Chemical Instrumentation of Atmospheric Wet ... - ACS Publications

Chapter 25

Chemical Instrumentation of Atmospheric Wet Deposition Processes Roger L. Tanner

Downloaded by CORNELL UNIV on August 11, 2016 | http://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch025

Environmental Chemistry Division, Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973

Field studies of wet deposition processes require the differentiation and determination of many reactive species at trace levels in clear-air gaseous and aero sol phases and in air containing clouds and precipita tion. These studies have placed extremely rigorous requirements on existing analytical techniques and, in several instances, required development of new approaches to sampling and determination of c r i t i c a l species. Measurement techniques for nitrogen oxides and oxyacids, SO2 and aerosol sulfate species, oxidant species including hydrogen peroxide and PAN, and vari ous organic species in the gas, aerosol and, where appropriate, aqueous phases from airborne platforms are reviewed. Emphasis is on recent developments in real-time and short-term integrated measurements which permit the differentiation of below-cloud, within -cloud ( i n t e r s t i t i a l ) , and aqueous phase species con centrations of oxidants, and of sulfuric and n i t r i c acids and their precursors. Recent developments in measurement techniques for nitrogen oxides and gaseous H2O2 applicable to airborne sampling are highlighted.

F i e l d s t u d i e s of wet d e p o s i t i o n processes r e q u i r e the d i f f e r e n t i a t i o n and d e t e r m i n a t i o n of many t r a c e r e a c t i v e s p e c i e s i n the s e v e r a l phases ( g a s e o u s , a e r o s o l , c l o u d water and p r e c i p i t a t i o n ) p r e s e n t i n the atmosphere. The requirements imposed on e x i s t i n g a n a l y t i c a l t e c h n i q u e s by these s t u d i e s have been e x t r e m e l y r i g o r o u s and, i n s e v e r a l c a s e s , have n e c e s s i t a t e d the development o f new approaches to the sampling and d e t e r m i n a t i o n of c r i t i c a l c h e m i c a l s p e c i e s . T h i s paper r e v i e w s these technique developments i n the c o n t e x t of t h e i r use i n a i r b o r n e sampling d u r i n g a t m o s p h e r i c f i e l d s t u d i e s . The focus o f the review i n c l u d e s t e c h n i q u e s f o r c l e a r a i r gases and a e r o s o l s , s p e c i e s i n c l o u d l i q u i d w a t e r , i c e m a t r i c e s and p r e c i p i t a t i o n , as w e l l as sampling techniques f o r gases and a e r o s o l s i n cloud i n t e r s t i t i a l a i r . 0097-6156/87/0349-0289$06.00/0 © 1987 American Chemical Society

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

THE CHEMISTRY OF ACID RAIN

290

The range of c h e m i c a l s p e c i e s to be determined i n c l u d e : • sub-ppb l e v e l s o f N 0 ( N 0 - n i t r o g e n o x i d e s and o x y a c i d s e x c l u d i n g N2O) determined i n r e a l - t i m e u s i n g ozone cherniluminescence o r by o t h e r a i r b o r n e i n t e g r a t i v e t e c h n i q u e s ; • sub-ppb l e v e l s of s u l f u r d i o x i d e (SO2) and a e r o s o l s u l f a t e determined i n r e a l - t i m e u s i n g the flame p h o t o m e t r i c d e t e c t o r , o r i n a i r b o r n e i n t e g r a t e d samples by o t h e r t e c h n i q u e s ; • o x i d a n t s (ozone, p e r o x y a c e t y l n i t r a t e [PAN], hydrogen p e r o x i d e ) i n gaseous and aqueous phases. Other c o n s i d e r a t i o n s r e l e v a n t to a i r b o r n e c o l l e c t i o n and d e t e r m i n a t i o n of atmospheric samples s p e c i a t e d by phase i n c l u d e : • c l o u d - and r a i n d r o p - f r e e a i r sampling; • c o l l e c t i o n o f aqueous l i q u i d and s o l i d samples; I n these d i s c u s s i o n s we w i l l thus use the f o l l o w i n g e x p l i c i t d e f i n i t i o n o f a c h e m i c a l measurement i n the atmosphere: the c o l l e c t i o n o f a d e f i n a b l e atmospheric phase as w e l l as the d e t e r m i n a t i o n of a s p e c i f i c c h e m i c a l moiety w i t h d e f i n a b l e p r e c i s i o n and accur a c y . T h i s d e f i n i t i o n i s r e q u i r e d s i n c e most atmospheric p o l l u t a n t s are n o t i n e r t gaseous and a e r o s o l s p e c i e s w i t h atmospheric concent r a t i o n s determined by source s t r e n g t h and p h y s i c a l d i s p e r s i o n proc e s s e s a l o n e . I n s t e a d they may undergo gas-phase, l i q u i d - p h a s e , o r s u r f a c e - m e d i a t e d c o n v e r s i o n s (some r e v e r s i b l e ) and, i n c e r t a i n c a s e s , mass t r a n s f e r between phases may be k i n e t i c a l l y l i m i t e d . A n a l y t i c a l methods f o r c h e m i c a l s p e c i e s i n the atmosphere must t r a n scend these c o m p l i c a t i o n s from c h e m i c a l t r a n s f o r m a t i o n s and microp h y s i c a l p r o c e s s e s i n order to be u s e f u l a d j u n c t s to atmospheric chemistry s t u d i e s . The d i s c u s s i o n t h a t f o l l o w s f i r s t d e s c r i b e s techniques f o r the a i r b o r n e c o l l e c t i o n o f a d e f i n a b l e atmospheric phase f o r subsequent d e t e r m i n a t i o n s , then c o n t i n u e s w i t h d i s c u s s i o n o f the a n a l y s i s t e c h n i q u e s themselves i n c l u d i n g those f o r n i t r o g e n o x i d e s and o x y a c i d s , s u l f u r o x i d e s , o x i d a n t s and s e l e c t e d o r g a n i c s p e c i e s . Emphasis i s on the m o d i f i e d i n s t r u m e n t a t i o n d e v i s e d and used by Brookhaven N a t i o n a l L a b o r a t o r y s t a f f to improve the s e l e c t i v i t y and lower the l i m i t s o f d e t e c t i o n o f these techniques (JL_).


