Applied Chemistry at Protein Interfaces

18 Bubble Scavenging and the Water-to-Air Transfer of Organic Material in the Sea DUNCAN C. BLANCHARD

Downloaded by CORNELL UNIV on October 16, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0145.ch018

Atmospheric Sciences Research Center, State University of New York, Albany, N.Y. 12222

Surface-active organic material (SAOM)in the sea tends to concentrate at the surface. It is brought there by diffusion, by Langmuir circulations, and by the surfaces of air bubbles rising through the water. These bubbles, produced primarily by breaking waves, not only carry SAOM to the surface but upon breaking eject it into the air. This process may account for the airborne droplets of sea water in the marine atmosphere which have SAOM concentrations several thousand times that found in the sea. The SAOM on the droplets has given marine meteorologists a tracer which enables them to understand better the role of these droplets in rain formation.

Although adsorptive bubble separation has been used commercially ^ for more than half a century [principally in froth flotation to sepa rate minerals from ores ( 1 ) ], oceanographers and marine meteorologists have only become aware of the importance of natural sea bubble processes in the past 20 years. Marine biologists consider the bubbles as a pos sible mechanism to convert dissolved organic material to particulate organic material (2); meteorologists consider the role of the bubbles both in the production of a sea-salt aerosol and the ejection of organic material into the atmosphere (3, 4). Organic Material in the Sea Composition of Sea Water. Sea water constitutes about 98% of all the water on the face of the earth and contains all of the naturally occurring elements known. It is about 96.5% water and 3.5% salt, most of which is in dissolved form. The salt content, or the salinity of sea 360 Baier; Applied Chemistry at Protein Interfaces Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

18.

Bubble

B L A N C H A R D

361

Scavenging

w a t e r , is defined as the t o t a l a m o u n t of s o l i d m a t e r i a l i n grams c o n t a i n e d i n 1 k g of sea w a t e r w h e n a l l the c a r b o n a t e has b e e n c o n v e r t e d to o x i d e , bromine a n d iodine have been replaced b y chlorine, a n d a l l organic m a t t e r has b e e n c o m p l e t e l y o x i d i z e d ( 5 ) .

D e p e n d i n g o n l o c a t i o n , the

s a l i n i t y w i l l b e a b o u t 32-38%< ( t h o u s a n d p a r t s / m i l l i o n ) , 35%c b e i n g t h e average. A l t h o u g h t h e s a l i n i t y m a y v a r y b y 1 0 % f r o m t h e average, i o n i c ratios r e m a i n constant w i t h i n n a r r o w l i m i t s for a l l oceanic w aters. T h i s c o n T

stancy is s u c h that oceanographers c a n p h y s i c a l l y d e s c r i b e sea w a t e r b y s i m p l y g i v i n g its t e m p e r a t u r e , pressure, a n d salinity. A s s h o w n i n T a b l e I ,


99.95%

of the s a l i n i t y of sea w a t e r is c o n t r i b u t e d b y e i g h t ions a n d

t h e l a r g e l y u n d i s s o c i a t e d b o r i c a c i d . T w o salts, N a C l a n d M g S 0 , ac4

c o u n t for 9 7 %

of the s a l i n i t y of sea w a t e r .

A s H o m e states i t

(6),

. . r o u g h l y s p e a k i n g , sea w a t e r is a n aqueous 0 . 5 M N a C l s o l u t i o n , 0 . 0 5 M i n M g S 0 , a n d c o n t a i n i n g a p i n c h o r trace of just a b o u t e v e r y t h i n g 4

imaginable." Concentration and Forms of Organic Material. C o n c e n t r a t i o n s of o r g a n i c m a t e r i a l i n sea w a t e r are orders of m a g n i t u d e less t h a n t h e s a l i n i t y , b u t the v a r i a t i o n s a r o u n d t h e m e a n are at least a n o r d e r magnitude more.

A l t h o u g h s a l i n i t y does n o t v a r y b y m o r e t h a n

of

10%

f r o m 35%c, the o r g a n i c content of sea w a t e r is o n l y o n the o r d e r of 1 p a r t p e r m i l l i o n ( a b o u t 1 m g / 1 ) , a n d v a r i a t i o n s of w e l l over 1 0 0 % are c o m mon.

T h e s e v a r i a t i o n s i n a s m a l l yet i m p o r t a n t c o n s t i t u e n t h a v e m a d e

studies difficult a n d tedious. O n l y w i t h i n the past 10 years h a v e d e t a i l e d studies b e e n m a d e o n the d i s t r i b u t i o n of o r g a n i c m a t e r i a l i n the sea. B e f o r e that r e l i a b l e i n s t r u m e n t a t i o n to d e t e r m i n e o r g a n i c c o n c e n t r a t i o n s , m u c h of w h i c h is b a s e d o n the o x i d a t i o n of the o r g a n i c m a t e r i a l a n d t h e

Table I .

Major Salt Constituents of Sea water Concentration,

Component ciNa+ S0 Mg + Ca K+ HC0 Br~ 4

c

2

2 +

b

c

H3BO3

Total

(115)

% of Total Salt 55.04 30.61 7.68 3.69 1.16 1.10 0.41 0.19 0.07 99.95

18.980 10.543 2.465 1.272 0.400 0.380 0.140 0.065 0.024 34.455

2

3

%

a

V a l u e s i n g per k g (%o) based on c h l o r i n i t y of 1 9 % . V a r i e s to give e q u i v a l e n t C 0 ~ d e p e n d i n g o n p H . V a l u e g i v e n is essentially true for p H 7.50 at 2 0 ° C . C o r r e s p o n d s to a s a l i n i t y of 3 4 . 3 2 5 % a

6

c

0

3

2

0

Baier; Applied Chemistry at Protein Interfaces Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

362

A P P L I E D

C H E M I S T R Y

A

T

P R O T E I N

I N T E R F A C E S

v a r i o u s means of d e t e c t i n g the resultant c a r b o n d i o x i d e ( 7 ) , h a d n o t b e e n developed. O r g a n i c m a t e r i a l exists i n one or m o r e forms ( d i s s o l v e d , p l a n k t o n i c , etc. ) at a l l depths i n the sea, b u t its mass c o n c e n t r a t i o n is greatest i n the e u p h o t i c z o n e w h i c h is the u p p e r 100 m of the sea w h e r e photosynthesis c a n occur.

T h i s r e g i o n is w h e r e most of the b u b b l e s are p r o d u c e d a n d

w h e r e t h e y i n t e r a c t w i t h the o r g a n i c m a t e r i a l .

I n this e u p h o t i c

o r g a n i c m a t e r i a l is f o u n d i n five different forms.

zone,

M o r e than 9 8 %

of

the mass of organics is n o n - l i v i n g i n e i t h e r d i s s o l v e d or p a r t i c u l a t e f o r m , w i t h the f o r m e r a b o u t 100 times the latter (8, 9).


p l a n k t o n , a n d fish c o n s t i t u t e less t h a n 2 %

P h y t o p l a n k t o n , zoo-

of the o r g a n i c m a t e r i a l i n

t h e sea. Phytoplankton. T h e c y c l e of o r g a n i c m a t e r i a l b e g i n s w i t h the p h y t o p l a n k t o n , u n i c e l l u l a r a n d c o l o n i a l p l a n t cells t h a t r a n g e i n size f r o m a b o u t 1 fx to 1 m m . O r g a n i c m a t e r i a l is p r o d u c e d i n the cells, p r i m a r i l y b y photosynthesis a n d the u t i l i z a t i o n of i n o r g a n i c n u t r i e n t s . T h e most i m p o r t a n t n u t r i e n t s a n d t h e i r respective concentrations a r e : s o l u b l e i n o r g a n i c phosphates ( 0 . 1 - 3 . 5 /xg a t o m s / 1 ) , nitrates ( 0 . 1 - 4 3 ) , a n d the n i trites

(0.1-3.5).

The

c o n c e n t r a t i o n of

p h y t o p l a n k t o n cells is h i g h l y

v a r i a b l e a n d , d e p e n d i n g o n the t i m e of y e a r a n d l o c a t i o n , m a y v a r y f r o m less t h a n 1 0 to m o r e t h a n 1 0 c e l l s / 1 . T h e n u m b e r of species m a y r a n g e 3

8

f r o m less t h a n 10 to greater t h a n 250 (10).

T h e a n n u a l net o r g a n i c c a r b o n

p r o d u c t i o n b y p h y t o p l a t i k t o n for a l l the oceans has b e e n e s t i m a t e d at about 2 Χ

10

10

m e t r i c tons ( I I ) .

T h e w e i g h t of the o r g a n i c m a t e r i a l

w o u l d b e t w i c e this v a l u e since it is c u s t o m a r y to m u l t i p l y b y t w o to c o n v e r t the w e i g h t of o r g a n i c c a r b o n to that of o r g a n i c m a t e r i a l .

As

s h o w n i n T a b l e I I , most of the m a t e r i a l is p r o d u c e d o n the o p e n ocean, a l t h o u g h the p r o d u c t i o n rate p e r u n i t c o l u m n of w a t e r ( c a r b o n / m / y r ) 2

c a n b e h i g h e r i n the coastal z o n e a n d i n u p w e l l i n g areas. Zooplankton and Fish. A p o r t i o n of the o r g a n i c c a r b o n

produced

b y the p h y t o p l a n k t o n is u s e d b y the z o o p l a n k t o n . T h e z o o p l a n k t o n c o n stitute a n a m a z i n g l y diverse g r o u p of a n i m a l s that differ w i d e l y i n size, shape, a n d c o n c e n t r a t i o n . T h e i r size ranges f r o m less t h a n a m i l l i m e t e r to m o r e t h a n a meter. T h e n u m b e r s of z o o p l a n k t o n i n the e u p h o t i c z o n e of the Sargasso Sea, for e x a m p l e , are of the o r d e r of 2 0 0 - 4 0 0 / m , 3

s t i t u t i n g a mass of 2 - 4 m g / m

3

(12)

or a b o u t 0.002-0.004 p p m .

con

T h i s is

f a r less t h a n the c o n c e n t r a t i o n of the n o n - l i v i n g d i s s o l v e d a n d p a r t i c u l a t e o r g a n i c m a t e r i a l . F i s h constitute o n l y 0 . 0 0 2 % of the t o t a l a m o u n t of or g a n i c m a t e r i a l i n the e u p h o t i c z o n e a n d for o u r purposes c a n b e n e g l e c t e d . Dissolved and Particulate Material.

It was e a r l i e r b e l i e v e d t h a t

the d i s s o l v e d a n d p a r t i c u l a t e o r g a n i c m a t e r i a l was p r o d u c e d p r i m a r i l y b y the d e a t h a n d b r e a k d o w n of the p h y t o p l a n k t o n a n d z o o p l a n k t o n .


