Measurement Methods for Peroxy Radicals in the Atmosphere

11 Measurement Methods for Peroxy

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Radicals in the Atmosphere Chris A. Cantrell, Richard E. Shetter, Anthony H. McDaniel, and Jack G. Calvert Atmospheric Kinetics and Photochemistry Group, Atmospheric Chemistry Division, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000

The measurement of peroxy radicals (RO ) in the atmosphere is an important and challenging problem. Determining the concentrations of HO and RO has been accomplished in the atmosphere and in the faboratory with systems that may be broadly grouped into two categories: chemical and spectroscopic. Several chemical conversion techniques and the use of spectroscopic methods in various wavelength regions are described. These approaches are critically evaluated for their potential use as atmospheric monitoring tools, primarily in the troposphere, although stratospheric applications are also mentioned. 2

2

2

THE CHEMISTRY OF THE TROPOSPHERE

is an i n t e r t w i n i n g o f cycles i n v o l v i n g gas-phase, condensed-phase, a n d m u l t i p l e - p h a s e reactions (1-8). I n o r d e r to u n d e r s t a n d the d i s t r i b u t i o n s (spatial a n d temporal) of a c h e m i c a l species, the i m p o r t a n t factors (i.e., sources, sinks, a n d c h e m i c a l reactions) that g o v e r n its b e h a v i o r m u s t b e u n d e r s t o o d . T h e roles p l a y e d b y free radicals i n the earth's atmosphere are m a n y a n d v a r i e d . O n e radical f a m i l y of p a r ticular interest is the o d d h y d r o g e n f a m i l y ( H O 4- H 0 ) . T h e organic p e r o x y a n d oxy radicals ( R 0 a n d R O ; R = C H , C H , etc.) are c h e m i c a l l y s i m i l a r to t h e o d d h y d r o g e n radicals a n d are i m p o r t a n t i n t e r m e d i a t e s i n the o x i d a t i o n of organic c o m p o u n d s i n the atmosphere (9). 2

2

3

2

5

P e r o x y radicals are f o r m e d i n the t r o p o s p h e r e t h r o u g h t h e i n t e r a c t i o n of s u n l i g h t w i t h c e r t a i n molecules or as p r o d u c t s of o t h e r radical reactions. 0065-2393/93/0232-0291$09.00/0

© 1993 American Chemical Society

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

292

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

I m p o r t a n t p e r o x y radical sources i n c l u d e the reactions o f the h y d r o x y l radical w i t h various c o m p o u n d s , for e x a m p l e , c a r b o n m o n o x i d e : HO + CO

>H + C0

2

(1)

HO + RH

>H 0 + R

(2)


alkanes:

R + 0

2

^ R 0

2

(3)

2

or alkenes:

HO + RCH=CHR — R C H ( O H ) C H R RCH(OH)CHR + 0

(4)

> RCH(OH)C(0 )HR

2

2

(5)

T h e s e equations demonstrate the l i n k that is e x p e c t e d b e t w e e n H O a n d R 0 i n the troposphere. H y d r o x y l radicals are f o r m e d i n processes i n i t i a t e d b y photolysis of various p r e c u r s o r s , for e x a m p l e , the u l t r a v i o l e t photolysis of 2

ozone ( 0 ) or nitrous a c i d ( H O N O ) . 3

0

3

+ hv

Ο CD) + H 0 2

H O N O + hv

> Ο

( D) L

+ 0

(6)

2

>2 HO

(7)

> HO + NO

(8)

P e r o x y radicals are also f o r m e d i n the troposphere t h r o u g h the photolysis of aldehydes (10, I I ) a n d t h r o u g h nitrate radical ( N 0 ) reactions (12-14). T h e h y d r o g e n a t o m a n d f o r m y l radical that are f o r m e d t h e n react w i t h m o l e c u l a r oxygen ( 0 ) (reactions 11 a n d 12) u n d e r t r o p o s p h e r i c c o n d i t i o n s . 3

2

C H 0 + hv 2

Η + HCO CH 0 + N0 2

HCO + 0

2

2

(9a)

>H

(9b)

2

+ CO

>HCO + HN0

3

Η + 0

>H + HCO

- ^

H0

2

>H0

2

3

(10) (11)

+ CO


(12)

CANTRELL ET AL.

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Measurement Methods for Peroxy

Radicak

293

T h e reactions o f alkenes w i t h ozone are also v e r y i m p o r t a n t sources o f t r o p o s p h e r i c peroxy radicals (15, 26):

CH

+ 0

2

• H COO*

3

2

—* H C 0 H *

CH OO* s

> 2H + C 0

2

—* H

HC0 H* 2

+ CO

2

(13) (14)

2

HC0 H*


+ CH 0

2

(15a)

2

(15b)

z

HC0 H* —

H 0 + CO

(15c)

HC0 H*

HC0 H

(15d)

2

2

2

2

T h e H atoms f o r m e d i n reaction 15a c a n react w i t h 0 (reaction 11) to f o r m H 0 . T h e s t a b i l i z e d C r i e g e e i n t e r m e d i a t e ( C H 0 0 ) can participate i n f u r t h e r reactions, some o f w h i c h w i l l result i n t h e f o r m a t i o n o f p e r o x y radicals. L a r g e r alkenes react w i t h ozone to p r o d u c e organic p e r o x y radicals. P e r o x y radicals p l a y m a n y roles i n t h e t r o p o s p h e r e . A reaction o f c r u c i a l i m p o r t a n c e is t h e oxidation o f n i t r i c oxide ( N O ) b y p e r o x y radicals. 2

2

2

R0

+ NO

2

>RO + N 0

2

(16a)

A second pathway i n this reaction results i n organic nitrate f o r m a t i o n (9, 1 7 , 16?). T h e size a n d structure of the organic group controls t h e y i e l d o f reaction 16b relative to 16a. R0

+ NO

2

RON0

(16b)

2

T h e s e a l k y l nitrate c o m p o u n d s have b e e n m e a s u r e d i n t h e t r o p o s p h e r e a n d c o n s t i t u t e d about 1.5% o f t h e total o d d n i t r o g e n b u d g e t at a r u r a l eastern U . S . site (19). R e a c t i o n 16a competes w i t h t h e oxidation o f N O b y ozone i n the t r o p o s p h e r e . 0

+ NO

3

>0

4- N 0

2

2

(17)

T h e 0 that is f o r m e d i n t h e troposphere is c o n t r o l l e d a p p r o x i m a t e l y b y the rate o f its generation b y N 0 p h o t o d e c o m p o s i t i o n (reactions 18 a n d 19) a n d b y t h e rate of its r e m o v a l b y reaction w i t h N O ; t h e c o n c e n t r a t i o n is r o u g h l y [ 0 ] = j o [ N 0 ] / ( f c [ N O ] ) (20). T h i s balance is i n f l u e n c e d b y t h e presence o f peroxy radicals because of t h e reaction s h o w n i n e q u a t i o n 16a. 3

2

3

N

2

2

17

N0

2

>NO + Ο

+'hv

o + o ^o 2

(18) (19)

3


294


(a)


15 f

«

.

H0 r2

,

Time, days

50,

,

(

Qf

0

,

s u m peroxy , , r

,

, t

1

,

,

I

2

.