X

X

I n t e r s t i t i a l A i r Sampling S t u d i e s of i n - c l o u d o x i d a t i o n and wet scavenging processes r e q u i r e the sampling o f gaseous s p e c i e s and/or f i n e a e r o s o l p a r t i c l e s i n a i r w h i c h c o n t a i n s c l o u d d r o p l e t s and/or p r e c i p i t a t i o n . I n order to a c c o m p l i s h t h i s sampling o f " i n t e r s t i t i a l ' a i r , i t i s necessary t o s e l e c t i v e l y remove c l o u d and r a i n d r o p l e t d i s t r i b u t i o n s ; these d i s t r i b u t i o n s have geometric mean diameters o f 10-20 \im and > 100 urn, r e s p e c t i v e l y . Removal i s u s u a l l y a c h i e v e d by the use of a c y c l o n e d e v i c e o r a c e n t r i f u g a l r o t o r apparatus (2) (see F i g u r e 1) f o r l i q u i d water c l o u d s and p r e c i p i t a t i o n . I n a d d i t i o n , these l a r g e r p a r t i c l e s a r e removed i n a d v e r t e n t l y by any curves i n the sampling i n l e t on the a i r c r a f t . Both c y c l o n e s and c e n t r i f u g a l r o t o r d e v i c e s remove p a r t i c l e s o f l a r g e r aerodynamic diameter by i m p a c t i o n induced by abrupt changes i n aerodynamic f l o w l i n e s , f o l l o w e d by c o l l e c t i o n o f the l i q u i d w a t e r o u t of the flow stream. Coarse a e r o s o l p a r t i c l e (> 2.5 urn d i a m e t e r ) d i s t r i b u t i o n s u s u a l l y o v e r l a p the lower end o f c l o u d dropl e t d i s t r i b u t i o n s , hence a r e removed to a s u b s t a n t i a l e x t e n t i n the 1


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Wet Deposition

Processes

291


ISO mm

SECTIONS VIEWED DOWNSTREAM

F i g u r e 1. C l o u d water sampler f o r a i r c r a f t e x p e r i m e n t s . ( R e p r i n t e d w i t h p e r m i s s i o n from r e f . 31. C o p y r i g h t 1979 C e n t r a l E l e c t r i c Research L a b o r a t o r i e s . )


THE

292

CHEMISTRY OF ACID RAIN

p r o d u c t i o n of " c l o u d - f r e e " a i r . However, f i n e a e r o s o l s and unscavenged gases ( e . g . , Νθ2» O3) may be sampled c o n v e n i e n t l y i n c l o u d f r e e a i r f o r subsequent sampling of a e r o s o l s (and SO2, HNO3, i f p r e s e n t ) u s i n g the BNL h i g h volume f i l t e r pack even a t sampling r a t e s > 0.5 m /min ( 3 ) . A n a l y s i s of cloudwater or p r e c i p i t a t i o n pre-removed by the r o t o r d e v i c e i s p r o b l e m a t i c a l , however, due to l i q u i d c o n t a m i n a t i o n from the i n l e t s u r f a c e s ( 3 ) ; i t i s p r e f e r a b l e to use the s p e c i f i c a l l y designed a t m o s p h e r i c water c o l l e c t o r s d e s c r i b e d below. 3


