Accommodation Coefficients of Ozone and SO2: Implications on SO2

Chapter 9

Accommodation Coefficients of Ozone and SO : Implications on SO Oxidation in Cloud Water 2

2

I. N.

Tang J. H. Lee

Downloaded by CORNELL UNIV on September 5, 2016 | http://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch009

Environmental Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973

Interfacial mass t r a n s f e r of t r a c e gases i n t o aqueous pnase is i n v e s t i g a t e d in a UV a b s o r p t i o n - s t o p flow apparatus. F o r the first t i m e , the mass accommodation coefficients are d e t e r m i n d f o r O 3 (5.3x10 ) and f o r SO2 (>2x10 ). The results are i n c o r p o r a t e d i n t o a s i m p l e model c o n s i d e r i n g the coupled interfacial mass t r a n s f e r and aqueous c h e m i s t r y in c l o u d d r o p s . I t i s shown t h a t dissolution of O 3 into a drop is f a s t com pared w i t h its subsequent o x i d a t i o n of d i s s o l v e d SO2. In addition, the c o n v e r s i o n r a t e of S ( I V ) to S ( V I ) i n aqueous drops by ozone r e a c t i o n s i s not limited by interfacial resistance. -4

-2

I n t e r f a c i a l mass t r a n s f e r i s an i m p o r t a n t c o n s i d e r a t i o n i n many dynamic p r o c e s s e s i n v o l v i n g the t r a n s p o r t of a gaseous s p e c i e s a c r o s s a gas-liquid interface. I n p a r t i c u l a r the r a t e of t r a c e gas i n c o r p o r a t i o n i n t o aqueous drops i n the atmosphere has r e c e n t l y r e c e i v e d much a t t e n t i o n because of i t s r e l e v a n c e to a c i d p r e c i p i t a t i o n ( 1 , 2 ) . I n the p r e s e n t paper, mass accommodation c o e f f i c i e n t measurements are r e p o r t e d f o r O 3 and S O 2 on water s u r f a c e s , u s i n g an UV a b s o r p t i o n stop flow t e c h n i q u e . The r e s u l t s are i n c o r p o r a t e d i n t o a s i m p l e model c o n s i d e r i n g the c o u p l e d i n t e r f a c i a l mass t r a n s f e r and aqueous c h e m i s t r y i n aqueous d r o p s . Some i m p l i c a t i o n s of the measured accom modation c o e f f i c i e n t s on the o x i d a t i o n of S O 2 by O 3 i n c l o u d water are d i s c u s s e d . Experimental A d e t a i l e d d e s c r i p t i o n of the apparatus shown i n F i g u r e 1 and the e x p e r i m e n t a l procedure w i l l be g i v e n e l s e w h e r e . H e r e , i t s u f f i c e s to summarize as f o l l o w s . The experiments were c a r r i e d out i n a thermos t a t e d r e a c t i o n c e l l c o n s t r u c t e d of a r e c t a n g u l a r P y r e x tube, 4 cm χ 8 cm i n c r o s s s e c t i o n and 38 cm i n l e n g t h , and p l a c e d w i t h i t s

0097-6156/87/0349-0109$06.00/0 © 1987 American Chemical Society

Johnson et al.; The Chemistry of Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

