The Chemistry of Acid Rain - ACS Publications - American Chemical

Chapter 2

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Subcontinental Air Pollution Phenomena G. M. Hidy Desert Research Institute, P.O. Box 60220, Reno, NV 89506

This paper discusses aspects of the accumulating body of observations characterizing deposition of airborne acid forming substances. Of p a r t i c u l a r interest are sulfur and nitrogen oxides species. The focus of the observations and interpretation is on subcontinental (or regional) scale phenomena extending over areas of 10 km . Spatial and temporal distributions of ambient sulfur oxide (or sulfate) and nitrogen oxide (or nitrate) concentrations and p r e c i p i t a t i o n chemistry are summarized as they r e f l e c t dry and wet deposition. Comparisons are given between conditions in the eastern and western United States. The importance of v a r i a b i l i t y in deposition exposure, within year and from year-to-year, is outlined. Evidence of linkage between source emissions and receptor measurements is included to complete the discussion. 6

2

f

S i n c e the m i d - 1 9 7 0 s , i n c r e a s i n g i n t e r e s t has emerged i n the e n v i r o n mental consequences o f the l a r g e s c a l e d e p o s i t i o n o f a t m o s p h e r i c contaminants. The d e p o s i t i o n o f a c i d - f o r m i n g c o n s t i t u e n t s , sulfate and n i t r a t e , i s o f p a r t i c u l a r c o n c e r n f o r p o t e n t i a l l y adverse ecological effects. These s p e c i e s d e r i v e from the o x i d a t i o n o f s u l f u r d i o x i d e (S0^) and n i t r o g e n o x i d e s (NO and N0^). Over most i f not a l l of the N o r t h American C o n t i n e n t , e m i s s i o n s o f t h e s e gases are b e l i e v e d to be dominated by man's a c t i v i t i e s , e s p e c i a l l y from f o s s i l f u e l c o m b u s t i o n , and the p r o d u c t i o n or r e f i n i n g o f m e t a l s . The " s c a l e " of exposure f o r d e p o s i t i o n c o v e r s exposure from p o l l u t a n t s i n l a r g e , s u b c o n t i n e n t a l a r e a s , the o r d e r o f 10^ km^. a l t h o u g h r e g i o n a l ambient a i r c o n c e n t r a t i o n s are w e l l below l e v e l s

0097-6156/87/0349-0010S06.00/0 © 1987 American Chemical Society

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

2.

11

Subcontinental Air Pollution Phenomena

HIDY

mandated by the U.S. C l e a n A i r A c t , d e p o s i t i o n c o n d i t i o n s may s t i l l be s u f f i c i e n t l y l a r g e i n some areas t o cause l o n g term e f f e c t s , p a r t i c u l a r l y i n remote, s u s c e p t i b l e a r e a s . There a r e no known d i r e c t p u b l i c h e a l t h e f f e c t s o f d e p o s i t i o n , and o t h e r r e g i o n a l e f f e c t s a r e h y p o t h e t i c a l , except p o s s i b l y f o r s u r f a c e water q u a l i t y . Thus, t h e r e i s g r e a t c o n c e r n f o r r e a l i z i n g " s i g n i f i c a n t b e n e f i t s " from l a r g e i n c r e a s e s i n i n c r e m e n t a l c o s t s o f p o l l u t i o n c o n t r o l t o address d e p o s i t i o n e x p o s u r e . S p e c i f i c a t i o n s o f " b e n e f i t " from r e d u c t i o n o f d e p o s i t i o n has r e q u i r e d a major investment i n s t u d i e s o f a i r b o r n e a c i d - f o r m i n g s u b s t a n c e s and t h e i r consequences t o a q u a t i c and t e r r e s t r i a l e c o systems. A major p a r t o f these s t u d i e s has r e s u l t e d i n c o n s i d e r a b l y improved knowledge o f the s u l f a t e and n i t r a t e d e p o s i t i o n c o n d i t i o n s i n N o r t h America, as w e l l as knowledge o f the a t m o s p h e r i c p r o c e s s e s a f f e c t i n g these d i s t r i b u t i o n s . In t h i s paper, a s p e c t s o f the c u r r e n t s t a t e o f knowledge i n d e p o s i t i o n p a t t e r n s a r e summarized, w i t h n o t e s about u n r e s o l v e d i s s u e s . D i s t r i b u t i o n of Deposition

Exposure

D e p o s i t i o n o f a t m o s p h e r i c c o n t a m i n a n t s takes p l a c e i n two p r i n c i p a l forms — d r y , by a b s o r p t i o n o f gases o r by p a r t i c l e c o l l e c t i o n at a s u r f a c e , and — wet, by s c a v e n g i n g and d e p o s i t v i a p r e c i p i t a t i o n . A t h i r d form ( " o c c u l t " d e p o s i t i o n ) i s sometimes c i t e d — the c o l l e c t i o n o f m a t e r i a l on s u r f a c e s v i a f o g o r m i s t . Of the t h r e e , the b u l k o f our knowledge c e n t e r s on wet d e p o s i t i o n . A l t h o u g h some ambient c o n c e n t r a t i o n d a t a have been a c q u i r e d , the d a t a a r e v e r y l i m i t e d i n t e m p o r a l and s p a t i a l c o v e r a g e . Data a r e v i r t u a l l y none x i s t e n t i n remote w e s t e r n a r e a s , p a r t i c u l a r l y i n a l p i n e areas where ecosystems a r e b e l i e v e d to be s u s c e p t i b l e . A few e x p l o r a t o r y measurements o f f o g d e p o s i t i o n have been o b t a i n e d a t mountain s i t e s , but no s y s t e m a t i c m o n i t o r i n g has been attempted. The f o g component i s not d i s c u s s e d f u r t h e r h e r e ; a v a i l a b l e o b s e r v a t i o n s suggest t h a t c l o u d s and f o g have h i g h e r c o n c e n t r a t i o n s o f a c i d formers than p r e c i p i t a t i o n ( b u t a p p a r e n t l y c a r r y a r e l a t i v e l y s m a l l p a r t o f the t o t a l burden i n most s i t u a t i o n s ) . E a s t e r n U n i t e d S t a t e s (EUS). The r e g i o n a l l y r e p r e s e n t a t i v e d i s t r i b u t i o n o f ambient s u l f u r o x i d e s and n i t r o g e n o x i d e s over the n o r t h e a s t e r n U n i t e d S t a t e s was f i r s t c h a r a c t e r i z e d i n the l a t e 1970*s from d a t a taken i n the S u l f a t e R e g i o n a l Experiment (SURE). The r e s u l t s have been r e p o r t e d i n s e v e r a l p u b l i c a t i o n s ( 1 ) . Concentra3 t i o n s o f S 0 i n t h e E a s t range from 6-26 yg/m , and p a r t i c u l a t e 3 s u l f a t e c o n c e n t r a t i o n s a r e 4-8 yg/m . The SURE NO and N0~ data 2are u n c e r t a i n i n q u a l i t y compared w i t h the SO^ (SO^ + S0^ ) d a t a 9

