The Aerosol Spectrometer and Its Application to Nuclear 
Condensation Studies 
A. Goetz AND O. PREINING 
California Institute of Technology, Pasadena, California 
Abstract—The aerosol spectrometer (A.S.) separates quantitatively airborne par- 
ticles in the diameter range 3 « — 0.03 uw from the atmosphere in the form of a size- 
spectrum, that is, a continuous band-shaped deposit. The position of a particle thereon 
is indicative of its ‘Stokes’ diameter’ while it was airborne, and independent of physical 
changes incurred after its separation from the suspending air. This size-classified sepa- 
ration results from the exposure of a laminar, continuous air flow to a large centrifugal 
field (up to 26,000 g), the flow rates vary between 3.3 and 74 lit/min. The size (and 
mass) distribution of the aerosol is derived from the typical variation of the deposit 
density along the spectrum, either by microscopic count (down to 0.1 «) or by micro- 
photometric recording of the light scattered by the particles under reflected dark-field 
illumination in a special micro-analyzer. 
A brief description of the instruments and the mathematical basis of the analytical 
procedure is presented, also its application to a ‘model” aerosol of polystyrene latex, 
consisting of equal-sized particles in various states of agglomeration. From the size 
definition in terms of the Stokes’ diameter, a relationship between the locus of deposi- 
tion of dry and hydrated hygroscopic nuclei is derived and subsequently supported ex- 
perimentally for NaCl aerosols. 
The A'S. has been applied to the analysis of natural and artificial aerosols in the sub- 
micron range. Samples of natural (off-shore and mountain) aerosol spectra are pre- 
sented; they follow in general the pattern determined by previous authors but show a 
fine structure which appears to be due to traces of organic matter. The artificial genera- 
tion and conditioning of NaCl aerosols and the so resulting size distribution is described, 
particularly the strong effect of the presence of traces of organic vapors (turpentine, 
pinene) during the hydration and dehydration of the salt nuclei. It is apparent that 
such traces prevent or delay the equilibrium of the nucleus when the humidity of its 
gaseous environment is altered. 
INTRODUCTION 
Our knowledge of the true constitution of aero- 
sol particles (that is, nuclei with or without con- 
densates) in the airborne state depends upon the 
degree by which the process of their conversion 
into a concentrate alters their nature. 
Separation by impingement or filtration expose 
the surface of the particle prior to and after its 
precipitation to the shearing action of the passing 
air stream. This shear may disturb seriously the 
dynamic equilibria of loosely sorbed or condensed 
components on the particle surface, whenever 
present. Hence minimal interference with the 
physico-chemical properties of the particles prior 
to their separation from the suspending air is as 
mandatory as a reliable size-classification in the 
diameter range (3 u > d > .03 uw), in order to ob- 
tain information about the size and mass distri- 
bution of natural and artificial aerosols. This is 
significant for at least two reasons: 
(1) Micro-analytic information about the con- 
stituents of unclassified precipitates can be seri- 
ously misled, as the presence of a few 1- 2-u par- 
ticles can obscure the true nature of thousands 
in the 0.1-u class because of the cubic relation 
between particle mass and diameter. 
(2) The study of the size-distribution variation 
with temperature, pressure, and the concentra- 
tion of gaseous components (HO, CO2, etc.), 
capable of association with such particles, prom- 
ises to enlarge present knowledge of the reaction 
kinetics of such systems. 
Of particular interest in this connection appears 
the role of organic components of the atmosphere, 
contributed by industry, plants, and conceivably 
by the biological components of the oceans, for 
they may influence significantly the dynamic 
equilibria of nuclear condensates, suspected 50 
years ago [Barus, 1907] and by numerous investi- 
gators since [Vonnegut and others, 1957]. 
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