THE CHEMISTRY OF ACID RAIN


110

l e n g t h i n a h o r i z o n t a l p o s i t i o n . The c e l l was equipped w i t h an o p t i c a l window on e i t h e r end, gas i n l e t and o u t l e t on the top, and l i q u i d i n l e t and o u t l e t on the bottom. L i q u i d water was c i r c u l a t e d through the lower p o r t i o n o f the c e l l by an a l l - T e f l o n diaphragm pump, whereas t r a c e q u a n t i t i e s o f O 3 o r S O 2 i n a h u m i d i f i e d c a r r i e r gas was f l o w i n g c o n c u r r e n t l y i n the upper space. The system was designed t o operate a t low p r e s s u r e , t h e r e f o r e , very p r e c i s e flow and p r e s s u r e c o n t r o l s were e s s e n t i a l to m a i n t a i n the r e q u i r e d s t a b i l i t y d u r i n g an experiment. An UV l i g h t beam, o b t a i n e d w i t h an i n t e n s i t y - r e g u l a t e d d e u t e r i u m lamp and a n a r r o w - s l i t monochromator, passed through the gas phase between two p e r f e c t l y a l i g n e d p i n h o l e s mounted i n f r o n t o f the o p t i c a l windows. An EMR 541-N PM tube, w i t h an a p p r o p r i a t e i n t e r f e r e n c e f i l t e r p l a c e d immediately b e f o r e i t , was used i n c o n j u n c t i o n w i t h a s s o c i a t e d e l e c t r o n i c s to c o n t i n u o u s l y m o n i t o r the UV i n t e n s i t y as a means of measuring changes i n c o n c e n t r a t i o n of the reagent g a s . I n a t y p i c a l e x p e r i m e n t , the system was pumped down to a s p e c i f i e d t o t a l p r e s s u r e , and a t the same time the f l o w r a t e s o f the aqueous phase and the h u m i d i f i e d c a r r i e r gas were c a r e f u l l y a d j u s t e d to m a i n t a i n a s t a b l e f l o w . O 3 or S O 2 from a r e s e r v o i r was l e a k e d i n t o the c a r r i e r gas through a p r e c i s i o n needle v a l v e . As soon as a steady l i g h t i n t e n s i t y was o b t a i n e d , two s o l e n o i d v a l v e s on the gas i n l e t and o u t l e t of the r e a c t i o n c e l l were c l o s e d and a t h i r d s o l e n o i d v a l v e on the by-pass l i n e opened. The gas phase i n the r e a c t i o n c e l l became stagnant and the l i g h t i n t e n s i t y i n c r e a s e d w i t h time as the reagent gas was being absorbed i n t o the aqueous phase. R e s u l t s and D i s c u s s i o n System A n a l y s i s . Because of the s i m p l i f i e d c e l l geometry and w e l l d e f i n e d o p e r a t i n g c o n d i t i o n s , a o n e - d i m e n s i o n a l m a t h e m a t i c a l model i s adequate f o r d e s c r i b i n g the mass t r a n s p o r t i n the gas phase. The d i f f e r e n t i a l e q u a t i o n i s g i v e n by Θ0

at

2

D

dC a 2

(1)

Y

where C i s the reagent gas c o n c e n t r a t i o n a t time t , y the v e r t i c a l c o o r d i n a t e e x p r e s s i n g the d i s t a n c e between the g l a s s w a l l (y = 0) and the g a s - l i q u i d i n t e r f a c e ( y = A ) . The i n i t i a l c o n d i t i o n i s r e p r e sented by C = C , a t t = 0; 0 < y < A 0

and

(2)

the boundary c o n d i t i o n s a r e ac

0, a t y = 0; t > 0

ay and


(3)

9.

Accommodation Coefficients of Ozone and

TANG AND LEE

111

S0

2

Here, D i s the d i f f u s i v i t y of the reagent gas i n the gaseous medium, V the mean m o l e c u l a r speed of a Maxwell-Boltzraann gas, and α the mass accommodation c o e f f i c i e n t . The standard s o l u t i o n i s r e a d i l y o b t a i n e d (3) as f o l l o w s : C(t,y) C

m

~ n-1

0

2Lcos(S yM)

^

n

(2 + L + 3

2

)cos3

L

η

where β

are p o s i t i v e r o o t s of the

η


3tan3

=

^

(5)