It is

18.

Bubble

B L A N C H A R D

363

Scavenging

n o w c l e a r t h a t t h e sequence of events is m o r e c o m p l i c a t e d . T h e r e is n o d o u b t that b o t h d i s s o l v e d a n d p a r t i c u l a t e o r g a n i c m a t e r i a l exists at a l l d e p t h s of the sea.

A l t h o u g h its c o n c e n t r a t i o n is less t h a n t h a t i n the

e u p h o t i c zone, the d i s s o l v e d o r g a n i c m a t e r i a l c a n b e f o u n d u n i f o r m l y d i s t r i b u t e d at depths greater t h a n 2 0 0 m (8, 13,14, 15).

This uniformity

i n the v e r t i c a l is not f o u n d i n the h o r i z o n t a l d i r e c t i o n , i n d i c a t i n g t h a t the o r g a n i c m a t e r i a l varies w i t h w a t e r mass. T h e o r g a n i c m a t e r i a l i n the d e e p waters represents f r o m 30 to 150 times the average a n n u a l p r o d u c t i o n of o r g a n i c m a t e r i a l i n the sea (16).

S i n c e most of the n e w l y p r o -

d u c e d o r g a n i c m a t e r i a l i n the e u p h o t i c z o n e is r a p i d l y u s e d or posed,

leaving only perhaps

1%

as a c o n t r i b u t i o n to t h e


organics i n the d e e p w a t e r s , it is possible that most of the

decom-

dissolved deep-water

m a t e r i a l ". . . m a y represent the g r a d u a l a c c u m u l a t i o n of s e v e r a l h u n d r e d s o r thousands of years . . . " ( 9 ) .

T h e l o n g - t e r m s u r v i v a l of o r g a n i c m a t e r i a l

i n the d e e p waters m a y o c c u r because the b a c t e r i a , w h i c h b r e a k d o w n the o r g a n i c m a t e r i a l i n the e u p h o t i c zone, c a n n o t d o so as easily i n t h e deep water

(17).

T h e p r o d u c t i o n of d i s s o l v e d m a t e r i a l i n the e u p h o t i c z o n e occurs not o n l y after the d e a t h of p h y t o p l a n k t o n organisms b u t also b y the d i f f u s i o n of o r g a n i c m o l e c u l e s f r o m the o r g a n i s m s d u r i n g n o r m a l g r o w t h (18,

19,

20, 21 ). " A l m o s t e v e r y p h y t o p l a n k t o n o r g a n i s m s t u d i e d has b e e n s h o w n to diffuse s m a l l o r g a n i c m o l e c u l e s i n t o its m e d i u m , a n d almost a n y o r g a n i c m o l e c u l e of b i o l o g i c a l interest has b e e n s h o w n to be g i v e n off b y some o r g a n i s m s " (22).

Z o o p l a n k t o n release e n o u g h a m i n o acids i n one m o n t h

to e q u a l the a m o u n t i n s o l u t i o n (18),

a n d the rate of release of d i s s o l v e d

m a t e r i a l b y g r o w i n g p h y t o p l a n k t o n is 1 0 - 3 0 % photosynthesis (19,

of t h a t b e i n g

fixed

by

21).

T h e c o m p o s i t i o n of the d i s s o l v e d o r g a n i c m a t e r i a l has b e e n the s u b ject of n u m e r o u s papers (23).

Jeffrey (24)

f o u n d that l i p i d s

extractable o r g a n i c c o m p o u n d s ) m a k e u p 1 0 - 2 0 %

(chloroform

of the d i s s o l v e d or-

g a n i c m a t e r i a l i n s e m i - t r o p i c a l waters a n d suggested i t m a y b e f r o m 40 to 5 5 % i n A n t a r c t i c waters. P r o b a b l y the b u l k of the d i s s o l v e d o r g a n i c m a t e r i a l is c o m p o s e d of proteins a n d p r o t e i n - d e r i v e d m e t a b o l i t e s ( 25, 26 ). T h e free a m i n o a c i d s , f a t t y a c i d s , sugars, a n d p h e n o l s represent less t h a n 1 0 % of the o r g a n i c m a t e r i a l i n sea w a t e r . T h e i d e n t i f i c a t i o n of i n d i v i d u a l proteins a n d p e p t i d e s has p r o v e d difficult, a n d d a t a h a v e b e e n p u b l i s h e d o n l y o n the a m i n o a c i d s p e c t r u m of the proteinaceous m a t e r i a l (25).

A

m a j o r i t y of the a m i n o acids o c c u r as c o m p o u n d s w i t h m o l e c u l a r w e i g h t s b e t w e e n 400 a n d 10,000 Concentration

(25).

of Organic Material at the Surface of the Sea

Although Benjamin Franklin

(27)

w a s the first to s h o w t h a t the

slicks c o m m o n l y o b s e r v e d o n the surface of the sea w e r e c o m p o s e d of


364

A P P L I E D

C H E M I S T R Y

A

T

P R O T E I N

I N T E R F A C E S

t h i n layers or m o n o l a y e r s of surface-active o r g a n i c m a t e r i a l , i t h a d b e e n w e l l k n o w n t h a t these slicks w e r e p r o d u c e d b y b i o l o g i c a l a c t i v i t y i n t h e sea. E u r o p e a n fishermen of t h e 18th c e n t u r y l o c a t e d schools of fish b y first l o o k i n g for h i g h concentrations of slicks o n t h e surface of the sea (-28).

T h e scientific c o m m u n i t y d i d not g i v e serious a t t e n t i o n to the

c o r r e l a t i o n of slicks to organisms w i t h i n the sea (29, 30)

u n t i l 250 years

later. Surface-Active Material and Surface Pressure.

If a s l i c k or a n y

surface-active film is to s p r e a d a n d increase i n area it m u s t exert a h o r i z o n t a l surface pressure. T h e r e l a t i o n b e t w e e n surface pressure a n d s u r -


face tension is ( b o t h m e a s u r e d i n d y n e s / c m ) :

Pi

where y

w

P

f

=

=

=

7w

—

7f

surface t e n s i o n of t h e w a t e r , y = f

surface t e n s i o n of the

film,

the surface pressure exerted b y the film. A p o s t i v e surface pressure

means t h a t the surface t e n s i o n of the film is less t h a n t h a t of t h e w a t e r , a n d i f the surface pressure increases for a n y reason, the surface t e n s i o n of t h e film m u s t decrease. O n a c l e a n , q u i e t , u n c o n f i n e d b o d y of w a t e r a film c a n s p r e a d u n t i l it becomes a m o n o l a y e r u n d e r zero surface pressure, at w h i c h p o i n t y

w

equals y . f

N a t u r e seldom provides such ideal condi-

tions, a n d a l l surface-active m a t e r i a l o n n a t u r a l b o d i e s of w a t e r do not s p r e a d to f o r m u n c o m p r e s s e d m o n o l a y e r s .

B o t h the w a t e r a n d the a i r

are u s u a l l y i n m o t i o n a n d p r o v i d e h o r i z o n t a l surface stresses w h i c h p r o d u c e surface pressure i n the

films.

S u r f a c e - a c t i v e o r g a n i c m a t e r i a l u n d o u b t e d l y exists o n the surface of n e a r l y a l l fresh w a t e r bodies, as was f o u n d o n a l l those s t u d i e d i n E n g l a n d (31) . T h e surface pressures p r o d u c e d b y this m a t e r i a l w e r e f r o m 1 to greater t h a n 30 d y n e s / c m . B y a p p e a r a n c e , the films w e r e a s s u m e d to b e c o m p o s e d of n a t u r a l p r o t e i n c o m p o u n d s .

W h e n the w i n d s w e r e suffi-

c i e n t l y strong a surface stress w a s generated w h i c h d r o v e the films d o w n w i n d to the lee side of the l a k e w h e r e they c o l l a p s e d . e x a m i n a t i o n of

Microscopic

this p r o t e i n m a t e r i a l r e v e a l e d m a n y s m a l l

organisms

i m b e d d e d i n the m a t e r i a l , p r e s u m a b l y t h e r e to eat it. I n d e e d , C h e e s m a n (32)

reports that a s n a i l uses his foot as a m i n i a t u r e L a n g m u i r t r o u g h

a n d is a b l e to compress a n d collapse surface-active films to o b t a i n c l u m p s of o r g a n i c m a t e r i a l that are e a s i l y eaten. V i s i b l e slicks are often seen w i t h i n 100 k m of l a n d .

P o s s i b l y these

are c a u s e d b y m a n - m a d e p o l l u t i o n ( o i l f r o m ships, sewage, etc.) by

seaweeds a n d h i g h e r p l a n k t o n concentrations n e a r t h e

( T a b l e I I ) . O n the o p e n sea, slicks are less c o m m o n (30).

and/or continent

T h i s does not

m e a n t h a t o r g a n i c m a t e r i a l is absent f r o m the surface b u t s i m p l y t h a t the surface-active m o l e c u l e s are not present i n sufficient q u a n t i t y to p r o -


Scavenging

365

d u c e a surface pressure that w i l l cause d a m p i n g of c a p i l l a r y w a v e s

(33)

18.

B L A N C H A R D

Bubble

w h i c h s m o o t h the sea a n d expose the s l i c k - l i k e or glassy a p p e a r a n c e of a m o n o l a y e r u n d e r pressure. Sea Surface Composition.

S a m p l i n g of the u p p e r 150 μΐη (34)

of

b o t h t h e A t l a n t i c a n d the P a c i f i c a n d i n b o t h s l i c k - c o v e r e d a n d n o n - s l i c k areas has r e v e a l e d surface-active m a t e r i a l s (35, 36).

T h e greatest q u a n

tities w e r e f o u n d i n the b i o l o g i c a l l y - r i c h areas, b u t e v e n the i n a c t i v e w a t e r s c o n t a i n e d some m a t e r i a l . T h e p r i m a r y surface-active, c h l o r o f o r m soluble organic components

were

alcohols, a n d h y d r o c a r b o n s .

A l t h o u g h these species w e r e b o t h i n a n d

free f a t t y acids, f a t t y esters, f a t t y


out of s l i c k areas a n d i n samples t a k e n a f e w meters b e n e a t h the surface, there w e r e i n d i c a t i o n s that a h i g h p r o p o r t i o n of t h e h i g h e r m o l e c u l a r w e i g h t a n d less w a t e r - s o l u b l e f a t t y acids a n d alcohols w e r e i n the s l i c k areas. B o t h G a r r e t t ( 3 5 ) a n d Jarvis et al. (36)

suggest that t h e surface

pressure i n the s l i c k areas forces the m o r e w a t e r - s o l u b l e a n d less surfacea c t i v e species f r o m the surface i n t o t h e u n d e r l y i n g w a t e r . T h e m i c r o l a y e r at the surface of the sea w a s f o u n d b y H a r v e y

(37)

to c o n t a i n b a c t e r i a , dinoflagellates, a n d other p l a n k t o n i n m u c h h i g h e r concentrations t h a n i n the w a t e r b e n e a t h .