C o l l e c t i o n o f Cloudwater and

Precipitation

The c o l l e c t i o n of atmospheric water samples by a i r b o r n e sampling i s , i n the c o n t e x t of t h i s paper, p r i n c i p a l l y concerned w i t h the d e f i n i t i o n of phase c o l l e c t e d , c o l l e c t i o n e f f i c i e n c y (as i n f l u e n c e d by the d r o p l e t s i z e s p e c t r a ) , p r e v e n t i o n of c o n t a m i n a t i o n d u r i n g sampling and s i m p l y a c q u i r i n g enough sample f o r a n a l y s i s d u r i n g the sampling time a v a i l a b l e ( i . e . , s p a t i a l / t e m p o r a l r e s o l u t i o n ) . A d e v i c e f o r sampling l i q u i d phases, designed by p e r s o n n e l a t the Atmospheric S c i e n c e s R e s e a r c h Center, SUNY a t Albany (4) o p e r a t e s by i m p a c t i n g d r o p l e t s onto s l o t t e d r o d ( s ) i n an apparatus (see F i g u r e 2) s i t u a t e d e x t e r n a l to the a i r c r a f t s k i n . The s i z e and number of the s l o t t e d r o d ( s ) and t h e i r o r i e n t a t i o n are determined by the phase to be c o l l e c t e d - c l o u d w a t e r or r a i n - and i t s s i z e s p e c t r a . Samples d r a i n from the rods to a c o l l e c t i o n v e s s e l i n s i d e the a i r c r a f t . Because the c o l l e c t o r i s made of p l a s t i c , i t i s somewhat f l e x i b l e and, as a r e s u l t , the c o l l e c t i o n e f f i c i e n c y even f o r a s i n g l e c l o u d d r o p l e t s i z e d i s t r i b u t i o n i s a f u n c t i o n of a i r c r a f t speed and angle o f a t t a c k (5^). T r a n s i e n t s p e c i e s ( e . g . H2O2, S ( I V ) ) must be s t a b i l i z e d w i t h i n a few hours of c o l l e c t i o n , whereas more s t a b l e s p e c i e s need o n l y be s t o r e d i n a c l e a n , c o o l p l a c e u n t i l a n a l y s i s can be performed. C o l l e c t i o n of supercooled l i q u i d w a t e r i n c l o u d s i s s i m p l e , u s i n g o n l y a p l a t e or screen exposed to RAM a i r ; the water i s l a t e r melted and s t o r e d p r i o r to a n a l y s i s ( 6 ) . C o l l e c t i o n of f r o z e n c l o u d p a r t i c l e s i s a l i t t l e more p r o b l e m a t i c a l s i n c e the l i q u i d water c o n t e n t can be low, and i n d i v i d u a l p a r t i c l e s are more s u b j e c t to bounce-off during impactive c o l l e c t i o n . C o l l e c t i o n of snow p a r t i c l e s aboard the a i r c r a f t i s most d i f f i c u l t of a l l due to the low aerodynamic d i a m e t e r e x h i b i t e d by these p a r t i c l e s i n RAM a i r streams. S u c c e s s f u l methods f o r the c o l l e c t i o n of snow and i c e c l o u d s a r e s t i l l i n an a c t i v e stage of development. Determination

of N i t r o g e n Oxides

The technique used f o r most r e a l - t i m e measurements of n i t r i c o x i d e (NO) and o t h e r n i t r o g e n oxides and o x y a c i d s i n the ambient atmo sphere i s based on the d e t e c t i o n of l i g h t from the c h e m i l u m i n e s c e n t r e a c t i o n of ozone w i t h NO (_7,8). T h i s l i g h t o r i g i n a t e s when a por t i o n of e x c i t e d s t a t e NO2 m o l e c u l e s formed i n the r e a c t i o n luminesce i n a continuum from ~600 nm i n t o the near i n f r a r e d r e g i o n . Ozonec h e m i l u m i n e s c e n t (CL) i n s t r u m e n t s f o r N 0 c o n s i s t of a chamber f o r m i x i n g ambient a i r w i t h excess ozone and a window f o r v i e w i n g the f i l t e r e d luminescence w i t h a p h o t o m u l t i p l i e r tube. N i t r o g e n s p e c i e s o t h e r than NO a r e converted t h e r e t o by passage over a heated X



25.

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Instrumentation

of Atmospheric

Wet Deposition

Processes

F i g u r e 2. ASRC c l o u d c o l l e c t o r . (Reprinted with permission from r e f . 4. C o p y r i g h t 1979 A t m o s p h e r i c S c i e n c e s R e s e a r c h Center.)


293

294


c a t a l y s t ( t y p i c a l l y molybdenum a t ~ 3 7 5 ° C ) . Two-channel i n s t r u m e n t s f o r s i m u l t a n e o u s d e t e r m i n a t i o n o f NO and N 0 are commercially a v a i l a b l e but g e n e r a l l y have l i m i t s of d e t e c t i o n (LODs) of *2 ppbv. Cons i d e r a t i o n has thus been g i v e n to improving the s e n s i t i v i t y and l o w e r i n g the LOD f o r measurements of NO and N 0 i n non-urban and background l o c a t i o n s a t which l e v e l s < 1 ppb are common. T h i s i s done by r e f e r e n c e to the response e q u a t i o n f o r the d e t e c t o r U,2,_10)» E q u a t i o n 1: X

