244 
1000f- vioev 
® BACKGROUND C TS MARCH 18,1959 
@ BACKGROUND C S MARCH23,1959 
YCOUNTS WITH 
MARCHIB 19 
ACCOUNTS WITH 
MARCH23.,) 
600+ 
NUMBER OF ICE CRYSTALS IN 10 LITERS 
Fic. 6—Comparison of temperature- 
spectrum counts of freezing nuclei at 
—20°C in air drawn from near the sur- 
face of agitated ocean water and exist- 
ing background values 
not be stated at this time. The threshold nu- 
cleation temperature of the sea-water aerosols 
has consistently been in the neighborhood of 
—14° to —16°C, in close agreement with the 
results of Birstein and Anderson [1953]. Trial 
runs with solutions of NaCl and MgCl, as well 
as other soluble substances appear to give posi- 
tive responses of comparable magnitude, while 
carefully distilled water shows a null effect. 
Concluding remarks—These results appear 
to offer strong circumstantial evidence, but not 
proof, of a marine source of aerosols active as 
freezing nuclei in the Washington, D. C., area. 
This evidence seems at variance with the ex- 
perience of others in this field, and raises some 
provocative questions regarding the physical na- 
ture of at least some of the freezing nuclei in 
the atmosphere. It is of course possible that the 
explanation for these results may reside in the 
inherent nature of the rapid-expansion tech- 
nique wherein soluble substances could initiate 
the freezing process before going into solution, 
but not in the slower condensation rates more 
representative of atmospheric processes. On the 
other hand, the mixing-chamber technique used 
in the earlier 1958 series of observations in the 
Washington, D. C., area showed similar ten- 
dencies for anomalous conditions to occur in 
air with a marine trajectory. The possibility 
of local sources of freezing nuclei cannot be 
ruled out completely. Hovsever, a series of ob- 
DWIGHT B. KLINE 
servations at various locations within a ten 
mile radius of the metropolitan area on 15 
separate days yielded no evidence that this might 
be the case. 
These results plus recent work of Papée 
[1959] pointing toward activation phenomena in 
soluble substances such as NaCl which may 
facilitate their role as a sublimation nucleus 
raises some intriguing and rather crucial ques- 
tions regarding the role of hygroscopic particles 
in atmospheric chemistry and cloud nucleation. 
They may have a bearing on the limited number 
of aircraft observations [Coons, Jones, and 
Gunn, 1949] indicating a tendency for the oc- 
currence of ice crystals in clouds at warm sub- 
freezing temperatures In marine air. 
Acknowledgments—The assistance of Gilbert 
D. Kinzer, Glenn W. Brier, and DeVer Colson 
in certain experimental and analysis phases of 
this study is gratefully acknowledged, along 
with help by Thomas H. Carpenter and Fred- 
erick Van Cleef in the laborious and tedious 
process of maintaining a daily freezing nuclei 
observational program. 
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