The Chemistry of Acid Rain - American Chemical Society

Chapter 27

Acid Clusters R. G. Keesee and A. W. Castleman, Jr.

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch027

Department of Chemistry, Pennsylvania State University, University Park, PA 16802

Gas phase molecular aggregates that contain acid molecules have been produced with free jet expansion techniques and detected by using electron impact ionization mass spectrometry. The clusters of aqueous nitric acid paralleled many properties of the condensed phase. Multiple nitric acid molecules were found in the clusters that were sufficiently dilute. The acid molecule was absent in the ionized clusters involving HCl and only water was evident. Experiments also demonstrated the reactivity of ammonia with aqueous nitric acid and sulfur dioxide clusters and of sulfur trioxide with water clusters. The natural occurrence of acid cluster negative ions offers a means to probe the gas phase acid loading of the atmosphere through laboratory and field studies of the ion chemistry.

The l a b o r a t o r y i n v e s t i g a t i o n of a phenomenon such as a c i d r a i n may p r o c e e d a l o n g s e v e r a l avenues. One extreme i n v o l v e s s i m u l a t i o n o f c o n d i t i o n s as i n smog o r c l o u d chambers i n which many p r o c e s s e s may o c c u r d u r i n g the c o u r s e of an e x p e r i m e n t . In t h i s manner, some i d e a of the o v e r a l l p i c t u r e may be g a i n e d . The o t h e r approach i s devoted to u n d e r s t a n d i n g b a s i c p r o p e r t i e s , such as a s p e c i f i c r e a c t i o n r a t e , upon which the l a r g e r p i c t u r e may be b u i l t . The l a t t e r t a c k i s t h a t which i s u n d e r t a k e n i n o u r l a b o r a t o r y . Specif i c a l l y , we a r e examining the c h e m i c a l and p h y s i c a l p r o p e r t i e s of molecular c l u s t e r s . M o l e c u l a r c l u s t e r s can be c o n s i d e r e d t o be the s m a l l e s t s i z e range of an a e r o s o l p a r t i c l e s i z e d i s t r i b u t i o n . N u c l e a t i o n from the gas phase t o p a r t i c l e s o r d r o p l e t s i n v o l v e s , i n t h e i n i t i a l s t a g e s , the f o r m a t i o n of c l u s t e r s . R e s e a r c h on c l u s t e r s p r o v i d e s a v a l u a b l e a p p r o a c h t o u n d e r s t a n d i n g , on a m o l e c u l a r l e v e l , the d e t a i l s of the t r a n s f e r of m o l e c u l e s from the gaseous t o the condensed s t a t e by e i t h e r new p a r t i c l e f o r m a t i o n o r heterogeneous p r o c e s s e s i n c l u d i n g a d s o r p t i o n onto o r d i s s o l u t i o n i n t o p a r t i c l e s .

0097-6156/87/0349-0317$06.00/0 © 1987 American Chemical Society

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

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The o x i d a t i o n of s p e c i e s such as S O 2 , N 0 , and o r g a n i c compounds v i a gas-phase r e a c t i o n s , aqueous phase r e a c t i o n s i n the s o l u t i o n s of d r o p l e t s and a e r o s o l p a r t i c l e s , or s u r f a c e r e a c t i o n s on p a r t i c l e s ( e i t h e r s u r f a c e - g a s or s o l i d - l i q u i d i n t e r f a c e s ) produce a c i d s i n the atmosphere. The r e l a t i v e i m p o r t a n c e of the v a r i o u s mechanisms depends on f a c t o r s such as the a e r o s o l l o a d i n g , r e l a t i v e h u m i d i t y , and s o l a r i n t e n s i t y . The s t u d y of c l u s t e r s i s most r e l e v a n t t o c o n d i t i o n s i n w h i c h gas-phase r e a c t i o n s dominate a c i d p r o d u c t i o n and g a s - t o - p a r t i c l e c o n v e r s i o n proceeds through n u c l e a t i o n . For i n s t a n c e , the p r o p e n s i t y of s u l f u r i c a c i d m o l e c u l e s t o form s m a l l h y d r a t e d c l u s t e r s i s i m p o r t a n t t o the n u c l e a t i n g a b i l i t y of s u l f u r i c a c i d ( 1 ) . A major impetus, w h i c h i s a p p l i c a b l e t o the heterogeneous c h e m i s t r y of the atmosphere as w e l l as o t h e r a r e a s of r e e a r c h , f o r the s t u d y of c l u s t e r s i s the p r o s p e c t t h a t such work w i l l l e a d t o a b e t t e r u n d e r s t a n d i n g of surface i n t e r a c t i o n s ( 2 ) . C o n s e q u e n t l y , knowledge of the p r o p e r t i e s and f o r m a t i o n of c l u s t e r s c o n t a i n i n g a c i d s c o n t r i b u t e s t o an u n d e r s t a n d i n g of some of the p r o c e s s e s i n v o l v e d i n the development of a c i d r a i n . T h i s paper p r e s e n t s an o v e r v i e w of r e s u l t s on the f o r m a t i o n and s t a b i l i t y of b o t h n e u t r a l and i o n i c acid clusters.


