Measurement Challenges in Atmospheric Chemistry - ACS Publications

12 Tropospheric Hydroxyl Radical A Challenging Analyte

Downloaded by CORNELL UNIV on August 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1993 | doi: 10.1021/ba-1993-0232.ch012

Robert J. O ' B r i e n and Thomas M. H a r d Chemistry Department and Environmental Sciences Program, Portland State University, Portland, O R 97207

The methods for the direct measurement of tropospheric hydroxyl radical, HO, are reviewed, and the technical hurdles that remain to be surmounted are discussed in the light of theoretical and experi mental results. Sensitivities, advantages, and disadvantages of several HO methods are compared, and a way to compare many of the existing HO methods experimentally is presented.

TROPOSPHERIC HYDROXYL RADICAL IS FORMED

i n s u n l i g h t a n d reacts r a p i d l y w i t h a v a r i e t y o f trace a t m o s p h e r i c constituents, u s u a l l y c o n v e r t i n g t h e m from w a t e r - i n s o l u b l e to water-soluble forms. ( H y d r o x y l r a d i c a l , h e r e t e r m e d H O , is w i d e l y c a l l e d O H i n the a t m o s p h e r i c science c o m m u n i t y . W e p r e f e r the I U P A C n a m e H O because it is consistent w i t h o t h e r h y d r o g e n oxides, H 0 , H 0 , a n d H 0 , a n d because it b e t t e r distinguishes H O f r o m O H " , a species often confused w i t h H O b y n o n a t m o s p h e r i c scientists.) H y d r o x y l greatly assists w e t d e p o s i t i o n or p r e c i p i t a t i o n - s c a v e n g i n g of natural a n d a n t h r o p o g e n i c a t m o s p h e r i c species. A l t h o u g h d a y t i m e H O concentrations are m u c h h i g h e r t h a n n i g h t t i m e levels (J), e v e n the d a y t i m e t r o p o s p h e r i c H O concentrations are l o w (e.g., a sea l e v e l n u m b e r d e n s i t y b e t w e e n 1 0 a n d 10 m o l e c u l e s p e r c u b i c c e n t i m e t e r o r a m o l e fraction o f 4 X 10~ to 4 X 10~ ). T h i s l o w concentration is p a r t l y a result of the h i g h c h e m i c a l r e a c t i v i t y of H O . D e p e n d i n g o n the a t m o s p h e r i c n i t r i c oxide ( N O ) c o n c e n t r a t i o n , H O is often r e g e n e r a t e d i n c h a i n sequences s u c h as this: 2

2

2

2

5

7

15

13

C O + ΗΟ· Η· + 0

2

> C0

2

+

Η·

> Η0 · 2

0065-2393/93/0232-0323$13.25/0 © 1993 American Chemical Society

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

(1) (2)

324

MEASUREMENT CHALLENGES IN ATMOSPHERIC CHEMISTRY

Η0 · + N O 2

2

0

(3)

* NO + Ο

(4)

2

Ν 0 · + hv Ο +

* Ν0 · + H O

0

2

(5)

3

I f the N O concentration is a p p r e c i a b l e , the efficiency o f trace gas o x i d a t i o n (free radical c h a i n length) is h i g h for this species. I n r e m o t e air masses, v e r y l o w N O concentrations l i m i t r e c y c l i n g of H O , a n d the H O o x i d a t i o n rate approaches the p r i m e H O generation rate, for instance b y these reactions:


0

3

4- hv

—> 0

2

(^Ag) + Ο (a D); l

Ο ( D) + H 0 X

\ < 320 n m

(6)

> 2ΗΟ·

2

(7)