Other workers have

found

b o t h o r g a n i c a n d i n o r g a n i c n i t r o g e n a n d p h o s p h o r u s c o n c e n t r a t e d i n the surface m i c r o l a y e r (38),

b a c t e r i a e n r i c h m e n t ( 3 9 ) , e n r i c h m e n t s of D D T

of u p to 1 0 times over that of sub-surface w a t e r (40),

a n d enrichments

5

of h e a v y metals i n the surface films of p e r h a p s 1 0

4

(41).

A l t h o u g h the

h e a v y m e t a l a n d D D T e n r i c h m e n t was f o u n d w i t h i n 50 k m of the U . S . E a s t e r n Shore, i t illustrates h o w m a t e r i a l i n j e c t e d or p r o d u c e d i n the m a i n b o d y of the sea c a n b e c o m e c o n c e n t r a t e d at t h e surface.

It is p o s s i b l e

that s o m e of this surface c o n c e n t r a t i o n , e s p e c i a l l y near l a n d , is f a l l o u t of m a t e r i a l f r o m the atmosphere. T h e c y c l e of surface-active m a t e r i a l i n the sea is s h o w n s c h e m a t i c a l l y i n F i g u r e 1.

M a n - m a d e sources are u n i m p o r t a n t . T h e a n n u a l i n p u t of

Table II.

Division of the Ocean into Provinces According to Their Level of Primary Organic Production a

Percentage of Ocean

Province O p e n ocean Coastal zone U p w e l l i n g areas Total 6

a

6

90 9.9 0.1

Area

Mean productivity (grams of carbon/ m /yr)

(km )

2

2

326 Χ 10 36 Χ 10 3.6 Χ 1 0

6 6 5

Total productivity (10 tons of carbon/yr) 9

50 100 300

D a t a f r o m R e f . 11. Includes offshore areas of h i g h p r o d u c t i v i t y .


16.3 3.6 0.1 20.0

366

A P P L I E D

C H E M I S T R Y

A

T

P R O T E I N

I N T E R F A C E S

WIND GENERATED AEROSOL I BURSTING BUBBLES I EVAPORATION BIOLOGICAL FEEDING J

SEA SURFACE- MONOLAYERS

DECOMPOSITION THROUGH PHOTOCATALYZED OXIDATION

|"

1

t

DISSOLUTION -»WAVE ACTION

UPWELLING DIFFUSION CONVECTION

BUBBLES

i

BIOLOGICAL SOURCES PRODUCTION


ADSORBED ON PARTICULATE MATTER DISSOLVED

MAN-MADE SOURCES

Figure

1. Cycle of surface-active material in the sea. with permission of the author.

From

Garrett (114)

o i l i n t o the seas b y m a n ( r i v e r discharges, spills f r o m ships, etc.)

plus

the a t m o s p h e r i c f a l l o u t o f p e t r o l e u m p r o d u c t s is several orders o f m a g n i t u d e less t h a n t h e o r g a n i c m a t e r i a l p r o d u c e d a n n u a l l y i n the e u p h o t i c z o n e ( 42, 43 ). E v e n w e r e i t to concentrate e n t i r e l y at the surface i t w o u l d p r o d u c e a film o n l y 58 A t h i c k , about t w i c e the thickness o f a m o n o l a y e r I n a d a y o r t w o , s u c h a film w o u l d b e r e m o v e d f r o m the sea b y

(42).

b a c t e r i o l o g i c a l d e g r a d a t i o n , m i x e d d o w n w a r d i n t o the sea a n d d i l u t e d , or t r a n s p o r t e d i n t o t h e atmosphere. Langmuir Circulations. M o s t o f the o r g a n i c m a t e r i a l i n a surface m o n o l a y e r reaches the surface b y m o l e c u l a r d i f f u s i o n , a i d e d g r e a t l y b y t u r b u l e n c e o r o r g a n i z e d c o n v e c t i v e m o t i o n s (33). T h e s e o r g a n i z e d m o tions, c a l l e d L a n g m u i r c i r c u l a t i o n s , a p p e a r to b e v e r y i m p o r t a n t i n the m i x i n g o f w a t e r i n the u p p e r f e w tens o f meters i n the sea. F i r s t n o t e d b y L a n g m u i r o n t h e sea, b u t s t u d i e d i n d e t a i l o n a fresh-water l a k e (44), t h e y are w a t e r m o t i o n s i n the f o r m o f alternate left a n d r i g h t h e l i c a l vortexes

( F i g u r e 2 ) that h a v e t h e i r axes a p p r o x i m a t e l y p a r a l l e l t o t h e

d i r e c t i o n o f the w i n d .

T h e s e vortexes p r o d u c e

alternate lines o f c o n -

v e r g e n c e a n d d i v e r g e n c e o n t h e surface w h i c h are also l i n e d u p w i t h the w i n d . O r g a n i c - r i c h w a t e r is c a r r i e d to the surface i n regions o f u p welling

(surface

divergence)

where

molecular

d i f f u s i o n enables t h e

surface-active m o l e c u l e s t o r e a c h the a i r - w a t e r interface.

T h i s surface

w a t e r p l u s the molecules is c a r r i e d i n t o the r e g i o n o f c o n v e r g e n c e w h e r e the w a t e r sinks l e a v i n g a n a c c u m u l a t i o n o f surface-active m a t e r i a l w h i c h is c o m p r e s s e d i n t o the v i s i b l e surface slicks o r w i n d r o w s that a r e a l i g n e d w i t h the w i n d (2, 45, 46). T h e s e l o n g lines o f c o m p r e s s e d

surface-active

films are 2 0 - 5 0 m apart o n the sea (47), a n d , a l t h o u g h not p r o v e d , the


18.

Bubble

B L A N C H A R D

367

Scavenging

L a n g m u i r c i r c u l a t i o n s that p r o d u c e the lines are t h o u g h t to e x t e n d that d e e p i n t o the sea. T h e l i t t l e - u n d e r s t o o d c o u p l i n g b e t w e e n the sea a n d the a i r c a n p r o d u c e c i r c u l a t i o n s t h a t transport o r g a n i c m a t e r i a l to t h e surface a n d compress i t i n t o v i s i b l e slicks t h a t f o r m lines p a r a l l e l to the w i n d .

A d e t a i l e d r e v i e w of the d y n a m i c s of L a n g m u i r c i r c u l a t i o n s

c a n be f o u n d i n a p a p e r b y F a l l e r

(48).

Formation and Concentration of A i r Bubbles in the Sea. A i r b u b bles i n t h e sea, the final m e c h a n i s m l i s t e d i n F i g u r e 2, a i d i n t r a n s p o r t i n g o r g a n i c m a t e r i a l a n d m a y c o n v e r t d i s s o l v e d o r g a n i c m a t e r i a l to p a r t i c u l a t e organic material.

F o r years i t h a d b e e n t h o u g h t t h a t the z o o p l a n k t o n ,


most of w h i c h are filter feeders a n d e s p e c i a l l y a d a p t e d f o r f e e d i n g

on

particulate organic material, fed primarily on phytoplankton a n d the par t i c u l a t e m a t e r i a l f o r m e d f r o m d e a d p h y t o p l a n k t o n . T h e r e w a s no k n o w n m e c h a n i s m b y w h i c h t h e z o o p l a n k t o n c o u l d u t i l i z e the vast store

of

d i s s o l v e d o r g a n i c m a t t e r i n the sea u n t i l 1963 w h e n b u b b l e s w e r e r e p o r t e d to p r o v i d e s u c h a m e c h a n i s m

(2).

B u b b l e s m a y b e p r o d u c e d b y several m e c h a n i s m s , a m o n g t h e m b i o l o g i c a l , w a t e r t e m p e r a t u r e changes, p r e c i p i t a t i o n , a n d w h i t e c a p s .

As a

result of the i n v e r s e r e l a t i o n b e t w e e n t e m p e r a t u r e a n d the s o l u b i l i t y of air i n w a t e r , i m m e n s e q u a n t i t i e s of a i r are g i v e n off b y t h e sea d u r i n g p e r i o d s of w a r m i n g . F o r e x a m p l e , d u r i n g M a r c h t h r o u g h O c t o b e r i n the G u l f of M a i n e , a b o u t 3 Χ 1 0 c m 5

3

of o x y g e n leave e a c h square m e t e r of

WINDROWS OR SLICKS AT

Figure 2.

Production

of slicks by Langmuir circulations of the sea

in the surface waters


368

A P P L I E D

the surface of the sea (49).

C H E M I S T R Y

A

T

P R O T E I N

I N T E R F A C E S

I f this w e r e i n the f o r m of a i r b u b b l e s of 50

μτη d i a m e t e r , the r a t h e r s t a g g e r i n g n u m b e r of 2.5 Χ 1 0

7

bubbles/m /sec

w o u l d r e a c h the surface of the sea! T h i s is not o b s e r v e d . a i r crosses t h e interface b y gaseous diffusion.

2

M o s t of this

T h e air supersaturation

n e e d e d for b u b b l e f o r m a t i o n i n the sea p r o b a b l y occurs o n l y u n d e r s p e c i a l situations n o t significant o n the g l o b a l scale

(3).

P r e c i p i t a t i o n b o t h i n the f o r m of r a i n a n d s n o w c a n p r o d u c e b u b b l e s o n the sea (3).

B u b b l e s f r o m m o d e r a t e r a i n intensities h a v e diameters

of less t h a n 200 μτη, a n d those p r o d u c e d b y s n o w are less t h a n 100 μΐη. H o w e v e r , since these b u b b l e s are p r o d u c e d o n l y i n t h e u p p e r f e w c e n t i meters of the sea a n d o n l y d u r i n g the p r e c i p i t a t i o n , t h e y d o not

con


stitute the m a j o r source of b u b b l e s i n the sea. T h e p r e c i p i t a t i o n of the c o n t i n e n t a l aerosol into the sea has b e e n suggested as a source of b u b b l e s ( 5 0 ) , b u t since this aerosol is c o m p o s e d of p a r t i c l e s p r i m a r i l y less t h a n 100 μτη d i a m e t e r , a n d since w a t e r drops of 100 μτη d i a m e t e r p r o d u c e n o b u b b l e s w h e n they f a l l i n t o t h e sea ( 3 ) , i t is u n l i k e l y t h a t the c o n t i n e n t a l aerosol p r o d u c e s a significant q u a n t i t y of b u b b l e s . T h e major source of b u b b l e s is t h e w h i t e c a p s or b r e a k i n g w a v e s w h i c h f o r m w h e n e v e r the w i n d speed exceeds 3 - 4 m / s e c ( 5 1 , 52).