I

3

4

· .

. •*

5

T i m e , days Figure 1. Hydroperoxy and organic peroxy radical concentrations as simulated for the marine boundary layer with (solid line) and without (dotted line) peroxy radical permutation reactions. (Reproduced with permission from reference 22. Copyright 1990 American Geophysical Union.)


11.

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295

Measurement Methods for Peroxy Radicals

E q u a t i o n s 17 t h r o u g h 19 g o v e r n the so-called N O - N 0 - 0 photostationary state system i n the atmosphere. M e a s u r e m e n t s o f k e y c o m p o n e n t s i n this system have b e e n u s e d to infer p e r o x y radical concentrations. 2

3


P e r o x y radicals are the o n l y gas-phase source o f p e r o x i d e c o m p o u n d s i n the t r o p o s p h e r e . H0

2

+ H0

2

> H 0

R0

2

+ H0

2

> ROOH + 0

2

2

+ 0

(20)

2

(21)

2

P e r o x y radicals are i n t e r m e d i a t e s i n the a t m o s p h e r i c o x i d a t i o n o f v i r t u a l l y a l l organic c o m p o u n d s . H 0 is soluble i n aqueous aerosols (21) a n d can participate i n a n u m b e r of oxidation reactions i n the aerosols. T h e o v e r a l l i m p o r t a n c e o f the aqueous-phase processes c o m p a r e d to t h e gas-phase c h e m istry is u n c e r t a i n . 2

T h e results o f c o m p u t e r simulations can be u s e d to estimate the degree o f s e n s i t i v i t y r e q u i r e d for m e a s u r e m e n t of the p e r o x y radicals i n the t r o p o s p h e r e . M a d r o n i c h a n d C a l v e r t (22) gave results o f 5-day s i m u l a t i o n s for free t r o p o s p h e r i c ("clean") a n d A m a z o n b o u n d a r y l a y e r ("moderately p o l l u t e d " ) conditions (Figures 1 a n d 2, respectively). T h e s o l i d a n d d o t t e d lines show the simulations w i t h a n d w i t h o u t reactions a m o n g the p e r o x y radicals s u m peroxy 5001

0

,

1

.

,

2

1

,

,

.

3

4

5

Time, days Figure 2. Organic peroxy radical concentrations as simulated for the moderately polluted Amazon boundary layer with (solid line) and without (dotted line) peroxy radical permutation reactions. (Reproduced with permission from reference 22. Copyright 1990 American Geophysical Union.)


296


t h e m s e l v e s , r e s p e c t i v e l y . F o r this d i s c u s s i o n , w e concentrate o n t h e s o l i d lines. T h e total m i d d a y m a x i m u m peroxy radical concentrations v a r y f r o m about 7 Χ 1 0 m o l e c u l e s / c m (28 parts p e r t r i l l i o n (ppt) b y v o l u m e m i x i n g ratio) for t h e clean conditions to a r o u n d 5 Χ 1 0 m o l e c u l e s / c m (200 ppt) for t h e m o r e p o l l u t e d case. T h e H 0 c o n c e n t r a t i o n is about o n e - t h i r d a n d one-fifth o f the total p e r o x y radical c o n c e n t r a t i o n for t h e t w o cases, respec t i v e l y . M o d e l s o f stratospheric concentrations o f H 0 (see F i g u r e 8, details discussed later) that use k i n e t i c s from t h e evaluations o f D e m o r e et a l . (23) indicate concentrations o f 3 Χ 1 0 , 1 Χ 1 0 , a n d 3 x 1 0 m o l e c u l e s / c m for altitudes o f 20, 4 0 , a n d 60 k m , r e s p e c t i v e l y . T h u s , v e r y sensitive t e c h n i q u e s w i l l b e r e q u i r e d to measure t h e d i u r n a l cycles a n d spatial variations of the p e r o x y radicals i n t h e troposphere o r i n t h e stratosphere. 8

3

9

3

2

2


6

7

6

3

Measurement Methods T h e discussion that follows is d i v i d e d i n t o t w o sections. T h e Spectroscopic M e t h o d s section i n c l u d e s those m e a s u r e m e n t t e c h n i q u e s that i n v o l v e t h e i n t e r a c t i o n o f a p h o t o n w i t h a peroxy radical. T h e C h e m i c a l C o n v e r s i o n M e t h o d s section describes t h e m e a s u r e m e n t o f another m o l e c u l e o r radical to w h i c h a p e r o x y radical has b e e n c o n v e r t e d .

Spectroscopic Methods.

H 0 a n d t h e other p e r o x y radicals have 2

characteristic absorptions d u e to various m o l e c u l a r processes. I n p r i n c i p l e , these spectroscopic features c o u l d b e u s e d to d e t e r m i n e a t m o s p h e r i c c o n centrations o f p e r o x y radicals. T h e discussion o f spectroscopic t e c h n i q u e s i n the m e a s u r e m e n t o f peroxy radicals is d i v i d e d into descriptions o f specific spectral regions. G e n e r a l issues r e l a t e d to t h e use o f spectroscopy for q u a n titative analysis are p r e s e n t e d next. T h e basis o f a b s o r p t i o n spectroscopy is straightforward. R a d i a t i o n o f the d e s i r e d w a v e l e n g t h is passed t h r o u g h a c e l l (or a t m o s p h e r i c a i r mass) c o n t a i n i n g t h e gas o f interest. T h e a m o u n t o f l i g h t a b s o r b e d is d e t e r m i n e d b y c o m p a r i s o n w i t h a reference m e a s u r e m e n t taken w i t h t h e c e l l e m p t y o r w i t h the gas o f interest r e m o v e d (more difficult for a t m o s p h e r i c sampling!). F r o m the B e e r - L a m b e r t l a w r e l a t i n g t h e c e l l p a t h l e n g t h (/), t h e a b s o r p t i o n cross section (σ) at w a v e l e n g t h λ , a n d t h e i n t e n s i t y o f l i g h t for t h e reference ( i ) a n d t h e sample (I), the concentration ( C ) c a n b e d e t e r m i n e d : 0

C =

^Ml σ(λ)/

(22)

T h i s p r o c e d u r e is r e p e a t e d for a l l t h e wavelengths o f interest. F o r this a p p r o a c h to b e effective, o n e m u s t e i t h e r b e able to find a r e g i o n o f t h e s p e c t r u m that is free o f interference d u e to absorption b y other a t m o s p h e r i c m o l e c u l e s o r o n e m u s t b e able to compensate for t h e i n t e r f e r i n g a b s o r p t i o n


11.

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297

b y some m e t h o d (spectral subtraction or spectral fitting, for example). T h e p r o b l e m o f spectral o v e r l a p w i t h o t h e r m o l e c u l e s often l i m i t s the use o f a b s o r p t i o n spectroscopy i n a t m o s p h e r i c m e a s u r e m e n t s . F o r a t m o s p h e r i c measurements w h e r e m e a s u r e m e n t o f i can b e difficult o r i m p o s s i b l e , dif ferential absorption can b e u s e d . H e r e the difference i n a b s o r p t i o n b e t w e e n the peak a n d v a l l e y i n a spectral feature is u s e d , a n d this a p p r o a c h e l i m i n a t e s the r e q u i r e m e n t for an accurate reference s p e c t r u m . T h e d e g r e e o f s p e c t r a l o v e r l a p can also be m i n i m i z e d b y n a r r o w i n g the spectral features. T h i s step is often a c c o m p l i s h e d t h r o u g h r e d u c i n g the total pressure b y p u m p i n g the sample to be m e a s u r e d into a m e a s u r e m e n t c e l l . A t l o w e r total pressures, not o n l y are the l i n e w i d t h s s m a l l e r , b u t the peak cross sections are also larger; thus, this approach y i e l d s a d o u b l e benefit.


