186 
stable compounds, such as HCI, HNO;, and H»SOu, on 
the growth of nuclei. In the presence of traces of such 
compounds there is equilibrium between the phase dis- 
solved in the nuclei and the gaseous phase. If gases, 
such as SO. and NH;, that form less stable solutions 
are involved, the dissolved part is minute (because of 
the smallness of the nuclei) so that practically every- 
thing goes into the gaseous phase. With increasing 
humidity a point will be reached (depending on the 
quantity and type of the substance) at which there is 
a sudden growth of nuclei. This growth will continue 
until the vapor pressure of the gas traces decreases 
perceptibly (Fig. 3, curve 5b). This process is likely to 
be of importance in the formation of dry fogs in indus- 
trial areas. 
Formation of Nuclei in Smoke and During Gas Re- 
actions 
The formation of nuclei in smoke is engendered by 
the growth of the smallest molecular particles by sub- 
limation, and by condensation of matter having a very 
high boiling point. Because of the high original nuclei 
concentrations, these particles continue to grow by 
coagulation, causing the formation of mixed nuclei. 
Among the important examples of this type of nuclei 
are soot particles and the tarry and hygroscopic par- 
ticles present in most combustion effluents. 
Gas reactions are likewise significant as a source of 
nuclei. Often they may become effective only at a 
great distance from the point where the gas traces 
originate, after mixing with other gas traces or through 
photochemical processes. According to Rothmund [82], 
nuclei are always produced if the products of such 
reactions are water-soluble and if the humidity of the 
air is sufficiently high. During the most varied reac- 
tions, he noted the formation of haze droplets of r & 
5 X 10 em. Details of the formation mechanism of 
these nuclei, particularly with reference to the sig- 
nificance of humidity, as well as the establishment of 
an equilibrium between the gaseous participants in the 
reaction and the reaction product (nuclei number, size), 
are not yet known. 
The concept of such equilibriums between the gaseous 
phase and the “nuclei phase” permits these two con- 
clusions: 
1. Many traces of matter will be found in the gaseous 
phase and also as condensation nuclei. This is corrobo- 
rated by the fact that in many instances the amount 
of matter appears to be too high to be explamed by 
plausible numbers and sizes of nuclei. For example, 
Cauer [5] was unable to establish a relationship between 
the concentration of Aitken-nuclei and traces of matter 
(NHi, NOz, SOs, and CT -contents). It must be 
pointed out, however, that quantitatively the large nu- 
clei, that is, the haze droplets and dust particles, may 
(in spite of their small number) prevail over the Aitken- 
nuclei, so that a more exact statement will have to wait 
until the entire nuclei spectrum is known. 
2. The change in external conditions (¢.g., m the 
humidity or the concentration of the gaseous phase) 
may cause the number of nuclei to change; the nuclei 
CLOUD PHYSICS 
concentration in a given mass of air would thus not 
represent a constant quantity. It 1s, for example, not 
unlikely that nuclei whose substances are only faintly 
volatile (such as oils and tars as well as the acids HNO3, 
H»SOx, and HCl) slowly evaporate if mixed with pure 
air. In areas which are far removed from human settle- 
ments and industry (e.g., mountains, oceans), one may 
not expect to find aerosols other than those consisting 
of solutions of nonvolatile salts or solid matter. In this 
connection mention should be made of an observation 
by Schlarb [84], according to which pressure changes in 
a climatic chamber between 14 and 114 atmospheres 
were found to result in a fluctuation of the number of 
nuclei in a proportion of about 1:50! 
In the following paragraphs a few meteorologically 
important gas reactions will be mentioned briefly [4, 
23, 29, 32]. 
Ozone, O3, while not in itself nucleogenic, enters into 
reaction with many other traces of matter, such as 
nitric oxides or SOs, thereby producing nuclei [81]; 
H.O2, which, however, according to Cauer [4], is found 
only close to flames, shows a similar behavior. 
Oxides of nitrogen, NO and NOz, originate simultane- 
ously with O; during electric discharges (thunderstorms) 
and as a result of the ultraviolet solar radiation [29], as 
well as in all incandescent processes and in flames, ac- 
cording to Coste and Wright [7]. These oxides are very 
effective in creating nuclei. 
Sulfur dioxide, SO2, which develops in all combustion 
processes, forms nuclei in combination with O03 or, 
according to Aitken [2], through photochemical proces- 
ses in combination with gas traces (formation of dry 
fogs after sunrise). These nuclei are probably composed 
of H.SO3 and. HSOu. 
Chlorine ions, Cl-, according to Cauer [6], escape from 
the surface of the sea as well as from droplets of ocean 
spray, and form HCl-nuclei photochemically. This ex- 
plains why a separate determination of the magnesium 
and chlorine contents of the air gives mass proportions 
which are entirely different from those found in sea 
water. Part of the chlorine content exists in the form of 
salts. 
Ammonium ions, NHf, result from all processes of 
combustion and from the decomposition of organic 
substances and, when reacting with the acids mentioned 
above, lead to the formation of nuclei. 
Todine, Iz, is dispersed into the air along the Atlantic 
Coast of Europe mainly by industrial charring of sea- 
weed for the extraction of iodine [4]. 
Some typical values, indicated by Cauer [4], will 
stress the significance of these traces (values are in 
10 g m-). 
Ozone: Tatra 30, Jungfraujoch 10; 
Nitrite: Tatra and Silesia 0.2; 
Sulfite: Berlin 200, Silesia occasionally 3, Tatra, 
faint traces, but very rarely; 
Chloride: Brittany 7, on the sea near Kiel 149, 
Silesia 32, Tatra 70; 
Ammonia: Silesia 17, Brittany 21; 
Todine: Average for Central Europe before No- 
vember 1, 1933, 0.6; thereafter 0.05 (be- 