(3)

When

a w a v e breaks l a r g e q u a n t i t i e s of e n t r a p p e d a i r are c a r r i e d i n t o the sea. T h i s a i r p r o d u c e s b u b b l e s t h a t range i n size f r o m less t h a n 100 μχη to several millimeters diameter ( 3 ) .

T h e b u b b l e - s i z e d i s t r i b u t i o n is h e a v i l y

w e i g h t e d t o w a r d t h e s m a l l e n d , v a r y i n g i n v e r s e l y w i t h the D p o w e r of d i a m e t e r D.

4

or t h e D

5

B e c a u s e of this s t r o n g inverse r e l a t i o n s h i p , a m a

j o r i t y of the b u b b l e s are less t h a n 200 μπι d i a m e t e r . A b o u t 1 0 / m 8

3

have

b e e n f o u n d i n the u p p e r m e t e r of t h e sea a f e w seconds after a w h i t e c a p has f o r m e d (3).

T h e o n l y d i r e c t measurements of b u b b l e - s i z e d i s t r i b u

t i o n i n the sea w e r e d o n e i n waters n e a r the shore a n d i n w h i t e c a p s t h a t w e r e r e l a t i v e l y s m a l l b y open-ocean

standards.

T h e a r e a l d i s t r i b u t i o n of b u b b l e s p r o b a b l y increases i n d i r e c t p r o p o r t i o n to the w h i t e c a p coverage, a n d a b o v e the c r i t i c a l w i n d s p e e d of 3 - 4 m / s e c the latter is a f u n c t i o n of the w i n d s p e e d ( 5 1 , 52, 53).

Know

i n g this f u n c t i o n , the c l i m a t i c w i n d s c a n b e i n t e g r a t e d over a n y g i v e n area of the sea to o b t a i n the average c o v e r a g e of w h i t e c a p s . b e e n d o n e (51)

a n d is a b o u t 3 . 5 %

o v e r the w o r l d ocean.

T h i s has Assuming

this p e r c e n t a n d t h a t the u p p e r meter of the sea d i r e c t l y b e n e a t h t h e w h i t e c a p area contains b u b b l e densities of a b o u t 1 0 / m , b u b b l e densities 8

of

at least 3.5

Xl0 /m 4

3

3

c o u l d b e m a i n t a i n e d t h r o u g h o u t the entire

e u p h o t i c zone i f m i x i n g processes c o u l d operate sufficiently fast. ever, this does not h a p p e n .

How

M o s t of the b u b b l e s p r o d u c e d b y b r e a k i n g

waves u n d o u b t e d l y rise to the surface w i t h i n 3 0 - 6 0 sees ( b u b b l e s of 100 a n d 200 μτη d i a m e t e r h a v e rise speeds of 0.5 a n d 1.5 c m / s e c , r e s p e c t i v e l y ) . H o w e v e r , t h r o u g h t u r b u l e n c e a n d the d o w n w e l l i n g i n the

convergent


Scavenging

369

regions of the L a n g m u i r c i r c u l a t i o n s w h e r e d o w n w a r d speeds of

3-6

18.

Bubble

B L A N C H A R D

c m / s e c (2)

h a v e b e e n o b s e r v e d a n d the p o s i t i v e l y b u o y a n t Sargassum

are h e l d b e n e a t h the surface

s m a l l a i r b u b b l e s c o u l d easily b e

(54),

c a r r i e d m a n y meters b e n e a t h the surface of the sea.

N o d a t a o n the

v e r t i c a l g r a d i e n t of b u b b l e s i n the sea are a v a i l a b l e to c o n f i r m this. With

Bubbles and the Formation of Particulate Organic Material.

s u c h h i g h concentrations of b u b b l e s i n the surface layers of the sea, i n t e r a c t i o n s b e t w e e n the b u b b l e s a n d the d i s s o l v e d o r g a n i c m a t e r i a l are significant. (3, 56)

Although bubble-organic

t e r i a l at a i r - w a t e r interfaces ( 5 5 ) Downloaded by CORNELL UNIV on October 16, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0145.ch018

i n t e r a c t i o n has

been

discussed

a n d m u c h b a s i c w o r k existed o n the a d s o r p t i o n of o r g a n i c m a

b u b b l e separation techniques

(I),

as w e l l as the l o n g p r a c t i c e of u s i n g i t was not u n t i l 1962 that d e t a i l e d

d a t a w e r e o b t a i n e d . I n that y e a r B a y l o r et al. (57)

found that inorganic

p h o s p h a t e ( P 0 ) , one of the n u t r i e n t s necessary for the g r o w t h of the 4

p h y t o p l a n k t o n , b e c a m e a t t a c h e d to a i r b u b b l e s i n sea w a t e r a n d w a s t r a n s p o r t e d to the surface.

B y c o l l e c t i n g the spray f r o m the b u r s t i n g

b u b b l e s t h e y w e r e a b l e to r e m o v e over 9 9 % of the P 0

4

f r o m the w a t e r .

W h e n t h e e x p e r i m e n t w a s r e p e a t e d i n a r t i f i c i a l sea w a t e r ( n o

organic

m a t e r i a l ) or i n P 0 - t a g g e d d i s t i l l e d w a t e r ( a b o u t 1 ^.g A / 1 ) , no P 0 4

4

was

r e m o v e d b y b u b b l i n g . T h i s i n d i c a t e d that s o m e of the d i s s o l v e d o r g a n i c m a t e r i a l i n n a t u r a l sea w a t e r w a s r e q u i r e d for P 0

4

removal.

surface-active a n i o n b i n d e r s b e c a m e a t t a c h e d to the P 0

4

Perhaps

a n d to the b u b

b l e , b o t h materials t h e n b e i n g c a r r i e d to the surface a n d ejected i n t o the atmosphere. P0

4

M e a s u r e m e n t s m a d e i n the u p p e r t e n meters of t h e v e r t i c a l

g r a d i e n t at sea s h o w e d that the g r a d i e n t i n c r e a s e d w i t h w i n d speed

i n a m a n n e r s u g g e s t i n g the associated increase i n b u b b l e p r o d u c t i o n w a s r e s p o n s i b l e for P 0

4

t r a n s p o r t to the surface. I n the f o l l o w i n g y e a r Sutcliffe

et al. (2) r e p o r t e d that the b u b b l e s p r a y was not o n l y r i c h i n P 0

4

b u t also

i n a n u n i d e n t i f i e d surface-active m a t e r i a l . T h e s e w o r k e r s (2, 57) also f o u n d a great d e a l of p a r t i c u l a t e o r g a n i c m a t e r i a l i n the spray. T h i s p a r t i c u l a t e m a t e r i a l w a s v i s u a l l y i n d i s t i n g u i s h able f r o m some of the p a r t i c u l a t e s f o u n d i n the sea. T h e p a r t i c l e sizes w e r e not r e p o r t e d b u t p r o b a b l y r a n g e d f r o m a b o u t a m i c r o n to a f e w tens of m i c r o n s i n d i a m e t e r .

T h e i r organic nature was indicated b y their

s o l u b i l i t y i n c y c l o p e n t a n e a n d the

fluorescence

O r g a n i c m a t e r i a l f o u n d i n sea w a t e r (58)

of the r e s u l t i n g s o l u t i o n .

behaves i n a s i m i l a r m a n n e r .

B r i n e s h r i m p w e r e f o u n d to t h r i v e o n the p a r t i c l e s (59).

The production

of the p a r t i c l e s b y b u b b l i n g a c t i o n was v e r i f i e d b y c a r r y i n g o u t

the

b u b b l i n g i n w a t e r w h i c h h a d b e e n passed t h r o u g h filters of 0.45 μΐη p o r e size. T h e m e c h a n i s m s of p a r t i c l e p r o d u c t i o n is not clear. T h e surfaceactive, o r g a n i c - P 0 and compressed

4

films m a y b e c a r r i e d to the surface b y the b u b b l e s

there b e y o n d

the collapse pressure to f o r m c o l l o i d a l


370

A P P L I E D

C H E M I S T R Y

A

T

P R O T E I N

I N T E R F A C E S

m i c e l l a e or l o n g fibers w h i c h c o u l d coalesce to generate the o r g a n i c p a r ticles. T h e b u b b l e s p r a y is t h e n generated f r o m this m a t e r i a l . L a t e r w o r k suggests U V r a d i a t i o n m a y also be i n v o l v e d i n the

film-to-particle

con

I f o r g a n i c p a r t i c l e s are generated f r o m the collapse of surface

films,

version

(60).

then particulate organic material should be found concentrated beneath the s l i c k areas ( c o n v e r g e n t z o n e s ) of the L a n g m u i r c i r c u l a t i o n s .

Light

s c a t t e r i n g e v i d e n c e i n these regions (2)

How

appears to c o n f i r m this.

ever, no a t t e m p t was m a d e to estimate t h e p e r c e n t increase i n p a r t i c l e s here as o p p o s e d to adjacent n o n - s l i c k c o v e r e d areas of the sea.


findings

have been reproduced

P a r t i c l e p r o d u c t i o n b y b u b b l i n g is c o n f i r m e d (62, 63) w o r k e r (17) (64)

These

(61). a l t h o u g h one

suggested the i m p o r t a n c e of b a c t e r i a i n the process. M e n z e l ' s

w o r k i n 1966, u s i n g w h a t a p p e a r e d to be a d e q u a t e controls, c o u l d

n o t find a n y o r g a n i c p a r t i c l e p r o d u c t i o n b y b u b b l i n g . s t u d y a p p e a r i n g i n 1969 ( 65)

A more

recent

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

earlier disagreements arose i n p a r t f r o m differences i n the size of

filters

u s e d a n d f u r t h e r t h a t the presence of p a r t i c l e s i n h i b i t s p a r t i c l e p r o d u c t i o n b y b u b b l i n g . A d d i t i o n a l p a r t i c l e s w i l l f o r m o n l y i f the steady-state c o n c e n t r a t i o n is r e m o v e d .

N o e x p l a n a t i o n f o r the p a r t i c l e i n h i b i t i o n exists.