X


S

=

(D

GAI

T h i s r e l a t e s the d e t e c t o r s i g n a l to the i n t e n s i t y ( I ) of l i g h t p r o duced, the e f f i c i e n c y of l i g h t c o l l e c t i o n ( G ) , and the PMT sensitivi t y (A). The i n t e n s i t y of l i g h t produced i s e x p r e s s e d as the p r o d uct of the NO c o n c e n t r a t i o n , the flow r a t e , and a s e r i e s of f r a c t i o n s ( f r a c t i o n o f NO2 formed i n the e x c i t e d s t a t e , f r a c t i o n o f N O 2 * d e c a y i n g r a d i a t i v e l y , and the f r a c t i o n of NO-O3 r e a c t i o n o c c u r r i n g w i t h i n view of the d e t e c t o r ) . O p t i m i z a t i o n of the response of the CL d e t e c t o r c o n s i s t s of m a x i m i z i n g each of the f a c t o r s i n the g o v e r n i n g response e q u a t i o n . C o l l e c t i o n e f f i c i e n c y (G) o f the d i f f u s e CL s o u r c e i s improved by p o l i s h i n g and g o l d - c o a t i n g the r e a c t i o n chambers. F a c t o r A i s maximized by s e l e c t i n g a l o w - n o i s e , i n f r a r e d s e n s i t i v e PMT. The i n t e n s i t y o f CL l i g h t , I , i s g i v e n i n E q u a t i o n 2: I

=

flf2(l

- exp[x /T ])(F/P)[NO] r

N O

(2)

I i s maximized by o p e r a t i n g a t h i g h flow r a t e s under reduced chamber p r e s s u r e i n an e n l a r g e d chamber volume u s i n g i n c r e a s e d c o n c e n t r a t i o n s of ozone. A summary of the improvements i n performance of the m o d i f i e d M o n i t o r Labs Model 8840 o x i d e s of n i t r o g e n m o n i t o r i s g i v e n i n T a b l e I (from Ref. 10) based on the i n s t r u m e n t d e s i g n shown i n F i g u r e 3. The p r e - r e a c t o r i s d e s i g n e d to f a c i l i t a t e the a c c u r a t e measurement of the " z e r o a i r " s i g n a l - the response o f the i n s t r u m e n t i n the absence of NO. A t the lowered LODs o b t a i n a b l e from the m o d i f i e d N0 i n s t r u m e n t the p o s s i b l e presence of q u e n c h i n g gases and i n t e r f e r i n g compounds i n ambient a i r n e g a t e s the use of tank a i r i n d e t e r m i n i n g the z e r o a i r s i g n a l . R e a c t i o n of ozone w i t h sampled a i r p r i o r to a d m i s s i o n to the r e a c t i o n chamber a l l o w s f o r " z e r o a i r " s i g n a l d e t e r m i n a t i o n under ambient a i r c o n d i t i o n s , hence d e t e r m i n a t i o n of low ambient NO l e v e l s can be made w i t h r e q u i s i t e p r e c i s i o n and a c c u r a c y . D e t e r m i n a t i o n of n i t r i c a c i d c o n c e n t r a t i o n s may be made u s i n g the m o d i f i e d N 0 a n a l y z e r by d i v e r t i n g a p o r t i o n of the a i r stream through a n y l o n f i l t e r and molybdenum c o n v e r t e r i n s e r i e s to o b t a i n a measure o f N0 -HN03; n y l o n removes a c i d i c gases i n c l u d i n g HNO3 and HC1, b u t g e n e r a l l y t r a n s m i t s o t h e r n i t r o g e n o x i d e and organonitrogen compounds w i t h the p o s s i b l e e x c e p t i o n of n i t r o u s a c i d . Nitric acid i s thus determined i n r e a l - t i m e from the d i f f e r e n c e between the 2 c h a n n e l s when the N0 /N0 -HN03 mode i s s e l e c t e d . D i f f i c u l t i e s i n o b t a i n i n g a c c u r a t e HNO3 l e v e l s have been observed u s i n g f i l t e r pack c o l l e c t i o n techniques. In F i g u r e 4 we show a comparison o f HNO3 c o n c e n t r a t i o n s observed a i r b o r n e i n samples c o l l e c t e d u s i n g the BNL X

X

X

X

X


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8840 Improved N0 Detector X

Sample Air In Teflon J Filter | N0 IConverterf

Nylon Filter ΝΟχ


X

IConverterl

Flow Restrictors Dry Air In

ΝΟ/ΝΟχ

- O Valve Ozone Source Background Valve

0

- Flow ' Restrictor

Background •O Valve

Pre-Reactor

Reaction Chamber

Reaction Chamber

V

To Vacuum Pump Figure 3. Improved chemiluminescent N0 with permission from r e f . 10. Copyright Laboratory.) X

detector. (Reprinted 1986 Brookhaven National


THE


296

CHEMISTRY OF ACID RAIN

Filter Pack HN03, ppb

F i g u r e 4. Comparison of a i r b o r n e f i l t e r pack and r e a l - t i m e H N O 3 data. ( R e p r i n t e d w i t h p e r m i s s i o n from r e f . 10. Copyright 1986 Brookhaven N a t i o n a l L a b o r a t o r y . )


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Table I .