X

Neutral Acid Clusters W i t h f r e e j e t e x p a n s i o n t e c h n i q u e s , we have produced c l u s t e r s of aqueous n i t r i c a c i d (_3 ), h y d r o c h l o r i c a c i d , s u l f u r i c a c i d (4^), pure a c e t i c a c i d ( 5 ) , and s u l f u r d i o x i d e ( 6 ) . For analogy t o b u f f e r i n g , the f o r m a t i o n of c l u s t e r s c o n t a i n i n g ammonia have a l s o been examined. These have i n c l u d e d ammonia w i t h aqueous n i t r i c a c i d (7_)> hydrogen s u l f i d e (7_), and s u l f u r d i o x i d e ( 8 ) . The b a s i c experiment i n v o l v e s e x p a n s i o n of vapor t h r o u g h a n o z z l e , c o l l i m a t i o n of the j e t w i t h a skimmer t o form a w e l l - d i r e c t e d m o l e c u l a r beam, and d e t e c t i o n of c l u s t e r s v i a e l e c t r o n impact i o n i z a t i o n and q u a d r u p o l e mass s p e c t r o m e t r y . Some v a r i a t i o n s i n c l u d e the i n t r o d u c t i o n of a r e a c t i v e gas i n t o vacuum near the e x p a n s i o n as d e s c r i b e d elsewhere (4,8) and the i m p l e m e n t a t i o n of an e l e c t r o s t a t i c q u a d r u p o l a r f i e l d t o examine the p o l a r i t y of the n e u t r a l c l u s t e r s . The e l e c t r i c d e f l e c t i o n t e c h n i q u e i s d e s c r i b e d by Kleraperer and coworkers ( 9 ) . Background on the p r o p e r t i e s of f r e e j e t e x p a n s i o n s i s d e s c r i b e d by Anderson ( 1 0 ) and Hagena ( 1 1 ) . A few i m p o r t a n t p o i n t s t o n o t e f o l l o w . The c l u s t e r d i s t r i b u t i o n s themselves a r e k i n e t i c a l l y quenched due t o a t r a n s i t i o n t o a f r e e m o l e c u l a r f l o w ( e s s e n t i a l l y c o l l i s i o n a l l e s s ) regime a f t e r a s h o r t d i s t a n c e (a few n o z z l e d i a m e t e r s ) from the n o z z l e t i p . The d i r e c t e d m o t i o n of the gas c r e a t e d by the e x p a n s i o n l e a d s t o a c o o l i n g of the t r a n s l a t i o n a l temperature due t o a n a r r o w i n g of the v e l o c i t y d i s t r i b u t i o n o f the m o l e c u l e s i n the j e t . R e l a x a t i o n of i n t e r n a l degrees of freedom a l s o o c c u r s but g e n e r a l l y the quenched temperatures are i n the o r d e r T ( v i b ) > T ( r o t ) > T ( t r a n s ) . W i t h c l u s t e r i n g , the l a t e n t heat of c o n d e n s a t i o n c o n t r i b u t e s t o an e x c i t a t i o n of the i n t e r n a l