1

X

because o f measurement a m b i g u i t i e s .

Estimated

average

ambient

3 nitrate believed

concentrations t o be n i t r i c

range from 0.3-0.5 yg/m , much o f which i s a c i d vapor.

NO

concentration

distributions


12

T H E CHEMISTRY OF ACID RAIN

were l e s s w e l l c h a r a c t e r i z e d , but range, m a i n l y as N0^. E v i d e n c e shows t h a t the

are

ambient

reported

i n the

concentrations

7-20

yg/m

o f SO

and NO χ χ i n the EUS are l i n k e d w i t h b r o a d areas o f h i g h e m i s s i o n d e n s i t y a s s o c i a t e d w i t h m e t r o p o l i t a n areas and heavy i n d u s t r i a l i z a t i o n . Comparison between r e g i o n a l l y r e p r e s e n t a t i v e c o n c e n t r a t i o n s i n w o r l d remote a r e a s , and those o b s e r v e d i n the EUS show e l e v a t i o n s o f a f a c t o r o f ten i n mean c o n c e n t r a t i o n s , w i t h s h o r t term average con c e n t r a t i o n s even h i g h e r than b a s e l i n e c o n d i t i o n s . E l e v a t e d c o n c e n t r a t i o n s o f s u l f a t e and n i t r a t e a s s o c i a t e d w i t h zones o f h i g h e m i s s i o n d e n s i t y are a l s o found i n p r e c i p i t a t i o n . The g e o g r a p h i c a l c o n c e n t r a t i o n d i s t r i b u t i o n s i n p r e c i p i t a t i o n are s i m i l a r to those found f o r a i r b o r n e s u l f a t e (SO and NO emission χ χ d i s t r i b u t i o n s are s i m i l a r i n the EUS). A l t h o u g h d i r e c t measurements are e x t r e m e l y l i m i t e d , the dry component of d e p o s i t i o n can be e s t i m a t e d q u a l i t a t i v e l y from d a t a n o t i n g t h a t the d e p o s i t i o n r a t e i s the product o f a d e p o s i t i o n v e l o c i t y and ambient c o n c e n t r a t i o n o b s e r v a t i o n s at ground l e v e l . F o r example, t a k i n g s u i t a b l e v a l u e s o f d e p o s i t i o n v e l o c i t y , l i s t e d f o r example i n T a b l e I I , and d a t a from the SURE ( 2 ) , e s t i m a t e s of the annual average dry d e p o s i t i o n r a t e f o r s u l f u r are the o r d e r o f 6-60 kgS/ha-yr i n the E a s t . T h i s i s compared w i t h v a l u e s of 4-16 kgS/ha-yr i n wet d e p o s i t i o n . A l t h o u g h dry d e p o s i t i o n l e v e l s o f Ν 0 have not been r e p o r t e d , they would be lower than s u l f u r , s i n c e the ambient c o n c e n t r a t i o n s are s i m i l a r but the d e p o s i t i o n v e l o c i t y i s smaller. Wet d e p o s i t i o n of n i t r a t e based on a v a i l a b l e d a t a i n 1980 i s 2-7 kgN/ha-yr. Comparison between d e p o s i t i o n components f o r c o n d i t i o n s i n EUS i n d i c a t e s t h a t the dry component w i l l s u b s t a n t i a l l y exceed the wet component near s o u r c e s . T h i s r e s u l t s from ambient c o n c e n t r a t i o n s f o r SO^ average about 10-20 ppb, and N0^ c o n c e n t r a t i o n exceed 10 ppb χ

near s o u r c e s (

m ο se m g oc »-} 2?

H SC

2.

HIDY


< S" 5 g £ £

80 70 60 50 40 30 g 20 S "0 8

|i|

I

ο

^

SC PF ΚΑ ΑΙ BO WS WH IT PS CV IL

35 4.30 Ζ 225 0 σ-

φ

NO;

il*

I

S 15 ëUJ .0 m

•à · r*i r*i f*l n f*l Ο 0 SC PF KA Al BD WS WH IT PS CV IL u SQ

' l i f t n ή FI

SC PF KA Al BO WS WH IT PS CV IL REMOTE AREAS EASTERN U.S.

F i g u r e 1. Comparison between 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 s i n w o r l d remote a r e a s ( l e f t ) , the e a s t e r n U.S. ( r i g h t ) and the w e s t e r n s t a t e s (WS). The range shown f o r the f i r s t two c a t e g o r i e s i s the s t a n d a r d d e v i a t i o n o f a n n u a l mean v a l u e s . For the w e s t e r n s t a t e s , the range i s the v a l u e s f o r d i f f e r e n t s i t e s o b s e r v e d between 1981 and 1984. (Data f o r the f i r s t two from G a l l o w a y e t a l , 3; f o r t h e w e s t e r n s t a t e s , H i d y & Young, E n v i r o n . Res. & T e c h . , u n p u b l . r e p o r t ) ( D a t a r e p r o d u c e d w i t h p e r m i s s i o n from R e f . 3. C o p y r i g h t 1984 AAAS).