Instrumentation

of Atmospheric

Wet Deposition

297

Processes

Comparison of C h a r a c t e r i s t i c s of M o d i f i e d and M o n i t o r Labs 8840 N 0 Detector

Unmodified

X

Modified 8840

Original 8840

Instrument Characteristic

NO and

Compounds

N0

NO and N 0 or N 0 and N0 -HN0

X

X

X

Detection L i m i t

(ppb)

2, NO and

N0

X

X

*0.2, NO and 0.3, HN0

3

N0

X

3


Lowest Range, ppb

f u l l scale

90% Response Time, seconds Sample Flow Rate,

seem

R e a c t i o n Chamber P r e s s u r e , R e a c t i o n Chamber Volume, cc Reactor I n t e r n a l Pre-reactor

(Y/N)

Surface

torr

50

8.7

120

10

1500

1500

250

10

15

225

Unpolished Stainless

P o l i s h e d and G o l d Coated S t a i n l e s s

No

Yes

* V a l u e s are based on s i g n a l / p e a k - t o - p e a k - n o i s e i n s t r u m e n t time c o n s t a n t of 5 seconds.

r a t i o of 2 a t an

f i l t e r pack and determined by the c o n t i n u o u s NO a n a l y z e r ( 1 0 ) . The median v a l u e i s near the LOD o f the r e a l - t i m e t e c h n i q u e , i n c o n s i d e r a t i o n of w h i c h the agreement between methods i s q u i t e good ( l e a s t squares s l o p e 0.72, r ^ = 0.83, i n t e r c e p t n o t s i g n i f i c a n t l y d i f f e r e n t than z e r o ) . F u r t h e r improvements i n d e t e c t o r performance are d e s i r a b l e , e s p e c i a l l y f o r d e t e r m i n i n g low a i r b o r n e c o n c e n t r a t i o n s of n i t r o g e n o x i d e s and H N O 3 i n remote areas or i n the presence of c l o u d s . As o u t l i n e d by K e l l y ( 1 0 ) , p o s s i b l e improvements i n c l u d e a s t i l l l a r g e r ozone s o u r c e , improved chamber d e s i g n , PMT c o o l i n g and photon counting s i g n a l p r o c e s s i n g . D i f f i c u l t i e s are encountered i n d e t e r m i n i n g NO2 u s i n g the ozone-chemiluminescence technique due to the n o n - s p e c i f i c c o n v e r s i o n of s e v e r a l n i t r o g e n o x i d e s / o x y a c i d s on the Mo c a t a l y s t . Use of FeS04 f ° N 0 2 - t o - N 0 c o n v e r s i o n has been d e s c r i b e d , but h u m i d i t y dependent s o r p t i o n / d e s o r p t i o n e f f e c t s have been r e p o r t e d , e.g., PAN (11). A l t e r n a t i v e l y , a commercial NO2 a n a l y z e r based on s u r f a c e chemiluminescence of NO2 i n the presence of a l u m i n o l s o l u t i o n , has been i n t r o d u c e d w h i c h e x h i b i t s the r e q u i s i t e s e n s i t i v e l y and selectivity. N i t r i c a c i d and c e r t a i n o t h e r gases can be sampled by d i f f u s i o n dénuder tubes, e l i m i n a t i n g a r t i f a c t s a s s o c i a t e d w i t h f i l t e r c o l l e c t i o n . The a n n u l a r dénuder v a r i a t i o n of t h i s t e c h n i q u e p e r m i t s samp l i n g a t h i g h e r f l o w r a t e s (5 to 10 L / m i n ) , hence may be u s e f u l f o r a i r b o r n e s a m p l i n g , a l t h o u g h no a i r b o r n e data have been r e p o r t e d to date. r


298



Determination

o f S u l f u r D i o x i d e and A e r o s o l

Sulfate

The most commonly used t e c h n i q u e f o r the a i r b o r n e d e t e r m i n a t i o n of ambient l e v e l s of s u l f u r d i o x i d e ( S O 2 ) and a e r o s o l s u l f u r i n r e a l time i n v o l v e s the use of a m o d i f i e d commercial flame p h o t o m e t r i c d e t e c t o r (FPD) ( 1 2 ) , a l t h o u g h r e c e n t v e r s i o n s o f p u l s e d f l u o r e s c e n c e i n s t r u m e n t a t i o n ( f o r S O 2 ) a r e becoming more p r o m i s i n g , A procedure f o r e n h a n c i n g the s e n s i t i v i t y of the commercial FPD by a d d i t i o n of a known background l e v e l of a s u l f u r compound, u s u a l l y S F 5 ( ~ 1 0 0 ppb) i n the hydrogen s u p p l y ( 1 2 ) , i s now w i d e l y used, r e s u l t i n g i n LODs a p p r o a c h i n g 0 . 2 ppbv ( 1 0 - s e c time c o n s t a n t ) . S u l f u r gases ( m o s t l y SO2 i n ambient a i r ) a r e determined a f t e r removal of a e r o s o l S p a r t i c l e s onto T e f l o n f i l t e r s . A e r o s o l s u l f u r c o n c e n t r a t i o n s are d e t e r mined a f t e r removal of gaseous s u l f u r compounds by passage through a d i f f u s i o n dénuder tube. Mass flow c o n t r o l l e r s are i n s e r t e d i n the hydrogen and exhaust gas l i n e s (the l a t t e r a f t e r removal of condens i b l e w a t e r ) to s t a b i l i z e gas f l o w s and hence, H 2 / O 2 r a t i o i n the burner. This modification i s required for airborne operation at sub-ppb s u l f u r c o n c e n t r a t i o n s ( 1 3 ) . C a l i b r a t i o n s show l i n e a r response w i t h < 1 0 ppb f u l l s c a l e s e n s i t i v i t y . Successful airborne use of a d u a l S 0 2 / a e r o s o l S i n s t r u m e n t f o r measurements i n c l e a r and c l o u d - i n t e r s t i t i a l a i r has been e x t e n s i v e l y documented ( 1 , 1 3 ) . Determination

