WMOD00399

• Title: WMO Training Workshop on Weather Modification for Meteorologists - Lecture Notes
• Date: 12/1/1979
• Weather Modification - Doc Type: Report

- 49 -

This is the so called Junge distribution (JungE), 1963). The general appearance of an aerosol
distribution is shown in Fig. 2. It is ssen that concentrations falloff very sharply with
increasing size. Therefore, the total number concentration is dominated by the Aitken nuclei
(< 0.2 pm). Continental aerosol larger than 0.2 pm (large nuclei) follow equation (5) quite
closely with a ; 3.

10'

~"'"

onE
~ 102
<U
Q.
:: Iei'

c

162

AI'luIn
-nuclei

---'--
162 101 100 10'
r,JLm

Fig. 2: General appearance of aerosJl distribution curve, showing the range of
Aitken, large and giant nuclei.

The purpose of applying supersaturation in particle counters is to cause particles to
grow to a size comparable to, or greater than, the wavelength of visible light, where they
can be more readily observed by optical, visual or photographic methods. To separate par-
ticles which possess critical supersaturations that are small enough to allow activation
and growth into drops in cloud formation, a much lower applied supersaturation is needed.
If a small expansion were to be used to produce the small supersaturation, comparatively
little growth would result because of th9 short-lived nature of supersaturation produced
by expansion, since it is quickly destroyed by heating of the temporarily cooled air by
conduction from the walls which remain at the original temperature.

Cloud Condensation Nuclei (CCN)

The supersaturation maximum during cloud formation typically ranges from a few hun-
dreths to about 1%, and depends not only on external variables like cooling rate and temper-
ature, but also on the particle content itself" In order to act as a CCN at 1% supersatura-
tion, a completely wettable but water insolublE: particle needs to be at least about 0.1 pm
in radius, whereas soluble particles can be as small as about 0.01 pm in radius. The
situation is described by the relation b9tween supersaturation and particle radius known
as the Kohler equation (see e.g. Mason, 1971, p.24) which relates the saturation vapour
pressure of a solution droplet of radius r, e~ to the saturation vapour pressure of a pure
liquid es(oo):

,
S=~
es(oo)

b 20'
(1 - 3) (exp R 'T)'
r vPL r

(6)

imMw
where b ; and a' ,PL are the sUrfaCE) energy and density of the solution, Mw the
(4n/3)PL ~
molecular weight of water, m and MN the mass and molecular weight of the dissolved salt and
i the van't Hoff factor (i ; 2 for diluts solutions). If we plot the variation of the rela-
tive humidity or supersaturation of the air adjacent to a solution droplet as a function of
its radius, we obtain what is referred tJ as Kcihler curve. Several such curves derived
from (6) are shown in Fig. 3. Consider a solution droplet in a situation reoresented by the
peak of curve 2 in Fig. 3, where the critical supersaturation (Sc - 1) a b-V2 a m-V2 of 0.36%
is reached at a droplet radius of about :2.10-5 cm. If the droplet evaporated slightly the
supersaturation would fall below that of the ambient air so the droplet would grow by con-
densation back to its original size. If, on the other hand, a droplet at the peak of the
curve should grow slightly, the supersatJration of the air adjacent to the droplet would
also fall below that of the ambient air and therefore the droplet would grow by condensation.
As a result of this growth, the supersaturation of the air adjacent to the droplet would