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modes o f t h e c l u s t e r w h i c h t h e c o l d c o l l i s i o n s may not f u l l y relax. A f u r t h e r n o t e i s t h a t f o r purposes o f mass i d e n t i f i c a t i o n and detection, the neutral c l u s t e r s are ionized. The i o n i z e d d i s t r i b u t i o n o f c l u s t e r s i z e s may n o t f a i t h f u l l y r e p r e s e n t t h e n e u t r a l s i z e d i s t r i b u t i o n i n a one-to-one correspondence due t o f r a g m e n t a t i o n upon i o n i z a t i o n , i o n s t a b i l i t y , i o n i z a t i o n c r o s s - s e c t i o n s , and mass d i s c r i m i n a t i o n i n t h e s p e c t r o m e t e r . F o r most o f t h e c l u s t e r s d e s c r i b e d h e r e w h i c h i n v o l v e hydrogen b o n d i n g , i o n i z a t i o n o f t h e neutral c l u s t e r usually leads t o a protonated c l u s t e r v i a a r e a c t i o n t y p i f i e d by +

(NH ) (NH ) Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch027

3

3

+

n

NH^N^Vi

+ NH

2

(1)

where t h e i o n i z e d m o l e c u l a r u n i t o f t h e c l u s t e r s p o n t a n e o u s l y reacts w i t h a neighboring molecule (12). The p r o d u c t i o n may be v i b r a t i o n a l l y h o t and undergo f u r t h e r d i s s o c i a t i o n . Attempts t o u n d e r s t a n d t h e s e d i s s o c i a t i o n p r o c e s s e s i n our l a b o r a t o r y (12) as w e l l as o t h e r s (13,14) a r e b e i n g made i n o r d e r t o b e t t e r i n t e r p r e t r e s u l t s such as those d e s c r i b e d below. W i t h t h e s e fundamental p r o c e s s e s i n mind, we can proceed t o d i s c u s s r e s u l t s c o n c e r n i n g acid clusters. I n t h e s t u d y o f aqueous n i t r i c a c i d (_3), d e u t e r a t e d s p e c i e s were used t o a v o i d a m b i g u i t y i n s t o i c h i o m e t r y . C l u s t e r s were produced by t h e e x p a n s i o n o f t h e vapor from heated c o n c e n t r a t e d aqueous n i t r i c a c i d . E l e c t r o n impact i o n i z a t i o n o f t h e c l u s t e r s produced i o n s o f t h e form D ( D N 0 3 ) ( U 2 0 ) y . I n t h e case o f t h e c l u s t e r s c o n t a i n i n g one n i t r i c a c i d m o l e c u l e , a d i s t i n c t l o c a l minimum o f t h e s i g n a l i n t e n s i t y i n t h e s i z e d i s t r i b u t i o n o c c u r s a t the c l u s t e r s i z e D ( D N 0 3 ) ( 0 2 0 ) 4 . S i n c e no e x p l a n a t i o n based on i o n s t a b i i t y seems f e a s i b l e , an a t t r a c t i v e e x p l a n a t i o n o f t h e p o s i t i o n of t h e minimum i s t h a t i t i s i n d i c a t i v e o f some r a t h e r abrupt t r a n s f o r m a t i o n o f t h e p r e c u r s o r n e u t r a l s i n t h i s s i z e range. Such a s i t u a t i o n might occur i f a complex became s u f f i c i e n t l y h y d r a t e d t o e n a b l e t h e f o r m a t i o n o f s o l v a t e d i o n p a i r s . A l a r g e change i n the charge d i s t r i b u t i o n w i t h i n t h e c l u s t e r s h o u l d o c c u r upon s o l v a t i o n t o form an i o n p a i r ΝΟβ-ζΙ^Ο^ΗβΟ*. A more p o l a r s p e c i e s s h o u l d have a l a r g e r c o l l i s i o n r a t e and hence a f a s t e r growth r a t e . T h i s e f f e c t s h o u l d m a n i f e s t i t s e l f i n t h e observed i n t e n s i t y d i s t r i b u t i o n as an i n c r e a s e i n i n t e n s i t y o f t h e l a r g e r s i z e d c l u s t e r s j u s t beyond t h e c l u s t e r t h a t underwent t h e i o n - p a i r formation. R e c e n t l y i o n - p a i r f o r m a t i o n i n c l u s t e r s has been suggested t o e x p l a i n t h e abrupt l i n e w i d t h broadening and s p e c t r a l changes i n t h e f l u o r e s c e n c e o f oi-naphthol(NH3) when η reaches f o u r a p p a r e n t l y t o y i e l d α-naphtholate · (ΝΪ^^ΝΗ^" " ( 1 5 ) . Ab i n i t i o c a l c u l a t i o n s on the h y d r a t i o n o f NH^F (16) and NaH P04 (17_) have shown t h a t as few as s i x water m o l e c u l e s a r e s u f f i c i e n t t o c r e a t e a s o l v a t e d i o n p a i r i n w h i c h t h e i o n s become s e p a r a t e d by s o l v e n t . For c l u s t e r s w i t h more than one n i t r i c a c i d m o l e c u l e , t h e s p e c i e s ( D N 0 3 ) ( D 2 0 ) (l l ) a r e n o t d e t e c t e d even though t h e HNO3 dimer has been o b s e r v e d ( 1 8 ) i n an e x p a n s i o n i n v o l v i n g anhydrous H N O 3 . The i n t e r e s t i n g a n a l o g y i n t h i s c a s e i s t h a t c o n c e n t r a t e d aqueous n i t r i c a c i d s o l u t i o n s a r e p h o t o c h e m i c a l l y and t h e r m a l l y u n s t a b l e and decompose v i a t h e s t o i c h i o m e t r y +