of Atmospheric

Oxidants

D e t e r m i n a t i o n of s e v e r a l a t m o s p h e r i c o x i d a n t s p e c i e s i s c r i t i c a l to u n d e r s t a n d i n g gaseous and aqueous p r o c e s s e s l e a d i n g to a c i d i c deposition. Hydrogen p e r o x i d e has a h i g h H e n r y s Law s o l u b i l i t y and must be measured i n gaseous and aqueous a t m o s p h e r i c samples to b e t t e r u n d e r s t a n d wet d e p o s i t i o n p r o c e s s e s . In c o n t r a s t , measurements of ozone and p e r o x y a c y l n i t r a t e s (PANs) (and p r o b a b l y a l k y l h y d r o p e r o x i d e s and p e r a c i d s ) u s u a l l y need to be made o n l y i n the gas phase due to t h e i r low aqueous s o l u b i l i t y ( 1 4 ) . 1

Measurement of ozone i n the gas phase i n r e a l - t i m e from a i r borne p l a t f o r m s a t atmospheric c o n c e n t r a t i o n s u s i n g the e t h y l e n e c h e m i l u m i n e s c e n c e t e c h n i q u e no l o n g e r p r e s e n t s s i g n i f i c a n t t e c h n i c a l difficulties. Measurement of ambient c o n c e n t r a t i o n s of PANs and o t h e r o r g a n i c n i t r a t e s can now be done w i t h automated apparatus from ground-based or a i r b o r n e p l a t f o r m s (15,16), u s i n g gas chromatography of p e r i o d i c , d i s c r e t e a i r samples w i t h e l e c t r o n c a p t u r e d e t e c t i o n . C a l i b r a t i o n s to e s t a b l i s h a b s o l u t e a c c u r a c y of d e t e r m i n a t i o n s a t low ppb l e v e l s a r e d i f f i c u l t , and u s u a l l y o n l y p e r o x y a c e t y l n i t r a t e data a r e r e p o r t e d . Advances i n t h i s a r e a u s i n g a new PAN p r e p a r a t i v e t e c h n i q u e (17) can be a n t i c i p a t e d . A n a l y s i s of aqueous-phase hydrogen p e r o x i d e can now be performed a t the μΜ-levels observed i n c l o u d w a t e r and r a i n samples u s i n g s e v e r a l r e c e n t l y developed and/or m o d i f i e d t e c h n i q u e s . The a u t h o r ' s p r e f e r e n c e i s the f l u o r e s c e n c e t e c h n i q u e u s i n g the p e r o x i d a s e - c a t a l y z e d d i m e r i z a t i o n r e a c t i o n of H 2 O 2 w i t h p - h y d r o x y p h e n y l a c e t i c a c i d (POHPAA t e c h n i q u e ) o f L a z r u s e t a l . ( 1 8 ) , as m o d i f i e d by K e l l y e t a l . (19) Data on a t m o s p h e r i c l e v e l s o f aqueous H 2 O 2 have i n c r e a s e d d r a m a t i c a l l y i n the p a s t two y e a r s w i t h these t e c h n i q u e developments, i n d i c a t i n g s e a s o n a l dependencies of H 2 O 2 s o u r c e s and n o n - c o e x i s t e n c e o f S O 2 and H 2 O 2 i n nonp r e c i p i t a t i n g clouds (3).


25.

TANNER

Instrumentation

of Atmospheric

Wet Déposition

Processes

299

Measurement o f gas-phase H 0 2 has lagged due to d i f f i c u l t i e s e x p e r i e n c e d i n c o l l e c t i n g H 2 0 2 ( g ) i n t o aqueous s o l u t i o n w i t h o u t gene r a t i n g " a r t i f a c t " p e r o x i d e by r a d i c a l s c r u b b i n g o r surface-mediated ozone d e c o m p o s i t i o n ( 2 0 ) . R e c e n t l y , i n a d d i t i o n to the r e p o r t e d d i r e c t o b s e r v a t i o n of H 2 0 2 ( g ) by diode l a s e r - b a s e d absorbance measurements ( 2 1 ) , three methods f o r a r t i f a c t - f r e e c o l l e c t i o n o f H 2 0 2 ( g ) have been r e p o r t e d based on prompt dérivatization and a n a l y s i s ( 2 2 ) or ozone pre-removal techniques (see F i g u r e 5 ) , w i t h d e t e r m i n a t i o n o f c o l l e c t e d H 2 0 2 ( a q ) by the POHPAA t e c h n i q u e (23) o r h e m i n - c a t a l y z e d l u m i n o l methodologies ( 2 4 ) . A i r b o r n e measurements f o r gaseous H 2 O 2 have been r e p o r t e d f o r o n l y one o f these techniques (25). Aqueous samples must be a n a l y z e d promptly o r e l s e f i x e d by a d d i t i o n o f reagent i n the case o f the POHPAA technique ( 2 6 ) . Downloaded by CORNELL UNIV on August 11, 2016 | http://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch025