15

16


A l a s k a ( P F ) , K a t h e r i n e , A u s t r a l i a (ΚΑ) , Amsterdam I s l a n d , I n d i a n Ocean, ( A I ) , and Bermuda, A t l a n t i c Ocean (BD). The e a s t e r n U.S. s i t e s i n c l u d e M u l t i s t a t e A t m o s p h e r i c Power P r o d u c t i o n P o l l u t i o n Study (MAP3S) s i t e s at W h i t e f a c e M o u n t a i n , NY (WH) I t h a c a , NY ( I T ) , P e n n s y l v a n i a S t a t e U n i v e r s i t y ( P S ) , C h a r l o t t e s v i l l e , VA (CV) and Urbana, IL ( I L ) . The w e s t e r n s t a t e s are N a t i o n a l A t m o s p h e r i c D e p o s i t i o n Program (NADP) s i t e s w i t h a c o n t i n u o u s r e c o r d between 1981-1984 ( i n C o l o r a d o , C a l i f o r n i a , Washington, Oregon and A r i z o n a ) . The

comparisons

equivalent

+

suggests that

[H ]

i n the WUS

to g l o b a l remote c o n d i t i o n s , but

precipitation is 2[SO^ ] and [NO^ ] are

i n t e r m e d i a t e between remote and EUS " p o l l u t e d " c o n d i t i o n s . In b o t h the E a s t and the West, p r e c i p i t a t i o n s u l f a t e tends to be l a r g e r than n i t r a t e c o n c e n t r a t i o n s , except near l a r g e s o u r c e s of NO i n the West, χ An i n t e r e s t i n g f e a t u r e o f WUS p r e c i p i t a t i o n i s t h a t the annual average s u l f a t e c o n c e n t r a t i o n s are w e l l c o r r e l a t e d w i t h c a l c i u m + 2(Figure 2). [H ] i s much l e s s w e l l c o r r e l a t e d w i t h e i t h e r [SO^ ] or

[NO^

ively.

] —

correlation

coefficient

This

is quite different

( r ) = 0.577 and

from EUS

0.546

respect

c o n d i t i o n s where the

anions

+

are w e l l c o r r e l a t e d w i t h [ H ] . The r e a s o n f o r the s t r o n g a s s o 2+ = c i a t i o n between [Ca ] and [SO^ ] i n w e s t e r n p r e c i p i t a t i o n may be a s s o c i a t e d w i t h s c a v e n g i n g o f gypsum r i c h s o i l d u s t , or may be r e l a t e d to r e a c t i o n s of scavenged l i m e s t o n e dust and s u l f u r i c a c i d . F i g u r e 2 suggests a range f o r the i n f l u e n c e of s o i l d u s t , and a v a l u e f o r a s u l f a t e background. A l i n e o f s l o p e u n i t y can be drawn t h r o u g h the d a t a

p o i n t s at SO^

This

concentrations

i s s t o i c h i o m e t r i c a l l y c o n s i s t e n t with 2+ . 2z e r o Ca c o n c e n t r a t i o n i s 5 y e q / l SO^ .

with

1:1

slope

line

suggesting

i n f l u e n c e d component

(excess

than 20

The

This value

a g l o b a l p r e c i p i t a t i o n background e x p e c t e d 2shown i n F i g u r e 1. Above 20 yeq/1 SO^ , there the

less

CaSO,.

peq/l.

i n t e r c e p t at . is consistent

from remote

sites

i s d e v i a t i o n from

the i n f l u e n c e of a n o n - s o i l dust 2SO^ ). T h i s component c o u l d be

i d e n t i f i e d w i t h u n n e u t r a l i z e d a c i d i c a i r p o l l u t i o n o v e r the WUS. From F i g u r e 1, the average o f the " e x c e s s " s u l f a t e over the West would be about 5 y e q / l , much s m a l l e r than l e v e l s found i n the EUS. Measurements of ambient c o n c e n t r a t i o n s of s p e c i e s i n remote l o c a t i o n s o f the West are v e r y l i m i t e d . However, the few t h a t e x i s t show the western r u r a l l e v e l s much lower than i n the E a s t . Typical examples are l i s t e d i n T a b l e I I .


2.

HIDY

Table I I .

Deposition Velocity (cm/sec)

1 ppb

2

S0

E s t i m a t e s o f Annual Dry D e p o s i t i o n F o r R u r a l / B a c k g r o u n d Areas i n the West ( A f t e r H i d y & Young, E n v i r o n . , Res. & Tech., unpublished report)

Typical Concentration*

Species S0

2 4

17


~

2

yg

/m

E s t i m a t e d Dry Deposition Rate** (kg/ha-yr)

0.5 3

2

0.2

0^3 Total

N0

1 ppb

2

HN0

0.5

3

N0 "

0.5

3

ppb ug/m

3

2.3

0.1

0.2

1.0

0.9 0Λ

0.2 Total

**As s u l f u r & These v a l u e s

nitrogen are used w i t h d e p o s i t i o n v e l o c i t i e s

o f w e s t e r n dry ively

are

1.1

deposition.