(!^)t 1

\

I

/

η

equation

L

=

—

(6) 4D

E q u a t i o n (5) i n d i c a t e s t h a t by m o n i t o r i n g the gas c o n c e n t r a t i o n change as a f u n c t i o n of time, the accommodation c o e f f i c i e n t may be deduced u s i n g a computer program. However, p r e c a u t i o n s must be undertaken to s a t i s f y the boundary c o n d i t i o n t h a t the s u r f a c e concen t r a t i o n of the d i s s o l v e d gas must be n e g l i g i b l e a t a l l times. This can be a c c o m p l i s h e d i n p r i n c i p l e by a g i t a t i o n and by a d d i t i o n of p r o p e r c h e m i c a l reagents i n the aqueous phase to remove the d i s s o l v e d gas as q u i c k l y as i t i s absorbed. In a d d i t i o n , the system must be operated a t s u f f i c i e n t l y low p r e s s u r e so t h a t the gas-phase r e s i s tance i s much s m a l l e r than i n t e r f a c i a l r e s i s t a n c e . R e s u l t s . For O 3 , experiments were made w i t h both n i t r o g e n and h e l i u m as c a r r i e r gas i n the p r e s s u r e range of 29 to 85 t o r r , c o v e r i n g an e f f e c t i v e d i f f u s i v i t y range of 1.46 to 5.61 cm^/sec. Data were taken a t three d i f f e r e n t temperatures, namely, 0, 10 and 19°C The e f f e c t s of added c h e m i c a l reagent on the apparent accommodation c o e f f i c i e n t , a , were s t u d i e d u s i n g pure w a t e r , NaOH and Na2S03 s o l u t i o n s . As shown i n F i g u r e 2, the decay of O 3 w i t h time under a g i v e n c o n d i t i o n behaves as expected from the mathematical s o l u t i o n and i s q u i t e reproducible. I n pure w a t e r , as shown i n F i g u r e 3, a has a s m a l l v a l u e of 1.7x10*7 r e s u l t of the water s u r f a c e being q u i c k l y s a t u r a t e d by O3. I t i n c r e a s e s o n l y s l i g h t l y to a v a l u e of 6 x l 0 ~ ? by the a d d i t i o n of 0.05N NaOH, i n d i c a t i n g the slow r e a c t i o n of OH" w i t h d i s s o l v e d O3. However, a i n c r e a s e s d r a m a t i c a l l y to a v a l u e of 4.5x10"^ upon adding o n l y 8xlO"^M Na2S03» I t c o n t i n u e s to i n c r e a s e w i t h i n c r e a s i n g Na2S03 c o n c e n t r a t i o n s u n t i l i t l e v e l s o f f a t a v a l u e of (5.3*±Ό.4)χ10"4, where the e r r o r s r e p r e s e n t one standard d e v i a t i o n e v a l u a t e d from a t o t a l of 79 measurements i n the p l a t e a u r e g i o n as shown i n F i g u r e 3. The f a c t t h a t a no l o n g e r changes w i t h s u l f i t e c o n c e n t r a t i o n i n d i c a t e s t h a t an u l t i m a t e or t r u e v a l u e of accommoda t i o n c o e f f i c i e n t f o r O 3 on water s u r f a c e s has now been reached. S i m i l a r experiments were c a r r i e d out w i t h S O 2 i n helium c a r r i e r gas. P r e l i m i n a r y r e s u l t s i n d i c a t e d t h a t a i n c r e a s e d from ~ 5 x l 0 ' i n pure water to ~ 2 x l 0 - ^ i n 0.05N NaOH s o l u t i o n . These v a l u e s of a are about two o r d e r s of magnitude l a r g e r than those of O 3 under i d e n t i c a l e x p e r i m e n t a l c o n d i t i o n s . When 0.2% H 2 O 2 s o l u t i o n ( a t pH = 13) was used, a i n c r e a s e d to about l x l 0 ~ 3 . F u r t h e r experiments w i t h s o l u t i o n s of h i g h e r H 2 O 2 c o n c e n t r a t i o n s showed t h a t the mass accommoda t i o n c o e f f i c i e n t of S02 would be g r e a t e r than 2 x l 0 " . No h i g h e r a

a

a s

a

a

a

5

a

a

a

3




F i g u r e 2.

O3 decay i n h e l i u m c a r r i e r gas.


9.

Accommodation Coefficients of Ozone and

TANG AND LEE

S0

2

113

v a l u e s c o u l d be o b t a i n e d f o r SO2 s i n c e H2O2 a t h i g h c o n c e n t r a t i o n s decomposed e x c e s s i v e l y on v e s s e l w a l l s , thereby i n t e r f e r i n g w i t h the measurement. However, i t was d i s c o v e r e d t h a t NaCIO was a v e r y e f f e c t i v e reagent f o r SO2 o x i d a t i o n i n aqueous s o l u t i o n s . U s i n g NaCIO, the v a l u e o f a f o r SO2 i n c r e a s e d to 2 x l 0 ~ and p o s s i b l y h i g h e r . The f i n a l v a l u e c o u l d n o t be measured w i t h good p r e c i s i o n s i n c e d i f f u s i o n a l p r o c e s s e s a l s o became i m p o r t a n t i n c o n t r o l l i n g mass t r a n s p o r t r a t e s a t such h i g h a v a l u e s . T h e r e f o r e , 2 x l 0 " r e p r e s e n t s the lower bounds of a f o r SO2. 2