I n the global t r o p o s p h e r e , the role of H O has b e e n to m a i n t a i n the l o w a n d r e l a t i v e l y constant trace gas c o m p o s i t i o n that a p p a r e n t l y has p e r s i s t e d for at least 10,000 years (2). M o r e r e c e n t l y , h o w e v e r , m e a s u r e d increases i n t r o p o s p h e r i c m e t h a n e (3-9) have l e d to c o n c e r n about h u m a n i t y ' s possible influence o n the natural abundance of H O (JO). I n r e m o t e regions of the t r o p o s p h e r e , n o n m e t h a n e h y d r o c a r b o n s a n d oxides o f n i t r o g e n are q u i t e l o w i n c o n c e n t r a t i o n i n large m e a s u r e because of H O - c a t a l y z e d r e m o v a l of these species d u r i n g the process o f long-range transport from m o r e p o l l u t e d regions. I n p o l l u t e d a t m o s p h e r i c regions r a n g i n g from c o n t i n e n t a l to u r b a n , these same natural c l e a n s i n g processes g e n erate concentrations o f i n t e r m e d i a t e p r o d u c t s that c o l l e c t i v e l y have b e e n c a l l e d p h o t o c h e m i c a l smog. T h e s e p r o d u c t s i n c l u d e o z o n e , p e r o x y a c e t y l nitrate ( P A N ) , H 0 , a n d o t h e r oxidants; aldehydes a n d ketones; s u l f u r i c , n i t r i c , a n d organic acids; a n d free radicals i n a d d i t i o n to H O (11-14). T h u s , h y d r o x y l radical plays a k e y role i n c h e m i c a l m o d e l s for e i t h e r the r e m o t e o r the p o l l u t e d troposphere, a n d accurate assessment o f the reactions that c o n t r o l its concentration is a p r e r e q u i s i t e for c r e d i b l e m o d e l i n g o f a t m o s p h e r i c processes. 2

2

A l t h o u g h H O was already k n o w n as an agent active i n c o m b u s t i o n , the 1960s b r o u g h t the r e a l i z a t i o n o f H O ' s t r o p o s p h e r i c r e a c t i v i t y . L e i g h t o n (14) speculated about the " n a t u r e a n d i n d e e d the r e a l i t y o f [the] apparent excess rate o f olefin [alkene] c o n s u m p t i o n " i n p h o t o c h e m i c a l s m o g studies. T h i s excess rate was soon i d e n t i f i e d w i t h the presence o f h y d r o x y l r a d i c a l i n a d d i t i o n to the r e c o g n i z e d reactive species o z o n e a n d o x y g e n atoms. I n fact, a c u r r e n t e s t i m a t i o n (15) o f the relative rates o f r e m o v a l o f s e v e r a l classes o f h y d r o c a r b o n s b y 0 , H O , a n d N 0 (Table I) indicates that (except for c e r t a i n alkenes) H O dominates the d a y t i m e r e m o v a l o f most h y d r o c a r b o n s . ( A l t h o u g h v e r y reactive, Ο atoms are r a p i d l y scavenged b y 0 to generate 3

3

2

0 .) 3

Interest i n t r o p o s p h e r i c H O concentrations has b e e n h i g h since at least the late 1960s, w h e n W e i n s t o c k (16) suggested that t r o p o s p h e r i c C O m i g h t



2

5.0 2.7 2.5 2.5 7.0 1.2 6.4 9.8 1.6 1.1

1.0

15

10

10

1 2

13

11

10-

10

10-

10

1 2

12

10

6

1 2

16

1 3

1 7

1 7

17

4

5

6

7

6

NO3 5

10

x 10 116 68 220 5787 NA 0.06 0.01 843 231 5 908 1448 671 0.05 77 0.04 1.2

Lifetimes (days)

1.7 x 10 3.0 X 10 5.6 X 10 NA 1200 1200 843 0.15 1200 6.2 1 1513 1.7 X 10 7.9 X 10 15 NA 0.37

Tropospheric

0.12 0.23 4 5 0.46 17 10 2 1 72 1

11

1362 42 1 12 43

HO

= 40 ppbv, [ H O ] = 1.0 Χ 10 molecules per cubic centimeter (daytime), and [NO.f] =

