recent advances in the chemistry and properties of atmospheric nucleants_a review

7/24/2019 Recent Advances in the Chemistry and Properties of Atmospheric Nucleants_a Review

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171

Re c e n t A d v a n c e s i n th e Ch e mi s t r y a n d Pr o p e r ti es o f A t m o s p h e r i c

N u c l e a n t s a R e v i e w

By ALBERTO C. MONTEFINALE, T r Io MONTEFINALE an d HENRY M . PAPI~E1)

Summary. A survey is carried out of recent progress in the investigation of weather-active

atmospheric nucleants. Emphasis is placed on the w ork done during the last decade, and som e resul-

ting im plications are discussed.

Introduction

T h e l a s t f e w y e a r s h a v e w i t n e ss e d a c o n s i d e r a b l e e x p a n s i o n o f t h e e x p e r i m e n t a l

w o r k w h i c h h a s t o d o w i t h t h e a t m o s p h e r i c n u c l e a n t s i n g e n e r a l , a n d i n p a r t i c u l a r

w i t h t h e t y p e s o f n u cl e i i n v o l v e d d i re c t l y i n c l o u d f o r m a t i o n a n d p r e c i p i t a t i o n p h e n o -

m e n a . T h i s e x p a n s i o n a p p e a r s t o b e l a r g e l y d u e t o a n i n c r e a s e d i n t e r e s t i n a t m o -

s p h e r i c s c i e nc e s , e n v i r o n m e n t a l p r o b l e m s , a n d a l s o t o a n i n c r e a s i n g n e e d f o r f r e s h

w a t e r . M u c h i n s ig h t w a s th u s g a i n e d i n t o t h e c h e m i s t r y a n d n a t u r e o f b o t h , n a t u r a l l y -

o c c u r r i n g a t m o s p h e r i c a e r o s o l a n d o f t h e p a r t i c u l a t e m a t t e r w h i c h o r i g in a t e s f r o m

h u m a n a c ti v it ie s . P r o g r e s s w a s a l s o m a d e i n t h e d e v e l o p m e n t a n d s t u d y o f ar ti f ic i a l

w a y s o f p r o d u c t i o n o f su i t ab l e n u c l e a n ts a n d i n t h e i r a p p l i c a t i o n t o w e a t h e r - c o n t r o l .

S c i e n ti f ic a n d e n g i n e e r i n g a s p e c t s o f a rt i fi c i al r a i n s t i m u l a t i o n , h a i l p r e v e n t i o n a n d

c l e a r in g o f f o g , a r e t h u s n o w i n a n a d v a n c e d s t ag e o f d e v e l o p m e n t a n d t e s ti n g [ 1 -3 ] 2)

a n d c o n c e r n a ri se s , o n t h e o t h e r h a n d , a b o u t t h e e f f e c ts o f a ir p o l l u t i o n a n d i n a d v e r -

t e n t w e a t h e r - m o d i f i c a t i o n [ 4 ] b y h u m a n a c t i v i t i e s .

I t w o u l d , t h e r e f o r e , a p p e a r t o b e o f i n t e r e s t to s u r ve y , b ri e fl y , t h e p r o g r e s s m a d e

d u r i n g t h e l a s t f e w y e a r s a n d s i n ce t h e t im e w h e n a n u m b e r o f m o n o g r a p h s w a s w r i t t e n

o n t h e s u b j e c t [ 5 - 7 ] . O w i n g t o t h e i m p o s s i b i l i t y o f m e n t i o n i n g , l e t a l o n e d e s c r i b i n g i n

t h is s h o r t r e vi e w , t h e d e t a i l e d fu n d a m e n t a l s o f b o t h a e r o s o l b e h a v i o u r , p h y s i c a l

a c t i o n a n d o f i ts r o l e i n w e a t h e r p h e n o m e n a , n o s u c h a t t e m p t w i ll b e m a d e h e r e .

T h o s e a s p e c t s w e r e a l r e a d y e x t e n s i v e l y d i s c u s s e d b y M A S O N [ 8 ] BYERS [9], ZETTLE-

M O Y E R [ 1 0 ] a n d o t h e r s ; q u o t a t i o n o f w o r k a n t e c e d e n t t o r e f e r e n c e s [ 5- 9 ] w i ll h e n c e

b e m a d e o n l y f o r t h e s a k e o f c o m p r e h e n s i o n .

1) Center for Aerosol Nucleation; N ational Research Council of Italy; V ia Vettore 4 M onte

Sacro), 1-00141 Rom a.

8) N umbers in brackets refer to References, pag e 201.



172 A .C . Montefinale, T. Montefinale and H. M. Pap (Pageoph,

1 Condensation nuclei

T h o s e a r e k n o w n t o c o n s i st , e s s en t ia l ly , o f a h y g r o s c o p i c o r w e t t a b l e s u b s t a n c e

w h i c h p r o m o t e s t h e g r o w t h o f a w a t e r c l u s t e r o r d r o p l e t a t s u i ta b l e v a p o r s a t u r a t i o n s

i n t h e a ir . I n t h e a b s e n c e o f c o a g u l a t i o n , a g g l o m e r a t i o n a n d c o a le s c e n c e , a n d a t

c o n d i t i o n s o f c o n s t a n t t e m p e r a t u r e a n d h u m i d i t y , t h e r a t e o f g r o w t h a n d t h e s i z e o f

t h e re s u l ti n g d r o p l e t w i ll o b v i o u s l y d e p e n d o n t h e c h e m i c a l n a t u r e a n d o n t h e d i m e n -

s i o n s o f t h e p r i m a r y n u c l e u s w h i c h , i f v e r y sm a l l , w a s s o f a r d e t e c t e d a n d s t u d i e d

a l o n e b y i n d u c i n g it s g r o w t h i n a s u p e r s a t u r a te d a t m o s p h e r e ( e x p a n s i o n t e c h n i q u e )

a n d , a l so , b y e l e c t r o s t a ti c c h a rg i n g a n d e v a l u a t i o n s o f it s m o b i l it y . C o n d e n s a t i o n

n u c l e i a r e t h u s s e m i - a r b i t r a r i l y d i v i d e d i n t o t h r e e c l a s s e s w h i c h w i l l h e r e a f t e r b e

s e p a r a t e ly d i sc u s s ed i n t e rm s o f r e c e n t w o r k t h e r e o n c a r r i e d o u t . N o c o m m e n t s w i ll

b e a t t e m p t e d o n t h e r o l e o f t h e s m a l l io n s ( r a d i u s l es s t h a n 5 x 1 0 - 2 m i c r o n ) i n n u c l e a -

t i o n p r oc e s s e s, b e c a u s e o f t h e l a c k o f i n f o r m a t i o n o n t h e p r i m a r y c h e m i c a l n a t u r e o f

t h o s e t r o p o s p h e r i c i o n s a n d h e n c e o n t h e i r m o d e s o f a c t i o n o t h e r th a n e l e c t r o s ta t ic .

T o a ll p r a c ti c a l e f fe c ts t h o s e i o n s m a y b e e x c l u d e d f r o m t h e p o p u l a t i o n o f a c ti v e

c o n d e n s a t i o n n u c le i , b e c a u s e o f th e h i g h w a t e r s u p e r s a t u r a t i o n s n e c e s s a r y f o r t h e i r

a c t i o n . H o w e v e r , s i n ce t h e y a re k n o w n t o e l e c t r o s t r ic t c o n s p i c u o u s a m o u n t s o f w a t e r

m o l e c u l e s [ 1 1 -1 2 ], t h e s t u d y o f a p o s s ib l e c o m p o n e n t o f th e v e r t i c a l t r a n s p o r t o f

w a t e r , c a u s e d b y e l e c t r ic g r a d i e n t s a n d d u e t o t h o s e c l u s te r s , c o u l d w i t h t im e b e c o m e

o f m o r e t h a n a c a d e m i c i n te r e s t.

a Aitken nuclei

( r a d i u s b e t w e e n 5 x 1 0 - 2 - 2 x 1 0 - 1 m i c r o n )

T h o s e s m a l le s t p a r ti c le s o f th e c o n d e n s a t i o n n u c l e i s p e c t r u m a r e commonly

detected a n d their concentrations evaluated u s in g in s t ru m en t s o f t h e A ITK E N , P O LL A K

o r R i c h t y p e , w h i c h o p e r a t e o n t h e p r in c i p l e o f e x p a n s i o n a n d o p t i c a l ( o r p h o t o -

e l e c tr i c ) m e a n s o f c o u n t i n g [8 ]. B e c a u s e o f t h e i r s m a l l s i ze it is g e n e r a l l y b e l i e v e d t h a t

t h e y m o s t l y c o n si s t o f h y g r o s c o p i c g as e s a n d o f p r o d u c t s o f t h e i r i n t e r a c t i o n a n d

p h o t o o x i d a t i o n [1 4]. P h o t o l y t i c t r a n s f o r m a t i o n , i n t o h y g r o s c o p i c m a t e ri a l , o f o ri g i-

n a l l y s p a r s e l y - s o l u b le o r g a n i c p r o d u c t s , i s a ls o p o s s ib l e . T h e c h e m i c a l n a t u r e o f t h o s e

n u c l e i h a s h e n c e , s o f a r a n d w i t h a f e w e x c e p t i o n s o n l y , l a r g e l y b e e n e s t i m a t e d b y

t e n t a t iv e d e d u c t i o n . C o n c e n t r a t i o n s o f u p t o a b o u t 1 7 • c m - 3 a r e k n o w n t o e x i s t i n

a i r - p o l l u t e d c i t ie s a t l o w a l t it u d e s , a n d t o d e c r e a s e t o a b o u t 10 3 • c m - 3 e i t h e r w i t h

e l e v a t io n ( a t a b o u t 2 .5 k m o v e r u n p o l lu t e d a r e a s) o r o v e r t h e o c e a n , u n d e r c o n d i t i o n s

o f n o r m a l v e r t i c a l m i x i n g a n d i n t h e a b s e n c e o f i n v e r s i o n s o r e x t e n s i v e s tr a t i fi e d

c l o u d s [ 8]. I t is n o w a c c e p t e d t h a t p r o c e s s e s w h i c h o c c u r o v e r l a n d a r e t h e m a i n s o u r c e ,

p e r u n i t a r e a , o f th o s e n u c l ei w h i c h d e r i ve f r o m c o m b u s t i o n s ( fi re s ), v o l c a n ic e r u p -

t i o n s , a i r p o l l u t i o n a n d b i o l o g i c a l a c t i v i t y . B u b b l e - f i l m d i s r u p t i o n s a n d R A Y L E I G H

j e t s d o n o t s e em t o a c c o u n t f o r t h e p r o d u c t i o n o f l a rg e a m o u n t s o f se a - sa l t n uc l ei o f

t h a t s i z e ; h a l o g e n g a s e s m a y , h o w e v e r , s e p a r a t e f r o m m a r i n e n u c l e i a l o f t a n d m a y

t h u s a l so b e e x p e c t e d t o r e a c t , e i t h e r d ir e c t ly o r p h o t o c h e m i c a l l y , w i t h o t h e r w i s e



Vol. 91, 1971/VIII) Chemistry nd Properties of Atmospheric Nucleants 173

inactive particulates, yielding small condensat ion nuclei. Halogens are also known to

react with water yielding acid, hygroscopic and oxidizing products.

A recent paper by WENT [15] points to processes of natural generation of AIT~CEN

nuclei in artificially-unpolluted areas, via photochemical reactions which involve

plant-emanations of both terrestrial and marine origin (mainly terpenes) and NO~

gas. NO~ is here to be regarded as NO, NO2, N203 or a mixture thereof and their

products of hydration, with possible traces of NO; transients photochemically gener-

ated when nitrate-containing substance is present in the aerosol. A number of earlier

contributions supports the hypothesis of the effect of natural and artificial short-wave

radiation on the production (or 'activation') of those condensation nuclei in humid

air which contains traces of impurities [16-23]. Pre-activation effects by sunlight have

recently also been reported [24], and may be due to a photoproduction of oxidants in

the irradiated air and their subsequent reactions [25] thus contribut ing to the persist-

ence of the An'KEN particulates during nighttime [26]. On the other hand ammonia,

and hence also the salts of ammonium, are known to be a common constituent of the

atmospheric aerosol, as they originate from human and animal activities [7, 27].

Photooxidation or ionization in moist air, of ammonia which by itself is hygroscopic,

yields even more hygroscopic NO~ [28] which is known to promote further photooxi-

dation of some aerosol components through a number of chemical steps [29-31],

where pernitrite transients [32], organic peroxides [14], and/or higher oxides of

nitrogen are probably involved. Strong nucleation effects of water vapor due to NO~

were, in fact, observed by several investigators [28, 33-35]. Similar considerations

may be applied, on a much larger scale, to artificially polluted areas [36-38] where

concentrations of photoreactive [3940] aerosol systems, which contain oxides of

nitrogen and sulfur compounds, are notoriously high [41-42]. NO~ generation is

there, per unit area, substantially above its natural rates of production by the oxida-

tion of biological NH3, auroras [43], thunderstorms (see comments by VI~MHSTER

with this regard, Ref. [44]), and upper-atmospheric reactions ([45] and others).

The above discussion is meant to provide a general qualitative idea about the

chemical processes which may account for the generation of AIT~ZEN particulates and

for their changes of concentration according to

geography and timing

The importance of

those nuclei, briefly, consists in providing plentiful centers of condensation which have

negligible sedimentation rates and which require low supersaturations of water vapor

(about 2~ at the most) to promote a 'smooth vapor-liquid transition of phase.

Supersaturations required are considerably higher in the case of small ions [8-10] or

for homogeneous nucleation [8-10], and the practical occurrence of those last two

processes, in the free atmosphere, thus appears to be very unlikely.

Chemical methods or the analysisof soluble components of the atmospheric aerosol

[46], and hence a quest for their hygroscopicity, may, necessarily be applied only to

the range of size-spectrum investigated, because of considerable changes of chemical

composition along this spectrum. Inasmuch this presents experimental difficulties in

the case o f AITI~EN nuclei, much progress was recently also achieved in this area of



174 A .C . Montefinale,T. Mo ntefinale and H. M. Pap6e Pageoph,

r e s e ar c h . I t w a s t h u s f o u n d t h a t a l a r g e p a r t o f t h o s e n u c l e i o f te n c o n s i st s , i n p o l l u t e d

a r e a s , o f a m m o n i u m s u l f a te [ 4 7 -4 8 ] w h o s e c o n c e n t r a t i o n i n f l u e n c e s t h e v i s i b il it y

v a lu e s [ 4 9- 50 ] d e p e n d i n g o n h u m i d i ty . T h e r e i s c le a r ly m u c h n e e d f o r m o r e m e a s u r e -

m e n t s o f t h i s ty p e , a n d f o r t h e i r e x te n s i o n t o v a r i o u s p a r t s o f t h e w o r l d , b e f o r e t h e

c h e m i s t r y a n d d i s t r i b u t i o n o f t h o s e n u c l e i i s f u l l y u n d e r s t o o d . B e c a u s e o f t h e s iz e o f

s o m e e c o n o m i c a l l y i m p o r t a n t , n a t u r a l l y f o g - b o u n d a r e a s [5 1 ] t h o s e i n v e s t i g a ti o n s a l s o

a p p e a r t o b e o f m o r e t h a n a c a d e m i c in t er e st . F o r t h e s a m e r e a s o n t h e r e is a ls o m u c h

n e e d f o r f u r t h e r w o r k o n m e c h a n i s m s b y w h i c h A IT K EN n u c le i b e c o m e i n a c t iv a t e d ,

c o a g u l a t e [ 7] , o r d is a p p e a r b y r a i n o u t o r w a s h o u t [ 5 2 ]. M o s t a v a il a b le i n f o r m a t i o n

o n t h o s e p a r t i c u l a t e s is m e r e l y b a s e d o n m e a s u r e m e n t s o f t h e i r m o b i l i t ie s w h e n e l ec -

t r ic a l l y c h a r g e d . T h e u p p e r l i m i t o f th e i r d i a m e t e r r a n g e c o i n c id e s , in f a c t , w i t h t h e

l i m i t o f t h i s t e c h n i q u e .

b Large nuclei r a d i u s b e t w e e n 0 . 2 - 1 . 0 m i c r o n )

B e c a u s e o f r e la t iv e l y l o w w a t e r - v a p o r s a t u r a t i o n s i n t h e a i r a n d n e c e s s a r y fo r t h e i r

a c t i o n a s c e n t e rs o f c o n d e n s a t i o n , b e c a u s e o f t h e i r la r g e c o n c e n t r a t i o n s i n t h e a t m o -

s p h e r e a n d t h e i r lo w s e d i m e n t a t i o n r a te s [ 5 3 ] , t h o s e n u c l e i a r e k n o w n t o p l a y a n

i m p o r t a n t r o l e i n t h e p r o c e s s e s o f n a t u r a l c l o u d - d r o p l e t f o r m a t i o n . T h e r e e x i s t s a

v a r i e t y o f d e v i c e s a n d methods of collection w h e r e b y t h e p a r t i c l e s c a p t u r e d m a y b e

e x a m i n e d u n d e r h i g h - r e s o l u t i o n m i c r o s c o p e s , o r a t l e a st c o u n t e d . I m p a c t o r c o l l e c t o rs ,

s p i d e r - t h r e a d m e s h e s , c o n i m e t e r s , t h e r m a l p r e c i p i ta t o r s , s o r t i n g d e v i c e s c o n i fu g e s ) ,

f l a m e c o u n t e r s , m i l li p o r e fi lt e rs a n d o t h e r f ie l d t e c h n i q u e s d e v e l o p e d b e f o r e 1 9 5 7 a r e

e x t e n s iv e l y d e s c r i b e d a n d c o m m e n t e d o n b y M A S ON [ 8 ]. M o r e e l a b o r a t e i n s t r u m e n t s

b a s e d o n p r i n c ip l e s o f c e n t r i f u g a l [ 5 4 ] o r e l e c tr ic [ 1 3] s e p a r a t i o n , h a v e r e c e n t l y a l s o

b e e n d e v e l o p e d . A u t o m a t i c s y s te m s w h i c h o p e r a t e o n p r in c i pl e s o f a c o u s t ic c o u n t i n g

[55] o r l i gh t - sca t t e r i ng [56-57] a r e , li kewi se , ava i l ab l e . S ince t he l a rge c on de ns a t i o n

n u c l e i a re p l e n t i fu l a b o u t 102 x c m - 3 , d e p e n d i n g o n l o c a t i o n s a n d a l t it u d e s ) t h e

d e t e r m i n a t i o n o f t h e i r c h e m i c a l n a t u r e i s c o m p a r a t i v e l y e a s ie l t h e n i n t h e c a s e o f th e

m u c h s m a l l e r AIT KE N a e r o s o l w h i c h a l s o s o m e t i m e s s e e m s t o c o n t a i n s i m i l a r c h e m i c a l

ing red ients [7, 48].

