Chemistry of Wintertime Wet Deposition - ACS Symposium Series

Chapter 21

Chemistry of Wintertime Wet Deposition

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 2, 2018 | https://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch021

Jean Muhlbaier Dasch Environmental Science Department, General Motors Research Laboratory, Warren, MI 48090

Four years of winter precipitation data from south eastern Michigan were examined to help understand the higher NO -, but lower SO --, concentrations in snow than in winter rain. The higher NO - levels i n snow could be attributed to the lower precipitation depths associated with snow events than with rain events. Conversely, SO -- was far higher in winter rain than in snow. The SO -- concentrations were highly corre lated with the temperatures of the cloud layers. The data suggests that SO i s incorporated and oxidized to SO -- i n clouds most efficiently when the hydrometeors are present as liquid droplets. The fact that NO does not show the same relationship suggests that incorporation of nitrogen species into cloud water followed by oxidation i s not as important a process for nitrogen as for sulfur. 3

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The S 0 - - / N 0 - - r a t i o of w i n t e r p r e c i p i t a t i o n i s lower t h a n t h a t of summer p r e c i p i t a t i o n i n t h e n o r t h e a s t e r n U n i t e d S t a t e s and e a s t e r n Canada ( 1 , 2 ) . P a r t o f t h i s d i f f e r e n c e can be a t t r i b u t e d t o d i f f e r ences between r a i n and snow, s i n c e snow has a lower S 0 - - / N 0 - r a t i o t h a n summer r a i n , o r even w i n t e r r a i n ( 2 , 3 ) . Several studies (3,4) have shown S 0 - - t o be lower i n snow than i n w i n t e r r a i n i n t h e northeastern United States. N 0 - , on the o t h e r hand, f r e q u e n t l y shows t h e r e v e r s e t r e n d w i t h h i g h e r c o n c e n t r a t i o n s i n snow t h a n i n winter r a i n ( 4 , 5 ) . The h i g h e r N 0 - c o n c e n t r a t i o n s i n snow t h a n i n r a i n c o u l d not be a t t r i b u t e d t o a i r t e m p e r a t u r e s , s y n o p t i c p a t t e r n s , p r e c i p i t a t i o n r a t e , wind d i r e c t i o n or wind speed i n an a n a l y s i s by Raynor and Hayes ( 5 ) . They s u g g e s t e d t h a t s i n c e b o t h w i n t e r r a i n and snow o r i g i n a t e from s u b - f r e e z i n g c l o u d s , t h e h i g h e r N 0 - c o n c e n t r a t i o n s found i n snow than r a i n must be due t o more e f f i c i e n t b e l o w - c l o u d s c a v e n g i n g of n i t r o g e n s p e c i e s i n the a i r by snowflakes t h a n by r a i n d r o p s . M o d e l i n g s t u d i e s by Chang a l s o suggest t h a t snowflakes s h o u l d scavenge g a s - p h a s e HN0 more e f f i c i e n t l y t h a n raindrops (6). In t h i s p a p e r , f o u r y e a r s o f w i n t e r p r e c i p i t a t i o n d a t a w i l l be examined t o p r o v i d e i n s i g h t s i n t o t h e mechanisms by which s u l f u r and 4

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0097-6156/87/0349-0242$06.00/0 © 1987 American Chemical Society

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

21.

Chemistry

DASCH

nitrogen t i o n s of i n terms t i o n and

of Wintertime

Wet

243

Deposition

species are incorporated into p r e c i p i t a t i o n . Concentra S 0 ~ - and N0 - i n w i n t e r r a i n and snow w i l l be c o n s i d e r e d o f p r e c i p i t a t i o n d e p t h , ambient c o n c e n t r a t i o n s , wind d i r e c cloud temperatures. 4