0

T h e r m a l e m i s s i o n spectroscopy can be u s e d i n m i d d l e - a n d f a r - i n f r a r e d spectral regions to m a k e stratospheric m e a s u r e m e n t s , a n d it has b e e n a p p l i e d to a n u m b e r of i m p o r t a n t m o l e c u l e s w i t h b a l l o o n - b o r n e a n d satellite-based d e t e c t i o n systems. I n this a p p r o a c h , the m o l e c u l e s of interest are p r o m o t e d to e x c i t e d states t h r o u g h collisions w i t h o t h e r m o l e c u l e s . T h e r e t u r n to the g r o u n d state is a c c o m p a n i e d b y the release o f a p h o t o n w i t h e n e r g y e q u a l to the difference b e t w e e n the q u a n t u m states of the m o l e c u l e . T h e r e f o r e , the e m i s s i o n s p e c t r u m is characteristic of a g i v e n m o l e c u l e . C a l c u l a t i o n o f the c o n c e n t r a t i o n can b e c o m p l i c a t e d because the e m i s s i o n m a y have o r i g i n a t e d f r o m a n u m b e r o f stratospheric altitudes, a n d this situation m a y n e cessitate the use of c o m p u t e r - b a s e d i n v e r s i o n t e c h n i q u e s (24-27) to r e t r i e v e a concentration profile. V i s i b l e a n d u l t r a v i o l e t fluorescence spectroscopy can also be u s e d i n certain instances. I n this case the m o l e c u l e is p r o m o t e d to an e x c i t e d state w i t h a p h o t o n of energy that matches a transition. A f t e r a t i m e , the m o l e c u l e returns to the g r o u n d state, sometimes b y e m i s s i o n of a p h o t o n that has b e e n red-shifted (is of l o w e r energy) from the w a v e l e n g t h o f the e x c i t i n g p h o t o n . T h i s shift is d u e to the v i b r a t i o n a l relaxation that occurs as the m o l e c u l e loses e n e r g y i n collisions, u s u a l l y p l a c i n g the m o l e c u l e i n the lowest v i b r a t i o n a l l e v e l o f an e x c i t e d e l e c t r o n i c state. F l u o r e s c e n c e spectroscopy can b e v e r y sensitive, b u t t h e r e can be p r o b l e m s w i t h q u e n c h i n g a n d w i t h differentiating photons scattered b y air m o l e c u l e s a n d b y aerosols f r o m those photons that are d u e to fluorescence. I n d e e d , the net q u a n t u m y i e l d for fluorescence can be a strong f u n c t i o n of total p r e s s u r e , a n d this o b s e r v a t i o n has p r o m p t e d research of fluorescence spectroscopy i n cells at r e d u c e d p r e s sure. Ultraviolet Spectral Region. H 0 a n d the other p e r o x y radicals have an absorption i n the m i d d l e u l t r a v i o l e t spectral r e g i o n d u e to a t r a n s i t i o n of the n o n b o n d i n g e l e c t r o n to a σ-antibonding o r b i t a l ( η - σ * ) . U n f o r t u n a t e l y , these absorptions show few structural features, as s h o w n i n F i g u r e 3. T h i s lack of structure p r o b a b l y arises f r o m the dissociative nature o f the a b s o r p t i o n 2


298


»

12

CH C(0)0 3

/

/

\

/

\

/

/

V

3

\

/

\

/

\

/


2

\

200

220

240

Wavelength

260

280

(nm)

Figure 3. Ultraviolet cross sections for hydroperoxy and acetylperoxy radicak of Moortgat et al. (51) compared with cross sections for ozone of Molina and Molina (134).

process. F r o m an a t m o s p h e r i c standpoint absorption d u e to ozone dominates the u l t r a v i o l e t r e g i o n f r o m 200 to 300 n m . T h e p r o b l e m is c o m p o u n d e d b y the fact that the concentration of ozone is t y p i c a l l y 50 to 2000 times greater than the peroxy r a d i c a l concentration, so e v e n at the m i n i m u m of the ozone s p e c t r u m (210 nm) the absorption d u e to ozone w o u l d be 7 to 300 times greater than that d u e to H 0 . O t h e r u l t r a v i o l e t - a b s o r b i n g c o m p o u n d s (e.g., various hydrocarbons) w i t h u n s t r u c t u r e d spectra i n this r e g i o n m a k e the use of absorption spectroscopy for the m e a s u r e m e n t of a t m o s p h e r i c p e r o x y r a d icals i m p r a c t i c a l . U l t r a v i o l e t absorption spectroscopy has b e e n u s e d suc cessfully i n laboratory studies ( I I , 28-57), w h e r e the reaction m i x t u r e can be c o n t r o l l e d (i.e., i n the absence of ozone) a n d I can easily be m e a s u r e d . 2

0

U l t r a v i o l e t absorption spectroscopy has b e e n a p p l i e d to the m e a s u r e m e n t of some t r o p o s p h e r i c m o l e c u l e s , i n c l u d i n g C H 0 , H O , 0 , N 0 , a n d H O N O (58-60), i n the spectral r e g i o n from about 300 to 400 n m . T h e s e measurements are most r e l i a b l e i f the r e s o l u t i o n of the i n s t r u m e n t is less than or e q u a l to the absorber l i n e w i d t h s for the g i v e n e x p e r i m e n t a l c o n ditions. F i g u r e 4 shows a section of the h i g h l y s t r u c t u r e d f o r m a l d e h y d e s p e c t r u m (61). T h i s structure is c r u c i a l i n o r d e r to u n a m b i g u o u s l y identify a n d quantify a t r o p o s p h e r i c m o l e c u l e . T h e p o t e n t i a l p r o b l e m s of spectral o v e r l a p w i t h i n t e r f e r i n g species can be i g n o r e d for the p u r p o s e of c a l c u l a t i n g a d e t e c t i o n l i m i t for H O i n an absorption e x p e r i m e n t c o n d u c t e d u n d e r i d e a l conditions. I n this case, for a 2 0 0 - m optical p a t h , a m i n i m u m m e a s u r a b l e absorbance of 10~ , a n d a w a v e l e n g t h of 210 n m , about 1 Χ 1 0 m o l e c u l e s / c m of H 0 c o u l d b e detected. T h i s n u m b e r is a p p r o x i m a t e l y 5 times the 2

3

z

3

3

10

2


2

11.