Direct chemical analyses o f la rg e , h y g r o s c o p i c nu c l ei s h o w t h a t N H + j a n d S O 2

predomina t e i n t hose pa r t i c l e s [7 , 41 , 47 , 48 , 49 , 58 , 59] t oge the r wi th some C1-

[7 , 48 , 49 , 58 , 59], ph osp ha t e s [60] , sm a l l e r am ou nt s o f N a + , M g + + , C a + + , N O 3

[4 9, 6 0] , f l u o r i d e [ 6 1 ] a n d o t h e r t r a c e c o n s t i t u e n t s . S u l fa t e , c h l o r i d e a n d a m m o n i u m

c o m p o n e n t s a l s o c o m p a r e i n A n t a r c t i c - a t m o s p h e r e a e r o s o ls [ 62 ] t o g e t h e r w i t h e x c es se s

o v e r O H - o f h y d r o g e n c a t io n s w h i c h s ee m t o b e a s s o c ia t e d w i t h p r o d u c t s o f su l f u r

o x i d a t i o n [6 3]. D e s p i t e a g e n e r a l b e l i e f t h a t c h l o r i d e i s o f m a r i n e o r i g i n , t h e r e i s

e v i d e n c e t h a t s o m e o f i t m a y o r i g i n a t e f r o m i n d u s t r i a l a n d v e h i c u l a r e m i s s i o n o v e r -

l a n d [6 0], v i ce - v er s a, d e sp i te a n o f f h a n d t h o u g h t t h a t a m m o n i a a n d h e n c e t h e a m -

m o n i u m c o n s t i t u e n t o f th e a e r o s o l m a y a l m o s t e n t i re l y b e o f te r re s tr ia l n a t u r e , m u c h

of i t i s gen era t ed by t he ocean s [64].



Vol. 91, 197 1/VIII) C he m istry nd Propert ies of Atmospheric Nucleants 175

W i t h s o m e a n a l o g y t o A I T K EN n u c l e i th e l a rg e , h y g r o s c o p i c , p a r t i c u l a t e s a r e

b e l i e v e d t o g e n e r a t e overland f r o m s m o k e , p o l l u t i o n , a n d a l s o f r o m c o a g u l a t i o n o f

s m a l l e r a e r o s o l f o r m e d b y g a s e o u s r e a c t i o n s a n d p h o t o l y t i c p r o c e s s e s . ver the sea

t h e y a r e m o s t l y p r o d u c e d b y d i s r u p t i o n [8 , 6 5] o f b u b b l e f il m s o n t h e s u r f a c e . T h e i r

c o n c e n t r a t i o n in t h e a t m o s p h e r e h e n c e d e p e n d s o n t h e w i n d f o rc e , a s f o u n d b y

WOODCOCK ( see Re f . [ 8 ] ) . As p rev io us ly m ent io ned , t he oce an-g enera t ed nu c l e i a l so

i n c l u d e s o m e o r g a n i c m a t e r i a l [ 1 5 , 6 6] o r p r o d u c t s o f i ts p h o t o a l t e r a t i o n [ 1 5 , 6 7] a n d ,

l ik e w i se , s o m e s a l ts o f a m m o n i u m . S i n c e p h o t o l y t i c re a c t i o n s s e e m t o p l a y a l a r g e

p a r t i n t h e c h e m i c a l c h a n g e s o f t h o s e a i r b o r n e s o l id s a n d l iq u i d s , i t is n o t s u r p r i s in g

t h a t r e l a t iv e c o n c e n t r a t i o n s o f t h e c h e m i c a l i n g r e d i e n t s o f t h e s ea a e r o s o l d i f fe r f r o m

tho se i n sea -wa te r [7 , 68-73] . Th i s d i f f e r ence [74-75] a l so appear s t o de pe nd on t he

i o n r a d i i in v o l v e d a n d o n t h e v a le n c e a n d a t o m w e i g h t o f t h e c o m p o n e n t s [7 6].

Cons i s t en t , l a rge - sca l e p ro j ec t s on

analyses of chemicals dissolved in precipitation

w e r e c a r r i e d o u t b y JU N G E o v e r t h e U n i t e d S t a te s [ 7 ]. T h o s e r e s u l ts a r e s u p p l e m e n t e d

b y d a t a o f th e S c a n d i n a v i a n sa m p l i n g n e t w o r k [ 7 7] a n d b y d a t a o b t a i n e d f r o m t h e

N o r t h , W e s t e r n E u r o p e a n a r e a [ 7 8 ] . J UN G E S m a p s o f d i s t r i b u t i o n i l lu s t r a t e t h e i n -

f lu e n c e o f t h e s e a o n t h e b a l a n c e o f c h l o r in e a n d s o d i u m i n t h e a t m o s p h e r e . T h e y a l s o

s h o w t h a t t h e r e is a f a i rl y u n i f o r m d i s t r i b u t i o n o f p o t a s s i u m o v e r th e U S A , a n i n c r e a s e

o f c a lc i u m a n d s u l f a te w i t h th e d i s t a n c e i n l a n d , a n d t h a t t h e s u l f a t e i n t h e a i r i s p r o b a -

b l y o f a n t h r o p o g e n i c o r i g in . B o t h , n i t r a te a n d a m m o n i a , a r e a s s o c i a te d w i t h h i g h e r

l a t i t u d e s t h e r e , p o s s i b l y e i t h e r d u e t o d e n s i t i e s o f v e g e t a t i o n c o v e r a g e , a u r o r a l p r o -

d u c t i o n [ 4 3] , i n d u s t r ia l a c ti v it ie s , a n d t h e p h o t o l y t i c d e p le t io n o f a m m o n i u m n i t ra t e

a t l o w e r l a t i t u d e s [ 3 2 ]. I t is o f i n te r e s t t o n o t e a t t h i s p o i n t a n d w i t h r e g a r d t o t h i s la s t

s t a t e m e n t t h a t , a c c o r d i n g t o JU N GE S a n a l y s e s c a r r i e d o u t a t R o u n d H i l l a n d i n F l o r i d a

[7 ], o n l y v e r y sm a l l a m o u n t s o f n i tr a t e a p p e a r t o c o e x i s t w i t h a m m o n i u m i n t h e l a r g e

n u c l e i ; th i s s a m e l a c k o f c o e x i s t e n c e i s a l s o a p p a r e n t f r o m m o r e r e c e n t d a t a o b t a i n e d

b y E GG LE TO N [ 4 9 ]. I t t h u s a p p e a r s t h a t b e s id e s a n a t u r a l p h o t o l y t i c d e p l e t i o n o f a m -

m o n i u m n i t ra t e [ 3 2 , 7 9], t h e a m m o n i u m n i t r a t e - N O x s y s te m s s c a v en g e e a c h o t h e r

i r r e v e r s i b l y w i t h a c o n c u r r e n t d e f i x a t i o n o f b i o l o g i c a l l y a s s i m i l a b l e a t m o s p h e r i c

c o m p o u n d s o f n i tr o g e n [ 80 ], a n d t h r o u g h s te p s w h i c h i n v o lv e t h e s p o n t a n e o u s d e c o m -

p o s i t io n o f a m m o n i u m n i tr it e .

A n u m b e r o f u p t o d a t e c o n t r i b u t i o n s i s f u r t h e r i l l u s tr a t iv e o f t h e geographic

extension o f la r g e n u c l e i [ 5 8 , 8 1]. C o n t i n e n t a l a ir m a s s e s u s u a l l y c o n t a i n m o r e o f

t h o s e p a rt ic l e s t h a n o c e a n i c a t m o s p h e r e s d o a n d s iz e d i s t r i b u t i o n s d o n o t a p p e a r t o

v a r y g r e a t l y f r o m c o n t i n e n t t o c o n t i n e n t [ 82 ] . T h e l i fe t i m e o f c l o u d n u c l e i g e n e r a l l y

s e em s t o b e o f a b o u t t h re e d a y s ; t h is n o t w i t h s t a n d i n g th o s e p a r t i c u la t e s a r e k n o w n t o

r e a c h a r e a s w h i c h a re u n i n f l u e n c e d b y a i r p o l l u t i o n [ 83 ] a n d a r e r e m o t e f r o m n a t u r a l

an d a r t i fi c i a l sources o f dus t s , su l f a t e s , ha l ides , sod ium , a m m on ia e t c . [7 , 70 , 84-87] .

The annual fluctuations o f t h e w a t e r - s o l u b l e c o m p o n e n t o f t h e t e r r e s t r i a l a e r o s o l

s t r o n g l y d e p e n d s o n h u m a n a c t iv i ti e s a n d o n b i o l o g i c a l p r o c e s se s . T h u s , i n t h e U n i t e d

S t at e s [ 7 ] , N H 3 a n d N O ~ g e n e r al ly a t ta i n c o n c e n t r a t io n m a x i m a d u r i n g t h e s p r in g -

s u m m e r p e r i o d i n u n p o l l u t e d a r e a s a n d i n a c c o r d a n c e w i t h t h e c y c le o f so il b i o lo g y .



176 A.C. Montefinale,T. Montefinaleand H. M. Pap6e (Pageoph,

This situation is reversed in urban areas, because of combustion processes. Similar

considerations may be applied to SO2 and SO2; very high negative correlation

between annual SO 2 concentrations in the air and temperature were, in fact, recently

found by KNAU~R [42] in the atmosphere of a big city. Median diameters of sulfate

particulates, and their concentrations in urban areas, generally also depend on humid-

ity values there [88]; humidity also appears to favor the oxidation process of SOz

to SO3 [89-90] thus indicating that this reaction largely occurs in solution. Overall

concentrations of CI-, both in precipitation and in the air overland, are generally

higher in winter than in summer [78, 91] although it is yet not clear whether this is

predominantly due to increased circulation patterns or to processes of combustion.

Many references so far quoted in this paper also contain results on s ize d is tr ibu

t io n s a n d co n c en t ra t io n s of large nuclei in the air, as a function of altitude. According

to recent findings [92], a marked discontinuity occurs in the well known logarithmic

distribution of large condensation nuclei, with elevation. This is explained in terms

of rainout and washout phenomena. Distributions of those nuclei are known to

change with the geographic position (which determines the origin of their bulk) and

hence, also with the time of the year, the latitude [93] and meteorological factors.

Large nuclei, hence, normally consist of a mixture of chemical compounds. The

components of this mixture are either hygroscopic or are of a mineral, insoluble but

wettable nature, or both, with a possibility of organic contamination from emanations

of vegetation and other biological phenomena.

C h em ica l a n a lyses

and investigations

of the large nuclei were therefore carried out, unti l recently, with the aim of a quali-

tative straightforward assessment of the inorganic anionic and cationic groups which

compare in those particulates and little attention was often paid as to whether the

particles captured were wet or dry . Much of the so called atmospheric chemistry

also consisted, essentially, of rainwater analyses for those species. It appears to be

somewhat unfortunate that interest in the chemical composition of those nuclei took

so much time to arise. Before about 1950, and with a few exceptions only, most in-

vestigators were largely concerned with determinations of size spectra of those parti-

cles, their concentrations in the air, and with the relative humidities necessary for

their activation i.e. their growth to an instrumentally detectable size, by delique-

scence or adsorption of water films on the surface. Deductions, on the role of those

chemical species in cloud-nucleation, are complicated in tha t particles captured in the

vicinity of the earth s surface may be expected to have different relative compositions

[94] from the effective nuclei of cloud formation and that the analysis of precipita-

tion is, on the other hand, likewise not quite representative of the chemical nature of

cloud nuclei [95].

Advances in the use of formvar for making replicas of liquid and solid aerosols [96]

recently enabled the development of continuous particle samplers [97] suitable for

mounting on airplanes and hence for extensive investigations of their geographic

distributions. This, together with modern techniques in applications of electron micro-

scopy [48] enabled better evaluations of the physical aspects of large nuclei to be




carried out. It also enabled to differentiate between solid nucleating substances and

their solutions. Much progress was also done in the application of various methods of

analytic chemistry to the investigation of large aerosols [98] and, aside of conventional

wet and chromatographic methods [46] the use of modern standardized instrumental

techniques enables the once tedious microanalyses to be performed promptly and

efficiently. Selective electrodes, neutron activation, absorption spectroscopy, flame

and emission spectroscopy and fluorescence methods are now routinely used by quali-

fied chemists and may shortly supplant some other laborious semi-qualitative ways of

small-particle detection and evaluation such as, for example, the use of gelatins and

Liesegang ring formation thereon. Although this latter method is still applied [99] and

useful for adaptat ion in rocket-borne sensors [100], it is subject to systematic errors

due to both, traces of impurities invariably coexistent with the aerosol component

for which the gelatin has been designed and to temperature variations [101-102]. This

method is, therefore, to be used with utmost caution in cases where chemical composi-

tions of particles to be detected or analyzed are not exactly known and, hence, the

gelatins not suitably calibrated for the particular purpose.

Because of difficulties encountered in the past, in evaluating the chemical nature of

large nuclei, not much fundamental work seems to have been done on the

c h e m i c a l

r e a c t i o n s

of the inorganic particles in the atmosphere. A large exception consists of

the work of several investigators whose interest in air pollution has recently contri-

buted to the large part of the present knowledge on the reactions in which those nuclei

participate. Much of this work has already been quoted in this paper. More fundamen-

tal research includes the effect of moist atmospheres on sodium chloride particulates

[103] whereby OH - impurities appear to replace CI- , and the photochemistry of

halide ion solutions with the release of halide atoms and solvated electrons whose

capture (and subsequent reactions) depends on scavenger impurities [104]. On the

other hand, following a paper by ROBBINS

e t a l

[105], work from this laboratory has

indicated that natural oxidative photolysis may be expected to induce alterations in

nitrate-bearing aerosol and may lead to the destruction of its NH4NO 3 component

[32] and also to the release of hygroscopic oxides of nitrogen from nitrate-bearing

particulates. There is little doubt that photolysis of the NO; anion in solution also

contributes to the release of halides from sea aerosol. This type of chemical reaction

does not require extreme conditions which were proposed by some earlier workers

and were recently discussed by DtJc~ [106]. As previously mentioned, nitrate is

plentiful in the atmosphere and large natural deposits thereof (whose origin is still

under discussion) are known to exist in several parts of the world [107]. More work

on processes which involve sodium chloride particulates will be discussed in the next

section.