X


2HN0

2N0

3

2

+ H 0 + 1/2 0 2

(2)

2

A p p a r e n t l y t h e e x o t h e r m i c i t y o f t h e c l u s t e r i n g o r e l e c t r o n impact i o n i z a t i o n s u p p l i e s t h e energy which i n i t i a t e s t h i s d e c o m p o s i t i o n i n t h e c l u s t e r s t h a t c o n t a i n t o o much n i t r i c a c i d compared t o t h e number o f s o l v e n t water m o l e c u l e s . When i n t r o d u c e d as a r e a c t a n t , ammonia appears t o be p r e f e r e n t i a l l y i n c o r p o r a t e d ( v i a H 0 replacement) i n t o c l u s t e r s c o n t a i n i n g HNO3 l e a v i n g t h e pure H 0 c l u s t e r s r e l a t i v e l y u n a f f e c t e d (7_). P r o d u c t i o n s o f t h e form Η ( Η Ν 0 ) ( Ν Η ) ( Η 0 ) w i t h x=0,l, y=0,l,2, and ζ up t o 7 a r e e a s i l y r e s o l v a b l e . Higher c l u s t e r s a r e a l s o o b s e r v e d , b u t w i t h low i n t e n s i t y ; t h e N H 3 / H 0 s t o i c h i o m e t r y was u n r e s o l v e d f o r t h e s e because o f p o o r e r mass r e s o l u t i o n . When t h e v a p o r from a b o i l i n g s o l u t i o n o f 6M HC1 i s expanded, t h e d e t e c t e d c l u s t e r i o n s l a c k HC1 as e v i d e n c e d by t h e absence o f the c h a r a c t e r i s t i c m+2 i s o t o p e due t o ^ ^ C l . On t h e o t h e r hand, t h e o b s e r v e d H ( H 0 ) d i s t r i b u t i o n i s n o t i c e a b l y d i f f e r e n t than t h a t o b t a i n e d i n t h e e x p a n s i o n o f pure water. For instance, the usual prominent d i s c o n t i n u i t i e s a t H ( H 0 ) 4 and H ( H 0 ) i a r e n o t e v i d e n t and a s l i g h t l o c a l minimum i n i n t e n s i t y o c c u r s a t Η ( Η 0 ) ΐ 3 · The i n d i c a t i o n i s t h a t HC1 i s i n i t i a l l y p r e s e n t i n t h e n e u t r a l c l u s t e r but t h a t i o n i z a t i o n l e a d s t o " b o i l i n g o f f " o f HC1. The m o l e c u l a r i o n s H ^ C 1 and H ^ C 1 a r e o b s e r v e d . E x p a n s i o n o f t h e vapor o v e r N H 4 S a l s o r e s u l t s i n an i o n i z e d c l u s t e r d i s t r i b u t i o n i n which one of t h e e x p e c t e d components, namely H S , i s absent ( 7 ) . Q u i t e u n l i k e t h e d i s t r i b u t i o n o b t a i n e d from t h e e x p a n s i o n o f pure ammonia, a s t r o n g l o c a l minimum s i m i l a r t o t h e H ( H N 0 3 ) ( H 0 ) d i s t r i b u t i o n i s present. In o r d e r t o e x p l o r e a d s o r p t i o n and heterogeneous p r o c e s s e s on a molecular s c a l e , i t i s of i n t e r e s t t o study the r e a c t i o n s of gases w i t h c l u s t e r s . We have performed one s e r i e s o f e x p e r i m e n t s i n which ammonia was expanded from t h e n o z z l e and S 0 was i n t r o d u c e d as a r e a c t a n t t h r o u g h t h e a n n u l a r o p e n i n g around t h e n o z z l e , and a n o t h e r s e r i e s i n w h i c h t h e r o l e s o f t h e gases were r e v e r s e d (8). W i t h ammonia i n t r o d u c e d v i a t h e n o z z l e (500 t o r r s t a g n a t i o n p r e s s u r e ) and when t h e S 0 p r e s s u r e b e h i n d t h e o u t e r a n n u l a r o p e n i n g i s 40 t o r r , i o n i z e d c l u s t e r s o f t h e form ( N H 3 ) S 0 and H ( N H 3 ) S 0 a r e d e t e c t e d . When t h e S 0 p r e s s u r e i s r e d u c e d t o 20 t o r r , no e v i d e n c e of S 0 i n c o r p o r a t i o n i n t o t h e ammonia c l u s t e r s i s found. A p e c u l i a r f e a t u r e i s that the unprotonated s p e c i e s e x h i b i t a normal s i z e d i s t r i b u t i o n , whereas t h e p r o t o n a t e d c l u s t e r s are s t r o n g l y peaked a t NH^+N^SO^ On t h e o t h e r hand, i o n i z a t i o n 2