Particulate Organic Material from Bubbles Going into Solution. A l t h o u g h w o r k o n this subject has ceased, e a r l y papers (2, 57)

suggest

that some of t h e s m a l l b u b b l e s p r o d u c e d b y b r e a k i n g w a v e s m i g h t go i n t o s o l u t i o n c o m p l e t e l y b e f o r e r e a c h i n g the surface, thus r e l e a s i n g a b s o r b e d o r g a n i c m a t e r i a l i n the f o r m of c o l l o i d a l m i c e l l e s w h i c h , u p o n a g g r e g a t i o n , c o u l d f o r m p a r t i c l e s of a size usable b y the

filter-feeding

z o o p l a n k t o n . T h e r e is no d o u b t that s m a l l b u b b l e s i n sea w a t e r , espe c i a l l y those less t h a n 100 μτη d i a m e t e r , go into s o l u t i o n r a t h e r q u i c k l y , e v e n i n w a t e r that is 1 0 0 %

s a t u r a t e d w i t h air. I n p r o p e r l y d e s i g n e d

l a b o r a t o r y experiments this c a n be o b s e r v e d w i t h the u n a i d e d eye.

The

reason is that a l l b u b b l e s h a v e a n i n t e r n a l pressure t h a t is h i g h e r t h a n that i n the w a t e r just o u t s i d e the b u b b l e b y 2y/R

w h e r e γ is the surface

tension a n d R the b u b b l e r a d i u s . B e c a u s e of this surface c u r v a t u r e effect w h i c h increases as b u b b l e size decreases, a l l b u b b l e s e v e n t u a l l y go into solution even i n air-saturated water.

B u b b l e s s m a l l e r t h a n 300 μτη d i

ameter w i l l be f o r c e d i n t o s o l u t i o n e v e n i n w a t e r that is u p to

102%

saturated, a n d b u b b l e s less t h a n 20 μπι w i l l dissolve i n w a t e r t h a t is u p to 1 1 5 % s a t u r a t e d (3).

T h i s pressure effect p r o d u c e d a r a p i d rate of

decrease a n d s u b s e q u e n t d i s a p p e a r a n c e of the s m a l l b u b b l e s w h e n r a i n d r o p s a n d snowflakes f e l l i n t o sea w a t e r

produced

(3).

It is l i k e l y that a significant p o r t i o n of the b u b b l e s p r o d u c e d b r e a k i n g w a v e s go i n t o s o l u t i o n b e f o r e t h e y r e a c h the surface.

by

Probably


18.

B L A N C H A R D

Bubble

371

Scavenging

at least 2 0 % of the b u b b l e s b e n e a t h a b r e a k i n g w a v e are less t h a n 100 / m i . B u b b l e s of 100 μτη a n d 50 μτη at a d e p t h of 1 m i n sea w a t e r 1 0 0 % saturated w i t h a i r w i l l go into s o l u t i o n i n 250 a n d 100 sec, r e s p e c t i v e l y ( 3 ) . S i n c e these b u b b l e s rise at speeds of o n l y 0.5 a n d 0.13 c m / s e c , t h e s m a l l e r ones w i l l never r e a c h the surface.

S i n c e b u b b l e r i s i n g speed decreases

w i t h size, c a l c u l a t i o n w o u l d no d o u b t s h o w that the 100 μτη

bubbles

w o u l d also go i n t o s o l u t i o n . I n a d d i t i o n , b u b b l e s l a r g e r t h a n 200

μτη

w h i c h are c a r r i e d s e v e r a l meters b e n e a t h the sea b y m e c h a n i s m s a l r e a d y discussed or are c a u g h t i n L a n g m u i r c i r c u l a t i o n cells ( 2 , 66)

w o u l d most

l i k e l y go i n t o s o l u t i o n .


E v i d e n c e suggests that the a d s o r p t i o n of o r g a n i c m a t e r i a l onto the surface of a b u b b l e r i s i n g t h r o u g h sea w a t e r occurs at s u c h a rate as to r e a c h a steady-state v a l u e w i t h i n 20 to 40 sec.

F o r c e - a r e a isotherms for

n u m e r o u s samples of n a t u r a l surface-active m a t e r i a l f o u n d o n the sea i n d i c a t e d that just at the p o i n t that a d e t e c t a b l e film pressure was n o t e d , the film area w a s of the o r d e r of 0.2 m / m g (36). 2

A s s u m i n g t h a t a 100

μπι b u b b l e attains this coverage, the v o l u m e of o r g a n i c m a t e r i a l i n the film

is a b o u t 40 μτη . 3

If this m a t e r i a l w e r e c o m p r e s s e d

as t h e b u b b l e

w e n t i n t o s o l u t i o n , i t c o u l d p r o d u c e a p a r t i c l e 4 μτη i n d i a m e t e r . W h e t h e r the p a r t i c l e is p r o d u c e d or not depends o n t w o factors.

F i r s t , as the

m a t e r i a l is c o m p r e s s e d b y the d i s s o l v i n g b u b b l e , the surface t e n s i o n d e creases, thus l o w e r i n g the i n t e r n a l pressure. S e c o n d l y , m a n y of the shorter c h a i n , less s t r o n g l y a d s o r b e d surface-active molecules w i l l be d i s p l a c e d f r o m the b u b b l e surface as it goes i n t o s o l u t i o n (35, 36).

Nevertheless,

the size d i s t r i b u t i o n of p a r t i c u l a t e m a t e r i a l at a d e p t h of 1 m i n the sea peaks i n the d i a m e t e r range 3 - 8 bubbles) were observed

μΐη w h e n w h i t e c a p s

(and

therefore

(61).

A n o r d e r - o f - m a g n i t u d e c a l c u l a t i o n of the flux of b u b b l e s e n t e r i n g the sea a n d d i s s o l v i n g gives 7 χ

1 0 b u b b l e s / m / m i n . T h e average t i m e 5

2

of a w h i t e c a p is a b o u t 1 m i n ; some 3 . 5 % w h i t e c a p s , a n d a b o u t 2 0 % of the 1 0

of the sea is c o v e r e d

s o l u t i o n . I n d e t a i l e d studies G o r d o n (67, 68) trations of f r o m 3 Χ

10

7

to 2.5 X

with

b u b b l e s p e r c u b i c m e t e r go i n t o

8

10 /m 8

3

has f o u n d p a r t i c l e c o n c e n

i n the surface waters of the

N o r t h A t l a n t i c . A m a j o r i t y of the p a r t i c l e s w e r e 20 μτη or less. A c o m p l e t e s u m m a r y of the w o r k d o n e o n the p e r p l e x i n g p r o b l e m of the o r i g i n of p a r t i c u l a t e m a t e r i a l i n the sea c a n be f o u n d i n R i l e y ' s r e v i e w (69).

H i s r e v i e w of the w o r k o n the b u b b l i n g makes i t clear that m e c h a

nisms other t h a n b u b b l i n g c a n p r o d u c e p a r t i c u l a t e m a t e r i a l . F o r e x a m p l e , particles a p p e a r to d e v e l o p m o r e or less s p o n t a n e o u s l y b y aggregation of smaller entities (70)

a n d b y the a d s o r p t i o n of d i s s o l v e d o r g a n i c m a t e r i a l

onto c a l c i u m carbonate p a r t i c l e s

(71).


372

C H E M I S T R Y

A

T

P R O T E I N

I N T E R F A C E S


A P P L I E D

Figure 3. (a) Composite view of high-speed motion pictures illustrating some of the stages in the formation of jet and jet drops upon the collapse of a 1.7 mm diameter bubble. The time interval between top and bottom frames is about 2.3 msec. The angle of view is horizontal through a glass wall. The surface irregularities are due to a meniscus, (b) Oblique view of the jet from a 1 mm diameter bubble.

The Water-to-Air

Transfer

of Organic

Material

T h e surface of the sea m a y rise a n d f a l l as w a v e s pass, a n d c a p i l l a r y w a v e s i n d u c e d b y w i n d gusts c a n c o v e r t h e s u r f a c e ; b u t u n t i l there is some p h y s i c a l d i s r u p t i o n of the surface t h e r e is n o k n o w n w a y i n w h i c h


18.

B L A N C H A R D

Bubble

373

Scavenging

particles or d r o p s of a n y k i n d c a n b e ejected i n t o t h e atmosphere. face disruption m a y occur i n several ways

( r a i n , s n o w , etc.)

Sur

b u t the

m a j o r w a y appears to b e b r e a k i n g w a v e s or w h i t e c a p s . W h e n w h i t e c a p s f o r m , a n d e v e n p r i o r to t h e i r f o r m a t i o n , fine s p r a y d r o p s are m e c h a n i c a l l y t o r n f r o m w a v e crests b y the w i n d .

It is u n l i k e l y that these d r o p s c o n

stitute the m a j o r m e c h a n i s m of d r o p ejection f r o m the sea (72).

They

are m u c h l a r g e r t h a n those c o m m o n l y f o u n d i n t h e m a r i n e atmosphere, a n d t h e y u n d o u b t e d l y f a l l b a c k to the sea before t r a v e l l i n g m a n y meters. T h e major m e c h a n i s m appears to b e the b r e a k i n g at the surface of a i r b u b b l e s p r o d u c e d b y the w h i t e c a p s . Dynamics of Bubble Breaking. W h e n a n a i r b u b b l e

breaks

the


b u b b l e c a v i t y q u i c k l y collapses p r o d u c i n g a jet of w a t e r w h i c h is ejected u p w a r d at v e r y h i g h speed. T o satisfy m o m e n t u m c o n s e r v a t i o n , a d o w n w a r d - m o v i n g jet ( u s u a l l y not v i s i b l e ) is also p r o d u c e d

(73).

T h e jet

becomes u n s t a b l e after r i s i n g a distance of a b o u t one b u b b l e d i a m e t e r a n d breaks u p i n t o one to five d r o p s , d e p e n d i n g o n b u b b l e size, w h i c h c o n t i n u e o n u p w a r d . T h e s e d r o p s are c a l l e d t h e jet drops. T h e bubble—jetd r o p m e c h a n i s m w a s first p r o p o s e d

i n 1937

(74)

to a c c o u n t for

the

sea-salt aerosol f o u n d i n m a r i n e atmospheres ( 7 5 ) , b u t it w a s not u n t i l 1953 that a h i g h speed c a m e r a w a s u s e d to c o n f i r m the details of this m e c h a n i s m (76).

F i g u r e 3 illustrates the f o r m a t i o n of the jet a n d the

jet d r o p s . F o r a g i v e n b u b b l e size the ejection h e i g h t a n d size of the d r o p s is r e m a r k a b l y constant, not v a r y i n g b y m o r e t h a n a f e w percent. A s s h o w n i n F i g u r e 4, c o m p i l e d w i t h d a t a f r o m several sources (3, 51, 77, 78),

the

ejection h e i g h t of the top jet d r o p f r o m b u b b l e s b r e a k i n g i n sea w a t e r increases w i t h b u b b l e size, r e a c h i n g a m a x i m u m of a b o u t 18 c m for a 2 m m b u b b l e a n d t h e n d e c r e a s i n g to near zero for b u b b l e s l a r g e r t h a n a b o u t 6 m m . T h e difference b e t w e e n the sea w a t e r c u r v e a n d the one for d i s t i l l e d w a t e r for b u b b l e s i n excess o f 1 m m is not significant. A p p a r e n t l y it is r e l a t e d to differences i n d e l a y times w h e n a b u b b l e reaches the surface a n d breaks ( 5 1 ) .