a

2

a


a

Model S t u d i e s . The o x i d a t i o n o f d i s s o l v e d SO2 i n water has been the s u b j e c t o f numerous s t u d i e s i n the p a s t decade, and i t s atmospheric s i g n i f i c a n c e r e g a r d i n g a c i d p r e c i p i t a t i o n needs no f u r t h e r e l a b o r a t i o n h e r e . However, most of the e a r l i e r s t u d i e s (4-7) have been devoted to aqueous c h e m i s t r y o n l y . R e c e n t l y , Schwartz and F r e i b e r g (8) have c o n s i d e r e d the importance of mass t r a n s f e r p r o c e s s e s i n l i m i t i n g the S(IV) o x i d a t i o n r a t e s i n aqueous drops. Chameides (9) has made a r a t h e r comprehensive model study o f the p h o t o c h e m i s t r y of a s t r a t i f o r m c l o u d i n a remote r e g i o n of the marine atmosphere. He concludes t h a t the r a t e o f SO2 c o n v e r s i o n to s u l f u r i c a c i d i s s e n s i t i v e to a v a r i e t y o f parameters i n c l u d i n g the accommodation c o e f f i c i e n t s o f the reagent gases such as S02, H2Û2, HO2 and OH. U n f o r t u n a t e l y , however, no accommodation c o e f f i c i e n t measurements have been r e p o r t e d i n the l i t e r a t u r e f o r these gases. I t would, t h e r e f o r e , be i n t e r e s t i n g to examine how i m p o r t a n t the newly measured accommodation c o e f f i c i e n t s would be i n the c o n v e r s i o n of S(IV) to S(VI) i n a water d r o p l e t . A simple model i s s e t up, which c o n s i d e r s o n l y aqueous c h e m i s t r y and gas-phase mass t r a n s f e r o f O3 and SO2 to a c l o u d d r o p l e t . A t t = 0, the d r o p l e t i s exposed to an atmosphere c o n t a i n i n g c o n s t a n t c o n c e n t r a t i o n s o f SO2 and O3. The aqueous c o n c e n t r a t i o n s of S(IV) and S(VI) a r e then c a l c u l a t e d as a f u n c t i o n o f time. The c h e m i c a l r e a c t i o n s c o n s i d e r e d i n the model a r e l i s t e d i n Table I , t o g e t h e r w i t h the a p p r o p r i a t e c o n s t a n t s used i n the c a l c u l a t i o n . The r a t e e x p r e s s i o n f o r gas-phase mass t r a n s f e r i s g i v e n by dC

3ϋγ

dt

a RT

, .

2

s

a

where a i s the d r o p l e t r a d i u s , R the gas c o n s t a n t , Τ the a b s o l u t e temperature, p the p a r t i a l p r e s s u r e of the reagent gas i n the b u l k gas phase and p a t the d r o p l e t s u r f a c e . Here, p i s r e l a t e d to the Henry's law c o n s t a n t , H, by s

a

a

Ρ

a

=

C — Η

(8)

and γ, a k i n e t i c c o r r e c t i o n f a c t o r to the Maxwell's e q u a t i o n (γ = 1 ) , can be e v a l u a t e d by one o f the s e v e r a l e x p r e s s i o n s proposed i n the l i t e r a t u r e ( 1 0 ) . F o r p r a c t i c a l purposes, however, these e x p r e s s i o n s y i e l d v e r y s i m i l a r v a l u e s . Consequently, the f o l l o w i n g e x p r e s s i o n due to Fukuta and W a l t e r (11) was used i n the p r e s e n t study:



114


•3

®-H

6

io- k

-

(5 0.05 Ν NaOH

Ο IO" ' 0 7

1

H 0 2

1 2

1

1 1 1 1 4 6 [no S0 ] χ ΙΟ M

1

" 8

2

2

3

F i g u r e 3 . Apparent accommodation c o e f f i c i e n t of O 3 as a f u n c t i o n of Na2S03 c o n c e n t r a t i o n i n aqueous s o l u t i o n .

10°

IO

1

IO TIME/sec 2

IO

3

IO

4

F i g u r e 4. C o n v e r s i o n of S ( I V ) to S ( V I ) i n d r o p l e t by ozone oxidation.



^S02(aq)

+

S02(g)

S02(aq)

^

HSO4-

NBS: ES:

+

=

+

=

+

2

H+

+ o

+ 02

I.