2

+

3

χ

3

ν

2

ζ

2

+

2

n

+

+

2

2

2

+

2

5

+

7

+

2

+

2

n

2

2

+

n

+

n

2

2

2


2

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of p u r e ammonia c l u s t e r s r e s u l t s a l m o s t e x c l u s i v e l y i n p r o t o n a t e d c l u s t e r s due t o the i n t e r n a l c l u s t e r r e a c t i o n ( 3 ) . When s u l f u r d i o x i d e i s i n t r o d u c e d t h r o u g h the n o z z l e and the amount of ammonia b e h i n d the a n n u l a r o p e n i n g v a r i e s f r o m 6 to 50 t o r r , the e x t e n t of ammonia i n c o r p o r a t i o n i n t o the c l u s t e r s d r a m a t i c a l l y i n c r e a s e s w i t h i n c r e a s i n g ammonia p r e s s u r e . Up t o two ammonia m o l e c u l e s were o b s e r v e d to be i n c o r p o r a t e d i n t o the c l u s t e r s w i t h 20 t o r r of ammonia b e h i n d the a n n u l a r o p e n i n g . With 40 t o r r , up t o f o u r N H 3 m o l e c u l e s were o b s e r v e d i n the c l u s t e r s . I n a d d i t i o n , c l u s t e r s c o n t a i n i n g one N H 3 m o l e c u l e become more p r e v a l e n t t h a n the pure ( S 0 ) clusters. The p r o b a b i l i t y t h a t a c l u s t e r of η S 0 m o l e c u l e s c o n t a i n e d one o r more NH3 m o l e c u l e s a p p r o x i m a t e l y d o u b l e d from n=2 to 8 i n a g r a d u a l manner. Based on o n l y the c l u s t e r h a r d - s p h e r e c o l l i s i o n c r o s s - s e c t i o n , a dependence o f n2/3 ( f a c t o r of 4 i n c r e a s e ) might be e x p e c t e d . However, s e v e r a l o t h e r e f f e c t s must a l s o be c o n s i d e r e d i n c l u d i n g the e x t e n t o f d i s s o c i a t i o n upon i o n i z a t i o n , the r e l a t i v e i n t e r n a l t e m p e r a t u r e s o f the c l u s t e r s , and the l i m i t e d number of i n t e r n a l d e g r e e s of freedom i n the c l u s t e r s . +

