AEROSOL SPECTROMETER AND ITS APPLICATION 
in the gravity field a, through a gas of the vis- 
cosity 7 (when neglecting its density) as 
7 1 n 
iia 2 = (5) 
i a pd. K(d) a: & 
where the Knudsen-Cunningham factor is rep- 
resented in the simplified form as A(d) = 1 + 
a/d, and @ represents the product of the mean 
free path (~8 xX 10~® cm) of the suspending gas 
and an empirical constant varying between 0.7 
and 1.2 [Wasser, 1933]. Hence two particle types 
of dy # dy, pi # p» will form the same deposit 
pattern C(d), for 6; = 6. when 
dip, (: + =) = dps (: ae =.) (6) 
dy (ly 
and their density ratio 
pi/p2 = di(dz + a)/di(d, + a), 
or if 
[di , d2 > a] = (d2/d,)? 
The particle diameter for p, # 1 can thus be 
converted in good approximation from di ~ pi = 
1 by 
i dy ihe os 1/2 
> Pp \ Ge aP 
Wee api \/ (7) 
This relation predicts also the variation of (6) 
for hygroscopic nuclei (for example, NaCl) be- 
cause the ratios of the densities ps/p, and of the 
geometric diameters before and after hydration 
can be calculated from the solubility S in g/em* 
water of the nuclear substance and its density. 
(The term ‘hydration’ is used for the almost 
spontaneous transition from the practically 
water-free crystalline state of the NaCl particle 
to that of a saturated solution. This transition 
occurs when the relative humidity (rh) reaches 
a critical value rh, above which the solid phase 
becomes unstable, partly because of the increase 
of solubility with decreasing particle size (Ost- 
wald-Freundlich), partly because of the decrease 
of vapor pressure of the resulting solution (Ra- 
oult). The volume increase (decrease of curvature 
of the droplet surface), caused by hydration of 
the crystal, acts in the same direction (Kelvin). 
This threshold value rh. should increase with in- 
creasing d and asymptotically reaches a maxi- 
mum (for NaCl at about 76% for d > 0.7 yu) 
or as 
175 
[Twomey, 1953; Orr, and others, 1958a]. Hence 
the coexistence of two phases (solute in equilib- 
rium with its saturated solution) is not likely to 
exist for any length of time in small airborne 
nuclei of water-soluble solids. The hydration of 
such nuclei is not strictly reversible when the 
rh is reduced below rh, , that is, a spontaneous 
‘dehydration’ may be delayed for a certain length 
of time, while the nuclei remain in a supersatu- 
rated state out of equilibrium with the vapor 
pressure of their gaseous environment.) 
This results in 
dy/dgs = (1 + ps/S}!8 
and 
pr/ps = (1 + 8)/(ps + 8) (8a) 
Hence 
s/tg = [Sete etal 7 
or, if 
[d,/ds > al = (dz/ds) V0, /0, (8b) 
For NaCl S = 2.64 10-1, pg = 2.163 which 
results in dz/ds = 2.09, p,/ps = 5.2 X 107}, 
6,/6s = 1.51 without A factor. Its inclusion 
renders 6,/6s; somewhat smaller depending on 
the absolute values of d, or ds. For a = 
AX =08 X 8 X 10-8 em, 6, /é6s is 1.82 ford, = 
0.2 uw, and 1.47 for d; = 1 pw. For most experi- 
mental work the complication involved by the 
inclusion of A(d) appears unnecessary; instead 
a corrected factor 6,/6s = 1.48 for 0.2 uw < 
6, < 1 wrange can be used. By this relationship 
the variation of a known C(d) of a salt aerosol 
can be predicted when the nuclei undergo hydra- 
tion or dehydration. 
NatTurRAL AEROSOLS 
The application of the A.S. to natural aero- 
sols provides an interesting comparison with the 
extended previous investigations on this subject 
[Junge, 1958] by a principally different method, 
as well as an extension into the submicron range. 
Only a few, though apparently typical, examples 
are presented in Figure 9 to demonstrate the 
results obtainable by this method. 
The selection of the sites and conditions of 
these field samples aimed at atmospheric condi- 
tions minimizing the chance of industrial and 
other pollution. The offshore marine aerosols 
(Fig. 9abd) were taken from a boat, anchored, 