A l t h o u g h the r e l a t i o n b e t w e e n b u b b l e a n d

jet d r o p d i a m e t e r is w e l l - k n o w n (51),

a g o o d r u l e of t h u m b is that the

jet d r o p d i a m e t e r is one-tenth that of the b u b b l e .

T h e top d r o p ejection

h e i g h t , at least for b u b b l e s of less t h a n 2 m m , is a b o u t 100 times the b u b b l e d i a m e t e r . F o r large b u b b l e s there is o n l y one d r o p ejected f r o m t h e jet w h i l e for s m a l l e r b u b b l e s s e v e r a l d r o p s appear.

T h e jet

drop

p h e n o m e n o n extends d o w n to the v e r y smallest b u b b l e s ; the p r o d u c t i o n of jet d r o p s as s m a l l as 2 μτη d i a m e t e r h a v e b e e n o b s e r v e d f r o m b u b b l e s s m a l l e r t h a n 50 μπι (79).

T h e d a t a of F i g u r e 4 d o not h o l d for a b u b b l e

c o a t e d w i t h surface-active o r g a n i c m a t e r i a l . T h e energy source for t h e ejection of the jet d r o p s c a n b e u n d e r s t o o d f r o m c a l c u l a t i n g the speeds of ejection r e q u i r e d to a t t a i n the


observed


374

A P P L I E D

C H E M I S T R Y

A

T

P R O T E I N

I N T E R F A C E S

BUBBLE DIAMETER (mm) Figure

4.

Jet drop ejection height as a function of bubble diameter, temperature and salinity

ejection heights ( 5 1 ) .

water

T h e s e c a l c u l a t i o n s , w h i c h took i n t o a c c o u n t t h e

h i g h d r a g forces associated w i t h l a r g e R e y n o l d ' s n u m b e r s , s h o w e d t h a t the ejection speeds i n c r e a s e d w i t h d e c r e a s i n g b u b b l e size a n d w e r e extra o r d i n a r i l y h i g h . F o r e x a m p l e , the t o p jet d r o p f r o m a 2 m m b u b b l e rises a b o u t 18 c m after b e i n g ejected f r o m the b u b b l e w i t h a s p e e d of 350 c m / s e c w h i l e t h e jet d r o p f r o m a 70 μτη b u b b l e i n w a t e r at 4 ° C rises o n l y 0.17 c m a n d has a n i n i t i a l s p e e d of 8 Χ

10 cm/sec. 3

W i t h o u t the

f r i c t i o n a l d r a g of the a i r , the latter d r o p w o u l d rise to a h e i g h t o f 335 m , about 2 χ

1 0 times h i g h e r t h a n is o b s e r v e d . 5

T h e k i n e t i c e n e r g y of the jet d r o p s is c a l c u l a t e d f r o m t h e ejection speeds a n d is p r o p o r t i o n a l to the s q u a r e of t h e b u b b l e d i a m e t e r .

The

source of the k i n e t i c e n e r g y is not the g r a v i t a t i o n a l p o t e n t i a l energy, w h i c h is a n o r d e r of m a g n i t u d e less t h a n the d r o p k i n e t i c energy, n o r is i t t h e p o t e n t i a l e n e r g y of the s l i g h t l y c o m p r e s s e d gas w i t h i n a b u b b l e at rest. T h e o n l y source of e n e r g y t h a t c a n a c c o u n t for t h e jet d r o p k i n e t i c e n e r g y is t h e surface free e n e r g y of the b u b b l e . T h i s varies w i t h the s q u a r e of the b u b b l e size, as does the k i n e t i c energy, a n d i n m a g n i t u d e is f r o m five to 10 times the k i n e t i c e n e r g y ( 5 1 ) .

W h e n an air bubble

breaks, some of the surface free e n e r g y is d i s s i p a t e d i n c i r c u l a r c a p i l l a r y w a v e s that m o v e o u t w a r d h o r i z o n t a l l y f r o m t h e b u b b l e . T h e rest is i n a


18.

B L A N C H A R D

Bubble

375

Scavenging

w a v e t h a t moves d o w n a l o n g t h e s u r f a c e of the b u b b l e . I t is this e n e r g y that p r o d u c e s the jet a n d jet d r o p s . A d e t a i l e d analysis of these w a v e s has b e e n m a d e b y M c l n t y r e ( 7 3 ) . A s k e t c h that shows b o t h these w a v e s

(80)

is g i v e n i n F i g u r e 5. T h i s sketch, b a s e d i n p a r t o n t h e h i g h - s p e e d m o v i e s s h o w n i n F i g u r e 3, shows 10 t i m e sequences o f the profiles of a c o l l a p s i n g b u b b l e . S e q u e n c e 1 shows the b u b b l e just after the b u b b l e film ( t h a t t h i n film of w a t e r w h i c h separates the a i r i n the b u b b l e f r o m t h a t a b o v e ) has b r o k e n , a n d sequence 10 shows the jet f u l l y f o r m e d a n d r e a d y to b r e a k into drops. Jet d r o p s a r e not the o n l y d r o p s p r o d u c e d b y a b u r s t i n g b u b b l e .


T h e r e are film d r o p s also. F i l m d r o p s are p r o d u c e d f r o m the b u b b l e

film

d u r i n g the t i m e i n t e r v a l f r o m b r e a k i n g to t i m e S e q u e n c e 2 i n F i g u r e 5. T h e y are m u c h s m a l l e r t h a n jet drops. M o s t are less t h a n a m i c r o m e t e r b u t some are as large as 20 or 30 ^ m . U n l i k e jet d r o p s , w h o s e n u m b e r s v a r y b e t w e e n one a n d five a n d decrease w i t h b u b b l e size, t h e n u m b e r of film drops p e r b u b b l e increases r a p i d l y w i t h size (51,81).

B u b b l e s of less

t h a n 0.3 m m d o n o t a p p e a r to p r o d u c e a n y film d r o p s at a l l , b u t a 2 m m b u b b l e p r o d u c e s a m a x i m u m of a b o u t 100, a n d a 6 m m b u b b l e a m a x i m u m of a b o u t 1000. F o r b u b b l e s b r e a k i n g i n d i v i d u a l l y , the presence of a surface-active m a t e r i a l at the b u l k a i r - w a t e r interface decreases the d r o p p r o d u c t i o n (51, 82).

film

H o w e v e r , i f t h e b u b b l e concentrations

are

h i g h e n o u g h to p r o d u c e c l u s t e r i n g at t h e surface, t h e presence o f

the

m a t e r i a l causes u p to a t h r e e f o l d increase i n film d r o p p r o d u c t i o n

(83).

Simple Experiments on Sea-to-Air Organic Transfer. B o t h jet a n d film

d r o p s , m i x e d u p w a r d i n t o the atmosphere b y t u r b u l e n c e , a c c o u n t

Figure 5. Ten stages in the time sequence of collapse of a 1.7 mm diameter bubble. Profiles are —1 /6000 sec apart. Data from Ref. 80.


376

A P P L I E D

C H E M I S T R Y

A T

P R O T E I N

I N T E R F A C E S

f o r the d i s t r i b u t i o n of sea-salt n u c l e i (also c a l l e d sea-salt p a r t i c l e s , the sea salt left w h e n w a t e r evaporates f r o m d r o p s of sea w a t e r ) t h a t is f o u n d i n the lowest 2 to 3 k m of the m a r i n e atmosphere ( 7 5 ) .

T h e r a t e of i n -

j e c t i o n of these d r o p s i n t o the a t m o s p h e r e is of the o r d e r of 1 0 tons of sea salt p e r year (51).

1 0

metric

T h e c o n c e n t r a t i o n of surface-active o r g a n i c

m a t e r i a l o n these d r o p s is c o n s i d e r a b l y h i g h e r t h a n that i n the sea, i n d i c a t i n g that the b u b b l e or the m e c h a n i s m of b u b b l e b r e a k i n g is c a p a b l e of c o n c e n t r a t i n g o r g a n i c m a t e r i a l . B y b u b b l i n g a i r t h r o u g h sea w a t e r surface-active m a t e r i a l c a n be s t r i p p e d f r o m the surface a n d c a r r i e d i n t o the a t m o s p h e r e b y jet a n d film d r o p s (2, 51, 83).

T o s t u d y jet d r o p s i n this process, b u b b l e s of k n o w n


size w e r e a l l o w e d to b r e a k , one at a t i m e , b e n e a t h a n o r g a n i c d u p l e x ( c a l l e d i n d i c a t o r o i l b y L a n g m u i r a n d Schaefer (84)

film

because the film w a s

t h i c k e n o u g h to g i v e i n t e r f e r e n c e colors a n d thus i n d i c a t e its p r e s e n c e ) , a n d t h e film c a r r i e d b y the t o p jet d r o p w h e n i t f e l l i n t o a c l e a n w a t e r surface

(51)

was o b s e r v e d .

T h i s e x p e r i m e n t , i l l u s t r a t e d i n F i g u r e 6,

s h o w e d t h a t the jet d r o p f r o m b u b b l e s larger t h a n 1 m m c a r r i e d some of the d u p l e x film w h i l e the drops f r o m s m a l l e r b u b b l e s d i d not.

These

films w e r e u n d e r zero pressure so a h y p o t h e s i s was f o r m u l a t e d t h a t r e l a t e d film t r a v e l d o w n t h e insides of the b u b b l e to the s p e e d at w h i c h jet drops f o r m e d . I n a s m u c h as o r g a n i c films o n the sea c a n b e u n d e r pressure, the experiments w e r e r e p e a t e d w i t h a n o l e i c a c i d film u n d e r pressure. E x p e r i m e n t a l difficulties o b s c u r e d the i n t e r p r e t a t i o n of this e x p e r i m e n t , b u t the surface pressure s e e m e d r e s p o n s i b l e f o r m o r e o r g a n i c m a t e r i a l o n t h e jet drops. L i t t l e is k n o w n of the m a n n e r i n w h i c h jet d r o p s a c c o m p l i s h the sea-to-air o r g a n i c transfer. P r e s e n t t e c h n i q u e s c a n detect o r g a n i c m a t e r i a l i n e x t r e m e l y s m a l l samples a n d p e r m i t a v a r i a t i o n of the F i g u r e 6 e x p e r i m e n t i n a r o t a t i n g tank (85)

w i t h a d e v i c e w h i c h c a n generate m o r e t h a n

1000 b u b b l e s p e r m i n of a n y d e s i r e d size. N o r e p o r t e d experiments detect surface-active o r g a n i c m a t e r i a l o n film

d r o p s , b u t several r e p o r t a n e n r i c h m e n t of v a r i o u s elements i n the

film

d r o p s (86, 87, 88).