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299


Wavelength (nm) Φ

4.0

Ε

3.0

320

315

C

.2 "ο Φ m

w

2 ο

32.0

31.5-

31.0

Wavenumber (10

3

1

cm"* )

Figure 4. Ultraviolet cross sections for formaldehyde from the study of Cantrell et al (61).

total peroxy radical c o n c e n t r a t i o n expected for a m o d e r a t e l y p o l l u t e d t r o p o s p h e r e (see F i g u r e 2). Researchers m a k i n g the a t m o s p h e r i c m e a s u r e m e n t s discussed for other i m p o r t a n t molecules have often u s e d o p t i c a l paths o f 5 k m or m o r e a n d u s e d i n s t r u m e n t a t i o n capable o f m e a s u r i n g absorbances i n the 10" range. U s i n g a 5 - k m p a t h l e n g t h results i n a c a l c u l a t e d d e t e c t i o n l i m i t of 4 Χ 10 m o l e c u l e s / c m of H 0 , sufficiently l o w to m a k e t r o p o s p h e r i c measurements u n d e r m o d e r a t e l y p o l l u t e d conditions. I n the use o f u l t r a v i o l e t absorption spectroscopy for the m e a s u r e m e n t o f t r o p o s p h e r i c H 0 , the p r o b l e m of spectral interference (overlap) is o f m o r e c o n c e r n t h a n the relatively small strength of the absorbance. T h i s spectral interference p r o b l e m p r e c l u d e s the use o f u l t r a v i o l e t absorption for t r o p o s p h e r i c H 0 a n d R 0 measurement. 4

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U l t r a v i o l e t fluorescence spectroscopy as d e s c r i b e d e a r l i e r cannot b e a p p l i e d to peroxy radicals because the a b s o r p t i o n leads almost e x c l u s i v e l y to dissociation. Photodissociation has b e e n e x p l o i t e d i n the laboratory to measure H 0 a n d C H 0 , because the fragments ( H O , C H 0 , etc.) f o r m e d i n the photodissociation can b e f o u n d i n e l e c t r o n i c a l l y e x c i t e d states, w h i c h can t h e n e m i t measurable r a d i a t i o n . T h i s so-called " p h o t o f r a g m e n t a t i o n " t e c h n i q u e has b e e n successfully a p p l i e d to the m e a s u r e m e n t o f a n u m b e r of m o l e c u l e s i n the atmosphere, i n c l u d i n g n i t r i c a c i d ( H N 0 ) , for w h i c h a d e t e c t i o n l i m i t of about 2.5 Χ 10 m o l e c u l e s / c m (0.1 ppb) has b e e n r e p o r t e d (62). T h e use o f photofragmentation to measure a t m o s p h e r i c p e r o x y radicals has not b e e n r e p o r t e d . 2

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Near-Infrared Spectral Region. A b s o r p t i o n s d u e to the p e r o x y radicals do n o t occur i n t h e v i s i b l e spectral r e g i o n . P e r o x y radicals d o have a weak near-infrared absorption d u e to a f o r b i d d e n t r a n s i t i o n to a l o w - l y i n g elec t r o n i c state, w h i c h i n H 0 is designated A' «— A " . T h e s e absorptions have b e e n o b s e r v e d b y H u n z i k e r a n d W e n d t (63) for H O a n d D 0 , as w e l l as for C H 0 a n d C H C H 0 (64) (see F i g u r e 5). T h e b a n d - p e a k cross sections seem to b e v e r y weak ( σ ~ 10~ c m / m o l e c u l e ) , a l t h o u g h t h e exact values are u n c e r t a i n . T h e weakness o f this a b s o r p t i o n m a y p r e c l u d e its use for a t m o s p h e r i c m o n i t o r i n g , a l t h o u g h t h e recent t e c h n o l o g i c a l i m p r o v e m e n t s i n n e a r - i n f r a r e d d i o d e lasers m a y make a b s o r p t i o n o r fluorescence w i t h n e a r infrared transitions viable options. T h e various organic p e r o x y radicals (e.g., C H 0 , C H 0 , etc.) m a y have slightly different n e a r - i n f r a r e d absorption features, a n d therefore speciation of various a t m o s p h e r i c p e r o x y radicals m a y also b e possible. 2

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If n e a r - i n f r a r e d d i o d e lasers have low-noise characteristics s i m i l a r to those o f m i d - i n f r a r e d d i o d e lasers, a n d thus m i n i m u m absorbances o f 10~ or less are possible, t h e n a n approximate d e t e c t i o n l i m i t c a n b e c a l c u l a t e d for a n absorption e x p e r i m e n t . F o r a 2 0 0 - m o p t i c a l p a t h , t h e calculated d e tection l i m i t is 5 Χ 1 0 m o l e c u l e s / c m , w h i c h is w e l l above levels o f H 0 e x p e c t e d to b e f o u n d i n t h e atmosphere. A n absorption e x p e r i m e n t i n this spectral r e g i o n a p p a r e n t l y w o u l d r e q u i r e e x t r e m e l y l o n g o p t i c a l p a t h lengths, a n d , i n d e e d , a calculation w i t h a 5 - k m p a t h y i e l d s a c a l c u l a t e d d e t e c t i o n l i m i t o f 2 Χ 1 0 m o l e c u l e s / c m , still r a t h e r h i g h for t r o p o s p h e r i c m e a s u r e m e n t s . O t h e r issues associated w i t h t h e use o f d i o d e lasers i n a b s o r p t i o n spectroscopy are discussed i n t h e next section. 5

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Middle-Infrared Spectral Region. T h e m i d d l e - i n f r a r e d spectral r e g i o n shows several features d u e to peroxy radicals, k n o w n at least since 1963 (6574). T h e v b a n d o f H 0 is s h o w n i n F i g u r e 6. I n f r a r e d a b s o r p t i o n spec troscopy has b e e n u s e d successfully i n laboratory m e a s u r e m e n t s o f H 0 kinetics (68, 75-78). D i o d e - l a s e r - b a s e d i n f r a r e d a b s o r p t i o n spectroscopy has also b e e n a p p l i e d to measurements o f a n u m b e r o f trace gases i n t h e at mosphere, including C H , N O , N 0 , N 0 , C H 0 , H N 0 , H C l , H 0 , 0 , H 0 , a n d C 0 (79-83). T h i s d i o d e laser a b s o r p t i o n t e c h n i q u e has advantages o v e r other infrared-based techniques because o f the l o w noise levels asso c i a t e d w i t h t h e laser diode as a l i g h t source, a l l o w i n g absorptions o f 1 0 " o r l o w e r to b e m e a s u r e d , a n d also because o f the l i n e n a r r o w i n g (and c o n c o m itant peak cross section increase) associated w i t h e i t h e r p u m p i n g t h e a i r sample into a c e l l to a total pressure o f a f e w torrs (millipaseals) o r m a k i n g m e a s u r e m e n t s i n t h e stratosphere d i r e c t l y . T h i s l i n e n a r r o w i n g leads to e n h a n c e d selectivity (and i n c r e a s e d sensitivity) because o f r e d u c e d o v e r l a p w i t h possible i n t e r f e r i n g absorptions. 3

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T h e i n f r a r e d s p e c t r u m o f H 0 is h i g h l y s t r u c t u r e d , a n d this feature p o t e n t i a l l y allows t h e absorption d u e to H O to b e differentiated f r o m o t h e r a t m o s p h e r i c species. S o m e infrared absorption data are also available for 2

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Figure 5. Near-infrared absorption spectra of HO2 (top), CH 0 (middle), and CH3CH2O2 (bottom) from the study of Hunziker and Wendt (64). (Reproduced with permission from reference 64. Copyright 1976 American Institute of Physics.) 3