2

n


2

o r

a

In g e n e r a l , the o b s e r v e d c l u s t e r d i s t r i b u t i o n s a r e smooth and i n n e i t h e r s e r i e s of e x p e r i m e n t s i s any p r e f e r e n c e f o r a p a r t i c u l a r s t o i c h i o m e t r i c r a t i o a p p a r e n t , e x c e p t f o r NH^+N^SO;? i n the protonated d i s t r i b u t i o n . These e x p e r i m e n t s a l s o d e m o n s t r a t e t h a t the n o z z l e d e s i g n r e s u l t s i n the r e a c t i o n of the s p e c i e s e x i t i n g f r o m the a n n u l a r o p e n i n g w i t h c l u s t e r s of the s p e c i e s i n t r o d u c e d t h r o u g h the i n n e r n o z z l e . The a d d i t i o n o f ammonia t o S 0 clusters was found t o be more e f f e c t i v e t h a n the a d d i t i o n of S 0 to NH3 clusters. Some e x p l a n a t i o n s i n c l u d e a h i g h e r p r o b a b i l i t y of S0 e v a p o r a t i o n upon i o n i z a t i o n , l e s s s e v e r e beam s c a t t e r i n g w i t h N H 3 c o l l i s i o n s on S 0 c l u s t e r s , o r a d i f f e r e n t r e a c t i v i t y (accommodation c o e f f i c i e n t s ) i n the two c a s e s . 2

2

2

2

A s t u d y of the r e a c t i o n of s u l f u r t r i o x i d e w i t h water c l u s t e r s has a l s o been made (4_)· The i o n c l u s t e r s o b s e r v e d i n t h e s e e x p e r i m e n t s were a s e r i e s of p r o t o n a t e d w a t e r c l u s t e r s H ( H 0 ) n w i t h η up to 14 and a n o t h e r l e s s abundant s e r i e s of the form S 0 3 ( H 0 ) H w i t h η up t o 9 . The H + ( H 0 ) s e r i e s c o u l d r e s u l t f r o m the i o n i z a t i o n of u n r e a c t e d w a t e r c l u s t e r s and a l s o from the i o n i z a t i o n of S 0 3 ( H 0 ) c l u s t e r s . The r e l a t i v e d i s t r i b u t i o n of the H (H 0) s e r i e s , however, was not a p p r e c i a b l y a f f e c t e d by t h e i n t r o d u c t i o n of S O 3 . The S03(H 0) H" " d i s t r i b u t i o n was v e r y s i m i l a r i n f o r m t o the H ( H N 0 3 ) ( H 0 ) d i s t r i b u t i o n o b s e r v e d f r o m the e x p a n s i o n of aqueous n i t r i c a c i d vapor i n t h a t a l o c a l i n t e n s i t y minimum ( a t n=4 f o r the S 0 3 ( H 0 ) H d i s t r i b u t i o n ) was o b s e r v e d . The S O 3 f l u x was such t h a t no masses c o r r e s p o n d i n g t o more t h a n one S O 3 m o l e c u l e i n a c l u s t e r were d e t e c t e d . E l e c t r o s t a t i c focusing d e m o n s t r a t e d t h a t the S 0 3 H 0 adduct, e x p e c t e d t o be the i n i t i a l product, r a p i d l y isomerized to H S04« +

2

+

2

n

2

2

n

m

+

2

n

1

2

n

+

2

n

+

2

n

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2

2

E l e c t r o s t a t i c d e f l e c t i o n e x p e r i m e n t s have shown the a c i d - w a t e r a d d u c t s t o be p o l a r . However, l a r g e r c l u s t e r s e x h i b i t d e f o c u s i n g b e h a v i o r i n d i c a t i v e of a p o l a r i z a b l e , but e s s e n t i a l l y n o n p o l a r , species. Only f o r the pure a c e t i c a c i d c l u s t e r s , s p e c i f i c a l l y the