N e w r e s e a r c h i n this area has b e e n p r e s e n t e d

(89). Adsorption of Organic Material on Bubbles Rising through the Sea. A l t h o u g h s o m e of the o r g a n i c m a t e r i a l o n jet a n d film drops originates i n m o n o l a y e r s o n the surface of the sea, most of the m a t e r i a l c a r r i e d i n t o t h e a i r is a d s o r b e d o n the surface of the b u b b l e w h i l e it is r i s i n g t h r o u g h the w a t e r . A d s o r p t i o n of d i s s o l v e d surface-active o r g a n i c m a t e r i a l (1,90, 91) a n d p a r t i c u l a t e m a t e r i a l (85, 92, 93, 94)

to a b u b b l e surface is w e l l

k n o w n b u t not i n sea-to-air transfer of o r g a n i c m a t e r i a l . F i g u r e 7 shows the process that takes p l a c e o n the surface of e v e r y a i r b u b b l e t h r o u g h the sea (90).

rising

M o l e c u l e s of d i s s o l v e d o r g a n i c m a t e r i a l diffuse

to the surface of the b u b b l e s w h e r e the d o w n w a r d d r a g of t h e w a t e r


18.

B L A N C H A R D

BREEZE FROM FAN

Bubble

377

Scavenging

e


MICROSCOPE TO MEASURE FILM DIAMETER

AIR BUBBLE

Figure 6.

DISTILLED WATER SURFACE

FILM FROM DROP ON CLEAN, DISTILLED WATER SURFACE

A method of detection of the removal of surface-active jet drops

films by

sweeps t h e m a r o u n d t o the l o w e r o r d o w n s t r e a m surface. T h u s a c o m p r e s s e d m o n o l a y e r w i l l b u i l d u p o n t h e b o t t o m o f the b u b b l e , t h e a m o u n t of c o m p r e s s i o n d e p e n d i n g i n p a r t o n b u b b l e rise speed, the t i m e i t is i n the w a t e r , a n d the c o n c e n t r a t i o n o f d i s s o l v e d o r g a n i c m a t e r i a l . A s the c o n c e n t r a t i o n o f surface-active m a t e r i a l o n a r i s i n g b u b b l e increases w i t h t i m e , the surface tension a n d thus the surface free energy decreases.

S i n c e the latter is the source o f e n e r g y f o r jet d r o p ejection,

the ejection heights o f the d r o p s decrease w i t h i n c r e a s i n g b u b b l e age. T h i s is o b s e r v e d b o t h w i t h b u b b l e s r i s i n g t h r o u g h aqueous

suspensions

of b a c t e r i a ( 8 5 ) a n d t h r o u g h samples of lake, r i v e r , a n d sea w a t e r . T h e decrease i n ejection h e i g h t is g e n e r a l l y p a r a l l e l e d b y a decrease i n d r o p size. A n e x a m p l e o f the ejection h e i g h t decrease for b u b b l e s i n sea w a t e r c o l l e c t e d i n L o n g I s l a n d S o u n d ( 9 5 ) is s h o w n i n F i g u r e 8. N o t e the r a p i d decrease i n the top jet d r o p ejection h e i g h t d u r i n g the first 20 sec of b u b b l e a g i n g a n d little c h a n g e after t h a t t i m e . T h e second jet d r o p shows l i t t l e effect o f the b u b b l e a g i n g . N o d a t a are r e p o r t e d o n the t i m e rate o f i n crease i n surface-active o r g a n i c m a t e r i a l o n the b u b b l e o r o n t h e top jet d r o p . T h e p r o b a b l e m a g n i t u d e is suggested b y the fact that the n u m b e r s of b a c t e r i a a t t a c h e d t o a r i s i n g b u b b l e c a n increase 1 0 times ( a n d i n 3

some cases 1 0 ) d u r i n g the first 2 0 sec o f b u b b l e life 4

(85).

T h e steady a d s o r p t i o n o f d i s s o l v e d o r g a n i c m a t e r i a l onto a r i s i n g b u b b l e , w i t h the r e s u l t i n g increase i n surface pressure, s h o u l d p r o g r e s s i v e l y f o r c e the m o r e w a t e r s o l u b l e a n d less surface-active species o u t o f the b u b b l e surface ( 3 5 ) .

T h u s , for a g i v e n s i z e d b u b b l e , t h e p r o p o r t i o n

of v a r i o u s species varies w i t h b u b b l e age.

T h i s is reflected i n the c o m -



378

A P P L I E D

ν

t

C H E M I S T R Y

A

T

P R O T E I N

I N T E R F A C E S

Figure 7. Adsorption of dissolved sur face-active material onto the surface of an air bubble rising through sea water. (A) The upstream region; (B) rough dividing line between compressed and non-compressed monolayer; (C) com pressed monolayer; (D) bubble wake. Data from Ref. 90.

p o s i t i o n of the m a t e r i a l c a r r i e d b y the jet drops.

N o t o n l y is the t o t a l

a m o u n t of o r g a n i c m a t e r i a l c a r r i e d b y a jet d r o p i n c r e a s e d w i t h b u b b l e age b u t so is the r e l a t i v e c o m p o s i t i o n .

T h e c o m p l e x i t y of the n a t u r e

a n d a m o u n t of surface-active o r g a n i c m a t e r i a l t r a n s f e r r e d f r o m sea to air a n d the interactions b e t w e e n a d s o r b e d o r g a n i c m a t e r i a l o n b u b b l e s a n d the d y n a m i c s of jet d r o p ejection has not b e e n i n v e s t i g a t e d a d e q u a t e l y . T h e r e l a t i o n b e t w e e n d r o p ejection h e i g h t a n d b u b b l e d i a m e t e r s h o w n i n F i g u r e 4, o b t a i n e d for b u b b l e s less t h a n 1 sec o l d , cannot b e a p p l i e d to the oceans or to a n y n a t u r a l bodies of w a t e r w h e r e b u b b l e ages of m o r e t h a n 20 sec are c o m m o n .

I n s u c h case the b e h a v i o r s h o w n i n F i g u r e 8 is

p r o b a b l y m o r e t y p i c a l t h o u g h p e r h a p s exaggerated.

These data were

obtained w i t h L o n g Island Sound water w h i c h may have h a d a higher d i s s o l v e d o r g a n i c content t h a n w a t e r o n the o p e n sea. Amount of Organic Material Ejected into the Atmosphere. A r o u g h estimate c a n b e m a d e of b o t h t h e e n r i c h m e n t of o r g a n i c m a t e r i a l o n jet d r o p s a n d film d r o p s a n d of the t o t a l a m o u n t of surface-active o r g a n i c m a t e r i a l ejected f r o m sea to a i r p e r year. A n a l y s i s of collections of seasalt p a r t i c l e s a n d sea w a t e r d r o p s i n t h e a t m o s p h e r e near H a w a i i (4,


96)

18.

B L A N C H A R D

Bubble

379

Scavenging

s h o w e d that t h e a m o u n t of surface-active m a t e r i a l o n t h e aerosol w a s a b o u t 5 0 % t h a t of t h e sea salt. O t h e r w o r k ( 9 7 ) i n t h e same area s u g gested i t w a s closer t o 1 0 % , b u t a p r o b a b l e error i n the c a l c u l a t i o n s m a d e i t too l o w b y a factor of t w o (98).

T h u s , t h e w e i g h t o f surface-active

o r g a n i c m a t e r i a l o n t h e m a r i n e sea-salt aerosol is f r o m 20 to 5 0 % that of t h e sea salt, c o n s t i t u t i n g a n e n r i c h m e n t o f 7 0 0 0 - 1 7 , 0 0 0 times w h a t is f o u n d i n the sea since sea w a t e r is 3 . 5 % sea salt a n d contains a b o u t 1 p p m organic material.

T h e e n r i c h m e n t is a t t r i b u t a b l e t o t h e b u b b l e m e c h a

nisms just discussed. B a r g e r a n d G a r r e t t ( 9 7 ) h a v e s h o w n t h a t the o r g a n i c m a t e r i a l f o u n d


o n t h e aerosol d i d i n d e e d c o m e f r o m t h e sea w i t h the jet a n d film d r o p s a n d n o t f r o m a n y c o n t i n e n t a l source.

T h e y f o u n d that t h e aerosol c o n

t a i n e d a m i x t u r e of surface-active c o m p o u n d s a n d n o n p o l a r h y d r o c a r b o n s , a n d s p e c i f i c a l l y i d e n t i f i e d five f a t t y acids ( C

1 4

- C i ) to b e i n t h e same 8

r e l a t i v e p r o p o r t i o n s as h a v e b e e n f o u n d i n sea surface slicks.

Further,

the film pressure vs. area curves f o r t h e surface-active m a t e r i a l o n t h e aerosol w e r e of the l i q u i d - e x p a n d e d t y p e , q u i t e s i m i l a r i n shape to those r e p o r t e d f o r sea surface samples

(36).

T h e t o t a l a m o u n t o f surface-active m a t e r i a l ejected i n t o t h e atmos p h e r e p e r year is 20 to 5 0 % o f the e s t i m a t e d 1 0

1 0

m e t r i c tons o f sea salt

• TOP DROP ο SECOND DROP

LU

20

40 60 80 100 BUBBLE AGE(SEC)

120

Figure 8. Decrease in ejection height of jet drops as a function of bubble age. After Lee (95) with permission of the author.


380

A P P L I E D

C H E M I S T R Y

A

t h o u g h t to b e i n v o l v e d i n t h e a n n u a l c y c l e ( 5 1 ) .

T

P R O T E I N

I N T E R F A C E S

T h i s a m o u n t s to 2 to

5 Χ 1 0 tons. P o s s i b l y i t is m o r e t h a n this since the organics-to-salt r a t i o 9

i n t h e a t m o s p h e r e n e a r H a w a i i is l i k e l y to b e less t h a n t h a t for the w o r l d o c e a n average. B i o l o g i c a l p r o d u c t i v i t y is l o w i n H a w a i i a n w a t e r s . P r o b a b l y most of this m a t e r i a l returns d i r e c t l y to the sea since o n l y a b o u t 1 0 % of the p a r t i c u l a t e m a t e r i a l ejected f r o m t h e sea falls out o n the c o n t i nents

(99).