Wet

0.8-4.6 kgS/ha-yr. and

deposition

to g i v e 2-

of SO^

0.7-2.3 kgN/ha-yr.

and

an NO^

estimate respect

These r a t e s

are

g e n e r a l l y w e l l below the l e v e l s i n the EUS. In the West, dry d e p o s i t i o n i s a s i m i l a r l e v e l as wet deposi t i o n f o r S and Ν i n many a r e a s , except near s o u r c e s and i n a r i d d e s e r t c o n d i t i o n s where wet d e p o s i t i o n i s n e g l i g i b l e . S i n c e measure ments o f wet d e p o s i t i o n a r e t a k e n i n v a l l e y s w i t h low p r e c i p i t a t i o n , they u n d e r e s t i m a t e the wet component somewhat f o r a l p i n e ( e c o l o g i c a l l y susceptible) conditions. The a l p i n e and s u b a l p i n e a r e a s i n the West have much h i g h e r a n n u a l p r e c i p i t a t i o n r a t e s than lower elevations. Variability C h a r a c t e r i z a t i o n of v a r i a b i l i t y i n d e p o s i t i o n i s important f o r bounding u n c e r t a i n t i e s i n exposure l e v e l s . A l s o t h i s a s p e c t o f depo s i t i o n i s c r u c i a l t o d e s c r i b i n g the " p r e d i c t a b i l i t y " o f s o u r c e receptor r e l a t i o n s h i p s (SRRs)(4). With a c q u i s i t i o n o f o b s e r v a t i o n s over a p e r i o d o f y e a r s , the v a r i a b i l i t y i n d e p o s i t i o n r a t e s has become b e t t e r known. L i k e the r e s u l t of a l l atmospheric processes, d e p o s i t i o n i s h i g h l y v a r i a b l e at a g i v e n s i t e and between s i t e s ; and w i t h i n y e a r , or from y e a r - t o year. V a r i a b i l i t y i n o b s e r v a t i o n s d e r i v e s from the measurement p r o c e s s i t s e l f , changes i n i n p u t ( e m i s s i o n s ) , and the " s t o c h a s t i c " c h a r a c t e r of a t m o s p h e r i c p r o c e s s e s i n f l u e n c i n g e m i t t e d m a t e r i a l b e f o r e i t i s r e t u r n e d to the e a r t h . The u n c e r t a i n t i e s i n the measurement p r o c e s s f o r p r e c i p i t a t i o n have been d e f i n e d


THE CHEMISTRY OF ACID RAIN

18

quantitatively. F o r wet d e p o s i t i o n , s a m p l i n g i s i d e n t i f i e d as the major a r e a o f measurement u n c e r t a i n t y , e s p e c i a l l y f o r snow. T y p i c a l t o t a l v a r i a b i l i t y i n concentration of species i n bulk samples i s shown f o r s u l f a t e i n F i g u r e 3. These d a t a c o n t a i n v a r i a t i o n s t h a t i n c l u d e a measurement component and the i n f l u e n c e o f a t m o s p h e r i c changes. The d a t a are r e p o r t e d f o r monthly samples i n the U.S. G e o l o g i c a l Survey (USGS) network of New York S t a t e . The d a t a r e p r e s e n t one of the l o n g e s t r e c o r d s a v a i l a b l e , from 1965 to 1984. The y e a r - t o - y e a r changes at the s i t e , show a s m a l l , s y s t e m a t i c downward d r i f t i n annual median s u l f a t e c o n c e n t r a t i o n on which i s superimposed a monthly v a r i a b l e component. The normal range of monthly ( w i t h i n y e a r ) v a r i a t i o n i s shown by the s t a n d a r d d e v i a t i o n , and the range o f v a l u e s i n a y e a r are a l s o shown. Two o u t l i e r p o i n t s are i n c l u d e d which cannot be r a t i o n a l i z e d by measurement u n c e r t a i n t y , but are out o f the s t a t i s t i c a l l y e x p e c t e d r a n g e . T h i s r e c o r d i l l u s t r a t e s w e l l t h a t s e v e r a l y e a r s of d a t a are needed to e s t a b l i s h a mean c o n d i t i o n , and p r o v i d e a b a s i s f o r d e s c r i b i n g "natural" variability. V a r i a b i l i t y i n dry d e p o s i t i o n i s p a r t l y r e f l e c t e d i n ambient c o n c e n t r a t i o n s , which v a r y o v e r a much wider range than e s t i m a t e s o f emissions. T h i s range i s a s c r i b e d to m e t e o r o l o g i c a l i n f l u e n c e ( 1 ) . A l a r g e and i l l - d e f i n e d v a r i a t i o n i n dry d e p o s i t i o n stems from surface conditions. Key to change i s the m o i s t u r e on a s u r f a c e as w e l l as b i o l o g i c a l a s s i m i l a t i o n c a p a b i l i t y ; both change d i u r n a l l y and s e a s o n a l l y ( 2 ) . I n t e r s i t e v a r i a b i l i t y has been s t u d i e d f o r both ambient c o n c e n t r a t i o n d a t a ( 1 ) , and f o r wet d e p o s i t i o n ( 5 ) . I n t e r s i t e 2c o r r e l a t i o n s f o r ambient SO^ and SO^ c o n c e n t r a t i o n s i n the EUS show d r a m a t i c d i f f e r e n c e s i n s p a t i a l

scale.