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Figure 6, Infrared absorption spectrum of v band of H0 holder (74). 3

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organic peroxy radicals (84). Z a h n i s e r et a l . (73) r e p o r t e d l i n e strengths for H 0 for n e a r l y c o i n c i d e n t l i n e pairs i n the v b a n d of H 0 at 1371.927 a n d 1411.180 c m " a n d suggest t h e y c o u l d possibly be u s e d for laboratory a n d field measurements of H 0 . T h e l i n e pairs have c o m b i n e d l i n e intensities of 1.2 X 10~ c m m o l e c u l e " c m " . A peak cross section of 5 Χ 1 0 " c m / m o l e c u l e is estimated for a D o p p l e r - w i d t h l i n e of this strength (the peak cross section c o u l d be s m a l l e r d e p e n d i n g o n the total p r e s s u r e ; this r e p r e sents the best case). T h i s estimate results i n a c a l c u l a t e d d e t e c t i o n l i m i t for H 0 of 2.5 X 1 0 m o l e c u l e s / c m (ambient concentration) i n a d i o d e laser absorption e x p e r i m e n t w i t h a m i n i m u m detectable absorbance of 10" , a c e l l pressure of 30 torr (3390 Pa), a n d a 2 0 0 - m o p t i c a l p a t h l e n g t h . T h i s c o n c e n tration is a p p r o x i m a t e l y w h a t is expected for a m i d d a y m a x i m u m v a l u e i n a m o d e r a t e l y p o l l u t e d atmosphere. C l e a r l y , l o n g e r o p t i c a l paths or l o w e r a b sorbance l i m i t s or b o t h w o u l d be r e q u i r e d to measure a t m o s p h e r i c H 0 b y diode-laser-based i n f r a r e d absorption spectroscopy w i t h reasonable s i g n a l to-noise ratios. T h e d e t e c t i o n l i m i t for a 5 - k m o p t i c a l p a t h is 1 X 10 m o l ecules/cm . 2

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s u r e m e n t s of H O have b e e n r e p o r t e d f r o m the m e a s u r e m e n t o f e m i s s i o n from t h e r m a l l y p o p u l a t e d rotational levels i n H 0 . T r a u b et a l . (85) u s e d a b a l l o o n - b o r n e far-infrared s p e c t r o m e t e r to m e a s u r e H 0 from an a l t i t u d e o f 19 to 49 k m . A b a l l o o n o r satellite p l a t f o r m is r e q u i r e d for this a p p r o a c h , w h i c h is a p p l i e d to stratospheric m e a s u r e m e n t s because t h e t r o p o s p h e r e is essentially opaque i n the far-infrared r e g i o n . T h e y e v a l u a t e d R - b r a n c h r o tational lines at a r e s o l u t i o n of 0.04 c m " from 142 to 147 c m " a n d c o m p a r e d each l i n e to a s y n t h e t i c s p e c t r u m generated from a l a y e r e d m o d e l a t m o s p h e r e ( F i g u r e 7). T h e a m o u n t of H 0 i n the m o d e l was adjusted u n t i l the leastsquares difference b e t w e e n the m o d e l a n d the m e a s u r e m e n t was m i n i m i z e d . E s t i m a t e d d e t e c t i o n l i m i t s for this t e c h n i q u e are about 1 Χ 1 0 m o l e c u l e s / c m (6 ppt) near 20 k m a n d about 5 Χ 10 m o l e c u l e s / c m (30 ppt) near 50 k m . T h e d e r i v e d day a n d n i g h t H 0 profiles w e r e c o m p a r e d w i t h results of a p h o t o c h e m i c a l m o d e l based o n k i n e t i c s from t h e evaluations of D e m o r e et al. (23) ( F i g u r e 8). T h e d a y t i m e m o d e l e d profile agrees w i t h m e a s u r e m e n t s u p to an a l t i t u d e o f 40 k m , a n d is 3 0 % b e l o w the m e a s u r e m e n t s above 4 0 k m . T h e n i g h t t i m e m e a s u r e m e n t s are m u c h less t h a n those f r o m t h e d a y t i m e , as e x p e c t e d from t h e o r y , a n d are i n g e n e r a l a g r e e m e n t w i t h the m o d e l . £

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Wavenumber (cm ) Figure 7. A, Atmospheric emission spectrum of region near that used by Traub et ah (85) to quantify stratospheric H0 concentrations. B , A laboratory H0 spectrum. C , The best-fit calculated spectrum. (Reproduced with permission from reference 85. Copyright 1990 American Association for the Advancement of Science.) 2


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de Zafra et a l . (86) m e a s u r e d H 0 b y u s i n g g r o u n d - b a s e d m i l l i m e t e r wave spectroscopy a n d three rotational e m i s s i o n lines near 265.8 G H z (~8.9 cm" ). T h e y c o m p a r e d t h e i r results to a p h o t o c h e m i c a l m o d e l b a s e d o n kinetics from the evaluations o f D e m o r e et al. (23) a n d f o u n d g o o d a g r e e m e n t above 35 k m . T r a u b et a l . (85) c o m p a r e d t h e i r far-infrared m e a s u r e m e n t s to the m i l l i m e t e r - w a v e m e a s u r e m e n t s a n d to t w o i n situ results, to b e d i s c u s s e d i n f o l l o w i n g sections. T h e y f o u n d the m i l l i m e t e r - w a v e m e a s u r e m e n t s to b e about 1 8 % larger than those of the t h e o r e t i c a l profile f r o m 37 to 67 k m . T h e e s t i m a t e d accuracy of this t e c h n i q u e is about ± 2 5 % t h r o u g h o u t t h e range from 37 to 67 k m . T h i s accuracy corresponds to an u n c e r t a i n t y o f about 4 X 1 0 m o l e c u l e s / c m at 37 k m to 6 x 1 0 m o l e c u l e s / c m at 67 k m . T h i s m e t h o d o f H 0 m e a s u r e m e n t is sensitive to the c o l u m n a b u n d a n c e a n d the b r o a d shape of the H 0 d i s t r i b u t i o n i n the u p p e r stratosphere a n d l o w e r 2

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mésosphère, b u t not to the details of the d i s t r i b u t i o n . A s seen i n F i g u r e 8, the m i l l i m e t e r - w a v e results m a t c h the shape of the t h e o r e t i c a l c u r v e w e l l .