Particles of diameters corresponding to the size of large nuclei were first detected

in the

s t r a t o s p h e r e

by JUNGE [7] and the present knowledge about their chemical

composition stems from the work of FRIEND

e t a l

[108], MossoP [109-110] and

FRIEND [111] who found, at altitudes between 12-18 km, concentrations of about

12 PAGEOPI-I 9l t971 ]vnI )



178 A. C, Montefinale,T. Montefinaleand H. M. Papee (Pageoph,

3 x 104 particles x m- 3. This material largely consists of ammonium sulfate and per-

sulfate, and is of terrestrial origin. Since ammonium nitrate readily decomposes

under ultraviolet radiation and since co-dissolved sulfate is partly oxidized to per-

sulfate under those conditions [32] this part of the aerosol may primarily originate

from the inter-reaction and oxidative photolysis of NH~ and SOz - bearing solution

droplets of the Aitken type size [112] and from their subsequent coagulation into

larger particulates. It has previously been mentioned that a large part of the Aitken

nuclei consists of hygroscopic and deliquescent (NH4)2SO 4 which obviously initially

carried NH 3 and NOx contamination. It would, therefore, appear difficult to view an

in situ formation of those stratospheric compounds from anhydrous SO2 and NH a

[l 13] and the photolytic mechanism, previously mentioned, offhand appears more

plausible. More discussion on those stratospheric particulates is given by CADLE

6 1

A feature of interest in the behavior of both, soluble and insoluble

solid

aerosol

substance, is their property of

electrification du ring humidification a nd desiccation

cycles. This obviously influences the activity of those nuclei, since it enhances their

power of coalescence with cloud droplets [114]; it also contributes to the balance of

atmospheric electricity and ion formation, because of the electric charge ejection by

those nuclei. A large amount of pioneering work on this subject is due to MOHLEISEN

[115-1171; the effect was independently confirmed by other workers who used pure

chemicals [118]. It is possible that this effect partly determines the charge-separation

phenomena which accompany the contact of volcanic lava with the sea [119], with

subsequent condensation of electrically-charged droplets and their disruptive evapo-

rat ion [120]. Charging effects of a qualitatively similar nature, but occurring on inso-

luble porous particulates during the release of gases therein adsorbed were, also,

more recently observed [121-124]. It was possible to establish, in this latter case, a

relationship between the rates of desorption and rates of electrical charging of those

particulates. It appears to be of added interest that the sign depended in this case on

the chemical nature and acidity of the adsorbing substance investigated, with most

interesting implications towards the generation of atmospheric electricity.

The above summary deals with some aspects of the present knowledge on the

nature, distribution and chemistry of large condensation nuclei. It also shows that

many problems thereon yet remain to be solved and that the information on their

nature, distribution, origin and chemistry needs much supplementing. The role of

those particulates in everydays occurrences is, however, better assessed, because of

their importance in cloud-formation processes. Growing clouds absorb (and probably

generate) large amounts of those nuclei; they release them upon their dissipation or

they reduce their concentrat ion during precipitation [125]. Changes of cloud-nucleus

content o f the air primarily determine the cloud-droplet concentrations and stability

[1261. Also, the number of cloud droplets is now known to agree with the number of

large, airborne cloud nuclei [127]. Those observations certainty warrant a further

extensive experimental effort aimed at investigating and understanding the chemistry

of this aerosol.



Vol. 91, 197 1/V III) Ch em ist ry nd Properties of Atmospheric Nucleants 179

c T h e g ia n t co n d ensa t io n n u c le i ( r a d i u s > 1 m i c r o n ) .

O w i n g t o t h e i r l a r g e s iz e a n d f a s t s e d i m e n t a t i o n r a te s , c o n c e n t r a t i o n s o f th o s e

n u c le i , i n th e a t m o s p h e r e , a r e m u c h s m a l l e r t h a n t h o s e o f t h e l a r g e p a r t ic u l a t e s. T h e

u s u a l ra n g e s a r e f r o m 1 - 1 0 0 p a r t i c l e s p e r l i t e r o v e r t h e o c e a n s ( d e p e n d i n g o n s i ze ,

a l t i t u d e a n d w i n d f o r c e ) , to 1 0 2 -1 0 4 o v e r t h e c o n t i n e n t s [ 8] . T h e i r c o n c e n t r a t i o n s i n

t h e a i r a n d t h e i r c h e m i c a l c o m p o s i t i o n s a l s o d e p e n d , p r i m a r i l y , o n t h e g e o g r a p h i c

p o s i t i o n a n d o n t h e m e t e o r o l o g i c a l c o n d i t i o n s ; c o n c e n t r a t i o n s a r e k n o w n t o c h a n g e

a c c o r d i n g t o t h o s e p a r a m e t e r s a n d f r o m d a y t o d a y , b y u p t o 2 - 3 o r d e r s o f m a g n i t u d e .

A s f a r a s th e n a t u r e o f th o s e n u c l e i i s c o n c e r n e d c h a n c e s a r e , f o r e x a m p l e , t h a t a g i a n t

p a r t i c le m a y c o n s i s t o f s p o r e s o r p o l l e n a t a s u i ta b l e t i m e o f t h e y e a r a n d o v e r a r e a s

o f e x t e n s i v e v e g e t a t i o n . I t m a y j u s t a s w e l l b e s i li c a o r c l a y a d v e c t e d f r o m a d e s e r t , o r

a p a r t i c l e o f s e a - s a l t o v e r t h e o c e a n . I t m a y a l s o b e a p i e c e o f s o i l o r r o c k e i t h e r d r y o r

c o a t e d w i t h a h y g r o s c o p i c s u b s t a n c e ( N a C 1, N H 4 C 1 , H 2 S O 4 e tc .) . T h e t r a n s i t i o n

f r o m o n e c h e m i c a l ty p e o f n u c l ei to a n o t h e r m a y , i n a s e n se , b e c o n t i n u o u s , t h e r e l a -

t iv e c o m p o s i t i o n s b e i n g d e p e n d e n t o n t h e i n t e r m i x i n g o f t h e o r i g in a l c o m p o n e n t s a n d

h en ce o n th e en v i r o n m en t w h ich g en era t es th em [12 8] . T h u s ME S ZA R OS [12 9] g ives

s p a n s o f p H r a n g e s b e t w e e n 3 .5 a n d 9 u n i ts f o r p r e c i p i t a t i o n in B u d a p e s t , d e p e n d i n g

o n c o n c e n t r a t i o n s o f c a l c iu m t h e r e i n . T h o s e p H l im i t s m a y , o f c o u r s e , a l so b e e x -

p e c t e d t o v a r y w i t h t h e g e o g r a p h i c p o s i t i o n a n d t i m e , a c c o r d i n g t o t h e s o u r c e o f

a c i d i t y a n d c h a n g e s o f h u m a n a n d b i o l o g i c a l a c t iv i ti e s [ 13 0] . S i n c e s u c h l a r g e c h a n g e s

o f p H d o o c c u r i n p r e c i p i t a t i o n i t is o b v i o u s t h a t i n d i v id u a l n u c l e i, w h i c h o f t e n c o n s i s t

o f c o n c e n t r a t e d s o l u t i on s , w i ll b e s u b j e c t t o m u c h w i d e r f l u c t u a ti o n s o f t h e i r h y d r o g e n

i o n c o n c e n t r a t i o n [6 3]. A l s o , s i nc e a t m o s p h e r i c n u c l e i a re m a d e o f m i x t u r e s o f v a r i o u s

c h e m i c a l s u b s t a n c e s , t h e u s e o f i n s t r u m e n t a l a n a l y s e s ( gl a ss m i c r o e l e c t r o d e s ) i s h e r e

p r e f e r a b l e to t i t r a t i o n m e t h o d s f o r p u r p o s e s o f a c id i t y de t e rm i n a ti o ns ~ b e c a u s e o f

p o s s i b l e b u f f e r i n g i n f lu e n c e s o f v a r i o u s a e r o s o l c o n s t i t u e n t s [ 63 ].

A rec en t p ap er b y N E L S O N an d G O K H A L E [13 1] d esc r ib es a se r i e s o f sam p l in g s

t a k e n i n f li g ht o v e r t h e N e w Y o r k S t a te . T h e A u t h o r s o b t a i n e d c o u n t s w h i c h a v e r a g e d ,

b e l o w o n e k m o f fl ig h t a l t it u d e , i n c l e a r a i r a n d w i t h o u t t u r b u l e n c e , a b o u t o n e n u c l e u s

o f 2 0 - 5 0 m i c r o n d i a m e t e r s i ze , p e r l it e r. N o n u c le i o f d i a m e t e r s a b o v e 5 0 m i c r o n

w e r e d e t e ct e d . A v e r a g e c o r r e s p o n d i n g c o n c e n t r a t i o n s o n a m o d e r a t e l y d i s t u r b e d d a y ,

w e r e 1 .5 n u c l e i p e r l i t e r o f a ir , a n d t h e r e w e r e a l s o a b o u t 8 0 n u c l e i p e r c u b i c m e t e r , o f

d i a m e t e r s a b o v e 5 0 m i c r o n . T h o s e p a r t i c le s c o n s i s t e d , a p p a r e n t l y , o f s o il o r r o c k

f r a g m e n t s w h o s e s u r f a c e w a s p o s s i b ly c o a t e d a n d / o r r e a c t e d w i t h m a n - m a d e m a t e ri a ls .

N o s o d i u m c h l o r i d e w a s d e t e c t e d i n t h is c a s e. T h e n u c l ei w e r e o f a m i x e d t y p e , a s

s u b s t a n t i a t e d b y h y g r o s c o p i c p a t c h e s t h e r e o n ; w a t e r fi lm s h e n c e f o r m e d o n t h o s e

p a r t i c l e s a t r e l a t i v e h u m i d i t i e s o f 1 00 K . T w o r e c e n t p a p e r s b y R AM AN A M U R T Y

et a l

[ 1 32 - 13 3 ] a r e i l lu s t ra t i ve o f p r o p o r t i o n s : ( n o n h y g r o s c o p i c ) / ( h y g r o s c o p i c ) p a r t i c u l a -

t e s i n th i s t y p e o f n u c le i a n d i n t h e a r e a o f D e l h i . R e f e r e n c e [1 33 ] gi v es , i n p a r t i c u l a r ,

d e t a i ls o n n u c l e i c o l l e c t e d i n f li g h ts a t e l e v a t i o n s b e t w e e n 3 0 0 a n d 3 0 0 0 m e t e r s . T h e

A u t h o r s g e n e r a ll y f o u n d a v e r a g e s o f a b o u t 5 0 0 hygroscop ic p a r t i c l e s p e r c u b i c m e t e r ,



180 A .C . Montefinale, T. Montefinale and H. M. Pap6e (Pageoph,

w i t h t h e e x c e p t i o n o f t u r b u l e n t m o n s o o n p e r i o d s w h e n t h is n u m b e r w a s t h r i c e a s

m u c h . A t t h e s a m e t i m e , n o n h y g r o s c o p i c p a r t i c l e s w e r e a b o u t 5 • 1 0 4 • m - 3 i n s u m -

m e r a n d 2 x 104 x m - 3 b o t h d u r i n g w i n t e r a n d t h e m o n s o o n . C h l o r i d e a v e r a g e s w e r e

o f t h e o r d e r o f 7 0 -2 5 0 p e r c u b i c m e t e r , w i t h m a x i m u m v a l u e s d u r i n g t h e m o n s o o n

a n d a t 1 50 0 m e t e r s o f e l e v a t i o n ; o t h e r s o l u b l e p a r t i c l e s p r e v a l e n t l y c o n s i s t e d o f n i tr a -

t e s a n d s u l fa t e s . M i x e d p a r t i c l e s o f n o n m a r i n e a n d p o s s i b l y p a r t l y s t r a t o s p h e r i c

o r i gi n , w h o s e s o l u bl e p a r t c o n t a i n e d s u lf a te s , p e r s u lf a t e s, a m m o n i u m a n d H +

c o m p o n e n t s , w e r e a l s o re c e n t l y f o u n d i n t h e a i r o f A n t a r c t i c a [62 ]. F u r t h e r i n f o r m a -

t i o n o n t h e g e o g r a p h i c d i s t r i b u t io n o f th o s e g i a n t a e r o s o l s o f la n d o r i g in m a y b e

i n f e r r e d f r o m s o m e o f t h e r e f e r e n c e s p r e v io u s l y q u o t e d i n t h i s p a p e r , e s p e c ia l ly f r o m

t h e w o r k o f J tllq G E [7 ] a n d h i s m a p s o f d i s t r i b u t i o n o f a t m o s p h e r i c c h e m i c a l s i n p r e c i -

p i t a t i o n . G e n e r a l l y s p e a k i n g , i t s ti ll , u n f o r t u n a t e l y , a p p e a r s t o b e f a s h i o n a b l e t o

m e a s u r e c o n c e n t r a t i o n s o f t h o s e n u c le i a n d t o s p e a k a b o u t t h e i r a c t i v it y w i t h o u t

p a y i n g m u c h a t t e n t i o n t o t h e i r c h e m i c a l n a t u r e a n d , h e n c e , t o t h e i r p o t e n t i a l a c t i o n

a n d r e a c t i o n s a l o f t . I t i s , i n d e e d , a l s o s u r p r i s i n g t o s e e h o w l i tt l e f u n d a m e n t a l w o r k

o n c o m p o s i t i o n s a n d r e a c t i o n s o f c o m p o u n d e d c h e m i c a l sy s te m s , s u c h a s m a y b e

e x p e c t e d t o o c c u r i n t h o s e n u c l e i, w a s d o n e i n t h e p a s t a n d i s b e i n g d o n e a t p r e s e n t ,

u n d e r c o n t r o l l e d l a b o r a t o r y c o n d i t i o n s .

I n a s m u c h t h e c h l o ri d e c o m p o n e n t o f t h e l a r g e n u c l e i m a y p a r t l y o r i g i n a t e o v e r -

l a n d , t h e g i a n t a e r o s o l s w h i c h w e r e c o l l e c t e d o v e r t h e o c e a n s o r in th e i r v i c in i ty , l a rg e ly

c o n s i s t o f s e a- sa lt . A l t h o u g h t h e y a l s o c o n t a i n s o m e m a t e r i a l w h i c h o f f h a n d m i g h t n o t

b e t h o u g h t t o b e a s s o c i a t e d w i t h t h i s s a l t [ 1 3 4 - 1 3 5 ] , t h e i r c o m p o s i t i o n a p p e a r s t o b e

l es s c o m p l e x a n d v a r i a b le t h a n t h a t o f th e g i a n t n u c l e i o f l a n d o r ig i n. T h e g i a n t m a r i n e

n u c l e i a r e p r o d u c e d , a c c o r d i n g t o W O OD CO CK a n d A s s o c i a t e s [ 7, 8 ], o v e r t h e o c e a n s

b y t h e a c t i o n o f a i r b u b b l e s e n t r a i n e d b y w h i t e c a p s ; t h o s e b u b b l e s , u p o n b u r s ti n g ,

p r o d u c e R a y l e i g h j e t s o f s e a w a t e r a n d h e n c e p r o j e c t t i n y d r o p l e t s i n t o t h e a i r [6 5,

1 36 , 1 37 ]. A s i m i l a r p r o c e s s o c c u r s , o n a v e r y la r g e s c a l e, w h e n m o l t e n l a v a e n c o u n t e r s

s e a - w a t e r [ 13 8] . A s s h o w n b y WOODCOCK h e c o n c e n t r a t i o n o f th o s e p a r t i c le s i n t h e

a i r i s t h u s a l s o d e p e n d e n t o n t h e a l t i t u d e a n d t h e f o r c e o f t h e w i n d w h i c h c a r r i e s t h e

e jec t ed d ro p le t s a lo f t [7 , 8 ] .

A l a r ge a m o u n t o f o u r k n o w l e d g e a b o u t th e g e o g r a p h i c d i s t r i b u ti o n o f m a r i n e

a e r o s o l s o v e r t h e a r e a o f th e U n i t e d S t a t e s , d e r iv e s f r o m t h e w o r k o f J U NG E [ 7].

E u r o p e a n d i s t r i b u t i o n s a r e b a s e d o n d a t a g a t h e r e d b y t h e S c a n d i n a v i a n n e t w o r k a n d

t h e s t a t io n s o f N o r t h - W e s t e r n E u r o p e [7 8]. D i s t r i b u t i o n s o n a g l o b a l s c a l e a n d p r o -

d u c t i o n r a t e s o f t h o s e p a r t ic l e s o v e r t h e o c e a n s w e r e r e c e n t ly e v a l u a t e d b y T O B A

[ 1 3 9 -1 4 0 ] . T h o s e r a t e s a r e f o u n d t o c h a n g e w i t h th e s e a s o n s o f th e y e a r s s in c e t h e y

d e p e n d p r i m a r i l y o n t h e d i s t r ib u t i o n s o f s u r f a c e w in d s a n d a r e a l s o i n fl u e n ce d b y

h u m i d i t i e s o v e r t h e s u r f a c e o f t h e s e a . E s t i m a t e s t h u s d e p e n d o n l o c a l c o n d i t i o n s , t h e

u s u a l o r d e r s o f m a g n i t u d e b e i n g b e t w e e n 1 0 - 2 a n d 1 0~ p a r t i c l e s • cm -2 • sec -1 .