SO^

concentrations

have

h i g h l y l o c a l i z e d p a t t e r n s of c o r r e l a t i o n t h a t e x c l u d e a r e g i o n a l character. In c o n t r a s t , a i r b o r n e s u l f a t e c o r r e l a t i o n s are h i g h l y regional in character. S p a t i a l v a r i a b i l i t y i n s u l f a t e i s dominated by two to t h r e e components t h a t can be i d e n t i f i e d w i t h p r e v a i l i n g , persistent meteorological conditions (1). The s p a t i a l s c a l e of i n t e r s i t e c o r r e l a t i o n d i f f e r s somewhat w i t h l o c a t i o n . However, the average p a t t e r n f o r wet s u l f a t e and n i t r a t e c o r r e l a t i o n i s shown w i t h s p a c i n g of s t a t i o n s i n F i g u r e 4. The graphs r e p r e s e n t an a g g l o m e r a t i o n o f c o r r e l a t i o n d a t a between 1981 and 1984 f o r more than 20 EUS s i t e s . The d i s t a n c e over which c o r r e l a t i o n o c c u r s f o r s u l f a t e extends almost to 2000 km, but c o r r e l a t i o n d e c r e a s e s r a p i d l y below l e s s than 0.5 w i t h i n 100 km. A s i m i l a r p a t t e r n i s found f o r n i t r a t e , except the s t a t i o n s p a c i n g r e f l e c t i n g no c o r r e l a t i o n i s l e s s than f o r s u l f a t e ( a p p r o x i m a t e l y 1200 km). The i n t e r s i t e c o r r e l a t i o n s i l l u s t r a t e w e l l the r e g i o n a l c h a r a c t e r of s u l f a t e and n i t r a t e d e p o s i t i o n i n the EUS. Circumstant i a l e v i d e n c e f o r a s i m i l a r s p a t i a l s c a l e o f i n f l u e n c e a l s o has been r e p o r t e d f o r the West ( 6 ) . V a r i a b i l i t y i n d e p o s i t i o n p a t t e r n s may be a s s o c i a t e d w i t h c e r t a i n dominant m e t e o r o l o g i c a l p a t t e r n s ( 1 ) . As noted above, ambient c o n d i t i o n s can be c l a s s i f i e d m e t e o r o l o g i c a l l y . Some e v i dence f o r such a c l a s s i f i c a t i o n a l s o e x i s t s f o r wet d e p o s i t i o n ( 5 ) .


2.

19


HIDY

30 • 26 • 26 24 · 22 · 20 H 16 · 16 14 · 12 10 8 6420-

r = Ν ζ

0.922 69

Sulfate Concentration (ueq/l) 1981

+

1982

Ο

1983

F i g u r e 2. C o r r e l a t i o n between c a l c i u m and s u l f a t e c o n c e n t r a t i o n i n western p r e c i p i t a t i o n . Data f o r 1981-1984 - a n n u a l a v e r a g e s o f NADP s t a t i o n s . (From H i d y & Young, E n v i r o n . Res. & Tech., unpubl. report.)

15.0 12.5

10.0

ε

7.5

Lu 2

5.0

2.5

0

-25 65

I 70

I 75

i 80

I

WATER YEAR BEGINNING

F i g u r e 3. Trend i n a n n u a l median v a l u e s of s u l f a t e c o n c e n t r a t i o n i n b u l k d e p o s i t i o n samples a t H i n c k l e y , NY ( c r o s s e s ) . V a r i a b i l i t y i s a l s o i n d i c a t e d i n the b o x p l o t by the s t a n d a r d d e v i a t i o n s (-) and the range ( v e r t i c a l b a r s ) and o u t l i e r s (0, ·)(Reproduced w i t h p e r m i s s i o n from Ref. 4. C o p y r i g h t 1984 APCA).



20

0.5

SULFATE 1980 0.4

0.3 -\

0.2

0.1

-0.1

1

'

1

I 800

400

I ' 1200

ι

'

-ι—ι—ι—ι—r-

I ' 1600

2000

2400

1NTERSITE DISTANCE (km)

-1 0

ι

ι

ι 400

ι

ι

ι

ι 800

ι

ι

ι

ι

ι

1200

ι

ι

ι

-

π 1 ι γ~~ι I

1600

2000

r~ 2400

INTERSITE DISTANCE (km)

F i g u r e 4. S c a l e o f r e g i o n a l phenomena i n p r e c i p i t a t i o n s u l f a t e and n i t r a t e a s i n d i c a t e d by i n t e r s i t e c o r r e l a t i o n s . Data a r e an average o f s e v e r a l s i t e s i n EUS. (Reproduced w i t h p e r m i s s i o n from R e f . 5. C o p y r i g h t 1985 E l e c t r i c Power R e s e a r c h I n s t . ) .


2.

HIDY

21


Source-Receptor

Relationships.

U l t i m a t e l y the m i t i g a t i o n t o the e n v i r o n m e n t a l e f f e c t s o f a c i d depo s i t i o n r e q u i r e s d e c r e a s e i n exposure through e m i s s i o n r e d u c t i o n . There has been c o n s i d e r a b l e debate about how much and where r e d u c t i o n s can be a c h i e v e d from p r a c t i c a l p l a n n i n g f o r SO^ and Ν 0 χ

e m i s s i o n r e d u c t i o n . An important a s p e c t o f the p r e d i c t a b i l i t y i s s u e l i e s i n the u n c e r t a i n t i e s i n the n o n - l i n e a r c h a r a c t e r o f a t m o s p h e r i c processes a f f e c t i n g d e p o s i t i o n rates ( 2 ) . Perhaps the b e s t e v i d e n c e f o r e s t a b l i s h i n g a d i r e c t l y p r o p o r t i o n a l r e l a t i o n s h i p between e m i s s i o n change i s comparison between long term t r e n d s l i k e t h o s e shown i n F i g u r e 3 w i t h e m i s s i o n changes over the same time p e r i o d . Such a d i r e c t r e l a t i o n i s suggested i n sulfate

(and p o s s i b l y NO^

) d a t a i n b u l k d e p o s i t i o n taken

a t Hubbard

Brook, NH ( e . g . , F i g u r e 5 ) . An average d e c r e a s e i n s u l f a t e o f about 2%/yr between 1970 and 1982 ( n o t shown i n F i g u r e 5) i s e s s e n t i a l l y the same as t h a t e s t i m a t e d from d e c r e a s e i n r e g i o n w i d e SO^ e m i s s i o n s (4,7). With the d i s c u s s i o n o f s p a t i a l c o r r e l a t i o n above, i t i s l o g i c a l to expect t h a t any r e l a t i o n between e m i s s i o n s and r e g i o n a l l y r e p r e s e n t a t i v e d e p o s i t i o n measurements s h o u l d be c o n s i s t e n t f o r s e v e r a l sites. To t e s t t h i s , t h e Hubbard Brook r e s u l t s were compared w i t h l i m i t e d d a t a from the USGS b u l k d e p o s i t i o n network, p r i m a r i l y l o c a t e d i n New Y o r k S t a t e ( 4 ) . The s t a t i o n s s e l e c t e d a r e r u r a l and are w i t h i n 550 km o f one a n o t h e r . Q u a l i t a t i v e comparison o f t r e n d i n d i c a t o r s i n t h e d a t a a r e summarized i n T a b l e I I I . Table I I I .