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Experimental Wet d e p o s i t i o n was c o l l e c t e d d u r i n g f o u r w i n t e r s a t a s i t e i n War ren, MI, a suburb n o r t h of D e t r o i t . The samples were c o l l e c t e d from l a t e December t o e a r l y A p r i l f o r f o u r w i n t e r s s t a r t i n g w i t h the 1981/82 w i n t e r . The water e q u i v a l e n t o f t h e w i n t e r t i m e p r e c i p i t a t i o n d u r i n g the f o u r w i n t e r s was 24, 12, 15 and 33 cm, r e s p e c t i v e l y . The s n o w f a l l d u r i n g the f o u r w i n t e r s was 147, 34, 86 and 124 cm. A n n u a l p r e c i p i t a t i o n d a t a w i l l a l s o be r e f e r r e d t o i n t h i s paper, which i s based on sampling a t t h i s s i t e from summer, 1981 t o summer, 1983. Wet d e p o s i t i o n was c o l l e c t e d on an event b a s i s i n p o l y e t h y l e n e b u c k e t s i n Aerochem M e t r i c c o l l e c t o r s s e t t o open o n l y d u r i n g p r e c i p i t a t i o n p e r i o d s . The p r e c i p i t a t i o n time was d e t e r m i n e d from a B e l f o r t recording rain-gauge. The p r e c i p i t a t i o n d e p t h (as water e q u i v a l e n t ) was d e t e r m i n e d as t h e volume o f p r e c i p i t a t i o n i n the b u c k e t d i v i d e d by t h e a r e a of t h e bucket o p e n i n g (638 c m ) . The p r e c i p i t a t i o n was f i l t e r e d t h r o u g h 0.4 Mm p o r e N u c l e p o r e f i l t e r s t o remove p a r t i c l e s and was then r e f r i g e r a t e d u n t i l time f o r a n a l y s i s . The i o n s , N0 - and S 0 - - , were a n a l y z e d by i o n chromatography. The c o n c e n t r a t i o n s o f p a r t i c l e s and g a s e s i n a i r were measured d u r i n g t h e l a s t two w i n t e r s t o a l l o w a c o m p a r i s o n of p r e c i p i t a t i o n c o m p o s i t i o n w i t h l e v e l s o f p o l l u t a n t s i n t h e a i r . Each s a m p l i n g p e r i o d l a s t e d 3 t o 5 days. A i r was sampled a t 10 L/min t h r o u g h a t r i p l e - s t a c k f i l t e r : a l-μπι p o r e - s i z e T e f l o n f i l t e r c o l l e c t e d p a r t i c l e s , a l-Atm p o r e - s i z e n y l o n f i l t e r c o l l e c t e d HN0 as w e l l as any N0 - t h a t v o l a t i l i z e d from t h e f i r s t f i l t e r , and d o u b l e c e l l u l o s e n i t r a t e f i l t e r s impregnated w i t h a 25% K C 0 , 10% g l y c e r o l s o l u t i o n c o l l e c t e d S0 . The T e f l o n f i l t e r was e x t r a c t e d i n 50 mL o f d e i o n i z e d water and t h e e x t r a c t was a n a l y z e d f o r N0 - and S 0 - - . The n y l o n f i l t e r was e x t r a c t e d i n 50 mL of t h e same b i c a r b o n a t e - c a r b o n a t e e l u a n t used i n the i o n chromatograph and t h e e x t r a c t was ana l y z e d f o r N0 -. The S 0 f i l t e r s were e x t r a c t e d i n 100 mL o f a 0.2% H 0 s o l u t i o n and t h e e x t r a c t was a n a l y z e d f o r S 0 " . In a d d i t i o n , N0 was c o l l e c t e d on a c a r t r i d g e c o n t a i n i n g d i p h e n y l a m i n e and ana l y z e d by t h e method o f L i p a r i ( 7 ) . M e t e o r o l o g i c a l d a t a were o b t a i n e d from L o c a l C l i m a t o l o g i c a l Data c o l l e c t e d a t t h e D e t r o i t M e t r o p o l i t a n A i r p o r t , 39 km SW of War ren. Upper a i r d a t a were based on rawinsonde, c o n s t a n t p r e s s u r e d a t a c o l l e c t e d t w i c e d a i l y a t F l i n t , MI, 75 km NW of Warren. The m e t e o r o l o g i c a l d a t a were o b t a i n e d from the N a t i o n a l C l i m a t i c Data C e n t e r i n A s h e v i l l e , NC. 2

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Results W i n t e r p r e c i p i t a t i o n was c o l l e c t e d i n Warren, MI over a f o u r - y e a r period. P r e c i p i t a t i o n was c l a s s i f i e d as r a i n , snow, o r mixed r a i n and snow based on t h e L o c a l C l i m a t o l o g i c a l D a t a from D e t r o i t M e t r o politan Airport. The volume-weighted mean c o n c e n t r a t i o n s a r e shown i n T a b l e I . The weighted s t a n d a r d d e v i a t i o n s were c a l c u l a t e d as