M u c h o f th e w o r k d o n e o n t h is s u b j e c t w a s s u m m a r i z e d b y JtlN G E [7 ] a n d f u r t h e r

d e v e l o p e d b y T O B A [ 1 4 1 -1 4 2 ] . I t i s, o f c o u r s e , a l s o t o b e e x p e c t e d t h a t v e r t i c a l d i s tr i -

b u t i o n s w i ll d e p e n d o n s u r f a c e w i n d s a n d h u m i d it ie s . P i o n e e r i n g c o n t r i b u t i o n s o n



Vol. 91, 197 1/VIII) Ch em istry nd Propert ies of Atmospheric Nucleants 181

t h i s s u b j e c t a re d u e t o W O OD CO CK , L O DG E a n d o t h e r s , a n d a r e s u m m a r i z e d i n R e f e r -

ences [7 , 8 ]. Th e gen era l l i nes o f t hos e l oga r i t hm ic d i s t r i bu t i ons were a l so r ecen t ly

d i s c u s s e d b y T O BA [1 41 ]; t h e p a r t i c l e n u m b e r s a r e f o u n d t o d e c r e a s e w i t h a l t i tu d e , t h e

p o p u l a t i o n o f

smaller

p a r ti c le s b e i n g p r o b a b l y r e d u c e d b y c o a g u l a t i o n a n d w a s h o u t ,

a n d t h a t o f t h e la r g e r o n e s b y r a i n o u t , w a s h o u t a n d s e d i m e n t a t io n . A l t h o u g h s e a s o n a l

e f fe c ts a n d l o c a l m e t e o r o l o g i c a l a n d c i r c u l a t i o n c o n d i t i o n s a r e d e t e r m i n i n g f a c t o r s i n

t h is c a se , d e c re a se s o f a b o u t a n o r d e r o f m a g n i t u d e m a y g e n e r a l ly b e e x p e c te d t o

o c c u r b e t w e e n t h o s e c o n c e n t r a t io n s w i t h in t h e f i rs t t e n m e t e rs f r o m t h e s u r fa c e o f th e

s e a a n d t h e e le v a t io n o f a b o u t 3 k i lo m e t e r s. I n t e r m s o f m a g n i t u d e o r d e rs t h e s a m e

a p p r o x i m a t e d i f fe r e n c e p a s se s , u p t o 3 k m o f e l e v a t io n , b e t w e e n c a t e g o r i e s o f p a r t i c le s

of d i am ete r s >__ 10 tl , 6 -10 g , 3 -6 Ix, an d be low 3 g w here ave rag e con cen t r a t i on s , a t

sea l eve l, a r e ro ug hly o f abo ut 5 x 10 z , 104 , 105 an d 106 pe r cu b i c m e te r , r e spec t i ve ly .

C u r r e n t t e c h n i q u e s a p p l ie d t o t h e

detection and capture

o f g i a n t n u c l e i w e r e a l s o

e x t e n s iv e l y r e v i ew e d b y M A S O N ; t h e y r a n g e f r o m a s t r a i g h t f o r w a r d u s e o f s u i t a b ly -

c o a t e d g l a s s s l i d e s [ 8 , 9 6 ] , t o k o n i m e t e r s , c a s c a d e i m p a c t o r s a n d a u t o m a t i c f l a m e

c o u n t e r s . M o r e e l a b o r a t e i n s t r u m e n t s [ 5 4 , 9 7] a r e a ls o a v a i la b l e . A r e c e n t p a p e r b y

LEE an d PATTERSON [60] show s tha t t he use o f ap pro pr i a t e im pac tor s [143] no w p erm i t s

b o t h s iz e d i s tr i b u t i o n s a n d c h e m i c a l a n a l y s e s o f g i a n t a e r o s o l s to b e c a r r ie d o u t w i t h

p r e c i si o n , o n a r o u t i n e b a s i s. T h e t e c h n i q u e s q u o t e d a b o v e a r e g e n e r a l l y a d a p t e d , b y

w o r k e r s i n v o l v e d , t o t h e p a r t i c u l a r p r e - re q u i s it e s o f th e i r p r o j e c ts a n d t o t h e i r e x p e r i-

m e n t a l c o n d i t i o n s [ 14 4 ]. C h e m i c a l a n a ly s is is s u b s e q u e n t l y p e r f o r m e d b y a d a p t i n g a

s u i ta b l e t e c h n i q u e a c c o r d i n g t o c i r c u m s t a n c e s ; t h o s e m e t h o d s w e r e a l r e a d y p re v i o u s ly

m e n t i o n e d u n d e r l a rg e n u c l e i .

T h e a b u n d a n c e o f m a r i n e s a lt a e ro s o ls , t h e a p p a r e n t s im p l i c it y o f t h e ir c h e m i c a l

n a t u r e a t t h e p o i n t o f o r ig i n , a n d t h e v a s t a m o u n t o f w o r k c a r r ie d o u t o n a l k a li n e

h a l i d e s , b y w o r k e r s i n s e v e ra l d is c i p li n e s o f s c ie n c e, e n a b l e s t o m a k e a n u m b e r o f

c o n s i d e r a ti o n s o n t h e b e h a v i o u r o f th o s e s u b s t a n ce s a n d o n r e l a te d m e c h a n i s m s o f

w a t e r n u c l e a t i o n t h e re b y . I t o f f h a n d a p p e a rs t o b e o b v i o u s t h a t m a r i n e a e r o so l ,

w h i c h e s s e n t ia l ly c o n s i s t s o f v e r y im p u r e s o d i u m c h l o r i d e , w il l a c t h e r e d i f f e r e n t l y

t h a n c h e m i c a l ly p u r e N a C1 . T h i s h a s r e c e n t l y a l so b e e n c o m m e n t e d o n b y P UESCH EL

et al

[ 1 45 ] w h o s e r e s e a r c h e m p h a s i z e s d i f fe r e n c e s i n r a t e s o f d r o p l e t g r o w t h o n n u c l e i

o f p u r e N a C 1 a n d m a r i n e s a lt , a t v a r i o u s v a l u e s o f r e l a ti v e h u m i d i t y . T h i s r e s e a r c h i s

i n d i c a t iv e o f c o n d i t i o n s w h i c h i n v o l v e r e l a ti v e l y f r e s h s e a - s p r a y n u c le i w h e r e t h e r o l e

o f o r g a n i c f il m s a d s o r b e d m a y b e s u b s t a n t ia l s in c e o r g a n i c im p u r i t ie s a r e k n o w n t o

a f f e c t b o t h , g r o w t h a n d e v a p o r a t i o n o f d r o p l e t s [1 46 -1 48 ]. T h e s i t u a t i o n b e c o m e s ,

h o w e v e r , m u c h m o r e c o m p l e x a n d i n te r e s ti n g w h e n p o s s i b le e n v i r o n m e n t a l ef fe c ts o n

m a r i n e p a r ti c le s , w h i c h fi n d th e m s e l v e s a t h i g h e r a l t i tu d e s a n d f o r p r o l o n g e d p e r i o d s

o f ti m e , a r e c o n s i d e r e d . T h e s u r f a c e o f t h o s e solid p a r t ic l e s m a y i n f a c t b e e x p e c t e d t o

a s s u m e , u n d e r t h e s e c i r c u m s t a n c e s , c o m p l e t e l y d i f f e r e n t p r o p e r t i e s f r o m t h o s e o f t h e

o r i g i n a l p r o d u c t , b e i t r e a g e n t - p u r i t y N a C 1 [ 1 0 3 ] o r s e a - s a l t . S o m e w o r k o n s u c h

c h a n g e s r e l a t e d t o pure s o d i u m c h l o r i d e , d a t e s b a c k t o s e v e ra l y e a r s a g o [ 1 4 9- 1 5 1 ];

N a C 1 p a r t i c u l a t e s w h i c h w e r e t re a t e d w i t h p l a s m a o r in e l ec t ri c a l d i s c h a rg e s , a r e k n o w n




to eject photoelectrons upon illumination with visible light [152] and since they thus

acquire, during their fall in the air, a conspicuous electric charge which favors coales-

cence, they were used in tests on warm-cloud dissipation [153-154]. The emission of

exo or photo-electrons from solid surfaces which were suitably suitably pre-stressed,

is today widely known to occur on a number of materials (cf. [153] for further ref-

erences and reviews on the subject). Those include appropriately pre-irradiated

sodium chloride [155-156] and they may also include cases of pre-stressing [157] as

may occur in desiccation processes. More of the earlier References on this subject were

pooled by one of us in 1962 [158]. Even the chemically pure chloride of sodium is now

known to be affected by ultraviolet light [159] and also to be

tribochemically

affected

by mechanical friction in air, with generation of nitrates [160]. Aside, therefore, of the

impurities which a sodium chloride particle of marine origin initially contains when

it is ejected from the sea, and besides the contamination which it may subsequently

acquire in the air, it can hardly be expected that a

solid

nucleus of marine salt will

remain aloft unaltered for long periods of time. Considerations of this nature are also

thought to be of help in understanding the M~HLEISEN effect [115-119]; the cycles of

hydration and dehydration mentioned, if accompanied by electric effects, are of

added interest in that they could lead to a multiplication of nuclei by the evaporative

ejection of small droplets from an electrically-charged, deliquescent particulate [120],

with the consequent formation of smaller nuclei.

liquid

particle of marine aerosol may also be expected to undergo chemical

change during its residence time in the air [104]. Some of those changes may lead to

the separation of chlorine from the drops. The more concentrated the solutions in

those drops, the more this separation would offhand appear to be favored. Various

hypotheses on the possible chemical aspects of this change were recently summarized

by DUCE [106]. It is known, however [7], that giant marine nuclei contain substantial

amounts o f nitrate. This is found to decompose under ultraviolet radiation through a

process which involves the formation of highly-oxidizing transients [161-163] capable

at suitable conditions, of liberating halogens from their solutions. Rates of liberation

will here obviously be I>B r>C1 ; the process of nitrate-photolysis has also been

found to proceed under natural sunlight illumination [164-165] and its kinetics to

depend on co-dissolved inorganic substance [166, 29]. Ammonium nitrate is also

affected, with the consequent defixation of biologically assimilable nitrogen [32]; since

the process is enhanced by chlorides this may explain the absence o f ammonium from

JUN6E S giant marine aerosol at Round Hill and in Florida [7]. Being this photolytic

reaction of a highly oxidizing nature, processes of consequent nitration and chlorina-

tion of organic matter contained in those systems are also to be expected to occur and

it is hoped that this discussion will stimulate more work along the line.

The few giant nuclei that normally [167-168] compare

in the stratosphere

are,

probably, of extraterrestrial origin, as substantiated by their high velocities of sedi-

mentation. Most available information about their chemical nature is, again, given

by JUN6E [7] and Associates. Elaborate methods of X-ray spectroscopy and fluore-




scence were used for the analysis of those particulates which were found to contain,

primarily, Si, Ca, Fe, A1 and K, with minor quantities of other elements. As previously

mentioned in this paper, most of the stratospheric aerosol consists, however, of large

hygroscopic ammonium sulfate and persulfate nuclei, hence the existence of

m i x e d

particulates is also possible in this case.

d. The role o f condensation nuclei in nature

Although the numerous Ai tken nuc le i provide excellent centers of water vapor

condensation in instruments designed for their study, it is now believed tha t the

large

nuclei

which require lesser saturations of water vapor for their activation , are largely

responsible for the formation of cloud droplets [9, 10]. A close agreement between

numbers of cloud droplets in a cloud, and of those nuclei outside the cloud and per

unit volume was, in fact, recently observed by TWOMEY and WARNER [127]. Those

large, mostly hygroscopic, particles are thus thought to be of primary importance in

processes o f natural cloud formation while the

giant pa r t ic les

appear to play a large

role in the precipitation of warm (above 0~ clouds, at otherwise suitable circum-

stances. It is generally agreed that the warm-cloud type of precipitation occurs through

coalescence processes which normally require droplets o f diameters larger than about

20 micron for an efficient capture of smaller droplets [1, 10]. Hence the practical

importance, in the atmosphere, of the giant nuclei on which those droplets grow. In

this area of research classic work includes, therefore, computations by B~ST [169] and

KEITH and ARONS [170] which deal with the growth of drops of salt solutions, and the

pioneering work of WOODCOCK and Associates [171-173] and of TURNER [174] on the

role of sea salt in precipitation. The importance of giant nuclei and, consequently, of

the large drops in precipitation mechanisms from warm clouds, has been demonstrated

in a series of contributions by BOWEN and by LUDLAM[1, 8] which thus integrated the

well known chain reaction mechanism of warm rain formation, proposed by LANG-

MUm [175], whereby an individual large drop which falls through a cloud, upon

reaching a radius of a few mm by coagulation and coalescence, disintegrates into

smaller drops which, in turn, behave likewise and so on. It is evident that in the case

of electrically-charged drops (or nuclei), the coalescence mechanism will be greatly

enhanced, as briefly reviewed by

PRUVVACHER

[176] and experimentally confirmed,

during the last decade, by a number o f other workers [177-184]. Collision efficiencies

between those droplets are found to increase by a few thousand percent, according to

diameters, electric fields and charges involved. It would thus appear tha t the use of

giant, electrically charged, nuclei, could lead to successful experiments on the modifi-

cation of warm clouds, and it could also be expected to yield encouraging results in

projects on hail suppression, since quantities relatively small (about one particle per

liter of air), in comparison to the possibilities of modern technology, here appear to

be needed [185] for a partial suppression of damage on the soil.

There is yet another effect to consider. A number of recent contributions is in-



184 A .C . Montefinale, T. Montefinale and H. M. Pap6e (Pageoph,

d i c a ti v e o f l o w e r i n g s o f w a t e r

supercooling

b y

small

a m o u n t s o f s o m e d is s o lv e d

e l e c t r o l y t e s [ 1 8 6 - t 8 7 ] . A l t h o u g h s o m e p r e v i o u s w o r k ( s e e R e f . [ 1 8 6 ] ) i n d i c a t e s t h a t

concentrated s o l u t i o n s b e h a v e d i f f e r e n tl y , t h i s e f f e c t, i f r e a l, c o u l d h a v e i n f l u e n c e o n

o v e r a l l g l a c i a ti o n p r o c e s se s i n s u p e r c o o l e d c l o u d s w h i c h c o n t a i n l a rg e a m o u n t s o f

i n o r g a n i c s a lt s, a n d b e e x p e c t e d t o a p p l y i n a r e a s o f h i g h i n o r g a n i c a e r o s o l c o n c e n -

t r a t i o n . A l s o , s in c e s m a ll a m o u n t s o f s o m e d i s s o l ve d s a lt s p r o v o k e i n c r e a s e s o f t h e

r a t e s o f d e n d r i t i c ic e - g r o w t h [ 18 8] , f o r m a t i o n o f s p o n g y [ 18 9] - a n d h e n c e l e ss d a m a g -

i n g h a il - c o u l d h e r e a l s o p o s s i b l y b e f a v o r e d .

2 The nuclei of ice formation

S i n ce p u r e l i q u i d w a t e r m a y u n d e r c o o l u p t o a b o u t - 4 0 ~ ( s o m e t i m e s r e f e r r e d t o

as S ClJA EF ER S p o in t [9] ), an d s in ce w a te r co n ta in e d in a c lo u d o f t en r ea ch e s t em p er a -

t u r e s o f - 1 0 ~ a n d s o m e t i m e s o f - 3 5 ~ [ 8], l a r g e a m o u n t s o f e n e r g y m a y b e e x p e c t e d

t o b e r e l ea s e d f r o m t h o s e s u p e r c o o l e d c l o u d s d u r i n g t h e i r i n c i p ie n t r e t u r n t o e q u i li -

b r i u m a n d a c c o r d i n g t o t h e i r s iz e. T h e h e a t o f f u s io n w h i c h i s re l e a s e d u p o n g l a c i a ti o n ,

f r o m t h o s e h y d r o m e t e o r s , i n c r e a se s t h e b u o y a n c y o f t h e g l a c ia t in g c l o u d m a s s a n d i t

h e n c e i n c r e a s e s i t s v e r t i c a l d e v e l o p m e n t ; g l a c i a t i o n , o n c e i n i t i a t e d , i s f a v o r e d b y t h e

t e m p e r a t u r e d e p e n d e n t d i f f er e n c es b e t w e e n w a t e r v a p o r p r e s s u r e s o n i c e a n d o n

s u p e r c o o l e d w a t e r, a n d t h e t r a n s f e r o f w a t e r o n t o t h e i c e p h a s e s e ts p r a c t ic a l l y in a s

s o o n a s g l a c i a t i o n b e g in s w i t h i n t h e c l o u d . T h i s is f o l lo w e d b y a f a s t g r o w t h o f t h e i c e

e m b r y o s i n t o s n o w c r y s ta l s , b y t h e s i ze i n c r e a s e o f th e l a t t e r a n d , c o n s e q u e n t l y , b y

t h e i n c r e a s e o f t h e i r v e lo c i ti e s o f s e d i m e n t a t i o n , t h e i r a c c r e t i o n w i t h w a t e r d r o p l e t s

a n d t h e i r u l ti m a t e m e l t i n g a n d f a l l o u t a s l i q u i d , a t s u it a b l e t e m p e r a t u r e s . T h e p r o c e s s ,

w h ich i s w id e ly k n o w n as th e B ZR GZ RO Nm e c h a n i s m o f p r e c i p i t a t i o n [ 19 0, 8 ] o b v i o u s l y

t h e r e f o r e r e q u ir e s t h e p r e s e n c e o f i ce p a r ti c u l a te s w i t h i n t h e c l o u d a n d , a s t h e h o m o -

g e n e o u s t r a n s i t i o n o f p h a s e n e c e s s i t a te s c o n s i d e r a b l e u n d e r c o o l i n g s [ 8, 1 0] a la r g e

e f f o r t w a s u n d e r s t a n d a b l y d i r e c t e d , s i nc e 19 46 , t o w a r d s t h e f i n d i n g o f s u i t a b l e

a r ti fi c ia l i n d u c t o r s o f t h i s p h a s e t r a n s it i o n , a n d t o w a r d s t h e i n d i v i d u a t i o n o f t h e

n a t u r a l o n e s .