Parameter

3

Summary o f Apparent T r e n d s i n Annual Median P r e c i p i t a t i o n C h e m i s t r y Data from the USGS S i t e s and Hubbard Brook (1965-1980) S i t e s (Reproduced w i t h p e r m i s s i o n from Ref. 4, C o p y r i g h t 1984 APCA)

Hubbard Brook Precipitation +

Hinckley Canton NY NY 0 0 0

2

SO, " 4_ N0 " 3

NH, 4 pH

+

Trend

Mays P t . NY

Salamanca NY 0

Athens PA 0

0

0

0+

+0

+

0

+

0

+

0

+

+

+

0

+

-

0

+

indicators

-

0

0

a r e : (+) upward, (-) downward, and (0) no t r e n d .

These i n d i c a t o r s a r e based on e s t i m a t e d s i g n i f i c a n t change by s t a t i s t i c a l t e s t i n g ( 4 ) . The summary i n d i c a t e s t h a t the t r e n d s a r e not c o n s i s t e n t f o r s u l f a t e , o r f o r o t h e r c o n s t i t u e n t s . S p a t i a l and temporal w e i g h t i n g a l s o have been used on the USGS d a t a t o o b t a i n y e a r l y averaged s u l f a t e d e p o s i t i o n f o r New Y o r k



1964-65

66-67

68-69

70-71

72-73

74-75

76-77

F i g u r e 5. Comparison between EUS S O 2 and ΝΟχ e m i s s i o n s and a n n u a l w e i g h t e d c o n c e n t r a t i o n s o f SO^-, N O 3 , and NH^ i n b u l k d e p o s i t i o n a t the Hubbard Brook E x p e r i m e n t a l F o r e s t from 1964 to 1977. L i n e t h r o u g h e m i s s i o n s i s drawn t h r o u g h e s t i m a t e s each f i v e y e a r p e r i o d from 1965. (From H i d y , 8 ) .


2.

HIDY

23


State. B i l o n i k ' s r e s u l t s (9) i n d i c a t e a maximum i n s u l f a t e d e p o s i t i o n i n the 1971-1973 p e r i o d as compared w i t h an apparent EUS SO^ e m i s s i o n s maximum between 1970 and 1975. Since d e p o s i t i o n r a t e l s a d i m e n s i o n a l l y c o n s i s t e n t parameter w i t h e m i s s i o n r a t e , these r e s u l t s q u a l i t a t i v e l y tend to s u p p o r t an SO emission-deposition r e l a t i o n s h i p f o r the EUS. 2The l a c k of s p a t i a l c o n s i s t e n c y i n r e l a t i n g SO^ concentration to SO^

emissions

may

be

the r e s u l t

of d i f f e r e n c e s

i n the

influence

o f s o u r c e s near a g i v e n s i t e , t o an i n a d e q u a t e q u a n t i t a t i v e d a t a base, or to m e t e o r o l o g i c a l d i f f e r e n c e s i n exposure. A t e s t of the f i r s t f a c t o r was attempted, c a l c u l a t i n g the e f f e c t o f r e g i o n a l l y d i f f e r e n t e m i s s i o n changes on d i f f e r e n t r e c e p t o r s , a c c o u n t i n g f o r r e d u c t i o n i n source e f f e c t w i t h d i s t a n c e from the UMACID model ( 5 ) . The r e s u l t s o f the c a l c u l a t i o n are g i v e n i n F i g u r e 6. The r e c e p t o r l o c a t i o n s are western New Y o r k S t a t e (WNY); N o r t h C e n t r a l P e n n s y l v a n i a (NCPA); Muskoka, Ont.; W h i t e f a c e M o u n t a i n , NY (WFM); and n o r t h e a s t e r n New Hampshire (VT-NH). There are c l e a r l y geo g r a p h i c a l d i f f e r e n c e s i n e x p e c t e d changes w i t h SO^ e m i s s i o n s a f t e r 1965, and the s u l f a t e d e p o s i t i o n change s h o u l d not n e c e s s a r i l y be l i n e a r s i n c e the apparent e m i s s i o n s were not c h a n g i n g l i n e a r l y over the p e r i o d . N e v e r t h e l e s s , a down t r e n d i n s u l f a t e s h o u l d have been o b s e r v e d c o n s i s t e n t l y i f m e t e o r o l o g i c a l v a r i a b i l i t y was s i m i l a r from site-to-site. These r e s u l t s are ambiguous. The a m b i g u i t y d e r i v e s i n p a r t from s u b r e g i o n a l d i f f e r e n c e s i n e m i s s i o n change over the EUS. The Hubbard Brook d a t a a r e c o n s i d e r e d h i g h e r q u a l i t y than the USGS s e t ( 7 ) . Thus, the Hubbard Brook d a t a are r e l i e d upon f o r supporting a linear source-receptor r e l a t i o n s h i p . A test tion

sulfate

for proportionality a l s o has

between SO^

been attempted

e m i s s i o n s have dominated SO^

emissions

f o r the West

emissions

i n the WUS.