N o data have b e e n p r e s e n t e d to date o n the u t i l i t y of far-infrared o r m i l l i m e t e r - w a v e spectroscopy for o t h e r p e r o x y radicals o r o n its possible use i n the troposphere w h e n c o m b i n e d w i t h a l o w - p r e s s u r e a b s o r p t i o n c e l l . Laser Magnetic Resonance and Electron Paramagnetic Resonance. T h i s category i n " s p e c t r o s c o p i c " techniques of p e r o x y radical m e a s u r e m e n t is somewhat different f r o m the others, i n that the sample m u s t b e p l a c e d i n a magnetic field i n o r d e r for the absorptions to occur. L a s e r m a g n e t i c resonance o f H 0 is based o n the absorption of far-infrared laser r a d i a t i o n (87). T y p i c a l l y , a gaseous sample is p u m p e d t h r o u g h a section of the c a v i t y of a c o n t i n u o u s - w a v e gas laser o p e r a t i n g at far-infrared w a v e l e n g t h s . A spect r u m is o b t a i n e d b y scanning the magnetic field s t r e n g t h to b r i n g the acc i d e n t a l near-resonance of the m o l e c u l a r transitions i n t o resonance w i t h a fixed w a v e l e n g t h laser l i n e t h r o u g h the Z e e m a n effect. T h e a m o u n t of a b sorption is d e t e r m i n e d b y m o n i t o r i n g the laser p o w e r w i t h an i n f r a r e d d e tector. T h i s t e c h n i q u e has b e e n l i m i t e d to laboratory studies of the spectroscopy a n d kinetics of H 0 a n d o t h e r radicals (88-92) a n d m a y not b e applicable to a m b i e n t measurements because o f the mass a n d p o w e r r e q u i r e d to generate the magnetic f i e l d . T h e d e t e c t i o n l i m i t r e p o r t e d for the H o w a r d a n d E v e n s o n study (88) was 2 Χ 1 0 m o l e c u l e s / c m , a l t h o u g h t h e y p o i n t out that this v a l u e is d e p e n d e n t o n the e x p e r i m e n t a l c o n d i t i o n s , s u c h as the gas pressure a n d the magnetic m o d u l a t i o n a m p l i t u d e . T h e p r o b l e m of b r i n g i n g a large magnet i n t o the field for a m b i e n t m e a surements has b e e n o v e r c o m e i n e l e c t r o n paramagnetic resonance ( E P R , also c a l l e d e l e c t r o n s p i n resonance, E S R ) b y M i h e l c i c , H e l t e n , a n d co w o r k e r s (93-99). T h e y c o m b i n e d E P R w i t h a m a t r i x isolation t e c h n i q u e to a l l o w the s a m p l i n g a n d radical quantification to o c c u r i n separate steps. T h e matrix isolation is also r e q u i r e d i n this case because E P R is not sensitive e n o u g h to measure p e r o x y radicals d i r e c t l y i n the a t m o s p h e r e . E P R spec troscopy has also b e e n u s e d i n laboratory studies of p e r o x y r a d i c a l reactions (100, 101). 2

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E P R is based o n the s p l i t t i n g o f m a g n e t i c e n e r g y levels caused b y the action of a magnetic field o n an u n p a i r e d e l e c t r o n i n a m o l e c u l e . T y p i c a l l y the magnetic field strength is o n the o r d e r of 3500 G , a n d s w e e p coils a l l o w the field to be v a r i e d over a small range so the r a d i a t i o n can be m a d e resonant w i t h the transition. T h e radiation is i n the m i c r o w a v e r e g i o n at about 9.5 G H z (0.3 cm" ). O t h e r techniques s u c h as m a g n e t i c field m o d u l a t i o n w i t h l o c k - i n d e t e c t i o n i m p r o v e the signal-to-noise ratio o f these m e a s u r e m e n t s . Because o f the i n t e r a c t i o n o f the e l e c t r o n s p i n w i t h n e a r b y n u c l e a r spins, h y p e r f i n e s p l i t t i n g patterns i n the spectra a l l o w differentiation b e t w e e n the various radicals present. T h i s p r o p e r t y has b e e n e x p l o i t e d i n the most r e c e n t of the a t m o s p h e r i c measurements of M i h e l c i c et a l . (99). 1


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T h e matrix isolation p r o c e d u r e relies o n the c o n d e n s a t i o n o f H 0 a n d C O o n a l i q u i d - n i t r o g e n - c o o l e d c o l d finger to f o r m a stable m a t r i x for radicals a n d other a t m o s p h e r i c species ( F i g u r e 9). T y p i c a l l y about 20 L of air are r e q u i r e d to achieve the d e s i r e d sensitivity for a m b i e n t m e a s u r e m e n t s . A matrix o f d e u t e r a t e d w a t e r ( D 0 ) n a r r o w e d the E P R l i n e w i d t h s a n d i m p r o v e d the signal-to-noise ratio, a n d thus this m a t r i x has b e e n u s e d for m e a s u r e m e n t s since O c t o b e r 1982. R e c e n t i m p r o v e m e n t s i n the use of this matrix i s o l a t i o n - E P R t e c h n i q u e have b e e n i n the analysis of the spectra. T h e analysis o f the s p e c t r u m is a m u l t i p l e - s t e p process. T h e first step is the r e m o v a l o f the c o n t r i b u t i o n to the s p e c t r u m d u e to the s a m p l i n g apparatus. T h e next step is subtraction o f the r e l a t i v e l y large c o n t r i b u t i o n d u e to N 0 . T h e r e m a i n i n g absorptions are d u e to H 0 , o t h e r organic p e r o x y radicals, a n d possibly other u n k n o w n radicals. A n u m e r i c a l f i t t i n g p r o c e d u r e 2

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Figure 9. Cold finger sampling apparatus used in matrix isolation-EPR measurements of Mihelcic et al. (97). (Reproduced with permission from reference 97. Copyright 1985 Yluwer Académie.)


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that has b e e n d e v e l o p e d successfully quantifies the amounts o f the H 0 , C H C ( 0 ) 0 , a n d the s u m of the o t h e r R 0 radicals (98). T h i s s e q u e n c e is s h o w n g r a p h i c a l l y i n F i g u r e 10. T h e early calibrations o f this m e t h o d w e r e p e r f o r m e d b y u s i n g the t h e r m a l d e c o m p o s i t i o n o f p e r o x y a c e t y l n i t r a t e ( P A N ) , w h i c h generates peroxyacetyl radicals, a l t h o u g h c a l i b r a t i o n o f o t h e r p e r o x y radicals ( H 0 , C H 0 , etc.) has since b e e n p e r f o r m e d . T h e P A N d e c o m p o s i t i o n reaction y i e l d s one peroxy radical a n d one N 0 m o l e c u l e that is also m e a s u r e d , a n d thus results, i n p r i n c i p l e , i n an absolute c a l i b r a t i o n . 2

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2


2

T h e m a t r i x i s o l a t i o n - E P R t e c h n i q u e ( M I E S R ) has p r o v e n to b e a useful m e t h o d o f peroxy radical m e a s u r e m e n t i n the a t m o s p h e r e . T h e m a t r i x isolation results s h o w n i n F i g u r e 8 are f r o m m e a s u r e m e n t s r e p o r t e d i n 1984 (96), a n d significant i m p r o v e m e n t s have since b e e n m a d e i n this t e c h n i q u e , a l t h o u g h n e w concentration profiles s u c h as those i n F i g u r e 8 have not yet b e e n r e p o r t e d . T h e drawbacks to this m e t h o d are a moderate d e t e c t i o n l i m i t (a few parts p e r t r i l l i o n b y v o l u m e m i x i n g ratio, a p p r o x i m a t e l y 1 0 m o l e c u l e s / c m ) , w h i c h l i m i t s its usefulness i n r e m o t e m e a s u r e m e n t situations; the r e q u i r e m e n t of r e m o v i n g the absorption d u e to N 0 , w h i c h l i m i t s m e a surements to situations w i t h r e l a t i v e l y l o w N 0 levels; a n d the fairly l o n g integration times r e q u i r e d (of the o r d e r of 0.5 to 2 h). 8