1 7

19

22

23

21

15

13

10-

1 7

10-

16

20

1 7

1 2

13

18

1 6

1 6

16

1 8

1 7

20

20

20

24

25

1 2

3

1 9

4.0 X 1 0 4.0 X 106.8 x 1 0 2.1 x 1 0 8.0 Χ ΙΟ" very small 8.2 Χ ΙΟ" 6.2 X 1 0 5.5 X 1 0 2.0 Χ ΙΟ" 9.5 X 1 Q 5.1 Χ ΙΟ" 3.2 X 1 0 6.9 X 1 0 9.7 X 1 0 6.0 X 101.2 X 1 0

24

7.0 x Ι Ο 4.0 x 1 0 2.1 Χ ΙΟ" very small 1.0 χ 101.0 χ 1 0 1.4 X 10~ 8.0 x 1 0 1.0 χ 101.9 Χ 1 0 1.1 χ 10 7.8 x Ι Ο 7.0 x 1 0 1.5 Χ Ι Ο 8.0 x 10very small 3.2 X 1 0

1 2

11

10 10

10-"

12

10-

10

1 3

1 2

1 3

10 10 10 -13

10-

NO3

1

s' )

03

3

Rate Constants (molecules cm

N O T E : Lifetimes are based upon [0 ] pptv (nighttime). N A , not available.

3

Methanol Ethane Propane Isoprene a-Pinene n-Butane Ethene Propene Acetylene Benzene Toluene (CH )aS Ammonia N0

2

CH 0

X X X X X X X X X X X X X X X X X

Methane CO

8.5 2.8 9.8 9.3 2.7 1.1

HO

Species

Table I. Trace Gas Rate Constants and Lifetimes for Reaction with Ozone, H y d r o x y l Radical, and Nitrate Radical


326

M E A S U R E M E N T C H A L L E N G E S IN A T M O S P H E R I C C H E M I S T R Y

be r e m o v e d b y reaction w i t h H O . Reactions 1 t h r o u g h 5 p r o v i d e a succinct illustration of H O catalysis. T h e result of such a cycle is to split m o l e c u l a r oxygen; H O is c o n s e r v e d . T h e net of reactions 1 - 5 is


CO + 20

2

+ hv

>C0

2

+ 0 ·

(8)