244

THE CHEMISTRY OF ACID RAIN

T a b l e I . C o n c e n t r a t i o n s o f Ions i n Winter

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29 41±4.9 26±3.5 0.63

25 34±3.1 55±5.2 1.6

31 31±3.3 66±4.6 2.1

3

Snow

Mixed

Rain No. Events NO (Meq/L) S0 " (Meq/L) S0 --/N0 -

Precipitation

d e s c r i b e d by T o p o l ( 4 ) . S 0 - ~ i s l o w e s t d u r i n g snow e v e n t s . N0 i s i n d e e d h i g h e r i n snow t h a n i n r a i n , as found i n o t h e r s t u d i e s ( 4 , 5 ) . The o p p o s i t e t r e n d i n S 0 - - and N 0 - l e a d s t o a s t r o n g downward t r e n d i n S0 --/N0 - r a t i o s from r a i n t o snow e v e n t s . The S0 --/N0 - r a t i o s a r e s i m i l a r t o t h o s e measured e l s e w h e r e i n t h e n o r t h e a s t e r n U n i t e d S t a t e s ( 4 ) . The t r e n d s i n N0 - and S 0 " w i l l be c o n s i d e r e d i n d i v i d u a l l y below. 3

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N i t r a t e C o n c e n t r a t i o n s . The h i g h e r c o n c e n t r a t i o n s o f N 0 - found i n snow t h a n i n w i n t e r r a i n s have been a t t r i b u t e d t o h i g h e r scavenging of HN0 i n t h e a i r by s n o w f l a k e s t h a n by r a i n d r o p s ( 4 , 5 , 6 ) . Another p o s s i b i l i t y w i l l be c o n s i d e r e d h e r e : t h a t c o n c e n t r a t i o n d i f f e r e n c e s can be e x p l a i n e d based on p r e c i p i t a t i o n d e p t h . C o n c e n t r a t i o n s o f i o n s such a s S 0 - ~ and N 0 - i n p r e c i p i t a t i o n have been shown t o v a r y i n v e r s e l y w i t h p r e c i p i t a t i o n depth ( 8 ) . During the w i n t e r p e r i o d s c o n s i d e r e d i n t h i s p a p e r , t h e average p r e c i p i t a t i o n d e p t h f o r w i n t e r r a i n e v e n t s was 1.3 cm compared t o 0.42 cm (as water e q u i v a l e n t ) f o r snow e v e n t s . The e f f e c t o f t h i s d i f f e r e n c e i s shown i n T a b l e I I 3

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T a b l e I I . P r e c i p i t a t i o n Events S e p a r a t e d by P r e c i p i t a t i o n Depth < 0.4 cm

No. Events Volume (mL) N0 ~ (Meq/L) S 0 " (Meq/L) S0 —/N0 3

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Rain

Snow

0.4 - 0.8 cm Snow Rain

> 0.8 cm Rain Snow

6 156 78 117 1.5

22 138 59 36 0.61

7 398 43 88 2.1

18 1179 27 61 2.3

5 347 44 28 0.64

3 1081 24 15 0.63

where e v e n t s a r e s e p a r a t e d b y p r e c i p i t a t i o n d e p t h . Based on t h i s d i v i s i o n , t h e N0 - c o n c e n t r a t i o n s a r e n o t h i g h e r i n snow events t h a n i n r a i n events. A m u l t i p l e r e g r e s s i o n a n a l y s i s was a l s o performed t o d e t e r m i n e t h e e f f e c t o f p r e c i p i t a t i o n depth and p r e c i p i t a t i o n t y p e (snow v s . r a i n ) on t h e N0 - c o n c e n t r a t i o n . C l o u d temperature was used as a measure o f p r e c i p i t a t i o n t y p e and was c a l c u l a t e d as d e s c r i b e d i n t h e next s e c t i o n . A l t h o u g h N 0 - c o n c e n t r a t i o n s were found t o be i n v e r s e l y c o r r e l a t e d w i t h p r e c i p i t a t i o n volume, t h e r e was no s i g n i f i c a n t c o r r e l a t i o n between N 0 - c o n c e n t r a t i o n s and t e m p e r a t u r e . T h e r e f o r e , a t t h i s l o c a t i o n , t h e lower water c o n t e n t o f snow events 3

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21.