3

2

2

T h e t e c h n i q u e o f s p i n - t r a p p i n g radicals has b e e n a p p l i e d to the m e a s u r e m e n t o f a t m o s p h e r i c h y d r o x y l b y W a t a n a b e et a l . (102), a l t h o u g h t h e r e are no reports of its use for peroxy radicals. T h e p r i n c i p l e i n v o l v e s the reaction of the radical of interest w i t h an organic n i t r o n e i m m o b i l i z e d o n a filter p a p e r o r o t h e r substrate. T h e sample is r e t u r n e d to the laboratory, a n d the n i t r o n e - r a d i c a l p r o d u c t is d i s s o l v e d i n a suitable solvent a n d m e a s u r e d w i t h E P R . T h e disadvantages o f the s p i n - t r a p p i n g t e c h n i q u e are difficulty i n f i n d i n g suitable organic n i t r o n e c o m p o u n d s a n d the fact that most of these molecules are p h o t o c h e m i c a l l y unstable. Mass Spectrometry. M a s s s p e c t r o m e t r i c d e t e c t i o n was u s e d i n e a r l y laboratory studies of H 0 (103, 104) a n d has also b e e n u s e d i n m o r e recent investigations (J05). T h e H 0 peak at the mass-to-charge ratio mie = 33 is useful i n laboratory identification a n d quantification, b u t i n the a t m o s p h e r e , most l i k e l y m u l t i p l e species w i l l interfere because of, for e x a m p l e , fragm e n t a t i o n of h y d r o c a r b o n s , h y d r o g e n p e r o x i d e , or oxygen isotopes. 2

2

Chemical Conversion Methods. Laser-Induced and Resonance Fluorescence of HO. C o n s i d e r a b l e effort has b e e n a p p l i e d to the m e a s u r e m e n t of H O i n the stratosphere a n d troposphere. U l t r a v i o l e t fluorescence t e c h n i q u e s based o n lasers or resonance lamps have r e c e i v e d a great d e a l o f attention a n d study. Because H O concentrations are t y p i c a l l y factors o f o n e t e n t h to o n e - h u n d r e d t h those o f H 0 i n the a t m o s p h e r e , the difficulties associated w i t h m a k i n g H O measurements b y u s i n g fluorescence [low s i g n a l to-noise ratio, laser-generated H O , b a c k g r o u n d fluorescence, etc.; see the 2


308

M E A S U R E M E N T C H A L L E N G E S IN ATMOSPHERIC CHEMISTRY

2.04284

1.98313

1

»

»


ι

2.01255

—J

I

I

100

I

I

300

1 f:9283.06 MHz

I

I

500

g-Value

1.9S457

I

700

1

1

900

Magnetic Field (Channels) Figure 10. Illustration of spectral analysis from the matnx isolation-EPR measurements of Mihelcic et al. (99). The curves correspond to A , an ESR spectrum of a sample collected on July 5,1986, from 11:39 to 12:09 (CET); Β, spectrum of sample holder; C, difference of A and B; D, NO2 reference spec trum; E, Difference of C and D magnified by a factor of 3; F, sum of H0 (13 ppt), CH C(0)0 (16 ppt), CH 0 (15 ppt), C H 0 (11 ppt), and C H 0 (60 ppt) as retrieved by the fit (magnified by a factor of 3); G through K, amounts of H0 , CH C(0)0 , CH 0 , C H 0 , and C H 0 , all magnified by a factor of 3, as retrieved by the multiple fit; and L, residuals after subtraction of F from E. (Reproduced with permission from reference 99. Copyright 1990 Kluwer Academic.) 2

3

2

2

3

3

2

3

2

2

2

2

5

2

4

5

9

2

4

9

2

2


11.



309

discussion of S m i t h a n d C r o s l e y (106)] b e c o m e m u c h less i m p o r t a n t w h e n a m e t h o d of c o n v e r t i n g H 0 to H O q u a n t i t a t i v e l y is f o u n d . T h e reaction o f H 0 w i t h N O is one means of a c c o m p l i s h i n g this c o n v e r s i o n . 2

2

H0

2

+ NO

> HO + N0

(23)

2

A n d e r s o n et a l . (107) r e p o r t e d a series of H 0 measurements i n the stratosphere b e t w e e n 29 a n d 37 k m ; they i n d u c e d reaction 23 a n d r e c o r d e d resonance fluorescence d e t e c t i o n of H O at 309 n m . T h e e x c i t i n g r a d i a t i o n for this m e a s u r e m e n t was generated b y m i c r o w a v e discharge of a H e - H 0 m i x t u r e . T h e m e a s u r e d m i x i n g ratios w e r e c o m p a r e d to the p h o t o c h e m i c a l m o d e l of L o g a n et a l . (I). T h e m e a n H 0 levels w e r e systematically h i g h e r than those o f the m o d e l , although the range o f values m e a s u r e d o v e r l a p p e d the m o d e l results. A c o m p a r i s o n w i t h c o m p u t e r simulations is also s h o w n i n F i g u r e 8 [ m o d e l results r e p o r t e d b y T r a u b et a l . (85)], again i n d i c a t i n g m e a s u r e d values h i g h e r than those of the m o d e l . O n e aspect of this t e c h n i q u e , w h i c h was apparently not r e c o g n i z e d i n the o r i g i n a l p a p e r , is that m e t h y l p e r o x y radicals are also c o n v e r t e d to H O b y the f o l l o w i n g sequence along w i t h reaction 24, although reaction 25 may be sufficiently slow u n d e r stratospheric conditions (low t e m p e r a t u r e a n d l o w 0 concentration) that its c o n t r i b u t i o n is n e g l i g i b l e . 2


2

2

2

CH 0 3

2

+ NO

CH3O + 0

>CH 0 + N0 3

>H0

2

2

(24)

2

+ CH 0

(25)

2

T h e use of l a s e r - i n d u c e d fluorescence ( L I F ) for t r o p o s p h e r i c H O a n d H 0 measurements was r e p o r t e d b y H a r d a n d co-workers (108-110), w h o d e v e l o p e d a fluorescence t e c h n i q u e based o n p u m p i n g the air sample into a l o w - p r e s s u r e c e l l ( F A G E ) a n d e x c i t i n g it w i t h a c o p p e r v a p o r l a s e r - p u m p e d dye laser w i t h a h i g h r e p e t i t i o n rate. T h e i r H 0 m e a s u r e m e n t s w e r e not made i n c o n j u n c t i o n w i t h e n o u g h other s u p p o r t i n g m e a s u r e m e n t s to a l l o w an accurate test of p h o t o c h e m i c a l models from the results. 2

2

Photofragment-Induced Emission. L e e a n d co-workers at San D i e g o State U n i v e r s i t y (111-113) used the photodissociation of H 0 at 147 n m to p r o d u c e excited H O radicals. T h e e m i s s i o n of r a d i a t i o n from H O has b e e n u s e d to quantify the H O concentration i n laboratory kinetics e x p e r i m e n t s : 2

a

H0

2

+ hv

HO (A 2 ) 2

+

> H O (A X ) + Ο

(26)

> H O ( Χ Π ) + hv ( 3 0 6 - 3 2 0 nm)

(27)

2

+

2

A s i m i l a r approach was d e s c r i b e d b y H a r t m a n n et a l . (114) for m e t h y l p e r o x y radicals i n laboratory photodissociation e x p e r i m e n t s . T h e e x c i m e r laser p h o -


310


tolysis at 248 n m p r o d u c e s H O ( A X ) , w h i c h emits r a d i a t i o n as s h o w n i n reaction 27. T h e s e photofragmentation t e c h n i q u e s m a y not b e a p p l i c a b l e to a t m o s p h e r i c measurements because of the large n u m b e r of possible i n t e r ferents at the p h o t o l y z i n g wavelengths, a l t h o u g h s u c h interferents have not yet b e e n d e m o n s t r a t e d .