(6).

and

precipita

Smelter

These

emissions

changed by more than annual

averaged

50% over the 1980-1984 p e r i o d . A n a l y s i s o f 2[SO^ ] d a t a from NADP f o r s e v e r a l w e s t e r n s i t e s

(mostly i n Colorado) provided c i r c u m s t a n t i a l evidence f o r a propor t i o n a l SRR. These r e s u l t s were c h a l l e n g e d by o t h e r s (10, 11). However, r e a n a l y s i s of the same d a t a f o r monthly v a r i a t i o n appears to g i v e a s t r o n g e r case f o r p r o p o r t i o n a l i t y (Oppenheimer, Μ., E p s t e i n , D., N a t u r e , i n p r e s s ) over d i s t a n c e s o f i n f l u e n c e beyond 1000 km. The new a n a l y s i s a l s o p l a c e s i n p e r s p e c t i v e w i t h i n y e a r ( s e a s o n a l ) , and y e a r - t o - y e a r v a r i a b i l i t y . The r e a s o n f o r the s t r e n g t h o f p r o p o r t i o n a l i t y r e l a t i o n s h i p i n the w e s t e r n d a t a r e l a t i v e to the E a s t i s not a p p a r e n t ; however, i t may be r e l a t e d to the s i z e o f the e m i s s i o n change " s i g n a l " i n the m e t e o r o l o g i c a l " n o i s e " compared w i t h h i s t o r i c c o n d i t i o n s i n the East. In e s t i m a t i n g the r e l i a b i l i t y o f t h e o r e t i c a l p r e d i c t i o n s , i t i s i m p o r t a n t to take i n t o account the y e a r - t o - y e a r v a r i a b i l i t y i n SRRs e s t i m a t e d from m e t e o r o l o g i c a l t r a n s p o r t . The p r e d i c t i o n - r e l i a b i l i t y q u e s t i o n i s c e n t r a l to c o n s t r u c t i n g a c o s t e f f e c t i v e p r a c t i c a l



24

Ol 1950

1

1

1

I

I

I

1955

1960

1965

1970

1975

1980

YEAR

F i g u r e 6. E s t i m a t e d c o n t r i b u t i o n to wet d e p o s i t i o n o f s u l f a t e from e m i s s i o n s i n the e a s t e r n U.S. and s o u t h e a s t e r n Canada based on 1978 m e t e o r o l o g y . The wet d e p o s i t i o n i n d e x i s the d e p o s i t i o n r a t e i f m e t e o r o l o g y were the same i n each y e a r . The p e r c e n t a g e change shown on each l i n e c o r r e s p o n d s to the c a l c u l a t e d r e d u c t i o n i n wet d e p o s i t i o n from e m i s s i o n change, w i t h m e t e o r o l o g i c a l c o n d i t i o n s assumed to be u n i f o r m from y e a r to year. The e x p e c t e d r a n g e of u n c e r t a i n t y i n e s t i m a t e o f wet d e p o s i t i o n u s i n g a c t u a l p r e c i p i t a t i o n r a t h e r than 1978 l e v e l s a r e i n d i c a t e d by squares and v e r t i c a l b a r s . (Reproduced w i t h p e r m i s s i o n from Ref. 4. C o p y r i g h t 1984 APCA).


2.

HIDY


25

design f o r s e l e c t i v e emission c o n t r o l improving d e p o s i t i o n i n d i s tant, s u s c e p t i b l e areas. A c t u a l o b s e r v a t i o n a l d a t a are v e r y l i m i t e d through which t e s t s of model v a l i d i t y and performance can be made. Data t o t e s t the s e n s i t i v i t y o f such c a l c u l a t i o n s are even l e s s available. An e x p l o r a t o r y a n a l y s i s o f the range o f v a r i a b i l i t y t h a t may be expected was attempted by Samson et a l (see H i d y e t a l , 5, p. I I - 2 98). T h e i r c a l c u l a t i o n s used a c t u a l m e t e o r o l o g i c a l d a t a t o e s t i m a t e a geographical d i s t r i b u t i o n of " n a t u r a l p o t e n t i a l " f o r emissions from a source a r e a to r e a c h a r e c e p t o r a r e a . I f wind c o n d i t i o n s e x i s t such t h a t wind blows a l l the time from the source over the r e c e p t o r , f o r example, t h i s p o t e n t i a l would be u n i t y . C a l c u l a t i o n s of " n a t u r a l p o t e n t i a l " were made u s i n g m e t e o r o l o g i c a l d a t a f o r several years. The y e a r - t o - y e a r v a r i a t i o n i n n a t u r a l p o t e n t i a l d i s t r i b u t i o n i s shown i n F i g u r e 7 f o r the EUS. The example c o n c e r n s the Upper Ohio R i v e r V a l l e y source complex. Near to the source a r e a and downwind ( e a s t w a r d ) , t h e r e i s a 20% v a r i a b i l i t y i n n a t u r a l p o t e n t i a l , w h i l e at g r e a t e r d i s t a n c e s and upwind (westward), the c o e f f i c i e n t o f v a r i a t i o n i n p o t e n t i a l i n c r e a s e s . These c a l c u l a t i o n s g i v e at l e a s t a q u a l i t a t i v e p i c t u r e o f the r e l i a b i l i t y i n a i r t r a n s p o r t c o n d i t i o n s i n f l u e n c i n g SRRs, p i c k i n g a s i n g l e t e s t y e a r . If the wind f i e l d and m i x i n g c o n d i t i o n s were the o n l y source o f v a r i a b i l i t y , the p r e d i c t i o n s c o u l d be r e a s o n a b l y r e l i a b l e i n c e r t a i n key a r e a s . However, o t h e r v a r i a t i o n such as a i r c h e m i s t r y , c l o u d s c a v e n g i n g and p r e c i p i t a t i o n are a d d i t i o n a l f a c t o r s unaccounted f o r i n such c a l c u l a t i o n s . A number of workers have become concerned about such q u e s t i o n s so t h a t r e s e a r c h i n u n c e r t a i n t i e s has expanded. U n f o r t u n a t e l y , the o b s e r v a t i o n s d e s c r i b i n g c h e m i c a l v a r i a b i l i t y i n both dry and wet d e p o s i t i o n are v e r y l i m i t e d f o r d i r e c t s t u d y . There i s need f o r a major investment i n f i e l d programs to a c q u i r e such d a t a . Summary. T h i s paper has summarized c e r t a i n f e a t u r e s o f o b s e r v a t i o n s c h a r a c t e r i z i n g dry and wet d e p o s i t i o n o f a i r b o r n e a c i d f o r m i n g s p e c i e s , i n p a r t i c u l a r , s u l f a t e and n i t r a t e . The r e g i o n a l or s u b c o n t i n e n t a l c h a r a c t e r o f d e p o s i t i o n d i s t r i b u t i o n s over the U n i t e d S t a t e s i s noted. Example d a t a are c i t e d showing d i f f e r e n c e s i n c h a r a c t e r i s t i c d e p o s i t i o n between the West and the E a s t . The West e x p e r i e n c e s s u l f a t e and n i t r a t e d e p o s i t i o n w e l l below t h a t found i n the E a s t , but l a r g e r than g l o b a l remote s i t e s are s u g g e s t e d . Relating acidity to s u l f a t e and n i t r a t e i n the West i s confounded by the s t r o n g a s s o c i a t i o n between c a l c i u m and s u l f a t e i n western p r e c i p i t a t i o n . Dry and wet d e p o s i t i o n have s i m i l a r r a t e s f a r from s o u r c e s ( b o t h i n the E a s t and the W e s t ) . S u l f a t e tends to dominate a c i d s p e c i e s i n p r e c i p i t a t i o n , except near l a r g e s o u r c e s o f Ν 0 . χ

