236 
freezing nuclei 
140 y 
20 
130 DNe 
0 
120] 
100 
Relative concentration 
(eZee) 
continental = mar. continental 
mar. continental 
HANS-WALTER GEORGII 
continental mar. 
Fre. 3—Comparison of the concentration of freezing nuclei at different 
locations (concentration at Frankfurt am Main equals 100%) (1) Frankfurt; 
(2) Taunus Observatory; (8) Zugspitze; and (4) Valentia Island 
liquid to solid improve with altitude in the 
atmosphere. This fact can be caused by the 
following reasons: (1) The particles of indus- 
trial and anthropogeneous origin in the ground 
layer of the atmosphere are poor ice nuclei. The 
concentration of these pollutants decreases 
rather rapidly with increasing altitude. (2) The 
high concentration of certain trace gases in the 
Frankfurt area causes an inactivation of the 
surface properties of the nuclei. In the free at- 
mosphere the concentration of these gases is neg- 
ligible. (8) Although the majority of our meas- 
urements including these at the high altitude 
station were carried out at outside air tempera- 
tures above the freezing level the influence of 
preactivated nuclei cannot be fully disregarded. 
The effect of an increased activity of freezing 
nuclei shows up also in the phenomenon de- 
scribed above, namely, the occurrence of very 
active freezing nuclei in the ground layer after 
showers. In a recent paper of Kassander and 
others [1957] the presence of preactivated nuclei 
in the free atmosphere is also indicated. During 
flights over the southwestern United States air 
samples from ice clouds were taken and warmed 
above freezing level in a cloud chamber. Later 
the air samples were cooled once more and pro- 
duced ice erystal concentrations of 100 per liter 
at only —10°C. Laboratory tests finally suc- 
ceeded in conserving the efficiency of the 
‘trained’ nuclei after they had kept above 0°C 
for several hours [Mason, 1959]. 
Inactivation of freezing nuclei by surface re- 
actions—In order to clarify the problem of in- 
activation of freezing nuclei as a first step, 
parallel measurements of the concentration of 
certain trace gases in the atmosphere of the heav- 
ily industrialized Rhem-Main Basin were taken. 
Applying the methods of trace-gas analysis by 
Junge we sampled SO., NH;, and NO., gases 
which were thought to be able to act as surface 
poisons. Table 1 gives a survey of the relation 
between the concentration of each of these gases 
and the activity of freezing nuclei. The latter is 
expressed as activity quotient, being the ratio of 
the number of large condensation nuclei and 
freezing nuclei active above —30°C. High fig- 
ures of the activity-quotient stand for low ‘qual- 
ity’ of the freezing nuclei. 
The figures indicate clearly that the increasing 
concentration of the three gases in question de- 
teriorates the activity of the freezing nuclei. 
However these parallel measurements of freez- 
ing nuclei and trace gases do not give an un- 
equivocal answer which of the three gases is the 
most effective in inhibiting the ice nucleation 
although it seems according to Table 1 that the 
nuclei react rather sensitive on the ammonia 