2

Determination

o f Organics

i n Atmospheric Samples

S e v e r a l c l a s s e s o f atmospheric o r g a n i c compounds a r e o f i n t e r e s t i n d e p o s i t i o n s t u d i e s . I n s t r u m e n t a t i o n f o r these s p e c i e s w i l l be b r i e f l y reviewed. C o n c e n t r a t i o n s of t o t a l hydrocarbons ( e x p r e s s e d as ppbv C) o r non-methane hydrocarbons can be determined from a i r b o r n e p l a t f o r m s u s i n g flame i o n i z a t i o n d e t e c t i o n , b u t t h e i r r e l e v a n c e to photochemic a l o x i d a n t p r o d u c t i o n , and hence to a c i d i c d e p o s i t i o n , i s i n d i r e c t . R e a c t i v e hydrocarbons have been determined i n s u r f a c e measurements u s i n g ozone chemiluminescence w i t h o p t i c a l f i l t e r s (27), b u t the i n s t r u m e n t a t i o n has n o t been developed f o r a i r b o r n e measurements. Aldehydes have been w i d e l y measured i n aqueous and gas phase samples, u s u a l l y u s i n g DNPH d e r i v a t i z a t i o n s and HPLC-UV d e t e r m i n a t i o n (28,29); however, few a i r b o r n e measurements have been r e p o r t e d . Organic a c i d s have been r e p o r t e d to be i m p o r t a n t s i n k s from the atmosphere f o r o x i d i z e d atmospheric carbon ( 3 0 ) , based on s u r f a c e measurements u s i n g i o n - e x c l u s i o n chromatography w i t h UV o r c o n d u c t i v i t y d e t e c t i o n s . L a s t l y , o r g a n i c p e r o x i d e s and p e r a c i d s have r e l a t i v e l y low aqueous s o l u b i l i t y , b u t have been h y p o t h e s i z e d to be i m p o r t a n t o x i d a n t s i n the gas phase. Methodologies f o r t h e i r unambiguous d e t e r m i n a t i o n i n atmospheric samples a r e s t i l l b e i n g developed. Summary S e v e r a l areas i n which c h e m i c a l measurement t e c h n o l o g i e s have become a v a i l a b l e and/or r e f i n e d f o r a i r b o r n e a p p l i c a t i o n s have been reviewed i n t h i s paper. I t i s a s e l e c t i v e review and many important m e t e o r o l o g i c a l and c l o u d p h y s i c s measurement c a p a b i l i t i e s of r e l e vance to atmospheric c h e m i s t r y and a c i d d e p o s i t i o n ( e . g . , measurement o f c l o u d l i q u i d water c o n t e n t ) have been i g n o r e d . I n p a r t i c u l a r , we have n o t d i s c u s s e d p a r t i c l e s i z e s p e c t r a measurements f o r v a r i o u s atmospheric condensed phases ( a e r o s o l s , c l o u d d r o p l e t s and p r e c i p i t a t i o n ) . F u r t h e r improvements i n c h e m i c a l measurement t e c h n o l o g i e s can be a n t i c i p a t e d e s p e c i a l l y i n the areas o f f r e e r a d i c a l s , o x i d a n t s , o r g a n i c s , and S O 2 and NO2 a t v e r y low l e v e l s . N e v e r t h e l e s s , major i n c r e m e n t a l improvements i n the u n d e r s t a n d i n g of a c i d d e p o s i t i o n processes can be a n t i c i p a t e d from the c o n t i n u i n g a i r b o r n e a p p l i c a t i o n o f the techniques d e s c r i b e d i n t h i s review.



4

ZERQ, AIR

2

2

Ν

ilr

MASS FLOW CONTROLLER

MIXING CHAMBER

1JT

2

= 2 t

DIH 0

ν 2

SCRUBBERS

PERISTALTIC PUMP^.

IMPINGERS

3

VENT

SAMPLE [COLLECTION VIALS

nei

F/M 3

®

F i g u r e 5. Schematic o f ambient a i r s a m p l i n g a p p a r a t u s f o r gaseous hydrogen p e r o x i d e . ( R e p r i n t e d from r e f . 23. C o p y r i g h t 1986 American C h e m i c a l S o c i e t y . )

Mn0 -TRAP

' F/M I

2

SOURCE

H 0

AMBIENT AIR


5 > 5

Ο τι

55 H

§

η s m

ο ο

25.

TANNER

Instrumentation

of Atmospheric

Wet Deposition

Processes

301

Acknowledgments The a u t h o r acknowledges many i n f o r m a t i v e d i s c u s s i o n s w i t h T.J. K e l l y and P.H. Daum. T h i s work was conducted under the a u s p i c e s o f the U.S. Department o f energy under c o n t r a c t No. DE-ÀC02-76CH00016.