3

I n r e m o t e t r o p o s p h e r i c air, w h e r e N O concentrations can be q u i t e l o w (17), the H O + C O oxidation m e c h a n i s m can follow o t h e r pathways, l e a d i n g to net ozone d e s t r u c t i o n r a t h e r t h a n formation (18, 19). Reactions 1 t h r o u g h 5 typify the m o r e c o m p l e x catalytic reactivity of H O w i t h h y d r o c a r b o n s , w h i c h p r o d u c e a c o m p l e x array of oxidation products w h i l e g e n e r a t i n g o z o n e p h o t o c h e m i c a l l y (11-13). Interest i n actually m e a s u r i n g the c o n c e n t r a t i o n of t r o p o s p h e r i c H O f o l l o w e d Weinstoek's (16) 1969 discussion o f the role of H O i n c o n t r o l l i n g t r o p o s p h e r i c C O a n d L e v y ' s (20-22) d e s c r i p t i o n o f the p r o d u c t i o n o f H O t h r o u g h o u t the troposphere i n reactions 6 a n d 7. T h i s m e c h a n i s m is b e l i e v e d to d o m i n a t e H O p r o d u c t i o n i n the least p o l l u t e d air a n d r e m a i n s a significant source i n p o l l u t e d air w h e r e a n u m b e r of other sources also exist. T h e i m p o r t a n c e of H O as a theoretically i n t e r e s t i n g small free radical a n d as an i m p o r t a n t catalyst i n c o m b u s t i o n a n d i n a t m o s p h e r i c c h e m i s t r y has l e d to measurements of rate constants for its reaction w i t h a w i d e array of o t h e r gases. T h e s e m e a s u r e m e n t s have a c h i e v e d , o v e r the last several decades, a h i g h degree of sophistication a n d accuracy, a n d t h e y are r e g u l a r l y r e v i e w e d a n d evaluated (13, 23). T h e role of H O as the "sole reactant" for m a n y t r o p o s p h e r i c trace gases has a l l o w e d the m e a s u r e m e n t , u n d e r s i m u l a t e d a t m o s p h e r i c c o n d i t i o n s , of the relative reaction rate constants of H O w i t h a host of h y d r o c a r b o n s , o t h e r organic c o m p o u n d s (24), a n d other trace gases, i n c l u d i n g N 0 (25). T h e s e rate constants are i n good agreement w i t h those m e a s u r e d b y the m o r e d i r e c t t e c h n i q u e s , a n d t h e y corroborate the c o n c e p t o f H O c o n t r o l o f h o mogeneous d a y t i m e t r o p o s p h e r i c c h e m i s t r y . T h i s e v i d e n c e for H O c o n t r o l of trace gas lifetimes can f o r m the basis for i n d i r e c t m e a s u r e m e n t of a m b i e n t H O concentrations a n d can serve as a means of c a l i b r a t i n g H O m e a s u r e m e n t devices (discussed later i n the chapter). C o n d e n s e d - p h a s e a t m o s p h e r i c oxidations (in aerosols, fogs, a n d clouds) that c o n v e r t S 0 (aq) to S O / " (aq), a l t h o u g h not d i r e c t l y i n f l u e n c e d b y H O , m a y be i n d i r e c t l y c o u p l e d to i t , because gaseous H O m a y d e t e r m i n e the c o n c e n t r a t i o n o f oxidants s u c h as o z o n e o r h y d r o g e n p e r o x i d e , w h i c h have significant aqueous s o l u b i l i t y . A l t h o u g h H O reacts r a p i d l y at m a n y surfaces a n d m a y b e t h e r e b y r e m o v e d , it m a y also b e absorbed or e m i t t e d f r o m a t m o s p h e r i c aerosols a n d d r o p l e t s . Z e l l n e r et al. (26) have m e a s u r e d aqueous H O g e n e r a t i o n rates f r o m solutions of n i t r a t e , n i t r i t e , a n d h y d r o g e n p e r o x i d e . O n the basis o f t h e i r m e a s u r e m e n t s t h e y suggested that i n situ H O generation rates w i t h i n aerosols m a y b e c o m p a r a b l e to H O d e p o s i t i o n rates. T h u s , e v e n the d i r e c t i o n of the H O 2

2


12.

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O'BRIEN & HARD

Radical

327

flux at aerosol surfaces may b e i n doubt. T h e h i g h heterogeneous a n d h o mogeneous reactivity of H O has i m p o r t a n t ramifications for H O m e a s u r e ments i n s a m p l e d a m b i e n t a i r , for relative-rate m e a s u r e m e n t o f H O rate constants u n d e r s i m u l a t e d a t m o s p h e r i c c o n d i t i o n s , for i n d i r e c t m e a s u r e m e n t of a m b i e n t H O concentrations, for c h e m i c a l r e m o v a l of H O f r o m s a m p l e d air d u r i n g H O b a c k g r o u n d signal m e a s u r e m e n t s , a n d for the c a l i b r a t i o n of ambient measurement instruments.