Chemistry

DASCH

of Wintertime

Wet

245

Deposition

compared t o r a i n events a p p e a r s s u f f i c i e n t t o e x p l a i n t h e h i g h e r NO3- c o n c e n t r a t i o n s found i n snow; t h e r e i s no e v i d e n c e t h a t HN0 i n t h e a i r i s scavenged more e f f i c i e n t l y by snow than by r a i n . 3

S 0 - ~ C o n c e n t r a t i o n s . Based on Tables I and I I , t h e r e c a n be no doubt t h a t S 0 - ~ l e v e l s i n w i n t e r r a i n a r e f a r h i g h e r t h a n i n snow a t t h i s l o c a t i o n , d e s p i t e d i f f e r e n c e s i n p r e c i p i t a t i o n d e p t h . Two p o s s i b l e sources o f the d i f f e r e n c e are t h e f o l l o w i n g : higher ambient S 0 and S 0 - - c o n c e n t r a t i o n s a v a i l a b l e f o r s c a v e n g i n g d u r i n g r a i n events o r h i g h e r S 0 t o S 0 - - c o n v e r s i o n d u r i n g r a i n e v e n t s . These p o s s i b i l i t i e s w i l l be c o n s i d e r e d f u r t h e r . Ambient c o n c e n t r a t i o n s o f p a r t i c l e s and gases were measured a t ground l e v e l d u r i n g t h e 1983-84 and 1984-85 w i n t e r s t o d e t e r m i n e i f h i g h e r c o n c e n t r a t i o n s were a v a i l a b l e f o r scavenging d u r i n g w i n t e r r a i n s and snows. S i n c e t h e ambient d a t a d i d not c o r r e s p o n d t o p a r t i c u l a r p r e c i p i t a t i o n e v e n t s , they were r o u g h l y grouped i n t o snow p e r i o d s and r a i n p e r i o d s . The r e s u l t s o f t h i s g r o u p i n g a r e seen i n T a b l e I I I . Based on a Student T - t e s t , t h e o n l y s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e s a t t h e 95% c o n f i d e n c e l e v e l i s f o r N 0 which i s A

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T a b l e I I I . C o n c e n t r a t i o n s o f P a r t i c l e s and Gases i n A i r d u r i n g R a i n P e r i o d s and Snow P e r i o d s Ug/m ) 3

Rain

so -4

HN03 2

2

Snow P e r i o d s 3.5±1.9 4.211.9 1.310.78 22110 24110

3.512.1 3.5±1.3 1.4±0.96 19±9.9 17±7.9

N03-

so N0

Periods

h i g h e r d u r i n g snow p e r i o d s . The s u l f u r s p e c i e s a r e a c t u a l l y somewhat h i g h e r d u r i n g snow p e r i o d s than d u r i n g r a i n p e r i o d s . Theref o r e , t h e h i g h e r S 0 - - l e v e l s i n r a i n cannot be a t t r i b u t e d t o h i g h e r l e v e l s o f ambient s u l f u r s p e c i e s a v a i l a b l e f o r s c a v e n g i n g . The o r i g i n o f t h e storm system c o u l d a l s o l e a d t o d i f f e r e n c e s i n p r e c i p i t i o n c o n c e n t r a t i o n s . S u l f u r e m i s s i o n s w i t h i n 480 km o f Warren a r e t w i c e a s h i g h from t h e south o r e a s t as from t h e n o r t h o r west, and N 0 e m i s s i o n s a r e a l m o s t t e n t i m e s h i g h e r from t h e e a s t , south o r west a s from t h e n o r t h ( 3 ) . S i n c e more snow events than r a i n events were from t h e c l e a n e r n o r t h , t h i s might e x p l a i n t h e lower S 0 - ~ l e v e l s i n snow t h a n i n r a i n . To e v a l u a t e t h i s , t h e ground l e v e l wind d i r e c t i o n was d e t e r m i n e d d u r i n g each p r e c i p i t a t i o n p e r i o d based on t h e L o c a l C l i m a t o l o g i c a l D a t a from D e t r o i t M e t r o p o l itan Airport. The d a t a was d i v i d e d i n t o N, E, S, and W q u a d r a n t s . E v e n t s w i t h a wind s h i f t o f more than 100° were e x c l u d e d from t h e analysis. The volume-weighted mean c o n c e n t r a t i o n s a r e shown i n T a b l e IV. Note t h a t t h e number o f events i s s m a l l from some d i r e c tions . The N O 3 - c o n c e n t r a t i o n s a r e lower i n r a i n than snow from a l l d i r e c t i o n s , b u t t h a t can be e x p l a i n e d based on t h e lower p r e c i p i t a t i o n depth i n snows than r a i n s . The S 0 - ~ l e v e l s a r e c o n s i d e r a b l y 4

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T a b l e IV.