T h e b e s t i c e n u c l e a n t s a v a i l a b l e in n a t u r e a r e t h e i c e c r y s ta l s t h e m s e l v e s ( o f w h i c h ,

f o r e x a m p l e , c i rr u s c l o u d s a r e m a d e ) . T h e y o b v i o u s l y d o n o t r e q u i r e l a g t i m e s f o r

t h e i r a c t i v a t i o n [ 1 9 1 -1 9 2 ] s i n c e t h e i n i t ia l i c e - e m b r y o i s a l r e a d y f o r m e d . A m e t h o d

o f p r o d u c i n g l a r g e a m o u n t s o f t h e s e n u c l e i i n a s u p e r c o o l e d c l o u d , b y s ti ll l a r g e r ,

lo ca l , u n d e rco o l in g s , w as f i r s t d ev e lo p ed a n d t e s t ed b y S CH AZ FZ R [19 3] a n d i s s ti ll

w i d e ly u s e d t o d a y f o r c l o u d - s e e d in g p u r p o se s . T h e m e t h o d c o n s is t s in d r o p p i n g s o li d

C O 2 i n t o t h o s e c l o u d s , w h e r e b y e n o r m o u s a m o u n t s o f ic e c r y s ta l s a r e f o r m e d i n t h e

w a k e o f t h e f a l l in g d r y - i c e p e l le t s . Q u a l i t a t i v e l y s i m i l a r r e s u lt s w e r e a l s o , m o r e r e c e n t -

l y , a c h i e v e d b y m e a n s o f s p r a y i n g o f s o m e h i g h l y - v o l a t i l e l i q u id s [ 19 4] a n d t h e s u b -

s e q u e n t p r o d u c t i o n o f v e ry l o w t e m p e r a t u r e s , u p o n t h e i r e v a p o r a t i o n .

E a r l i e r i d eas w h ich f i r s t led V ON N EG U T o th e d i sco v ery o f s i lv e r - io d id e as a p ra c -

t ic a l , i c e - f o r m i n g c o m p o u n d [1 95 ], w e r e b a s e d o n t h e p r in c i p l e

ofepitaxy

( l a tt i c e p a r a -



Vol. 91, 197 1/V III) C he m istr y nd Properties of Atmospheric Nucleants 185

m e t e r s i m i la r it y ) b e t w e e n ic e a n d t h e n u c l e a t o r . T h u s a r o s e t h e c o n c e p t o f sub l imat i on

( h e r e t o b e r e g a r d e d a s th e o p p o s i t e o f w h a t t h i s t e r m u s u a l l y m e a n s ) w h i c h im p l i e d a

d i r e c t tr a n s f e r o f w a t e r v a p o r o n t o t h e s o l id , e p i ta x i a ll y f a v o r e d , n u c l e a t o r s u b s ta n c e .

T h i s a t t r a c ti v e c r it e r i o n n o w a p p e a r s t o b e b u t o n e o f t h e m a n y m e c h a n i s m s b y w h i c h

n u c l e a t i o n m a y t a k e p l a c e a n d t h e e a r l i e r w i d e s p r e a d a c c e p t a n c e o f it s e x c lu s i v it y

s e em s t o h a v e d e l a y e d t h e p r o g r e s s o f t h is b r a n c h o f s c ie n c e f o r s o m e y e a rs . O n t h e

o t h e r h a n d , t h e c o n c e p t o f f r e e z i n g ( a n e a r li e r - p o s tu l a t e d m e c h a n i s m i n v o l v i n g t h e

s p o n t a n e o u s f r e e z i n g o f m i n u t e d r o p s ) w a s s u b s e q u e n t l y a ls o f o u n d t o b e i n c o m p l e t e .

M o r e r e c e n t r e s e a r c h [1 9 6 -1 9 8] e m p h a s iz e s t h e p r e d o m i n a n c e o f t h e c o n t a c t n u cl e a -

t i o n m e c h a n i s m ( m e c h a n i c a l c o n t a c t b e t w e e n a s u p e r c o o l e d d r o p a n d i ce , o r a s u it a -

b l e c a t a l y z i n g s u b s t a n c e ) a n d n u c l e a t i n g m a t e r i a l s a r e h e n c e n o w m o r e b r o a d l y

t e r m e d i c e - f o r m i n g n u c l e i .

T h e m e c h a n i s m s a t w h i c h a s u b s t a n c e m a y n u c l e a t e i ce a t a g iv e n te m p e r a t u r e

d e p e n d o n a v a r i e t y o f p h y s i c a l f a c t o r s a n d t h e y a l s o d e p e n d o n t h e s t r u c t u r e a n d

s h a p e o f t h e n u c l e a t o r . A b r i e f r e v i e w b y S ~N G E R [1 99 ] p r o v i d e s a n e a r l y d i s c u s s i o n

a n d i t a l s o r a is e s t h e p o i n t t h a t t h e s u r f a c e o f a n e f f i c ie n t , p u r e , so l i d n u c l e a t o r s h o u l d

p r i m a r i l y b e i n s o l u b l e . T h i s s u r f a c e ( a n d a l s o t h e w a t e r t o b e n u c l e a t e d ) s h o u l d ,

m o r e o v e r , b e u n c o n t a m i n a t e d b y s p e c if ic p o i s o n s [ 2 00 - 20 1 ], b o t h o f t h e a b o v e

p r e r e q u i s i t e s b e f i t t i n g a n y c a t a l y z i n g s o l i d s u r f a c e w h i c h i s u s e d i n c h e m i c a l

p r o c e s s e s .

I c e - n u c l e a t i n g p r o p e r t i e s o f a s o l id h e n c e a l s o d e p e n d o n i t s s u r f a c e a r e a [ 2 0 2 - 20 3 ]

a n d s h a p e [2 04 ], s i n c e t h e a m o u n t o f a c t i v e s i te s o n a n u c l e u s m a y b e e x p e c t e d t o b e a

f u n c t i o n o f th o s e p a r a m e t e r s [2 05 ]. O n t h e o t h e r h a n d , r e c e n t i n v es t ig a t i on s o n t h e

f r e e z in g o f s u p e r c o o l e d w a t e r i n c l e a n a n d r e l a ti v e l y large g lass cap i l l a r i e s ap p ear s to

s h o w t h e a b s e n c e o f c a p i l l a r y ef f e c ts alone o n t h e t e m p e r a t u r e o f p h a s e t r a n s i t i o n [2 06 ].

T h i s n o t w i t h s t a n d i n g , c a p i l l a r y e f fe c t s s e e m t o d e t e r m i n e t h e s o - c a l le d

pre-activation

t o w a r d s i c e - n u c l e a t i o n , w h i c h s o m e p a r t i c u l a t e s p o s s e s s a n d w h i c h , e s s e n t i a ll y , c o n -

s is ts o f in c r e a s in g t e m p e r a t u r e t h r e s h o l d s o f i c e - f o r m a t i o n a f t e r a p a r t i c l e h a s a l r e a d y

a c t e d a s a n i c e - n u c le u s a n d , n o r m a l l y , in t h e a b s e n c e o f in t e r m e d i a t e t h a w i n g [ 2 0 7 -

2 0 8] . T h e r e is a l s o e v i d e n c e , h o w e v e r , t h a t i n s o m e c a s es m e m o r y e f f e c ts m a y b e

e x p e c t e d t o p e r s i s t e v e n a f t e r t h e n u c l e i w e r e h e a t e d u p t o a s m u c h a s 6 . 5 ~ [ 20 9] .

M o r e r e c e n t w o r k p o i n t s , a t t h e s a m e t i m e , t o t h e i m p o r t a n c e o f c r ys t a ll i ne m o n o -

l a y e r f o r m s o f w a t e r a d s o r b e d o n s u i t a b le s u b s t r a t e s [ 2 10 - 21 1 ] a s a n e x p l a n a t i o n o f

t h o s e m e m o r y e f f e c t s .

N e c e s s a r y p r e r e q u i s it e s f o r t h e i n i t i a t io n o f t h e p h a s e t r a n s i t io n o f w a t e r , o n a

s u i ta b l e s o l id , w e r e a l s o f o u n d t o i n c l u d e t h e m o l e c u l a r s t r u c t u r e o f th e n u c l e a t o r

m a t e r i a l [ 2 1 2 ] a n d t h e s u r f a c e d i s l o c a t i o n s c o n t a i n e d o n i t s s u r f a c e ( [ 2 1 3 ] a n d B . J .

M A s oN , a s d e s c r i b e d i n R e f . [ 21 4] ). T h o s e a r e w e l l k n o w n t o p r o m o t e c h e m i c a l r e a c -

t i v i ty [ 2 1 5 - 2 17 ] . A d d e d t o t h i s a r e t h e e f f e ct s o f h y d r o p h i l i c i m p u r i t y s i te s o n e s s e n -

t ia l ly h y d r o p h o b i c s u b s ta n c e s [21 4, 2 1 8 -2 1 9 ] w i t h c o n s e q u e n t f o r m a t i o n o f l a r ge

w a t e r c l u s te r s a n d t h e i r s u b s e q u e n t f r e e z i n g a t s u i ta b l e c o n d i t i o n s [ 2 2 0 -2 2 1 ] . A n o t h e r

f a c t o r i s e le c t r o f re e z i n g [2 2 2- 2 23 ] w h i c h m a y b e e x p e c t e d t o b e f a v o r e d b y p h o t o -



186 A .C . Montefinale,T. Mo ntefinale and H. M. Papde Pageoph,

s e n s it iv e , h e t e r o g e n e o u s a n d l a r g e ly h y d r o p h o b i c s u r f a c e s, s u c h a s t h o s e o f i m p u r e

s il v e r i o d i d e . L a s t ly , b u t n o t t o t h e l e a s t e x t e n t , n u c l e a b i l it y d e p e n d s o n a m b i e n t a l

c o n d i t i o n s s u c h a s v a p o r s u p e r s a t u r a t i o n s a n d r a t e s o f c o o l i n g [ 22 4].

a. Natural ice- forming nucle i

I t i s t o b e e x p e c t e d t h a t n a t u r a l i c e - f o r m i n g n u c le i o f m i n e r a l o r i g i n w i ll n o t b e

m a d e o f c h e m i c a l l y - p u r e p a r t i c u l a te s b u t t h a t t h e y w i l l o f t e n c a r r y s o l u b l e , h y g r o -

s c o p i c c o n t a m i n a t i o n s , t h u s a l s o e n t e r i n g th e b r a c k e t o f c o n d e n s a t i o n n u c l e i. C o r r e l a -

t io n s b e t w e e n c o n c e n t r a ti o n s o f large c o n d e n s a t i o n n u c l e i , a n d i c e - f o r m i n g p a r t i c u -

l a te s i n t h e a i r a r e, i n f a c t, w e l l k n o w n [2 25 ]. I t is a ls o k n o w n t h a t t h e p r e s e n c e o f i c e -

n u c l e a n t s a t g r o u n d l e ve l is o f t e n a s s o c i a t e d w i t h p r e c i p i ta t i o n [2 25 -2 27 ], t h a t t h e y

f r e q u e n t l y s h o w w e l l- d e fi n e d g e o g r a p h i c b a n d s o f c o n c e n t r a t i o n [ 2 2 6 ] a n d t h a t th e

so-ca l l ed i ce-nucleus s torms t e n d t o o c c u r d u r i n g u n s t a b l e m e t e o r o l o g i c a l s i t u a t i o n s

[227-230] . I nve s t i ga t i on s o f t he nature o f in o r g a n i c , i c e - f o r m i n g s u b s t a n c e s i n t h e

a t m o s p h e r e h a v e , u n d e r s t a n d a b l y , b e e n t h e o b j e c t o f e x t e n s iv e w o r k . E a r l i e r s tu d i e s

by SCHAEFER [ see Ref . 8 )] w ho es t ab l i shed t h a t som e c l ay , so i l an d san d p a r t i c l e s

w e r e a c ti v e a t t e m p e r a t u r e s a b o v e - 2 0 ~ w e r e f o l lo w e d b y m o r e d e t a il e d w o r k o f

severa l i nv es t i ga to r s [8 ] i nc lu d ing M ASON and M AYBANK [231] and M ASON [232].

I c e - f o r m i n g p r o p e r t i e s o f c la y s a n d s t o n y m e t e o r i te s w e r e t h u s i n v e s t i g a t e d i n d e t a i l

an d m ore r ecen t ly , FUKUTA [233] and SANO, FUJITANI an d M AENA [203] a l so t e s t ed

v a r i o u s i n o r g a n i c c h e m i c a ls f o r t h e i r i c e -n u c l e a t in g p r o p e r ti e s . F i e l d w o r k b y K U M AI

[234] an d KUM AI an d FRANCIS [235] has l a t e r sho w n t ha t c l ay m inera l pa r t i c l e s a cc ou nt

f o r a l a r g e p e r c e n t a g e o f n a t u r a l n u c l e a t o r s i n th e a t m o s p h e r e , i n s o m e a r e a s o f t h e

w o r l d . T h e s e a r c h f o r n a t u r a l i c e - n u c l e a n t s s t i l l a p p e a r s t o c o n t i n u e a n d w a s a l s o

r e c e n t l y e x t e n d e d t o o r g a n i c s u b s t a n c e s , f o l lo w i n g t h e d i s c o v e r y o f t h o s e a c t iv i ti e s i n

som e o f t hese m a te r i a l s [236-237] .

I n a s m u c h K U M AIw e n t t o g r e a t p a i n s t o e v a l u a t e t h e c h e m i c a l n a t u r e o f h i s n u c le i ,

m u c h o f th e individuation of t hose pa r t ic l e s , i n na tu re , i s s ti ll ca r r i ed ou t by sea rch ing

f o r t h e ef fec t a n d d i s r e g a r d i n g t h e composi t ion o f t h o s e s u b s t a n c e s . T h u s t h e r e s e e m s

h e r e to b e s o m e a n a l o g y w i t h th e a p p r o a c h w h i c h l ea d s t o t h e e v a l u a t i o n o f c o n d e n s a -

t i o n n u c l e i c o n c e n t r a t i o n s i n t h e a i r. B r ie f ly , t h e p a r t i c u l a te s a r e d e t e c t e d b y t h e i r a b i l i t y

t o f o r m i c e , a t s u i t a b l e s a t u r a t i o n s , a n d t h e i c e - p a r t i c l e s g r o w n a r e t h e n c o u n t e d .

Var iou s m e th od s t o ach i eve t h i s were r ec en t ly r ev i ewed by BOUCHER [10] and , ac -

c o r d i n g t o t h e p r i n c i p l e o f o p e r a t i o n i n v o l v e d , u s e i s m a d e o f c loud chambers o f t h e

m i x i n g , d i f f u s i o n o r e x p a n s i o n t y p e s . B ri e fl y , a s u i t a b l e v o l u m e o f a d e q u a t e l y m o i s t

a i r w h i c h c o n t a i n s n u c l e i , i s i n t r o d u c e d i n t o t h e c h a m b e r w h e r e t e m p e r a t u r e a n d

h u m i d i t y a re p r e a d j u s t e d a c c o r d i n g t o r e q u i r e m e n t s. T h e n u m b e r o f n u c le i i s th e n

e v a l u a t e d e i t h e r b y d i r e c t o b s e r v a t i o n o f i ce c ry s t a ls t h e r e o n f o r m e d a n d e a s i ly

d e t e c t a b l e v i su a l l y b y t h e i r c h a r a c t e r i s t ic g l i tt e r, o r b y t h e c o u n t i n g o f c e n t e r s o f

c r y s t a l g r o w t h i n d u c e d b y t h o s e f a l li n g i ce c r y s ta l s i n a s a t u r a t e d s u c r o s e s o l u t i o n

s i tu a t e d a t th e b o t t o m o f t h e c h a m b e r . P a r a m e t e rs m e a s u r e d b y t h o s e m e t h o d s



Vol. 91, 197 1/VIII) C he m istry nd Propert ies of Atmospheric Nucleants 187

u s u a l ly i n c l u d e t h e n u c l e a ti o n s t h r e s h o l d ( th e m a x i m u m t e m p e r a t u r e a t w h i c h a s u b -

s t a n c e i n d u c e s th e c h a n g e o f p h a s e o f s u p e r c o o l e d w a t e r d r o p s ) a n d t h e a c t i v i t y ( t h e

f r a c t i o n o f n u c le i a ct iv e ) a t a s p e c if ic t e m p e r a t u r e . O t h e r m e t h o d s w h i c h a r e e s s en -

t i a ll y b a s e d o n v i s u a l o r o p t i c a l e x a m i n a t i o n , a t c o n s t a n t t e m p e r a t u r e s , o f p h a s e

t r an s i t i on i n wa te r d ro p l e t s [192 , 224, 238] o r sup ercoo l ed fi lms [191] con den sed on

h y d r o p h o b i c s u r fa c e s, w e re a l so e x t e n si v el y u s e d b y a n u m b e r o f A u t h o r s . H a n d y ,

s t a n d a r d i z e d m e t h o d s o f i c e -f o r m i n g - n u c l e i - c a p t u r e b y m i l l i p o r e fi lt e rs , f o l l o w e d b y

t h e i r d e v e l o p m e n t a n d c o u n t i n g u s i n g s u c r o s e o r s o d i u m s il ic a te s o l u t io n s [ 2 39 - 24 2 ],

w e r e a ls o m o r e r e c e n t l y b r o u g h t f o r t h a n d e f fi ci e nt , m o d e r n , a u t o m a t e d s y s t e m [ 24 3]

n o w o f f e r s e v e r a l a d v a n t a g e s o v e r t h e o l d e r m e t h o d s .