HO Chemical Lifetime: Implications for Measurement A s i d e f r o m those species that react w i t h a t m o s p h e r i c o x y g e n , n i t r o g e n , o r water vapor (e.g., Η, Ο ( P), Ο (*D), or organic radicals), h y d r o x y l r a d i c a l is a p p a r e n t l y the atmosphere's most reactive species, a n d its short c h e m i c a l lifetime has various i m p l i c a t i o n s for m e a s u r e m e n t strategies. I n r o u g h t e r m s , a single H O radical survives for about 1 s i n the cleanest regions of the troposphere b u t o n l y about 1 ms i n the most p o l l u t e d regions. T h e s e lifetimes are i n v e r s e l y p r o p o r t i o n a l to the concentrations a n d H O reactivities of those species w i t h w h i c h H O reacts. I n r e m o t e m a r i n e air [nonmethane h y d r o c a r b o n concentrations less than 25 parts p e r b i l l i o n b y v o l u m e (ppbv)] (27), C O is the d o m i n a n t reactant w i t h H O a n d m e t h a n e is s e c o n d , whereas i n p o l l u t e d air an i m m e n s e array of anthropogenic a n d b i o g e n i c h y d r o c a r b o n s d o m i n a t e H O r e m o v a l . T h e l i f e t i m e of an i n d i v i d u a l H O r a d i c a l s h o u l d b e d i s t i n g u i s h e d from the decay of H O concentration w h e n the p h o t o l y t i c p r o d u c t i o n terms (e.g., reactions 6 a n d 7) are r e d u c e d — f o r instance, w h e n a c l o u d passes i n front of the s u n — o r s t o p p e d — a s at sunset o r w h e n air enters a dark s a m p l i n g system. T h e c h e m i c a l l i f e t i m e of an i n d i v i d u a l H O m o l e c u l e is g i v e n b y ( X i k ^ T j ) " , w h e r e [T ] indicates the c o n c e n t r a t i o n o f an a t m o s p h e r i c trace gas that reacts w i t h H O a n d k is the rate constant. I n contrast to this short l i f e t i m e , the concentration response t i m e , c h a r a c t e r i z i n g the d e c l i n e of H O concentration w h e n darkness is i m p o s e d , d e p e n d s u p o n the reactivity of the i n d i v i d u a l H O e n t i t y , the r e g e n e r a t i o n of H O b y c h a i n processes, a n d the collapse of l a b i l e H O reservoirs, w h i c h i n g e n e r a l store a m u c h larger concentration of H O than is present i n the free r a d i c a l state. S i m i l a r considerations a p p l y to the increase i n H O c o n c e n t r a t i o n w h e n i r radiance is increased. 3

1

{

{

I n contrast w i t h the t y p i c a l 1-s lifetime of H O i n the cleanest t r o p o s p h e r i c air, a significant fraction of the reacting H O can b e regenerated v i a reactions 2 a n d 3, so the o b s e r v e d decay of H O c o n c e n t r a t i o n is p r o l o n g e d . T h e i m p o r t a n c e of the regeneration process d e p e n d s o n the relative c o n centrations of the various H O radical sinks. O n e p r e d o m i n a n t radical sink is N 0 , w h i c h s t o i c h i o m e t r i c a l l y converts H O to the stable n o n r a d i c a l species H N 0 as follows: 2

3

ΗΟ· + Ν 0 · 2

> ΗΝ0

3


(9)

328


I m p o r t a n t H O reservoirs are H 0 , H N 0 , P A N , a n d others. I n clean air, the concentration of H 0 is about 100 times that of H O (1), a n d H 0 continues to generate H O i n darkness b y reaction 3 a n d o t h e r steps. L a b i l e n o n r a d i c a l reservoirs dissociate to p r o d u c e H O i n several steps. F o r instance, 2

4

2


HN0

2

4

> Η0 · + Ν0 · 2

(10)