I n terms of the t o t a l p r o d u c t i v i t y of the oceans ( T a b l e I I ) the a m o u n t of a i r b o r n e surface-active m a t e r i a l is o n l y 5 to 1 2 % o f 2 Χ carbon per year (about 4 Χ

10

10

10

10

tons of

tons of o r g a n i c m a t e r i a l ) .


T h e w a t e r - t o - a i r transfer of o r g a n i c m a t e r i a l b y b u b b l e s occurs not o n l y i n the sea b u t also i n b o d i e s of f r e s h w a t e r . B a i e r (100),

i n a study

of o i l films p r o d u c e d o n L a k e C h a u t a u q u a , N e w Y o r k , b y h u m a n a c t i v i t y , c o n c l u d e d that n a t u r a l b u b b l e m e c h a n i s m s w e r e l a r g e l y r e s p o n s i b l e for c l e a n i n g the l a k e surface b y ejecting the films i n t o the atmosphere. f u t u r e w o r k confirms this

finding,

bubbles

c o n v e r t i n g w a t e r p o l l u t i o n to a i r p o l l u t i o n

m a y be

a mechanism

If for

(101).

Other Methods for Organic Removal from the Surface of the Sea. T h r e e other m e c h a n i s m s are s h o w n i n F i g u r e 2 for the r e m o v a l of o r g a n i c m a t e r i a l f r o m the surface of the sea. T h e first is b i o l o g i c a l c o n s u m p t i o n . H i g h concentrations of z o o p l a n k t o n a n d b a c t e r i a ( 3 9 )

are f o u n d i n the

surface m i c r o l a y e r of the sea i n d i c a t i n g the use of t h e film as f o o d .

How

significant this is for film r e m o v a l is not k n o w n . T h e second m e c h a n i s m , p h o t o c h e m i c a l o x i d a t i o n b y U V i r r a d i a t i o n , acts o n c h e m i c a l l y - u n s a t u r a t e d c o m p o n e n t s of t h e surface films to b r e a k t h e m i n t o s m a l l e r , m o r e s o l u b l e fragments w h i c h are easily d i s p l a c e d f r o m the surface (102).

It

has also b e e n suggested that U V i r r a d i a t i o n c a n b e the first step i n a series of reactions w i t h l i p i d films to p r o d u c e p a r t i c u l a t e m a t e r i a l

(60).

T h e t h i r d m e c h a n i s m , the b r e a k i n g u p of the surface film a n d m i x i n g b a c k i n t o the sea b y t u r b u l e n c e a n d L a n g m u i r c i r c u l a t i o n s , is l i t t l e u n d e r stood. T h e r e l a t i v e significance of these m e c h a n i s m s is not k n o w n except that i t varies w i t h w i n d , t e m p e r a t u r e lapse rates b o t h i n a n d a b o v e the sea, a n d b i o l o g i c a l a c t i v i t y i n the sea. Organics from the Sea and the Formation

of Rain

A t a g i v e n c l o u d d e p t h , r a i n forms m o r e easily i n c l o u d s over the sea t h a n i n c l o u d s over the c o n t i n e n t (103).

T h e reason lies i n the differences

i n the aerosol d i s t r i b u t i o n s b e t w e e n t h e m a r i n e a n d c o n t i n e n t a l atmos pheres (104).

A l t h o u g h aerosol p a r t i c l e s less t h a n 0.1 μχη i n d i a m e t e r

m a y not c o m e f r o m the sea, a significant p o r t i o n of those greater t h a n 0.1 μτη a n d essentially a l l l a r g e r t h a n 1 μπ\ o r i g i n a t e at the surface of the sea (3, 51, 75,105).

P a r t i c l e s c o m i n g f r o m the sea c a r r y o r g a n i c m a t e r i a l


18.

B L A N C H A R D

Bubble

381

Scavenging

w h i c h c a n i n f l u e n c e r a i n - f o r m i n g a n d other w e a t h e r m e c h a n i s m s i n t h e a t m o s p h e r e over t h e sea. Organic Films and Evaporation. O n e s u c h m e c h a n i s m is the rate of e v a p o r a t i o n of surface-active phere.

film-coated

w a t e r d r o p s i n t h e atmos

I t is often suggested that t h e films c a n s i g n i f i c a n t l y r e t a r d t h e

e v a p o r a t i o n , b u t i t has b e e n p o i n t e d out ( 9 7 ) that this is b a s e d o n l a b o r a t o r y w o r k w h e r e m o n o m o l e c u l a r films of h i g h l y a d l i n e a t e d , s t r a i g h t c h a i n m o l e c u l e s w e r e used.

S o m e of the surface-active m a t e r i a l f o u n d

o n t h e sea a n d i n t h e atmosphere contains p e r m a n e n t l y b e n t ( c h e m i c a l l y u n s a t u r a t e d ) h y d r o c a r b o n sections w h i c h prevents close p a c k i n g i n c o m


pressed films. F i l m s c o n t a i n i n g these m o l e c u l e s h a v e holes a n d are thus u n a b l e to p r e v e n t e v a p o r a t i o n (106).

T h e v a l i d i t y of this i d e a has b e e n

s h o w n c o n v i n c i n g l y i n experiments w h e r e t h e d r o p l e t s i n w a t e r fogs w e r e c o a t e d w i t h v a r i o u s surface-active m a t e r i a l s . C e t y l a l c o h o l , c o m p o s e d of l i n e a r m o l e c u l e s , a p p r e c i a b l y r e t a r d e d t h e e v a p o r a t i o n of t h e f o g w h i l e o l e y l a l c o h o l , c o m p o s e d of n o n - l i n e a r m o l e c u l e s , d i d not (107).

Thus,

the surface-active m a t e r i a l o n jet a n d film drops h a v e l i t t l e effect o n d r o p evaporation.

E v a p o r a t i o n leaves

a h i g h r a t i o of o r g a n i c m a t e r i a l to

sea salt. Organic Surface-Active Material and the Formation of Rain. O r g a n i c - l a d e n drops f r o m t h e sea p r o v i d e a c l u e to t h e m e c h a n i s m for t h e f o r m a t i o n of m a r i n e r a i n . R a i n forms i n m a r i n e clouds w h e n a sufficient n u m b e r of giant ( those l a r g e r t h a n 1 μχη ) sea-salt p a r t i c l e s are c a r r i e d i n t o c l o u d s w h i c h h a v e t h e p r o p e r u p d r a f t s a n d v e r t i c a l thickness. P r e s u m a b l y the g i a n t particles p r o v i d e t h e n u c l e i f o r the f o r m a t i o n of r a i n d r o p s , t h e r a i n d r o p s f o r m i n g b y a coalescence process as the n u c l e i f a l l t h r o u g h t h e c l o u d of smaller c l o u d drops (108, 109).

down

Recently, evidence

has b e e n p r e s e n t e d w h i c h suggests that i t is n o t t h e g i a n t salt p a r t i c l e s that p r o v i d e the n u c l e i for r a i n f o r m a t i o n , b u t t h e s o - c a l l e d l a r g e sea-salt p a r t i c l e s , those i n t h e range 0 . 1 - 1 μτη

(110).

T h e e v i d e n c e f o r this m e c h a n i s m is p r o v i d e d b y t h e surface-active o r g a n i c m a t e r i a l o n the sea-salt p a r t i c l e s . T h e i o d i n e - t o - c h l o r i n e r a t i o of t h e sea-salt p a r t i c l e s is 1 0 0 - 1 0 0 0 times t h a t of sea w a t e r a n d varies i n v e r s e l y w i t h p a r t i c l e size (111, 112).

A s t h e p a r t i c l e s rise f r o m t h e sea

as jet o r film drops, a f r a c t i o n a t i o n process occurs as t h e y are ejected i n t o the a t m o s p h e r e r e s u l t i n g i n a r e l a t i v e increase i n i o d i n e . T h i s i o d i n e is o r g a n i c a l l y b o u n d i n surface-active m a t e r i a l a n d is thus ejected i n t o t h e a t m o s p h e r e w i t h the o r g a n i c m a t e r i a l (96).

F r o m a knowledge of the con

c e n t r a t i o n of i o d i n e i n o r g a n i c m a t e r i a l i n t h e sea a n d t h e a m o u n t of o r g a n i c m a t e r i a l o n t h e a i r b o r n e salt p a r t i c l e s , one c a n d e d u c e i o d i n e - t o c h l o r i n e ratios i n the range a c t u a l l y o b s e r v e d

(96).

T h e inverse d e p e n d e n c e o f these ratios w i t h p a r t i c l e size is also consistent w i t h t h e o r g a n i c film hypothesis.

S i n c e t h e i o d i n e is b o u n d


382

A P P L I E D

C H E M I S T R Y

A

T

P R O T E I N

I N T E R F A C E S

i n t h e film w h i c h is o n t h e surface o f t h e d r o p , i t varies d i r e c t l y w i t h surface area, R . T h e c h l o r i n e i n t h e d r o p itself varies w i t h R . T h e r e 3

2

fore, t h e i o d i n e - t o - c h l o r i n e r a t i o s h o u l d v a r y as R " w h i c h is a b o u t w h a t 1

is o b s e r v e d (112).

A l t h o u g h i t seems l i k e l y that these ratios are c a u s e d

b y o r g a n i c a l l y - b o u n d i o d i n e i n surface films, a n a l t e r n a t i v e h y p o t h e s i s has b e e n p r e s e n t e d w h i c h suggests t h a t t h e i o d i n e is n o t o n t h e aerosol w h e n i t leaves t h e sea b u t diffuses to i t i n t h e f o r m of gaseous i o d i n e f r o m the a t m o s p h e r e

(113).

T h e inverse r e l a t i o n b e t w e e n p a r t i c l e size a n d I / C l p r o v i d e d a tracer b y w h i c h t h e r o l e of t h e sea-sal^ particles i n t h e r a i n - f o r m i n g process c a n


be studied.

C o m p a r i s o n of t h e I / C l ratios i n t h e sea-salt particles i n

m a r i n e a i r to those i n r a i n d r o p s f r o m clouds f o r m e d i n this a i r i n d i c a t e d that o n l y t h e s m a l l e n d o f t h e sea-salt p a r t i c l e s p e c t r u m plays a n y role i n the r a i n f o r m a t i o n

(110).

Acknowledgments A l l , or sections of, t h e first draft of this p a p e r w e r e r e a d b y W i l l i a m Sutcliife, Jr., D o n a l d G o r d o n , Jr., John Wheeler, a n d Peter Wangersky, of t h e B e d f o r d Institute i n N o v a S c o t i a a n d b y B r u c e P a r k e r of t h e V i r g i n i a P o l y t e c h n i c Institute a n d State U n i v e r s i t y . I t h a n k t h e m f o r t h e i r helpful constructive criticism.

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