M e t e o r o l o g i c a l p r o c e s s e s , e m i s s i o n changes and measurement u n c e r t a i n t y l e a d to v a r i a b i l i t y i n d e p o s i t i o n r a t e s . Deposition measurements are u n c e r t a i n m a i n l y from a m b i g u i t i e s i n sampling t e c h niques. M e t e o r o l o g i c a l v a r i a b i l i t y produces p o t e n t i a l l y l a r g e w i t h i n y e a r and y e a r - t o - y e a r d i f f e r e n c e s i n exposure to d e p o s i t i o n .


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F i g u r e 7. V a r i a t i o n i n y e a r - t o - y e a r t r a j e c t o r y c a l c u l a t i o n s b e g i n n i n g from t h e upper Ohio R i v e r V a l l e y . The c u r v e s r e p r e s e n t the c o e f f i c i e n t of v a r i a t i o n i n the d i s t r i b u t i o n s of annual " n a t u r a l p o t e n t i a l " o f m a t e r i a l r e l e a s e d i n t h e upper Ohio R i v e r V a l l e y r e a c h i n g l o c a t i o n s downwind from t h i s s o u r c e a r e a . (Redrawn and r e p r o d u c e d w i t h p e r m i s s i o n from R e f . 5. Copyright 1985 E l e c t r i c Power R e s e a r c h I n s t . ) .


2.

HIDY

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The r e l i a b l e e s t i m a t i o n of source impact on r e c e p t o r c o n d i t i o n s i s d i f f i c u l t from t h e o r y because of undetermined u n c e r t a i n t i e s . I n f e r e n c e from comparison between e m i s s i o n s and measurements o f f e r s an a l t e r n a t i v e to c a l c u l a t i o n s . Measurements i n the E a s t have y i e l d e d ambiguous s o u r c e - r e c e p t o r r e l a t i o n s h i p s . However, e v i d e n c e s u g g e s t s t h a t r e c e n t changes i n s u l f a t e d e p o s i t i o n i n the West are l i n k e d w i t h r e l a t i v e l y l a r g e changes i n SO^ e m i s s i o n s from n o n f e r r o u s m e t a l s m e l t e r s , e s p e c i a l l y i n New Mexico and A r i z o n a . the

M e t e o r o l o g i c a l v a r i a b i l i t y needs to be c o n s i d e r e d i n r e l i a b i l i t y of source-receptor c a l c u l a t i o n s .

estimating

Acknowledgments P a r t o f t h i s s t u d y was d e r i v e d from r e s e a r c h sponsored by the E l e c t r i c Power R e s e a r c h I n s t i t u t e and West A s s o c i a t e s , I n c .

Literature Cited 1.

The Sulfate Regional Experiment: Report of Findings, EA-190(3), Electric Power Research Institute: Palo Alto, CA, 1983. 2. Acid Deposition; Atmospheric Processes in Eastern North America, National Academy of Sciences, 1983. 3. Galloway, J . N.; Likens, G. E.; Hawiey, M. E. Science: 1984, 226, 829-831. 4. Hidy, G. M.; Hansen, D. Α.; Henry, R. C.; Ganesan, K.; Collins, J . J . Air Poll. Contr. Assn. 1984, 31, 333-354. 5. Feasibility and Design of the Massive Aerometric Tracer Experiment (MATEX), EA-4305 (2), Electric Power Research Institute: Palo Alto, CA, 1985. 6. Oppenheimer, M.; Epstein, C.; Yuhnke, R. Science 1985, 229, 854-858. 7. Acid Deposition. Long Term Trends, National Academy of Sciences, 1986. 8. Hidy, G. M. Proc. 2nd Nat'l. Symposium on Acid Rain, Pittsburgh Chamber of Commerce, Pittsburgh, PA, 1982. 9. Bilonik, R. A. Atmos. Environ. 1985, 19, 1829-1845. 10. Hidy, G. M. Science 1986, 233, 10. 11. Newman, L . ; Benkovitz, C. Science 1986, 233, 11-12. RECEIVED February 3, 1987


The Chemistry of Acid Rain - ACS Publications - American Chemical

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