2

a n d this reaction can t h e n be followed b y reaction 3 to p r o d u c e H O . Short H O radical lifetimes i n the dark p r e v e n t the c o l l e c t i o n o f a m b i e n t air samples for later analysis, except perhaps for the case of filter c o l l e c t i o n techniques that use a s p i n - t r a p p i n g reagent (28). F u r t h e r m o r e , r a p i d reac t i v i t y of H O w i t h surfaces mandates careful attention to the s a m p l i n g t r a i n u s e d to m o v e a m b i e n t air into a m o r e amenable analysis e n v i r o n m e n t . T w o goals can be e n v i s i o n e d w h e n a m b i e n t H O m e a s u r e m e n t s are c o n t e m p l a t e d , b o t h r e l a t e d to the r e c i p r o c a l c o n t r o l that H O maintains u p o n o t h e r trace gases a n d that they, i n t u r n , m a i n t a i n u p o n the H O c o n c e n t r a t i o n . T h i s d i c h o t o m y arises because H O comes into r a p i d e q u i l i b r i u m w i t h its c h e m i c a l surroundings a n d at the same t i m e alters this c h e m i c a l e n v i r o n m e n t o n a m u c h longer t i m e scale r a n g i n g f r o m hours to years. T h u s H O e q u i l ibrates w i t h C O a n d C H i n the cleanest t r o p o s p h e r i c air i n a m a t t e r of seconds b u t continues to r e m o v e C O a n d C H , w h i c h have c h e m i c a l lifetimes of about 2 months a n d 7 years, respectively. I n p o l l u t e d air H O bears the same relationship w i t h a m u l t i t u d e of h y d r o c a r b o n s , N O , a n d N 0 , w h i c h have c h e m i c a l lifetimes that range f r o m less than 1 h o u r for the most reactive w i t h H O to the 7-year l i f e t i m e of m e t h a n e , the least reactive h y d r o c a r b o n . T h e simplest goal i n H O determinations is to obtain H O concentrations that are b r o a d l y representative of the air mass b e i n g s a m p l e d . E v e n i n the r e m o t e troposphere, r e m o v e d f r o m local e m i s s i o n sources, [ H O ] m a y v a r y signifi cantly i f there are fluctuations i n its c o n t r o l l i n g c h e m i c a l species ( C O , 0 , H 0 , N 0 , N O , a n d so forth) because of v e r t i c a l c o n v e c t i o n . I f the m o r e a b u n d a n t trace gases have r e l a t i v e l y constant concentrations, for instance i n the m i x e d layer, H O w o u l d be expected to have a stable c o n c e n t r a t i o n over large regions of the atmosphere. I n p o l l u t e d regions, the p r o x i m i t y o f sources a n d surface sinks results i n m u c h greater v a r i a b i l i t y i n these c o n t r o l l i n g c h e m i c a l entities; this v a r i a b i l i t y is expected to result i n larger v a r i a b i l i t y i n H O concentrations, so m o r e care m u s t b e u s e d i n d e f i n i n g a " r e p r e s e n t a t i v e " H O concentration. N e v e r t h e l e s s , it is useful to have m e a s u r e d values of [ H O ] representative of the " m i d l a t i t u d e free t r o p o s p h e r e " , the " r e m o t e c o n t i n e n t a l m i x e d l a y e r " , the layer " b e l o w the i n v e r s i o n h e i g h t i n d o w n t o w n L o s A n g e l e s " , or elsewhere. 4

4

2

3

2

2

A second goal i n H O measurements is m o r e specifically o r i e n t e d t o w a r d c o m p a r i n g the predictions of c h e m i c a l m o d e l s w i t h m e a s u r e d H O c o n c e n trations. N o w , the concentrations of a l l relevant c h e m i c a l species, l i g h t fluxes, a n d so forth m u s t be c o n c u r r e n t l y m e a s u r e d . T h e r a p i d e q u i l i b r a t i o n