E f f e c t o f Wind D i r e c t i o n on P r e c i p i t a t i o n C o n c e n t r a t i o n (Meq/L) S0 --

NO 3


4

North East South West

Rain

Snow

Rain

Snow

44 ( 1 ) * 63 (9) 72 (10) 48 (3)

20 15 39 47

16 (1) 35 (9) 31 (10) 28 (3)

20 46 54 48

(5) (4) (7) (2)

(5) (4) (7) (2)

* V a l u e s i n p a r e n t h e s e s a r e t h e number o f events r e p r e s e n t e d by each mean.

h i g h e r i n r a i n t h a n i n snow f o r t h r e e d i r e c t i o n s . T h e r e f o r e , t h e g r o u n d - l e v e l wind d i r e c t i o n cannot e x p l a i n t h e h i g h e r s u l f a t e l e v e l s i n r a i n than snow. I t i s a l s o p o s s i b l e t h a t snow scavenges p a r t i c u l a t e S 0 - - l e s s e f f i c i e n t l y t h a n r a i n , b u t t h i s cannot be d e t e r m i n e d from t h i s d a t a s e t . However, i n d i c a t i o n s from t h e l i t e r a t u r e suggest t h a t t h e r e v e r s e i s t r u e . Knutson e t a l . (9) r e v i e w e d s e v e r a l s t u d i e s showi n g t h a t snow scavenged p a r t i c l e s f a s t e r t h a n r a i n . Chan and Chung (10) a l s o found a h i g h e r s c a v e n g i n g r a t i o f o r S 0 - - p a r t i c l e s by snow than r a i n . The o t h e r p o s s i b i l i t y t o be c o n s i d e r e d i s t h a t o f g r e a t e r S 0 t o S 0 - - c o n v e r s i o n i n c l o u d water d u r i n g r a i n e v e n t s t h a n d u r i n g snow e v e n t s . The c o n v e r s i o n r a t e w i l l depend on a v a r i e t y o f f a c t o r s i n c l u d i n g t h e S0 c o n c e n t r a t i o n , the c o n c e n t r a t i o n of oxidants such as ozone o r hydrogen p e r o x i d e , and t h e i n c o r p o r a t i o n and r e a c t i o n o f t h e s e s p e c i e s i n c l o u d hydrometeors. The c o n c e n t r a t i o n s o f S 0 and o x i d a n t s a r e u n l i k e l y t o show l a r g e v a r i a t i o n s from December t o e a r l y A p r i l when t h e s e samples were c o l l e c t e d . More l i k e l y , t h e f a c t o r of importance i s t h e s t a t e of the p r e c i p i t a t i o n i n the c l o u d , whether f r o z e n o r l i q u i d , o r t h e r e l a t i v e l e n g t h o f t i m e i n each s t a t e . The s t a t e o f t h e p r e c i p i t a t i o n would a f f e c t b o t h t h e i n c o r p o r a t i o n and r e a c t i o n o f s u l f u r s p e c i e s i n c l o u d d r o p s . F i r s t , d u r i n g t h e f r e e z i n g p r o c e s s , most o f t h e d i s s o l v e d S 0 i s l o s t from t h e d r o p as i n d i c a t e d by experiments o f I r i b a r n e e t a l . ( 1 1 ) . S e c o n d l y , t h e o x i d a t i o n o f t h e r e m a i n i n g d i s s o l v e d S 0 w i t h i n an i c e c r y s t a l w i l l be r e t a r d e d compared t o r e a c t i o n w i t h i n a d r o p l e t . I t has been argued t h a t t h e p r e c i p i t a t i o n s t a t e i s unimportant i n w i n t e r s t o r m s , because a l l c l o u d m o i s t u r e would be e x p e c t e d t o be f r o z e n a t c l o u d l e v e l s , whether i t appeared a s r a i n o r snow a t ground l e v e l ( 5 ) . However, S c o t t found h i g h e r S 0 ~ - l e v e l s i n rimed s n o w f l a k e s where growth o c c u r e d by a c c r e t i o n o f water d r o p l e t s than i n unrimed snowf l a k e s where growth o c c u r r e d by vapor d e p o s i t i o n ( 1 2 ) . We i n v e s t i g a t e d t h e e f f e c t o f t h e temperature i n t h e c l o u d s f o r t h e storms o f t h e f i r s t two w i n t e r s u s i n g t h e upper a i r d a t a from F l i n t , MI. F o r each p r e c i p i t a t i o n e v e n t , t h e c l o u d r e g i o n was r o u g h l y d e f i n e d as t h e a l t i t u d e s w i t h r e l a t i v e h u m i d i t i e s g r e a t e r t h a n 90%. The median temperature i n t h i s a l t i t u d e range was n e x t d e t e r m i n e d . The temperature f o r t h e snow events ranged from -14° C 4

