AEROSOL SPECTROMETER AND ITS APPLICATION 
TasLe 2—Test data pertinent to Fig. 9 
| | 
Date Location Aiea) Weather | Time HD oO 
pe An hr 2c | -% | rpm mm 
a | July 1, 1959 St. Barbara Harbor shore line | SSW-2 on | haze 09-10} 19 | 87 | 18 | 1 
shore | 
b | Aug. 22, 1959 About 40 mi WSW of St. Bar- | SW-4 clear 16-18} 25 | 55 | 18} 1 
bara Island, kelp beds | 
c | Nov. 12,1958 | Mt. Wilson Observatory |A| NO-1 heavy | 12-13} 8 | 85 | 24 | 0.75 
(6000 ft), 100 ft above haze 
ground at tree top level IB| SW-3 | fog 13-16) 6 | 90 | 24 | 0.75 
d | Aug. 25, 1959 S. shore Catalina Island | SSW 1-2 haze 08-10) 22 | 84 | 18] 1 
relation indicated by the hatched curve. Quite 
surprising ishere the practical absence of particles 
for 6 > 0.8 up. A repeatedly observed exception 
is Figure 9b caused by the presence of a second 
maximum. If the first represents a dehydrated 
salt aerosol with a maximum at ds = 0.21 uw + 
0.01, the second a corresponding hydrated frac- 
tion at d, = 0.46 uw + 0.01, d,/ds = 2.2 + 0.2 
results, identical with the value 2.09 as derived 
from Stokes law for this ratio. This can be inter- 
preted as the coexistence of a dehydrated and a 
hydrated fraction of the same salt aerosol, either 
because of its different ‘age’ or because of factors 
delaying the evaporation of the condensate. 
Figure 9d represents a comparison of two com- 
plete counts of the same size spectrum, one (A) 
taken immediately after the exposure, the other 
(B) after a 48-hr storage in an air-tight container 
in the presence of silica gel as desiccant. The 
latter count is somewhat less in the range of small 
d although no evidence of coagulation was no- 
ticed. (The natural aerosol spectra taken in for- 
ests, desert, and the High Sierra (since this paper 
was written) confirm this finding without excep- 
tion. It appears thus that a fraction of the aerosol, 
predominantly the smallest sizes, can evaporate 
at a larger or lesser rate without leaving a visible 
trace. This would indicate an aerosol fraction 
consisting of volatile substances with very low 
vapor pressure, that is, small compared with 
H.0.) 
In all diagrams the ordinate scale is relative, 
however, the counts were normalized with respect 
to the sampling time, in order to render the curves 
comparable. The total particle concentration 
ranged between 10? and 10% per cm’. 
ARTIFICIAL AEROSOLS 
As initially indicated, the influence of atmos- 
pheric traces of gaseous organic materials upon 
the nuclear condensation (or dehydration) proc- 
ess presents an important problem about which 
little appears to be known at present; here the 
A.S. method could be expected to yield informa- 
tion. 
The basic effect of such traces can be postulated 
approximately as follows: Provided their vola- 
tility is relatively low, these molecules are likely 
to become associated (adsorbed) with the nuclei. 
At high relative humidity they will form the sur- 
face, because for components of lesser surface 
tension this represents the configuration of least 
surface energy (Gibbs theorem), and most organic 
materials have surface tensions much smaller 
than water or electrolytes. If the vapor pressure 
of such a surface layer is smaller than that of the 
condensate (for example, of the saturated NaCl 
solution), the rate of evaporation will be less for 
a droplet carrying an organic surface layer than 
without such [Archer and La Mer, 1954], if the 
relative humidity decreases. This would result in 
the delay or prevention of equilibrium states, 
such as occur in persistent fogs and hazes. If, on 
the other hand, the nuclei were dehydrated when 
contacted by the organic molecules, subsequent 
condensation, due to an increased relative hu- 
midity, may also be delayed. If this assumption 
is valid, at least in principle, aerosols of hygro- 
scopic nuclei should produce different size spectra 
in the A.S., depending on the presence or absence 
of organic traces in the gas phase, if intercepted 
within a relatively short time after their exposure 
to a change of humidity. Such tests require arti- 