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Radical

329


of H O i n photostationary e q u i l i b r i u m w i t h its s u r r o u n d i n g s facilitates s u c h comparisons. T h u s , i f the p r o x i m i t y o f the g r o u n d o r another surface has a l t e r e d t h e atmospheric albedo o r t h e total c h e m i c a l c o m p o s i t i o n o f a n air sample so that i t is n o t representative o f any p a r t i c u l a r air mass, m e a n i n g f u l m o d e l comparisons can still b e made as l o n g as ancillary m e a s u r e m e n t s are m a d e o f the relevant c h e m i c a l entities w i t h w h i c h H O has e q u i l i b r a t e d . A n a t m o s p h e r i c c h e m i c a l m o d e l m a y t h e n b e tested for its a b i l i t y to c o r r e c t l y p r e d i c t H O concentrations from a n i n p u t o f the r e l e v a n t a t m o s p h e r i c c o n ditions (light fluxes a n d d o m i n a n t trace gas species concentrations). A k e y e l e m e n t i n m e a s u r i n g such a l o w - c o n c e n t r a t i o n , s h o r t - l i v e d , c h e m i c a l species is t h e averaging t i m e r e q u i r e d to o b t a i n acceptable signalto-noise ratios. T h e u l t i m a t e desire is to achieve s a m p l i n g t i m e s shorter t h a n the H O concentration response t i m e itself. S u c h sensitivity w o u l d a l l o w f o l l o w i n g s h o r t - t e r m fluctuations i n H O b r o u g h t about b y naturally o c c u r r i n g or d e l i b e r a t e l y i n t r o d u c e d p e r t u r b a t i o n s . E x c e p t for t h e c o m p l e x i t y o f atm o s p h e r i c c o m p o s i t i o n , these e x p e r i m e n t s w o u l d b e t h e o p e n - a t m o s p h e r e e q u i v a l e n t o f relaxation kinetics w i d e l y e m p l o y e d i n the laboratory b y c h e m ical kinetics scientists. I n t h e laboratory, a s i m p l e c h e m i c a l c o m p o s i t i o n allows u n a m b i g u o u s c o r r e l a t i o n o f reaction rates w i t h rate constants. I n t h e o p e n atmosphere, t h e m o r e difficult goal is to relate m e a s u r e d H O fluctuations w i t h t h e e n t i r e suite o f trace gases w i t h w h i c h H O m a y react o r from w h i c h i t c o u l d b e f o r m e d . Sufficient sensitivity has y e t to b e a c h i e v e d i n a m b i e n t H O measurements because t h e continuous m e a s u r e m e n t s have r e q u i r e d l o n g averaging times a n d t h e p o i n t m e a s u r e m e n t s have n o t b e m a d e w i t h sufficiently h i g h frequency.

Tropospheric HO: A Challenging Analyte A l t h o u g h t h e focus o f this chapter is t r o p o s p h e r i c H O m e a s u r e m e n t s , i t is w o r t h w h i l e to m e n t i o n techniques that have p r o v e n useful i n the laboratory or i n o t h e r regions o f the atmosphere. A s a s m a l l m o l e c u l e i n the gas phase, H O has a m u c h - s t u d i e d a n d w e l l - u n d e r s t o o d discrete a b s o r p t i o n s p e c t r u m i n t h e near U V (29), s h o w n i n F i g u r e 1, that lends i t s e l f to a v a r i e t y o f absorption a n d fluorescence t e c h n i q u e s . T h e total a t m o s p h e r i c H O c o l u m n d e n s i t y has b e e n m e a s u r e d (30-32) from absorption o f solar U V r a d i a t i o n , o b s e r v e d w i t h a h i g h - r e s o l u t i o n scanning F a b r y - P e r o t spectrometer. L o n g p a t h measurements o f stratospheric H O from its t h e r m a l e m i s s i o n spectra i n the far i n f r a r e d have b e e n r e p o r t e d (33-35). L o n g a b s o r p t i o n paths i n t h e a t m o s p h e r i c b o u n d a r y layer have b e e n u s e d for H O d e t e c t i o n f r o m its U V absorption (36-42). F l u o r e s c e n c e measurements o f H O have b e e n a c o m m o n feature o f laboratory kinetics studies of the reaction-rate coefficients o f H O w i t h various molecules a n d o f studies o f this free radical i n c o m b u s t i o n systems (24). I n fact, a l t h o u g h d i r e c t t r o p o s p h e r i c fluorescence H O m e a s u r e m e n t s w e r e first