322

THE CHEMISTRY OF ACID RAIN

t r i m e r and i n d i c a t i o n s a l s o f o r t h e pentamer o r l a r g e r was p o l a r i t y o f l a r g e r c l u s t e r s e v i d e n t ( 5 ) . Ionic Acid

Clusters

E x a m i n a t i o n o f t h e c l u s t e r i n g o f n e u t r a l m o l e c u l e s onto i o n s i s a n o t h e r approach t o t h e s t u d y o f a c i d c l u s t e r s . Cluster ions observed i n t h e atmosphere r e f l e c t t h e r o l e o f a c i d s . Strong a c i d s a r e p r e f e r e n t i a l l y c l u s t e r e d t o n e g a t i v e i o n s w h i c h a c t as bases. Under normal a t m o s p h e r i c c o n d i t i o n s , ambient i o n s o f t h e type HS04"(H S04) (HN0 ) or N03"(HN0 ) (the l a t t e r a l s o probably h y d r a t e d i n t h e lower t r o p o s p h e r e ) have been observed throughout the l o w e r atmosphere (19,20). L a b o r a t o r y s t u d i e s have been made o f t h e thermodynamic s t a b i l i t y o f N 0 ~ ( H N 0 ) c l u s t e r s (21,22) and t h e r e a c t i v i t y o f n i t r i c a c i d ( 2 3 ) and s u l f u r i c a c i d "Ç24) w i t h n e g a t i v e i o n s . Through t h e s e s t u d i e s , an u n d e r s t a n d i n g o f the pathways t o t h e p r o d u c t i o n s o f t h e a c i d c l u s t e r s and t h e i r r e l a t i o n s h i p t o t h e gas-phase a c i d l o a d i n g o f t h e atmosphere has been d e v e l o p e d . Consequently, i n c o n j u n c t i o n w i t h these l a b o r a t o r y r e s u l t s , i n s i t u d e t e c t i o n of t h e r e l a t i v e ambient abundance o f t h e s e i o n i c c l u s t e r s a l l o w s e s t i m a t e s t o be made o f t h e gas-phase c o n c e n t r a t i o n o f these a c i d s . Heitman and A r n o l d (19) e s t i m a t e from t h e i r measurements t h a t t h e gaseous a c i d i c s u l f u r c o n c e n t r a t i o n i n t h e 12 t o 8 km a l t i t u d e range i s 1 0 t o 10^ m o l e c u l e s cm""^. Once a g a i n , t h e l a r g e s t u n c e r t a i n t y i n t h e s e r e s u l t s i s f r a g m e n t a t i o n ; i n t h i s case t h e f r a g m e n t a t i o n t h a t o c c u r s upon e x t r a c t i n g ambient i o n s i n t o t h e mass s p e c t r o m e t e r . Recent s t u d i e s i n d i c a t e t h a t o b s e r v a t i o n s o f S 0 o r HS0 a s s o c i a t e d w i t h ambient c l u s t e r i o n s o f t h e s t r a t o s phere p r o b a b l y a r e n o t due t o c l u s t e r i n g w i t h t h e ambient g a s e s , but r e s u l t from f r a g m e n t a t i o n o f s u l f u r i c a c i d i n t h e c l u s t e r i o n s (25). The observed p o s i t i v e i o n s a r e p r o t o n a t e d c l u s t e r s c o n t a i n i n g w a t e r and h i g h p r o t o n a f f i n i t y s p e c i e s such as a c e t o n i t r i l e i n t h e l o w e r s t r a t o s p h e r e (26) o r ammonia i n t h e l o w e r t r o p o s p h e r e ( 2 0 ) . Other h i g h p r o t o n a f f i n i t y s p e c i e s such as p y r i d i n e and p i c o l i n e s may e n t e r i n t o t h e p o s i t i v e i o n c h e m i s t r y o f t h e lower t r o p o s p h e r e ( 2 7 , 2 8 ) . F u r t h e r d i s c u s s i o n o f t h e s e s t u d i e s and t h e e x p e r i m e n t a l t e c h n i q u e s can be found elsewhere ( 2 8 , 2 9 ) . 2


clusters,

x

3

y

3

3

3

n

n

6

3

3

Acknowledgments Support by t h e Department of Energy, Grant No. DE-ACO2-82-ER60055, and t h e N a t i o n a l S c i e n c e F o u n d a t i o n , Grant No. ATM-82-04010, i s g r a t e f u l l y acknowledged.