7

w

K

6

K K

5

K

=

=

=

=

=

H 4

=

-

8

exp[1120(l/T

= l/298])M

l/298)]M

1x10-14 e x p [ - 6 7 1 6 ( l / T

-

0.018222T)]M

NBS

ES

NBS

NBS

NBS

(4)

(4)

NBS

References

B u t t e r w o r t h and C o . , L t d . ,

l/298)]M

e x p [ 2 . 3 0 2 6 ( - 4 7 5 . 1 4 / T + 5.0435 -

6xl0~

-

atm-1

sec-1

sec"l

l/298)]M

exp(-10,500/RT)M~l

exp[3120(l/T

1 7

1.7x10-2 e x p [ 2 0 9 0 ( l / T

1.23

l.OxlO

1.0x1014 e x p ( - 1 1 0 0 0 / R T ) M - l

1 / 2 9 8 ) ] M atm-1

=

-

S o l u b i l i t y Constants

1.15x10-2 e x p [ 2 3 6 0 ( l / T

Rate,

=

*3

*2

Hi

Equilibrium,

S O 2 O x i d a t i o n b y Ozone i n D r o p l e t

N a t i o n a l B u r e a u o f S t a n d a r d s T e c h n i c a l Note 2 7 0 - 1 , 1965. " E l e c t r o l y t e S o l u t i o n s b y R . A . R o b i n s o n and R . H . S t o k e s , " 1959.

H+

+ H

=

5F=^H+ + O H -

S 0 4

=F=^ S 0 3

HSO3-

H2O

S04

HSO4""

H2O ^ H S O 3 -

+ S03= —>

—>

03(aq)

HSO3-

+

^ 0 3 ( a q )

0 3 (aq)

03(g)

Reactions

Table


116


Y

(9)

-

a + (4D/Va) Note t h a t the terra c o n t a i n i n g α i n the denominator accounts f o r interfacial resistance. E q u a t i o n (7) was combined w i t h a p p r o p r i a t e chemical r e a c t i o n r a t e e x p r e s s i o n s to y i e l d a s e t of coupled d i f f e r e n t i a l e q u a t i o n s e x p r e s s i n g r a t e s of change i n the d i s s o l v e d O 3 and S(IV) c o n c e n t r a t i o n s . The equations were then s o l v e d n u m e r i c a l l y w i t h the u s u a l c o n s t r a i n t s of e l e c t r o n e u t r a l i t y and the a p p r o p r i a t e i o n i c e q u i l i b r i a g i v e n i n Table I . F i g u r e 4 shows the r e s u l t s of a case c a l c u l a t i o n performed f o r the f o l l o w i n g c o n d i t i o n s : a « 10 um; Τ - 283 Κ; ρ(0β) = 300 ppb; p(S02> - 1 ppb; α(0 ) = 5χ10~ ; a ( S 0 ) - 2 x l 0 ~ , l x l O " * , l x l O " ; and i n i t i a l d r o p l e t pH = 7. I n F i g u r e 4 [ S ( I V ) ] * i s the s a t u r a t i o n molar c o n c e n t r a t i o n of S(IV) i n the absence of O 3 and, t h e r e f o r e , the curves r e p r e s e n t the time e v o l u t i o n of [ S ( I V ) ] and [ S ( V I ) ] n o r m a l i z e d to [S(IV)1* s o l e l y f o r the convenience of comparing and p l o t t i n g the calculated results. I t i s i n t e r e s t i n g to observe t h a t i n a l l cases [ S ( I V ) ] always r i s e s to a peak and then f a l l s o f f g r a d u a l l y . I n c o n t r a s t , [ S ( V I ) ] c o n t i n u e s to i n c r e a s e as the o x i d a t i o n r e a c t i o n goes on. It indi c a t e s a dynamic process i n which the s t a t i o n a r y - s t a t e concept does not seem to apply to the s u l f u r s p e c i e s . On the c o n t r a r y , the c a l c u l a t i o n ( r e s u l t s not shown here) i n d i c a t e s t h a t the d r o p l e t q u i c k l y becomes s a t u r a t e d w i t h d i s s o l v e d O 3 and, s h o r t l y a f t e r , m a i n t a i n s a s t e a d y - s t a t e [ O 3 ] c l o s e to s a t u r a t i o n f o r a long time, even though the accommodation c o e f f i c i e n t of O 3 used i n the c a l c u l a t i o n i s as low as the measured v a l u e , 5 x l 0 " . I n a d d i t i o n , the e f f e c t of i n t e r f a c i a l S O 2 mass t r a n s f e r on o x i d a t i o n i s not c o n s i d e r e d a p p r e c i a b l e s i n c e the curves c a l c u l a t e d f o r ot(S02) = 1 are almost i d e n t i c a l to those c a l c u l a t e d f o r a(S02) = 2 x l 0 ~ . Only when

Accommodation Coefficients of Ozone and SO2: Implications on SO2

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