330


J0

a b s o r p t ί on 300 Κ

I if 281

282

283

ι .il ι 284

R1 Q| I I 1 1 i R2I Q2tl—I—I I L Pi I LII I I 1 IR2 Q2 Q 1LL4^J|

J


0 0

L 1

1

! 2

285

I

P2 Pf2

absorption 300 Κ

t

307

308

318

309

31 1

LL_L 1 I » Rl i l l ! R2! l_l

Ql » » » ι » » •21 » » I 1 "I » Pî L L J R2 Q2 L L U Ql2 III I 1 I

1

ί

J J

P2 P12

Figure 1. Portions of the UV excitation spectra of HO ( N " = 1 to 6) at 300 K; line positions are from Dieke and Crosswhite (29) and Einstein Β coefficients are from Chidsey and Crosley (47). (Top) 1 "-1

BEAM EXPANDER

I

(Γ

ι

\ Downloaded by CORNELL UNIV on August 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1993 | doi: 10.1021/ba-1993-0232.ch012

ν—

MODELOCKEO

C3

REFLECTOR

λ

YY::1 POWER SUPPLY PM-HV

PM S.DiSC

MONITOR MOTOR [D6PLAY

DOUBLE MONOCHROMATOR

4«H TRIGGER

ADC

LSI 11/2 COMPUTER

PLOTTER FLOPPY DISC

Figure 6. Diagram of LPA instrument. The reflector is at a distance of up to 5 km from the rest of the instrument. The slotted disk-photomultiplier (PMS.DISC) has been replaced by a diode array. (Reproduced with permission from reference 38. Copynght 1984.) L a m b e r t - B e e r e q u a t i o n (equation 14). W i t h the p r o v i s i o n of a reference H O absorption s p e c t r u m , a n d w i t h care to a v o i d local i n s t r u m e n t a l artifacts that affect the two beams differently, this design allows the r e m o v a l of extraneous a t m o s p h e r i c absorption features w i t h o u t r e q u i r i n g assignment to k n o w n a b s o r b i n g species. T h e F r a n k f u r t L P A i n s t r u m e n t (51-53) departs from b o t h of these i n struments i n two p r i n c i p a l ways: it achieves the necessary p a t h l e n g t h w i t h i n a 6 - m folded-path c e l l , a n d i t r a p i d l y scans a n a r r o w - b a n d f r e q u e n c y - d o u b l e d d y e laser across the spectral r e g i o n of interest (the Qi(2) l i n e group) i n a process sometimes c a l l e d differential o p t i c a l a b s o r p t i o n s p e c t r o m e t r y ( D O A S ) . T h e scanning rate is sufficient to ensure that the o b s e r v e d air v o l u m e is c h e m i c a l l y a n d p h y s i c a l l y stationary d u r i n g each scan (the baseline standard d e v i a t i o n is less t h a n 2 x 10" for a 0.2-ms scan). T h e laser o u t p u t is actively feedback-stabilized to p r o v i d e a flat spectral baseline, a n d a d e t e c t i o n l i m i t b e t t e r t h a n 10" i n o p t i c a l d e n s i t y has b e e n c l a i m e d . A s u m m a r y of p u b l i s h e d L P A configurations is g i v e n i n T a b l e I I . 4

5



41

88

54

Julich

NOAA

Frankfurt

Fast-scanning feedbackstabilized frequencydoubled ring dye laser pumped by A r ion laser

+

Frequencydoubled broadband tunable dye laser pumped by continuous wave A r ion laser XeCl pulsed

Laser Source

+

21

H 2

Measurement Challenges in Atmospheric Chemistry - ACS Publications

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