D e s p i t e o b s e r v a t i o n s w h i c h c o r r o b o r a t e t h e e x i s t e n c e o f a c l o s e r e l a t i o n s h i p , i n

s o m e t y p e s o f c l o u d s , b e t w e e n f a l l - o u t r a t e s o f s n o w o r g r a u p e l , a n d t h e c o n c e n t r a -

t i o n s o f t h o s e a t m o s p h e r i c i c e -n u c l e i [2 4 4 ] w h i c h a r e a c t iv e a t c l o u d t o p t e m p e r a t u r e s ,

i t w o u l d a p p e a r t h a t t h e n u m b e r o f th o s e p a r t ic l es i s n o t a s i m p l e fu n c t i o n o f th e t o t a l

n u m b e r o f ic e c ry s ta l s c o n t a i n e d i n s o m e o t h e r t y p e s o f c l o ud s . T h e r e a s o n f o r t h is

a p p a r e n t d i s c re p a n c y s e em s t o b e d u e t o naturalprocesses o f ice-multiplication w i t h a

c o n s e q u e n t i n c r e a s e o f t h e n u m b e r o f i c e p a r t ic l e s o v e r t h e n u c l ei . S e v e r a l m e c h a n i s m s

w e r e p r o p o s e d a n d t e s te d i n th e l a b r a t o r y t o e x p l a i n th i s p h e n o m e n o n w h i c h s o m e -

t imes sho ws up i n excesses , b y severa l o rde r s o f m agn i tude , o f ice -c rys t a l s ove r i ce-

n u c l e i w i t h i n a n d a r o u n d th e c l o u d r e s p ec t iv e l y , a n d a c c o r d i n g t o i t s s u p e r c o o l i n g

[245-248] . Im pl i ca t i ons an d poss ib l e causes were r ecen t ly d i scussed by M osso p [249]

o n t h e b a s i s o f p a s t e x p e r i m e n t a l e v id e n c e . P o s s i b le m e c h a n i s m s in c l u d e f r a g m e n t a -

t i o n a n d s h a t t e r i n g o f l a r g e f re e z i n g d r o p s d u r i n g t h e i r f a ll [ 2 50 -2 51 ], e v a p o r a t i v e

i c e - fi la m e n t f o r m a t i o n o n a v a i l a b l e ic e p a rt i c le s a n d t h e i r s u b s e q u e n t s h a t t e r in g

[252-253] , i ce -whi ske r fo rm at io n i n e l ec t r i c f i elds [254] an d t he i r sh a t t e r i ng [255-258] ,

r im ing a nd sha t t e r i n g an d i ce -c rys t a l gene ra t i on i n e l ec t r i c f i e lds [259]. I t i s, the re fore ,

o f ad de d i n t e r es t t o r eca l l a t t h i s po in t som e o f REITER S w ork [260-261 ] on t he e l ec tr i-

c a l e f fe c ts w h i c h a c c o m p a n y t h e f re e z i n g o f N O x - c o n t a m i n a t e d w a t e r a n d a l so h i s

i m p l i c a t i o n o n t h e m u l t i p l ic a t i o n o f ic e c r y s t a ls w h i c h c o u l d t h u s b o t h p a r t l y b e d u e

t o , a n d f u r t h e r g e n e r a t e , i n a fr e e z i n g h y d r o m e t e o r , o x i d e s o f n i t r o g e n w h i c h a r e w e l l

k n o w n f o r t h e i r e l e c t ri c i ty , c a r r y i n g p r o p e r t ie s . W h i l e th e r e s e e m s t o b e l i tt le d o u b t

t h a t p r o c e s s e s s u c h a s t h o s e d e s c r i b e d m a y o c c u r i n n a t u r e , t h e r e i s a l s o e v i d e n ce to

t h e e f f ec t t h a t t h o s e p h e n o m e n a r e q u i r e t h e p r e se n c e of pr imacy nuc l e i fo r t h e i r i n i t ia -

t i o n . T h e i r p r e s e n c e s e e m s a l s o t o b e n e c e s s a r y f o r t h e p r o d u c t i o n o f ic e p a r ti c u l a t e s

b y mechanical shock [2 62 -2 64 ] s u c h a s , f o r e x a m p l e , th u n d e r c l a p s in a t h u n d e r c l o u d

[2 65 ], t h o u g h t h e e l u c i d a t i o n o f th i s m e c h a n i s m o f f h a n d a p p e a r s t o b e s t il l t h e o b j e c t

o f s o m e c o n t r o v e r s y [ 26 6] .

De sp i t e a n um be r o f r ev i ews on t he su b j ec t [8 -10 , 267] t he re s t i ll i s a va r i e ty o f

o p i n i o n o n t h e source o f n a t u r a l , a t m o s p h e r i c i c e - f o r m i n g n u c l e i . I t i s u n f o r t u n a t e

t h a t , b e c a u s e o f t h e c o n t i n u i n g i n c r e as e o f a ir p o l l u t i o n a l l o v e r th e w o r l d a n d b e c a u s e

of i t s e f f ec t on t he co nc en t r a t i o n o f t hos e nuc l e i i n t he a i r [268-269], t h i s s i t ua t i on

m a y h a r d l y b e e x p e c t e d to i m p r o v e . T h e c o u n t s o f t e n in c r ea s e d u r i n g r a in s h o w e r s



188 A .C . Montefinale,T. M ontefinaleand H. M. Pap6e (Pageoph,

a n d d u r i n g f o g d i s s i p a ti o n in d i c a ti n g , a s t h e y d o , t h e i r w e ll k n o w n d e p e n d e n c e o n t h e

v e r t ic a l e x c h a n g e o f a i r m a s s e s a s w e l l a s t h e p r e a c t i v a t i o n m e m o r y e f fe c ts w h i c h

d e p e n d o n t h e s t a te o f th e s u rf a c e o f t h o s e p a rt ic l es a n d o n t h e ir c o n t a m i n a t i o n . T h e

sc i en ti f ic l i t e r a tu re i s i nd i ca t i ve o f a p re do m ina n t ly l and -or ig in [270-271] o f t hose

p a r t i c u l a t e s , a l t h o u g h t h e r e a r e a l s o s o m e h i n t s t o t h e c o n t r a r y [ 2 7 2] . D e s p i t e t h e

p r e s e n c e o f c l a y i n a l a r g e p e r c e n t a g e o f s n o w c r y s t a ls c o ll e c te d in u n p o l l u t e d a t m o -

s p h e r e s [2 34 -2 35 ] t h e r e w o u l d a p p e a r t o b e n o o f f h a n d a s s o c i a t i o n o f i c e - f o r m i n g

nuc l e i w i th t e r r es t r i a l o r vo l ca n i c du s t ove r Aus t r a l i a (226, 273] . Som e paper s app ear ,

i n f ac t , t o be i nd i ca t i v e [228 , 274] o f a r equ i r em ent by a nu c l ea to r pa r t i c l e , o f a

r e s id e n c e t im e i n t h e c l e a n e r, u p p e r s t r a t a o f th e a t m o s p h e r e , p o s s i b l y s i n ce t h is m a y

c a u s e i t s s u r fa c e t o d e c o n t a m i n a t e s u f f ic i e n tl y b y r a d io l y s is , p h o t o l y s i s a n d o u t g a s s i n g ,

t o r e - a c ti v a t e it f o r p u r p o s e s o f i c e - f o r m a t i o n .

b. The extra- terres trial nuclei o f ice form atio n

T h e r e b e i n g a n i n f l u x r a t e o f a b o u t 1 04 t o n s [ 2 7 5 ] o f i n t e r p l a n e t a r y d u s t , p e r d a y ,

o n E a r t h , i t w o u l d s e e m t o b e e v i d e n t t h a t t h e s e a r c h f o r i c e - f o r m i n g p a r t i c u l a t e s

s h o u l d n o t b e c o n f i n e d t o e a r t h l y s o u r c e s a l o n e . T h u s , o n t h e b a s i s o f r a i n f a l l- p e a k

d i s t r i b u t i o n s o v e r a n u m b e r o f y e a r s , B OW E N [ 27 6 ] p u t f o r w a r d t h e h y p o t h e s i s o f i ce -

n u c l e a t i n g e f fe c ts d u e t o

meteor showers

w i t h a t i m e l a g o f a b o u t 3 0 d a y s w h i c h a i e

n e c e s s a r y f o r t h e s m a l l p a r t ic u l a t e s , th u s g e n e r a t e d i n t h e u p p e r a t m o s p h e r e , t o r e a c h

t h e l e v el s o f t h e c l o u d s . E a r l i e r a r g u m e n t s f o r a n d a g a i n s t t h i s h y p o t h e s i s , a s w e ll a s

BOWEN s sub sequ en t w ork were ex t ens ive ly com m en ted on by FLETCHER [277-278] ,

M o s s o v [ 2 67 ] a n d o t h e r s , a n d w e r e fo l l o w e d b y f u r t h e r r e s e a r c h w h i c h d e a l t w i t h t h e

l u n a r i n f lu e n c e o n b o t h , t h e i c e - n u c l e u s c o n c e n t r a t i o n [2 26 , 2 7 9 - 2 80 ] a n d t h e w i d e -

s p r e a d p r e c i p i t a t i o n p h e n o m e n a [ 2 81 -2 83 ]. T h e i n t e r e s t w h i c h a r o s e b e c a u s e o f

B o w E N s t h e o r y h a s u n t i l r e c e n t l y y i e l d e d in t e r e s t in g d i s c u s s io n s o n t h e s u b j e c t

[284-286] a l t h ou gh the co n t ro ver sy s ti ll app ear s t o pe r s i s t .

Severa l p r ev ious ly -qu oted s tud i es [7 , 108-111, 144] be a r ev idence t ha t t he s t r a to -

s p h e r i c a e ro s o l l a y e r c o n t a i n s l a rg e a m o u n t s o f in s o l u b l e m a t e r i a l w h i c h p r e v a l e n t l y

a p p e a r s t o b e o f e x t r a t e rr e s t ri a l o r i g i n . I t w a s p o i n t e d o u t t h a t t h e s t r a t o s p h e r i c

s u l fa t e l a y e r [7 ] m a y p o i s o n t h e i c e - n u c l e a t i n g a c t i v it y o f t h o s e s o l i d p a r t i c u l a t e s o f

ex t r a t e r r es t r i a l o r i g in [226] , un l es s t he me teor i t i c ma te r i a l t r ave r sed t h i s r eg ion ve ry

q u i c k l y . D e s p i t e a n u m b e r o f r e c e n t p a p e r s w h i c h s u g g e s t b u t a l o w i c e - fo r m i n g

a c t i v it y o f t h e m e t e o r i t i c m a t e r i a l , a n d o f d u s t s f o u n d a t h i g h e r a l t it u d e s i n s o m e

p l a c e s o f t h e w o r l d [2 87 -2 88 ], t h e r e i s e v i d e n c e t h a t n u c l e a t i n g p ro p e r t i e s d e p e n d o n

t h e c h e m i c a l c o m p o s i t i o n o f t h o s e m e t e o r i te s [ 2 8 9 ] a n d t h a t s a m p l e s c o ll e c te d a t h i g h

e l eva t i ons a t o the r l oca t i on s ma y exh ib i t l a rge r ice - fo rm ing ac ti v i ti e s [290-293]. A l so ,

p a r t ic l e s o b t a i n e d f r o m m e t e o r i t e s w h i c h w e r e b o i le d u n d e r p a r t i a l v a c u u m i n a s o l a r

f u r n a c e [ 2 9 4] w e r e s h o w n t o b e a c ti v e a t - 1 5 ~ t h e d i s c re p a n c i e s m e n t i o n e d a b o v e

e v i d e n t l y a r i s i n g f r o m t h e t r e a t m e n t w h i c h t h e s u r f a c e o f t h o s e p a r t i c l e s u n d e r w e n t .

I t is c le a r t h a t a p r e h e a t e d , o u t g a s s e d a n d h e n c e u n c o n t a m i n a t e d a n d p r e q u e n c h e d




surface of a meteoritic particle, as it appears in nature, and also of suitable chemical

composition and size, will behave differently towards the catalysis of ice-formation,

than the

s a m e

material would after a prolonged contamination and/or hydration. It

is also clear that conditions towards both activation and poisoning of those surfaces

do seem to exist in the upper regions of the atmosphere and that consequently the

apparent discrepancies of the results obtained by various workers are here due to

differences in environmental influences to which those particles were subjected during

their history.

c . On som e com monly used ar t if i c ia l nuc l e i o f i ce f ormat ion and on

the resul t ing impl icat ions

Notwithstanding the large number of artificial inorganic solid substances which

have, until now, been investigated in the laboratory for their ice-forming properties,

the study of silver iodide, the first-discovered [295-296] and tested ice nucleator for

weather modification [195], continues to yield information, in this area of research,

towards the nature of processes which occur on solid surfaces, and towards their

prerequisites for ice-embryo formation. Thus, although the concept of the epitaxial

fit first stimulated VONNECUT to carry out his pioneering work, and although the im-

portance of this fit, between the ice-lattice and that of a nucleator [297], was observed

to favor, under some circumstances [298], the transition of supercooled water to

ice, it has also been shown that pure, unheated and unquenched silver iodide substance,

which does not contain hygroscopic impurities, is a very poor ice-forming agent

[299-300]. The compound may, however, be activated by light (see observations by

MASON, Ref. [230] and [191,301]), probably following the formation of surface in-

homogeneities which are known to promote the nucleability of ice on AgI [302]. Also,

and aside of surface contamination and irregularities, AgI normally used for purposes

of ice-nucleation may always be expected to be

intrinsically

impure, due to the pre-

sence of more than one phase, therein generated by thermal dispersion [303-307] and

the fast quenching which follows its introduction into the air. It was pointed out,

moreover, that silver iodide is essentially hydrophobic [308, 218] and that its ice-

nucleating properties depend on the number of hydrophilic sites, due to impurities.

Those sites favor the formation of primary water clusters [204,309] which subsequently

extend into ice, at suitable conditions. Since silver iodide is photochemically sensitive

[310-311] and since it yields photogalvanic effects in the presence of soluble contami-

nations [312-313] it is be expected that those factors may also contribute to the forma-

tion of those separated polar sites at which water clusters will develop. Some of the

above mentioned properties also apply to the case of lead iodide [314-318] which is

known to be an excellent ice-forming substance [319, 213] and whose surface, for the

purpose of its ice-forming effects, probably consists of a mixture of mono and di-

iodides, and of the basic iodide.

Despite its extensive use in cloud physics and weather-control research, silver




iodide is expensive and its use is therefore limited to very small (submicron) particles.

Although this yields large surface areas per unit weight, it also exposes the material

to faster photodeteriorat ion [320-322] and hydrodecay [323-325]. The first factor in-

duces a collapse of the surface structure because of a primary photodissociation into

elements and their subsequent reactions, the second provokes the leaching of active

sites and a decrease of the overall surface area, especially in presence of complexing

KI or NaI [326] salts. The cumulative decay of the activity, due to both of those fac-

tors, may reach up to about three orders of magnitude per hour.

Instruments designed with the purpose of dispersion, from the ground or from

aircraft, o f silver iodide aerosol, range from burners of various design - where a

suitably complexed (with NaI, amines, ammonia etc.) solution (in acetone, amines etc.)

undergoes combustion in a supporting fuel (propane etc.) [327] - to the use of AgI-

impregnated charcoal or gunpowder [8] and of detonat ing fuses named Weathercord

which incorporate this iodide [328]. Pyrotechnic devices which operate on the prin-

ciple of alumino or magnesiothermic reduction of silver iodate and other oxidants, or

which contain adequate self-sustaining combustible chemical mixtures and include AgI

to be dispersed, were also, more recently developed in the United States [329-332]

following similar general ideas which were elsewhere previously brought forth [333-336].