Literature Cited 1. Heist, R. H.; Reiss, H. J. Chem. Phys. 1974, 61, 573. 2. Whetten, R. L . ; Cox, D. M.; Trevor, D. J.; Kaldor, A. Surf. Sci. 1985, 156, 8. See also other papers in that volume.



27.

KEESEE & CASTLEMAN

Acid

Clusters

323

3. Kay, B. D.; Herman, V.; Castleman, A. W., Jr. Chem. Phys Lett. 1981, 80, 469. 4. Sievert, R.; Castleman, A. W. Jr. J. Phys. Chem. 1984, 88, 3329. 5. Sievert, R.; Cadez, I.; Van Doren, J.; Castleman, A. W., Jr. J. Phys. Chem. 1984, 88, 4502. 6. Castleman, A. W., Jr.; Kay, B. D. Int. J. Mass Spectrom. Ion Proc. 1985, 66, 217. 7. Kay, B. D.; Hofmann-Sievert, R.; Castleman, A. W., Jr. Chem. Phys. 1986, 102, 407. 8. Keesee, R. G.; Kilgore, K.; Breen, J. J.; Castleman, A. W., Jr. J. Aerosol Sci. Tech., in press. 9. Falconer, W. E.; Buchler, Α.; Stauffer, J. L . ; Klemperer, W. J. Chem. Phys. 1968, 48, 312. 10. Anderson, J. B. In "Molecular Beams and Low Density Gas Dynamics"; Wegener, D. P., Ed.; Marcel Dekker:New York, 1974, pp. 1-91. 11. Hagena, O. F. In "Molecular Beams and Low Density Gas Dynamics"; Wegener, D. P., Ed.; Marcel Dekker:New York, 1974, pp. 93-181. 12. Echt, O.; Dao, P. D.; Morgan, S.; Castleman, A. W., Jr. J. Chem. Phys. 1985, 82, 4076. 13. Buck, U.; Meyer, H. Phys. Rev. Lett. 1984, 52, 109. 14. Kamke, W.; Kamke, B.; Kiefl, H. U.; Hertel, I. V. J. Chem. Phys. 1986, 84, 1325. 15. Cheshnovsky, O.; Leutwyler, S. Chem. Phys. Lett. 1985, 121, 1. 16. Odutola, J. Α.; Dyke, T. R. J. Chem. Phys. 1978, 68, 5663. 17. Kollman, P.; Kuntz, I. J. Am. Chem. Soc. 1976, 98, 6820. 18. Lee, W. K.; Prohofsky, E. W. Chem. Phys. Lett. 1982, 85, 98. 19. Heitmann, H.; Arnold, F. Nature. 1983, 306, 747. 20. Perkins, M. D.; Eisele, F. L. J. Geophys. Res. 1984, 89, 9649. 21. Davidson, J . Α.; Fehsenfeld, F. C.; Howard, C. J. Int. J. Chem. Kinet. 1977, 9, 17. 22. Lee, N.; Keesee, R. G.; Castleman, A. W., Jr. J. Chem. Phys. 1980, 72, 1089. 23. Fehsenfeld, F. C.; Howard, C. J.; Schmeltekopf, A. L. J. Chem. Phys. 1975, 63, 2835. 24. Viggiano, Α. Α.; Perry, R. Α.; Albritton, D. L . ; Ferguson, E. E.; Fehsenfeld, F. C. J. Geophys. Res. 1982, 87, 7340. 25. Schlager, H.; Arnold, F. Planet. Space Sci. 1986, 34, 245. 26. Arijs, E.; Brasseur, G. J. Geophys. Res. 1986, 91, 4003. 27. Eisele, F. L . ; McDaniel, E. W. J. Geophys. Res. 1986, 91, 5183. 28. Keesee, R. G.; Castleman, A. W., Jr. J. Geophys. Res. 1985, 90, 5885. 29. Keesee, R. G.; Castleman, A. W., Jr. Ann. Geophys. 1983, 1, 75. RECEIVED May 5, 1987


The Chemistry of Acid Rain - American Chemical Society

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