- 
midity (see pp. 187 f.) reflects primarily the growth of 
the large nuclei (r > 10~ em) with humidity, whereas 
the dependence of P on humidity (see pp. 187 f.) 
chiefly illustrates the growth of nuclei in the size range 
below r = 10 cm. A few isolated, direct size measure- 
ments lead to the same conclusions (Fig. 8, curve 1). 
From these results we see that as a general rule, the 