4

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t o -4° C w i t h a median o f -10° C whereas t h e r a i n o r mixed e v e n t s ranged i n t e m p e r a t u r e from -15° C t o 11° C w i t h a median o f 1° C. Based on t h e s e t e m p e r a t u r e s i t appears t o be u n t r u e t h a t most c l o u d l a y e r s a r e f r o z e n i n the w i n t e r a t t h i s l o c a t i o n , s i n c e drops c a n e a s i l y e x i s t i n a supercooled s t a t e a t these temperatures (13). F i g u r e 1 shows a p l o t o f the S 0 - - / N 0 - r a t i o i n the p r e c i p i t a t i o n as a f u n c t i o n o f c l o u d t e m p e r a t u r e . A h i g h l y s i g n i f i c a n t , p o s i t i v e c o r r e l a t i o n e x i s t s (r=0.75) between the S0 --/N0 - r a t i o and the t e m p e r a t u r e i n the c l o u d . The r a t i o o f S0 --/N0 - i s used, r a t h e r than S 0 - ~ c o n c e n t r a t i o n s , t o n o r m a l i z e f o r t h e e f f e c t o f p r e c i p i t a t i o n d e p t h ; S0 -- i n p r e c i p i t a t i o n a l s o c o r r e l a t e d w i t h c l o u d t e m p e r a t u r e but t o a l e s s e r d e g r e e (r=0.35). I t i s i m p o s s i b l e t o draw a d i v i s i o n between s o l i d - p h a s e and l i q u i d - p h a s e hydrometeors based on t h e c l o u d t e m p e r a t u r e because o f s u p e r c o o l i n g and because c l o u d drops most l i k e l y go t h r o u g h s o l i d and l i q u i d phases as the water c i r c u l a t e s from the low, warmer a l t i tudes t o t h e h i g h , c o o l e r a l t i t u d e s . However, t h i s graph s h o u l d p r o v i d e an i n d i c a t i o n o f t h e s t a t e o f t h e system, w i t h hydrometeors a t the low t e m p e r a t u r e s e x i s t i n g a s i c e c r y s t a l s and hydrometeors a t t h e h i g h e r t e m p e r a t u r e s e x i s t i n g a s l i q u i d drops and a g r a d a t i o n o f c o n d i t i o n s in-between. T h e r e f o r e , t h i s e v i d e n c e s u g g e s t s t h a t t h e h i g h e r S0 ~- l e v e l s i n r a i n than i n snow i s due t o t h e g r e a t e r d i s s o l u t i o n and r e a c t i o n o f S 0 i n l i q u i d d r o p s than i c e c r y s t a l s . C o n v e r s e l y , t h e f a c t t h a t N 0 - c o n c e n t r a t i o n s a r e t h e same i n r a i n and snow i n d i c a t e s t h a t t h e d i s s o l u t i o n o f NOx i n t o d r o p s f o l l o w e d by o x i d a t i o n i s a l e s s i m p o r t a n t p r o c e s s t h a n f o r S 0 . 4