Inasmuch the use of generators, which burn complexed AgI solutions, would

readily imply that silver iodide particulates thus produced will be strongly conta-

minated by other hygroscopic products [337], interest in the role of those contamina-

tions seems to have arisen only very recently [338-341]. The reason for this is that the

aerosol thus prepared worked in most of the cases anyway, although with different

yields and according to the method used. Also, concern about the chemistry of AgI

surface action took several years to materialize mainly because of the happy coinci-

dence that, as previously mentioned, AgI should indeed be contaminated in order to

be active as an ice nucleator.

As a result of an increasing interest in the surface properties of inorganic ice-

nucleator materials, and following the elaboration of more precise techniques for

their investigation [342-343], the field of study of those substances understandably

extended, during the last decade, to an impressive array of

organic compounds.

This

trend was started in 1957 by BASHKIROVAand KRASIKOV [344] with their report on

phloroglucinol s ice-forming activity at about -6~ and with its subsequent testing

in the field [345]. Added impulse was provided by the work of HEAD [346-348] who

investigated a variety of steroids and other polycyclic compounds and found tha t the

ice-forming activity of some of them depended both on the freshness of the surface

exposed by grinding and on its hydrogen-bonding properties [349]. Parallel investiga-

tions which pointed to similar conclusions are due to KOMABAYASIand IKEBE [350];

other work on arninoacids some of which were found to be active at about -4~ is

due to POWER and POWER [351]. Several of the most important points raised by those

investigations were subsequently confirmed by FUKUTAand MASON [236]. The finding

that aminoacids nucleate ice assumes importance because of more recent work by



Vol. 91, 197 1/VIII) C he m istr y nd Propert ies of Atmospheric Nucleants 191

S ID LE w h o c o n f i r m e d , e x p e r i m e n t a l ly , th e p r e s e n c e o f s o m e o f t h o s e c o m p o u n d s i n

t h e a t m o s p h e r e [ 3 5 2 ] . M o r e i n v e s t i g a t i o n a n d d i s c u s s i o n a l o n g t h i s l i n e i s d u e t o

o t h e r s [3 53 ]; i n d i c a t i o n w a s t h u s o b t a i n e d o f a c o r r e l a t i o n b e t w e e n t h e c h e m i c a l

prop er t i e s o f a subs t ance a nd th i s t ype o f su r f ace ac t i on [354-355] , an d the in f luence

o f b o t h , t h e a g e o f a s a m p l e [ 35 6] a n d o f th e e x p e r i m e n t a l o r e n v i r o n m e n t a l c o n d i t i o n s

i n v o l v e d [ 3 5 7 ] , o n i t s i c e - f o r m i n g n u c l e a b i l i t y . A s t h i n g s a r e t o d a y i t w o u l d a l s o

a p p e a r t o b e o f i n t e r e s t t o i n v e s t i g a te s o m e o f t h o s e o r g a n i c m a t e r i a ls f o r t h e i r s u s -

c e p t i b il i ty t o a c t i v a t i o n [3 58 ] o r i n a c t i v a t i o n b y c h e m i c a l r e a c t i o n o r s u r f a c e p o i s o n i n g

d u e t o p r o d u c t s o f a t m o s p h e r i c n a t u r a l a n d a r ti f ic i a l a i r p o l lu t i o n . I t w o u l d a l so b e o f

i n t e r e s t t o t e s t t h e i r a c t i v i t i e s a f t e r h a v i n g t h e m a d s o r b e d o n s u i t a b l e m i n e r a l s u b -

s tr a te s , s u c h a s o c c u r in n a t u r a l a e r o s o ls . W o r k o f th i s k i n d w o u l d a d d t o t h e k n o w -

l ed g e a b o u t t h e r o l e o f th e s e n u c l e a t o r s in n a t u r e , a s t h e a c t i v it y o f m a n y o f t h e m is

k n o w n t o b e s t ro n g l y d e p e n d e n t o n t h e m o d e o f t h e ir p r e p a r a t i o n [3 59 -3 60 ].

D e s p i t e a w i d e s p r e a d u s e o f s i l v e r i o d i d e i n w e a t h e r m o d i f i c a t i o n e x p e r i m e n t s ,

s o m e o t h e r r e c e n t l y - f o u n d a r t if i c ia l n u c l e a t o r s ( a n d t h e m e t h o d s o f t h e i r d i s p e r s io n )

m e r i t a t t e n t i o n b e c a u s e o f t h e i r s i m p l i c it y , v e r s a t i li t y a n d l o w c o s t . O n e o f t h e m i s

s o l id m e t a l d e h y d e [ 3 6 1 ] w h i c h , w h e n fr e s h l y g r o u n d , w a s f o u n d t o n u c l e a t e i ce a t

a b o u t - 0 . 4 ~ S u i ta b l e g e n e r a t o r s o f m e t a l d e h y d e s m o k e s w e r e t h e r e fo r e d e v e l o p e d

[362] an d t e s t ed i n t he f ie ld fo r i t s ac t i on o n su perco o led c lo uds [363] . Th e su bs t anc e

p r o b a b l y a c t s o n t h e b a s i s o f t h e p r i n c i p le o f e p i t a x y ; i ts d r a w b a c k c o n s i s ts o f t h e

h i g h r a t e s o f e v a p o r a t i o n o f th e m i c r o c r y s t a ls d i s p e rs e d a n d o f th e i r p o s s ib l e d e t e r i o r a -

t i o n b y e x p o s u r e t o h u m i d i t y a n d l i g h t . L i k e s i l v e r i o d i d e , i t s h o u l d t h e r e f o r e b e

d i r e c t ly i n t r o d u c e d i n t o s e e d a b l e c l o u d s . T h e o t h e r s u b s t a n c e i s d r y u r e a [ 3 64 ] w h i c h ,

t h o u g h v e r y h y g r o s c o p i c a n d r e a d i l y s o l u b le , a c t s o n a s u p e r c o o l e d c l o u d b y l o c a l

u n d e r c o o l i n g d u e t o i t s e n o r m o u s l y e n d o t h e r m i c h e a t o f s o lu t io n . T h i s p r o v o k e s a

r a p i d f r e e z in g o f a w a t e r f i lm f o r m e d o n a p a r t i c le o f th i s m a t e r i a l . S u i t a b l e d i s p e r si n g

u n i t s o f t h i s c o m p o u n d w e r e h e n c e a l s o d e v e l o p e d [ 36 5] a n d t h e s u b s t a n c e t e s t e d in

t h e a t m o s p h e r e [3 64 , 3 66 ]. B e c a u s e o f i ts h y g r o s c o p i c i t y u r e a a l s o o f fe r s , w h e n

d i s p e r s e d a s a p o w d e r o f a d e q u a t e s iz e , t h e a d v a n t a g e o f a c t in g a s c o n d e n s a t i o n n u c l e i.

P a r a d o x i a l l y , i ts d r a w b a c k l ie s i n i t s h i g h s o l u b i l i ty a n d h y g r o s c o p i c i ty , b o t h p r e v e n t i n g

i ts r e g e n e r a b i l i t y i n t h e a t m o s p h e r e , f o r i c e - f o r m i n g p u r p o s e s , b y d e s i c c a t i o n . I t is a l so

k n o w n t o b e a f fe c te d b y N O x a n d i ts p r o p e r ti e s o f a c t in g a s a c o n d e n s a t i o n n u c l eu s

c o u l d t h e r e f o r e b e a f f e c t e d b y a i r p o l l u t i o n . I n o r d e r t o e x p l e te i ts i c e - f o r m i n g p r o p e r t ie s

i t s h o u l d h e n c e a l s o b e i n t r o d u c e d d i r e c tl y i n t o a s u p e r c o o l e d c l o u d t o b e s e e d e d .

T h e t h i r d v e r s a t il e p r o d u c t is t h e s u l fi d e o f a l u m i n u m . T h e i c e - f o r m i n g a c t iv i t y o f

t h i s s u b s t a n c e , w h e n f re s h l y p r e p a r e d f r o m t h e e l e m e n t s , s t a r ts a t a b o u t - 3 . 5 ~ T h i s

p r o p e r t y o f A 1 2 S a w a s d i s c o v e re d i n 1 96 2 a n d p a t e n t s c o v e r i n g i ts u s e f ir s t a p p e a r e d i n

1 96 5. G i a n t n u c l e i o f a l u m i n u m s u l fi d e a r e u s e d , a s r e c e n t ly d e s c r i b e d [3 67 ], i n a f o r m

e n c a p s u l a t e d in s p h e r u le s o f a l u m i n a a n d s o d i u m a l u m i n a t e . T h e y h a v e a l s o b e e n

t e s te d , a s o f l a t e , f o r p u r p o s e s o f w e a t h e r m o d i f i c a t i o n [3 68 -3 69 ] a n d a l t h o u g h

a l u m i n i n u m s u l f i d e , a l i k e A g I , h a s a l s o a h e x a g o n a l c r y s t a l l i n e f o r m , i t s a c t i o n i s

p r i m a r i l y t h o u g h t t o b e d u e t o t h e c r e a t i o n , d u r i n g i t s s l o w r e a c t i o n w i t h w a t e r , o f



192 A.C. Montefinale,T. Montefinaleand H. M. Pap6e Pageoph,

a mosaic structure which involves polar sulfidic forms intermixed with sparsely

soluble and insoluble material. The use of bulky apparatuses is unnecessary for the

dispersion of AlzS3-containing smoke, since monodisperse particulates about 1.5

micron of diameter) are generated by the combustion of suitable self-sustaining solid

chemical mixtures. Those mixtures covert themselves almost quantitatively into the

aerosol which is driven out of the flame by a current of simultaneously-generated

nitrogen gas. Since those nuclei also possess some hygroscopicity due to the presence

of both, sodium aluminate and A12S3 which is known to react slowly with water yield-

ing hydrogen sulfide and the hydroxide of aluminum, this type of aerosol also

belongs to the class of condensation nuclei. It hence is also an example of what is

commonly termed

m ixe d nucleator .

Some of the analyt ical chemis try methods which were previously mentioned in

connection with the analyses of condensat ion nuclei, may also be used to analyze the

ice-forming particulates. The obvious difficulty consists however here both, in the in-

dividuation of the ice-forming particle at the center of the snow crystal, and in the

subsequent evaluation of its chemical nature. This task calls for high-precision,

tedious and time-consuming work, and for the use of expensive and complicated in-

strumentation; it is thus not surprising that not many contributions are available

along this line. Outstanding research includes the use of electron-microscopic techni-

ques and electron-diffraction patterns [234, 235, 370]; those enabled the identification

of natural clay minerals in crystals of snow.

Since silver iodide continues to be regarded by many workers as a very fashionable

ice-forming agent, much effort was, understandably, devoted to the search for traces

of this material both in ice-crystals and in the precipitation collected on the ground,

following cloud-seeding experiments. Techniques based on the use of electron micro-

scopy on suitably pre-treated samples [371-372] were hence also applied to this case.

Methods based essentially on the extraction of silver iodide from water samples, and

on its subsequent re-use as ice-forming substance, in adequately calibrated cloud

chambers, are also available [373-376]. Those advances were later followed by the

development of techniques which involve neutron activation procedures [377-378]

thus leading to the increase of both, precision and consistence of the results. Finally,

atomic absorption spectroscopy [379] and techniques of inducing crystal growth in KI

solutions supersaturated with respect to AgI [380] have very recently also been used

as a tool for the quantitative analysis of silver in seeded snow.

It would appear to be somewhat unfortunate that a much larger effort was thus

devoted, in the past, to the development of methods applied to the monitoring of silver

iodide submicron particles in precipitation, than to the search for the nature and

modes of action o f natural, ice-forming, nucleators.

3. The induction, inhibition an d alteration o f natu ral hyd rom eteor proc esses

Natural, weather-active, condensation and ice-forming nuclei in the air are thought



Vol. 91, 197 1/V III) Ch em ist ry nd Properties of Atmospheric Nucleants 193

t o e x e r t a l a rg e i n f lu e n c e o n e v e r y d a y s li fe o f a l a r g e f r a c t i o n o f t h e p o p u l a t i o n o f t h e

w o r l d . T h e e m e r g i n g a r t o f ar ti fi c ia l c l o u d - m o d i f i c a t io n h e n c e s t im u l a t e s t h e h o p e s

o f b e i n g a b l e t o i n c r e a se t h e n a t u r a l p r e c i p i t a t io n i n a r i d a r e a s, o f r e li e v in g d r o u g h t

a n d a l so o f a v o i d i n g e x c es s iv e a n d d a m a g i n g d o w n p o u r s a n d c o n s e q u e n t f lo o d s. A i r-

l in e a u t h o r i t i e s a n d i n h a b i t a n t s o f c h r o n i c a ll y f o g - b o u n d t e r r i t o r ie s a w a i t t h e d a y

w h e n e f fi c ie n t f o g - d is p e r s a l p r o c e d u r e s w i ll b e c o m e o p e r a t i o n a l , a n d m o d e r n p r o -

g re s se s i n h a i l - su p p r e s s i o n w a r r a n t o p t i m i s m t o w a r d s a n e c o n o m i c a l a p p l i c a t io n ,

o n a l a r g e s c a le , o f t h e r e s u l t s o f a n u m b e r o f f i e ld t e s ts , n o w u n d e r e x p e r i m e n t a l s t u d y

i n s e v er a l p a r t s o f t h e w o r l d . M o s t e x p e r i m e n t a t i o n c a r r i e d o u t i n t h e s e t h r e e b r a n c h e s

o f w e a t h e r c o n t r o l i n v o l ve s t h e u s e o f s u it a b le n u c l e a t o r p a r t ic u l a t e s , a n d o f d e v ic e s

d e s i g ne d f o r t h e i r e c o n o m i c a l a n d e f fi c ie n t p r o d u c t i o n .

a T h e a c ti o n b y c o n d e n s a ti o n n u c l e i

A s p r e v i o u s l y m e n t i o n e d , t h e p r i n c ip l e o f o p e r a t i o n c o n s is t s h e r e o f d is p e r si n g

a d e q u a t e h y g r o s c o p i c c h em i c a l s , i n c o n d i t i o n s w h i c h f a v o r t h e i r p e n e t r a t i o n w i t h in

o r a b o v e t h e c l o u d m a s s o r f o g t o b e s e e de d . T h e p a r t i c le s s h o u l d b e e s s e n ti a ll y m o n o -

d i s pe r s e a n d o f a d i a m e t e r w h i c h c l as si fi es th e m a m o n g t h e

g ian t nuc lea tors

T h e i r a c t i o n o n

w a r m

a b o v e 0 ~ c o n s is t s o f r e m o v i n g t h e s t a g n a n t d r o p l e t s

b e c a u s e o f t h e h y g r o s c o p i c i t y [ 3 81 - 38 3 ] o f t h e n u c l e a n t a n d b y s u b s e q u e n t c o a l es c e n c e .

P i o n e e r i n g w o r k o n t h e s u b j e c t d a t e s b a c k t o 1 93 8 [ 3 84 ] ; c o n c e n t r a t e d s o l u t i o n s o f

c a l c i u m c h l o r i d e w e r e t h e n u s e d in f i e ld - te s ts a n d i n a m o u n t s o f a b o u t 2 . 5 g in s p e r

c u b i c m e t e r o f f o g. A m o r e r i g o r o u s e x p e r i m e n t a t i o n , b a s e d o n t h e u s e o f m o n o d i s -

p e r s e a b o u t 1 - 4 m i c r o n i n d i a m e t e r ) s o d i u m c h l o r i d e w a s r e c e n t l y p e r f o r m e d b y

JIU ST O a n d A s s o c i a t e s [3 85 ] in c l o u d c h a m b e r s w h e r e c o n d i t i o n s w e r e p r e - s e t a c c o r d -

i n g t o n a t u r a l r e q u i r e m e n t s . S i n c e t h e o b j e c t o f t h i s r e s e a r c h w a s t o i n c r e a s e v is i bi li -

t ie s , e s t im a t e s w e r e c o m p u t e d f r o m t h e w e ll k n o w n l a w s w h i c h g o v e r n th i s p a l a m e t e r

i n a c l o u d o f p a r t i c u l a t e s [ 3 8 6 - 38 7 ] . F o l l o w i n g t h e d e v e l o p m e n t o f e f f ic i e n t g e n e r a t o r s

o f m o n o d i s p e r s e d i a m e t e r a b o u t 1.5 m i c r o n ) a n d e l e c t r o a c t iv e c o n d e n s a t i o n n u c le i

[ 3 35 - 33 6 ] e x p e r i m e n t s w e r e a l so r e c e n t l y c a r r i e d o u t i n t h e P o - V a l l e y w h i c h is k n o w n

t o b e c h r o n i c a l l y - i n f e s t e d b y s h a l l o w f o g s [5 1] d u r i n g t h e c o l d s e a s o n . T h e r e s u l t s

o b t a i n e d y i e l d e d o v e r a l l a v e r a g e i n c r e a s e s o f g r o u n d v i s ib i li t y o f a b o u t 3 0 0 m e t e r s

o v e r a v a s t a r e a [ 36 8] . T h e r e s e e m s , t h e r e f o r e , t o b e n o d o u b t t h a t s u c c e s s f u l o p e r a -

t i o ns o f f o g -c l e a ri n g , b y t h e u s e o f g i a n t n u cl e i, a r e n o w o n l y a m a t t e r o f p r o p e r

log is t ics .