References 1. Tanner, R.L.; Daum, P.H.; Kelly, T.J. Intern. J. Environ. Anal. Chem., 1983, 13, 323-335. 2. Walters, P.T.; Moore, M.J.; Webb, A.H. Atmos. Environ., 1983, 17, 1083-1091. 3. Daum, P.H.; Kelly, T.J.; Schwartz, S.E.; Newman, L. Atmos. Environ., 1984, 18, 2671-2684. 4. Winters, W.; Hogan, Α.; Mohnen, V.; Barnard, S. "ASRC airborne cloud water collection system", Atmospheric Sciences Research Center, State University of New York at Albany, ASRC-SUNYA Publication No. 728, 1979. 5. Huebert, B.J.; Baumgardner, D. Atmos. Environ., 1985, 19, 843-846. 6. Scott, B.C.; Laulainen, N.S. J. Appl. Meteor., 1979, 18, 138-147. 7. Fontijn, Α.; Sabadell, A . J . ; Ronco, R.J. Anal. Chem., 1970, 42, 575-579. 8. Stedman, D.H.; Daby, E.E.; Stuhl, F.; Niki, H. J. Air Poll. Contr. Assoc., 1972, 22, 260-263 (1972). 9. Delany, A.C.; Dickerson, R.R.; Melchoir, F.L., Jr.; Wartburg, A.F. Rev. Sci. Instrum., 1983, 53, 1899-1903. 10. Kelly, T.J. "Modifications of Commercial Oxides of Nitrogen Detectors for Improved Response", Brookhaven National Laboratory, Upton, NY, Report No. BNL-38000, 1986. 11. Tanner, R.L.; Lee, Y.-N.; Kelly, T.J.; Gaffney, J.S. "Ambient HNO3 Measurements-Interference from PAN and Organo-Nitrogen Compounds", presented at the 25th Rocky Mountain Conference, Denver, CO, 1983. 12. D'Ottavio, T.; Garber, R.; Tanner, R.; Newman, L. Atmos. Environ., 1981, 15, 197-203. 13. Garber, R.W.; Daum, P.H.; Doering, R.F.; D'Ottavio, T.; Tanner, R.L. Atmos. Environ., 1983, 17, 1381-1385." 14. Lind, J . ; Kok, G.L. J. Geophys. Res., 1986, 91, 7889-7895. 15. Spicer, C.W.; Holdren, M.W.; Keigley, G.W. Atmos. Environ., 1983, 17, 1055-1058. 16. Singh, H.B.; Salas, L . J . Nature, 1983, 302, 326. 17. Gaffney, J.S.; Fajer, R.; Senum, G.I. Atmos. Environ., 1984, 18, 18215-18218. 18. Lazrus, A.L.; Kok, G.L.; Gitlin, S.N.; Lind, J.A.; McLaren, S.E. Anal. Chem., 1985, 57, 917-920. 19. Kelly, T.J.; Daum, P.H.; Schwartz, S.E. J. Geophys. Res., 1985, 90, 7861-7871. 20. Heikes, B.G. Atmos. Environ., 1984, 18, 1433-1445. 21. Schiff, H.I.; Mackay, G.I. "The Development of a Method for Measuring H2O2 in Real Air Using a Tunable diode Laser Absorption Spectrometer", Final Report of Research Project RP 2023-5, prepared by Unisearch Associates, Inc., for the Electric Power Research Institute, 1984.



302


22. Lazrus, A.L.; Kok, G.L.; Lind, J.A.; Gitlin, S.N.; Heikes, B.G.; Shetter, R.E. Anal. Chem., 1986, 58, 594-597. 23. Tanner, R.L.; Markovits, G.Y.; Ferreri, E.M.; Kelly, T.J. Anal. Chem., 1986, 58, 1857-1866. 24. Groblicki, P.J.; Ang, C.C. "Measurement of H2O2 without Ozone Interference", Proceedings of the Symposium on Heterogeneous Processes in Source-Dominated Atmospheres, New York, 1985, 86-88. 25. Heikes, B.G.; Lazrus, A.L.; Kok, G.L. "Measurements of H2O2 in the Lower Troposphere", presented at the 17th International Symposium on Free Radicals, Granby, CO, 1985. 26. Kok, G.L.; Thompson, K.; Lazrus, A.L.; McLaren, S.E. Anal. Chem., 1986, 58, 1192-1194. 27. Kelly, T.J.; Gaffney, J.S.; Phillips, M.F.; Tanner, R.L. Anal. Chem., 1983, 55, 135-138. 28. Grosjean, D.; Fung, K. Anal. Chem., 1982, 54, 1221-1224. 29. Tanner, R.L.; Meng, Z. Environ. Sci. Technol., 1984, 18, 723-726. 30. Keene, W.C.; Galloway, J.N.; Holden, J.D. J. Geophys. Res., 1983, 88, 5122-5130. 31. Kallend, A.S. "The Fate of Atmospheric Emissions along Plume Trajectories over the North Sea: First Annual Report to EPRI, March, 1979", Central Electric Research Laboratories, Leatherhead, Surrey, England, Report No. RO/L/R 1998, 1979. RECEIVED March 10, 1987


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