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Discussion Four y e a r s o f w i n t e r p r e c i p i t a t i o n e v e n t s were a n a l y z e d i n terms o f S0 ~- and N 0 - c o n c e n t r a t i o n s t o p r o v i d e i n f o r m a t i o n on the mechanisms by w h i c h t h e s e i o n s a r e i n c o r p o r a t e d i n t o p r e c i p i t a t i o n . N 0 was h i g h e r i n snow than i n w i n t e r r a i n , a s suggested by o t h e r s t u d ies. However, i n t h i s s t u d y the d i f f e r e n c e c o u l d be a t t r i b u t e d t o t h e lower p r e c i p i t a t i o n d e p t h s a s s o c i a t e d w i t h snows than w i t h w i n ter rains. There was no e v i d e n c e t h a t snow scavenged HN0 more e f f i c i e n t l y than r a i n a t t h i s l o c a t i o n . C o n v e r s e l y , S0 -- was f a r h i g h e r i n w i n t e r r a i n s t h a n i n snow. T h i s c o u l d not be e x p l a i n e d i n terms o f t h e ambient l e v e l s o f s u l f u r s p e c i e s o r t h e scavenging o f S0 -- p a r t i c l e s . However, the c l o u d temperatures were h i g h enough i n the c a s e o f r a i n t o suggest t h a t the c l o u d hydrometeors c o u l d have been p r e s e n t as l i q u i d d r o p l e t s r a t h e r than i c e c r y s t a l s . The S0 ~- c o n c e n t r a t i o n s o f the p r e c i p i t a t i o n were c o r r e l a t e d w i t h w i n t e r c l o u d l a y e r t e m p e r a t u r e s . The d a t a suggests t h a t S 0 i s i n c o r p o r a t e d and o x i d i z e d t o S0 -- i n c l o u d s when t h e hydrometeors a r e p r e s e n t as l i q u i d d r o p l e t s . The f a c t t h a t N 0 - l e v e l s a r e t h e same i n b o t h r a i n and snow suggests t h a t i n c o r p o r a t i o n o f n i t r o g e n s p e c i e s i n t o c l o u d water f o l l o w e d by o x i d a t i o n i s l e s s important a process f o r n i t r o g e n than f o r s u l f u r . 4

3

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American Chemical Society Library 1155 15th of St., Johnson et al.; The Chemistry AcidN.W. Rain Washington, D.C. Washington, 20036 ACS Symposium Series; American Chemical Society: DC, 1987.



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F i g u r e 1. The i n f l u e n c e of c l o u d t e r m p e r a t u r e on the S O 4 — / N O 3 - r a t i o of w i n t e r p r e c i p i t a t i o n . (Reprinted p e r m i s s i o n from r e f . 14. C o p y r i g h t 1987 Pergamon.)


with

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Acknowledgment s I thank Ken Kennedy f o r c o l l e c t i n g samples, Frank L i p a r i , W i l l i a m S c r u g g s , Pat Mulawa, and Rene Vandervennet f o r sample a n a l y s i s and George W o l f f and Sudarshan Kumar f o r h e l p f u l d i s c u s s i o n s .


Literature Cited 1. Bowersox, V.C.; Stensland, G.J.,"Seasonal Patterns of Sulfate and Nitrate in Precipitation in the United States," 74th Air Pollution Control Meeting, Paper 81-6.1, June 1981. 2. Summers, P.W.; Barrie, L.A., "The Spatial and Temporal Varia tion of Sulphate to Nitrate Ratio in Precipitation in eastern North America," presented at Muskoka Conference, September, 1985. 3. Dasch, J.M.; Cadle, S.H. Atmos. Environ. 1985, 19, 789. 4. Topol, L.E. Atmos. Environ., 1986, 20, 347. 5. Raynor, G.S.; Hayes, J.V., In Precipitation Scavenging, Dry Deposition, and Resuspension, Pruppacher, H.R., Semonin, R.G., Slinn, W.G.N., Eds., Elsevier Press, 1983, p 249. 6. Chang, T.Y. Atmos. Environ. 1984, 18, 191. 7. Lipari, F. Anal. Chem. 1984, 56, 1820. 8. Barrie, L.A. J. Geophys. Res. 1985, 90, 5789. 9. Knutson, E.O.; Sood, S.K.; Stockham, J.D. Atmos. Environ. 1976, 10, 395. 10. Chan, W.H.; Chung, D.H.S. Atmos. Environ. 1986, 20, 1397. 11. Iribarne, J.V.; Barrie, L.A.; Iribarne, Α., Atmos. Environ. 1983, 17, 1047. 12. Scott, B.C. J. Applied Met. 1981, 20, 619. 13. Pruppacher, H.R., In Chemistry of the Lower Atmosphere, Rasool, Ed., Plenum Press, NY, 1973, pp 1-67. 14. Dasch, J. M. Atmos. Environ. 1987, 21, 141. RECEIVED

March 25, 1987


Chemistry of Wintertime Wet Deposition - ACS Symposium Series

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