A s m e n t i o n e d e a r l i e r i n th i s p a p e r , s e e d i n g o f

c louds

w i t h g i a n t c o n d e n s a t i o n n u c le i

h a s , a s i t s o b j e c t , th e i n d u c t i o n o f c o a l e s c e n c e p r o c e s s e s a n d t h e c o n s e q u e n t in c r e a s e

o f p r e c i p i t a t i o n o n t h e g r o u n d a t a p r e d e t e r m i n e d p l a c e . S e e d i n g m a y b e c a r r i e d o u t

f r o m a p la n e , o r f r o m t h e g r o u n d , b u t i n t h is l a t t e r c a s e u n d e r c o n d i t i o n s w h i c h f a v o r

t h e f o r m a t i o n o f s u it a b le u p d r a f t s , w h e r e b y n u c l ei m a y p e n e t r a t e i n t o t h e a v a i l a b l e

o r p r o s p e c t i v e h y d r o m e t e o r s . E a r l i e r re s u lt s o b t a i n e d b y t h e u s e o f th i s t e c h n i q u e a r e

q u o t e d i n R e f e r e n c e [ 1 ]; t h e p r o c e d u r e w a s , m o r e r e c e n t l y , u s e d i n P u e r t o R i c o [3 88 ],

13 PAGEOPIt91 1971]vnI)



194 A .C . Montefinale, T. Montefmale and H. M. Pap6e Pageoph,

I n d i a [ 3 8 9 ] a n d I t a l y [ 3 6 9 ] w i t h c o n s e q u e n t i n c r e a s e s i n p r e c i p i t a t i o n v a l u e s u p t o

a b o u t 5 0 ~ .

S i n c e n o a p p r e c i a b l e g r o w t h o f e l e c t r i c a l l y - u n c h a r g e d w a t e r d r o p s , w h i c h a r e

s m a l l e r t h a n 1 0 - 20 m i c r o n i n d i a m e t e r , a p p e a l s t o o c c u r u n d e r n a t u r a l c o n d i t i o n s ,

a n d s i n c e hail m a y b e d u e t o t h e f r e e z i n g o f a f e w l a r g e d r o p l e t s w h i c h a r e a v a i l a b l e

i n a c o n g e s t e d c l o u d , it h a s b e e n p r o p o s e d i n t h e p a s t t h a t t h e i n t r o d u c t i o n o f la r g e

a m o u n t s > 1 02 x m - 3 ) [ 1 85 ] o f g i a n t c o n d e n s a t i o n n u c l e i i n t o t h e b a s e o f a c l o u d ,

c o u l d l e a d t o t h e f o r m a t i o n o f su f f ic i e n t e x c e s se s o f t h o s e p o t e n t i a l h a i l e m b r y o s , t o

r e d u c e t h e i r i n d i v i d u a l a v e r a g e s i z e a n d t h u s p r e v e n t t h e i r r e a c h i n g t h e g r o u n d i n

s o l i d f o r m . T h i s v e r y i m p o r t a n t s t u d y w a s s t r e n g h t e n e d b y t h e w o r k o f A PPLE MA N

[3 90 ] w h o p o i n t e d o u t t o t h e p o s s i b i l i t y o f e x i s t e n c e o f a n a t u r a l h a i l s u p p r e s s i o n

m e c h a n i s m i n t h e c o a s t a l r e g i o n s o f t h e U n i t e d S t a te s . T h i s , o b v i o u s l y , i s a r e g i o n

w h e r e s e a - s a lt g i a nt c o n d e n s a t i o n n u c l e i a b o u n d . A l s o , a n a d d e d p o i n t o f i n t e re s t

c o n s i st s , i n t h e c a s e o f h a i l - f o r m a t i o n , a s r e l a t e d t o n u c l e i , o f t h e s u g g e s t i o n t h a t s i n c e

s m a l l a m o u n t s o f s o m e d i s s o l v e d s a lt s f a v o r d e n d r i t i c g r o w t h o f i c e , l es s d a m a g i n g

s p o n g y h a i l [1 89 ] c o u l d f a l l f r o m a c l o u d t r e a t e d w i t h a s u i t a b le a m o u n t o f e f fi c ie n t

n u c le i o f c o n d e n s a t i o n , t h a n i t w o u l d f r o m a n u n t r e a t e d c l o u d a n d a t t h e s a m e

c o n d i ti o n s . T h e a m o u n t o f p a r t ic l e s r e q u i r e d t o f u lf il l t h e r e q u i r e m e n t s o f a n y o f

t h o s e p r o c e ss e s is b y n o m e a n s p r o h i b i t i v e a n d i t w o u l d i n d e e d a p p e a r t o b e o f la r g e

i n t e r e s t t o t e s t t hi s d o u b l e q u e r y w i t h o n e f i e ld e x p e r i m e n t a t i o n t o b e c a r r i e d o u t i n a n

a r e a w h e r e h a i l s t o r m s r e g u l a r l y r e c u r , a n d w h e r e t h e p e r i s h a b l e p r o d u c e i s s u f f ic i e n tl y

d e n s e a n d i m p o r t a n t t o w a r r a n t t h e d i s p o n i bi l it y o f re l ia b l e d a t a o n p a s t , p r e - e x p e r i-

m e n t a l l o s se s , a n d a l so o n t h e c o n t e m p o r a n e o u s l o ss e s o f s o m e s u i t a b le a d j a c e n t c o n -

t r o l a r e a s .

b. The action by nuclei o f ice formation

T h e a r ti fi c ia l s t im u l a t i o n o f r a i n f r o m s u p e r c o o l e d c l o u d s is n o r m a l l y b a s e d o n t h e

a s s u m p t i o n t h a t t h e n u m b e r o f n a t u r a l i c e - n u c l e i i s i n s u ff i c ie n t , i n t h o s e c l o u d s , t o

i n d u c e t h e i r p r e c i p i t a t io n a t t e m p e r a t u r e s w i t h in p r e va i li n g . O r g a n i z e d a t t e m p t s a t t h e

induction o f precipitation

b y s e e d i n g w i t h a d e q u a t e i c e - f o r m i n g n u c l e a n ts , a r e b e i n g

t h e r e f o r e c a r r ie d o u t i n v a r i o u s p l a ce s o f th e w o r l d , s i n ce a n u m b e r o f ye a r s . T h e

a r i s in g d i f f i cu l t ie s a r e m o s t l y d u e t o t h e n e c e s s i t y o f c a l c u l a t in g t h e s i g n if i c a n c e o f t h e

s u c c e ss a c h i e v e d , i f a n y ; s i g n i f ic a n t r e s u l ts w e r e o b t a i n e d b y a n u m b e r o f w o r k e r s i n

t h e p a s t d e s p i t e th e f a c t t h a t , o n t h e o t h e r h a n d , t h e u s e o f v e r y s im i l a r m e t h o d s y i e l d ed

i n o t h e r c a s e s r e s u l t s w h i c h w e r e e i t h e r n o n s i g n i f i c a n t o r , s o m e t i m e s , e v e n n e g a t i v e

s ig n i f i can t

decreases

o f p r e c i p i t a t i o n ) . N u c l e i u s e d f o r t h e p u r p o s e s ti ll u s u a l l y c o n -

s i st o f s i lv e r i o d i d e w h i c h i s e i t h e r d i sp e r s e d , a t s u i t a b l e m e t e o r o l o g i c a l c o n d i t i o n s ,

f r o m g r o u n d , b a s e d b u r n e r s o r f r o m a i r c r a ft . S o m e o f t h o s e t e c h n i q u e s a r e k n o w n t o

h a v e p r o d u c e d i n c r e a se s o f p r e c i p it a t i o n o f u p t o 2 0 ~ , e s p e ci a ll y i n m o u n t a i n o u s

a r e a s a n d u s u a ll y d u r i n g n o r m a l l y r a i n y se a s on s . T h e r e s ul ts o b t a i n e d a r e e v a l u a t e d

e i t h e r o n t h e b a s is o f a h i s t o ri c a l p e r i o d , o r a r e f e r e n c e a r e a a d j a c e n t t o t h e t a r g e t



Vol. 91, 1971/viii) C he m is tr y nd Properties of Atmospheric Nucleants 195

t e r r i t o r y , o r b o t h . B e t t e r s ti ll , u s e is o f t e n m a d e o f r a n d o m l y c h o s e n n o - s e e d i n g ,

p e r i o d s o n t h e t a r g e t a r e a i t s e l f .

T h e p o s s i b i li t y o f decreasingprecipitation by overseeding h a s a l s o b e e n d i s c u s s e d

i n th e p a s t [ 8] . A n e x p e r i m e n t a l i n v e s t i g a t i o n o f t h i s p o s s i b i li t y w o u l d o f f h a n d

a p p e a r t o b e a t t r a c ti v e i f c a r r ie d o u t i n s o m e m a r i t i m e a r e a s w h e r e l a rg e , s e a s o n a l

a n d l o c a l i z e d p r e c i p i t a t i o n s a r e k n o w n t o o c c u r . B r i e fl y , a n d f o r e x a m p l e , a t r a n s -

f o r m a t i o n o f a s u p e r c o o l e d l i qu i d c lo u d i n t o a n i c ed f o r m , b y a very large ex cess

o f s u i t a b l y - i n t r o d u c e d n u c le i m a y r e s u l t i n a n a b s e n c e o f p r e c i p i t a ti o n o n t h e

g r o u n d a n d a n a l m o s t q u a n t i t a t i v e t r a n s f o r m a t i o n o f t h a t c l o u d m a s s i n t o s m a ll

i ce p a r t i c l e s .

O n t h e b a s is o f p ri n c ip l e s m e n t i o n e d e a r l ie r i n t h is p a p e r dissipation o f supercooled

fogs w a s c a r r i e d o u t i n t h e p a s t b y m a n y w o r k e r s , u s i n g e i t h e r s o l i d C O 2 o r A g I o r

v o l a ti l e h y d r o c a r b o n s . T h e a i m w a s , a s u su a l , t o p r o v o k e a t r a n s i t i o n o f p h a s e e i t h e r

b y t h e u s e o f n u c l e i o r b y l o c a l u n d e r c o o l i n g s , t o a v a i l o n e s e l f o f t h e i r g r o w t h a t t h e

e x p e n s e o f t he r e m a i n i n g l i q u id , a n d t o i n d u c e t h e i r s u b s e q u e n t f a l l o u t. R e s u l t s o b -

t a i n e d b y t h e u s e o f i c e - f o r m i n g n u c l e i a n d / o r s u p e r c o o l i n g ) in f o g - c l e a r i n g a n d a r t i -

f ic i al s t im u l a t i o n o f r a i n w e r e , i n t h e p a s t , t h e o b j e c t o f a m u l t i t u d e o f t e c h n i c a l r e p o r t s

a n d a l s o o f s e v e r a l scientific p a p e r s . A l a r g e a m o u n t o f t h e m o s t u p - t o - d a t e r e li a b l e

w o r k o n t h i s s u b j e c t i s e it h e r d ir e c t ly d e s c ri b e d i n t h re e r e c e n t v o l u m e s o n w e a t h e r

m o d i f i c a t i o n p r o b l e m s [ 3 - 4 , 3 9 1 ] o r i s t h e r e i n c o n t a i n e d a s r e f e r e n c e s .

P r i n c ip l e s o f o v e r s e e d in g , u s i n g i c e - f o r m i n g n u c l ei i n a m o u n t s l a r g e r t h a n a b o u t

1 0 - 10 2 p e r c u b i c m e t e r o f a i r [ 3 9 2 - 3 9 3] w e r e a l s o a p p l i e d t o

hail suppression

C l e a r - c u t ,

s u c c e s s f u l a n d o u t s t a n d i n g w o r k i n c l u d e s r e c e n t o p e r a t i o n s c a r r i e d o u t i n K e n y a

w h e r e s il v er io d i d e w a s d i r e c tl y i n t r o d u c e d f r o m a i r c r a f t i n to d e v e l o p i n g t h u n d e rc e l l s

T . J . H E N DE RS ON , R ef . [4] p . 4 7 4 ). I t a l so in c lu d es th e u s e o f A g I an d P b I 2 w h ich i s

d e l iv e r e d i n t o s e l e c te d s p o ts o f a g a t h e r in g h a i l s t o r m b y m e a n s o f p r o p e r l y a i m e d

m i s s il e s a n d a t a s u i t a b l e t i m e o f t h e d e v e l o p m e n t o f t h e s t o r m [ 3 9 4 -3 9 6 ] . S i g n if i c a n tl y ,

o p e r a t i o n s b a s e d o n t h e u s e o f m i x e d g i a n t n u c l e i o f q u i t e s t r a i g h t f o r w a r d c o m p o s i -

t i o n , s u c h a s w h i t e c la y a n d p o r t l a n d c e m e n t , a r e a l s o k n o w n t o h a v e i n d u c e d c h a n g e s

a n d d i s s i p a ti o n o f t h u n d e r s t o r m s w h e n a d e q u a t e a m o u n t s a b o u t 4 0 k g ) o f t h o s e

p o w d e r e d s u b s t a n c e s w e r e t h e r e i n d r o p p e d f r o m a i r c r a f t [ 3 9 7 ] .

4 The inadvertent mod ification o f weather

T h e r e c e n t s p r e a d o f a u t o m o b i l e s , i n d u s tr i e s a n d u r b a n d e v e l o p m e n t s ha s l e d to a n

i m p r e s s i v e i n c r e a s e o f m i s c e l l a n e o u s p a r t i c u l a t e m a t t e r i n t h e a i r o n a g l o b a l s c a l e

[3 98 ]. I n a s m u c h m o s t o f t h is m a t e r i a l m a y , f r o m t h e h u m a n a n d e c o l o g ic a l p o i n t o f

v i e w , b e m e r e l y c l as s if ie d a s o f f e n s i v e a n d d i s t u r b i n g d i r t , i ts r o l e w i t h r e g a r d t o

w e a t h e r i s n o w k n o w n t o r a i s e c o n c e r n o f t h e m e t e o r o l o g i c a l p r o f e s s i o n . A c l as s ic

p a p e r o n t h e s u b j e c t w a s w r i t t e n b y S C nA EF BR i n 1 96 8 a n d w a s a c c o m p a n i e d b y a

n u m b e r o f p a r a l l e l c o n t r i b u t i o n s o n a s c a le o f o n e f u l l s c ie n t if i c s e s s io n [ 4]. B r i ef l y ,

t h e p o l l u t io n o f a i r i n t r o d u c e s e n o r m o u s a m o u n t s o f b o t h , i c e f o r m i n g a n d c o n d e n s a -



196 A.C. Montefinale, T. Montefinale and H. M. PapOe (Pageoph,

tion nuclei, into the atmosphere, and the consequences of this, though o ffhan d alarm-

ing, are now still difficult to foretell.

Aside of some offenders well-known since many years, such as the smoke from

extensive natural fires and industr ial activities, which may be expected to change cloud-

droplet populations [399-400], distributions, ice-nuclei contents in the air [401] and

precipitation patterns [402], and besides the ammonia in the atmosphere [27] which

could yield Aitken nuclei but also noxious transients [403] by chemical photooxida-

tion,

lead metal

from vehicular emissions is now thought to influence weather pheno-

mena to some extent. Modern, large cities are known to generate this assimilable [404]

poison in submicron-sized [405-410] particulates, at rates of some hundred weight

per day, and steady-state amounts of lead which floats in the atmosphere of an un-

ventilated city of a few million people, may be estimated in terms of tons. Since lead-

iodide is known to be an excellent ice-nucleator material, it is not surprising that

mixtures of automobile exhausts and gasoline vapors, with iodine vapor, were also

found to nucleate ice [411-414]. Large amoun ts of nat ura l ice-forming nuclei are

though t to be due to this reaction [415], depending also on the availability of iodine in

the air and hence on the geographic location of a city. The kinetics of photo and

hydrodec ay of those nuclei, which probably consist of

large

lead particulates whose

surface contains sites reacted with iodine, were not yet fully determined. There is no

doubt, however, that atmospheric lead may thus not only represent a direct hazard to

health but that it may also influence humans because of its interconnection with

weather phenomena.

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Received 28th January 1971)

recent advances in the chemistry and properties of atmospheric nucleants_a review

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