2

+

Near-Infrared Chemiluminescence. T h e t e c h n i q u e of c h e m i l u m i n e s cence has b e e n most successfully a p p l i e d i n the a t m o s p h e r e to the m e a s u r e m e n t of N O a n d other oxides of n i t r o g e n t h r o u g h various c o n v e r s i o n p r o c e d u r e s . G l a s c h i c k - S c h i m p f et al. (115) a n d H o l s t e i n et a l . (116) r e p o r t e d results of laboratory k i n e t i c studies i n w h i c h the H 0 c o n c e n t r a t i o n was d e t e r m i n e d t h r o u g h the c h e m i l u m i n e s c e n c e reaction system s h o w n i n e q u a tions 28 a n d 29. It involves the same m o l e c u l a r transitions i n the n e a r infrared as discussed p r e v i o u s l y (for H 0 : e m i s s i o n f r o m the A ' state). 2

2

2

H0

2

+ 0 H0

( Δ) Χ

2

(Α Α') 2

2

•

>H0

2

(Α Α') + 0

>H0

2

(A X")

2

(28)

2

+ hv (1.43 μπι)

2

(29)

T h u s , i n a constant concentration of 0 ( Δ) that is m u c h greater that the concentration of H 0 , the radiation i n t e n s i t y at 1.43 μηι is p r o p o r t i o n a l to the H 0 concentration. T h i s i n f o r m a t i o n has not b e e n a p p l i e d to a t m o s p h e r i c m e a s u r e m e n t s or to other peroxy radicals to date. 2

ι

2

2

Chemical Amplification. T h e m e a s u r e m e n t of a s m a l l e l e c t r i c a l signal is often a c c o m p l i s h e d b y amplification to a larger, m o r e easily m e a s u r e d one. T h i s t e c h n i q u e of amplification can also be a p p l i e d to c h e m i c a l systems. F o r peroxy radicals, C a n t r e l l a n d S t e d m a n (117) p r o p o s e d , as a " p o s s i b l e " t e c h n i q u e , the c h e m i c a l c o n v e r s i o n of p e r o x y radicals to N 0 w i t h a m p l i fication (i.e., m o r e than one N 0 p e r peroxy radical). T h i s m e t h o d has also b e e n used for laboratory studies of H 0 reactions o n aqueous aerosols (21). T h e f o l l o w i n g c h e m i c a l s c h e m e was p r o p o s e d as the basis of the i n s t r u m e n t : 2

2

2

H0

2

+ NO

> HO + N0

HO + CO

> H + C0

(23)

2

(1)

2

T h i s c h a i n reaction converts N O a n d C O to N 0 a n d C 0 at a rate p r o p o r t i o n a l to the s u m of the H 0 a n d H O concentrations. T h e t e c h n i q u e also measures c e r t a i n organic peroxy radicals, because t h e y are c o n v e r t e d to H 0 after a few steps. P e r o x y radicals are c o n v e r t e d to H 0 a c c o r d i n g to the f o l l o w i n g reactions for p r i m a r y peroxy radicals: 2

2

2

2

2

RCH 0 2

2

+ NO

RCH 0 + 0 2

2

>RCH 0 + N0 2

> RCHO +

H0

2

2


(30) (31)

11.


CANTRELL ET AL.

311

a n d for secondary p e r o x y radicals: RR'CH0

+ NO

2

RR'CHO + 0

2

> RR'CHO + N 0 > RR'CO + H 0

(32)

2

(33)

2

T e r t i a r y p e r o x y radicals have no a l p h a h y d r o g e n that can be abstracted b y 0 i n the second step. H o w e v e r , a u n i m o l e c u l a r d e c o m p o s i t i o n of the alkoxy radical results i n the formation of a peroxy radical, w h i c h can b e m e a s u r e d .


2

RR'R"C0

+ NO

2

RR'R"CO R + 0

• RR'R"CO + N 0

(34)

2

> R + R'R"CO R0

2

(35) (3)

2

A r y l peroxy radicals m a y not He as effectively c o n v e r t e d because t h e analogous u n i m o l e c u l a r d e c o m p o s i t i o n reaction m a y not occur. A c y l p e r o x y radicals are c o n v e r t e d to H 0 i n a s i m i l a r fashion, s h o w n for acetylperoxy radicals: 2

CH C(0)0 3

+ NO

2

CH C(0)0 3

CH

3

+ 0

> CH C(0)0 + N 0 3

>CH

3

CH 0

2

3

+ C0

2

(36) (37)

2

(38)

2

T h e m e t h y l peroxy radical that is f o r m e d converts to H 0 i n the sequence for p r i m a r y peroxy radicals shown i n equations 30 a n d 31. T h e c h a i n r e a c t i o n is subject to t e r m i n a t i o n steps that i n c l u d e the f o l l o w i n g : 2

OH + NO H0

2

HONO

(39)

+ wall

» n o n r a d i c a l products

(40)

> H 0

(20)

H0

2

+ H0

2

H0

2

+ N0

2

2

2

+ 0

H0 N0 2

2

2

(41)

F o r most conditions e m p l o y e d i n the c u r r e n t i n s t r u m e n t , the c h a i n is t e r m i n a t e d b y reactions 39 a n d 40, a l t h o u g h reaction 41 can b e i m p o r t a n t for certain circumstances. F o r o p t i m u m conditions of about 3 p p m b y v o l u m e N O , 1 0 % v / v C O , a n d 5 s of reaction t i m e , c h a i n lengths (amplification factors) o f 500 to 1000 are possible (118-122). T h e m e a s u r e m e n t of N 0 (typically p p b b y v o l u m e levels) i n the p r e s ence of large N O a n d C O concentrations is the greatest analytical challenge 2


312


i n the use of this t e c h n i q u e . E a r l y attempts i n v o l v e d the use of optoacoustic spectroscopy i n w h i c h a c h o p p e d 4 8 8 - n m argon i o n laser b e a m i n d u c e s a pressure p u l s e p r o p o r t i o n a l i n strength to the N 0 c o n c e n t r a t i o n (see ref erences 123-125). H o w e v e r , this approach was not sensitive e n o u g h for t r o p o s p h e r i c peroxy radical m e a s u r e m e n t s . T h e m e a s u r e m e n t s r e p o r t e d i n the l i t e r a t u r e are a l l based o n the m e a s u r e m e n t of N 0 w i t h l u m i n o l c h e m i l u m i n e s c e n c e (126, 127). A l t h o u g h the p r e s e n c e of N O a n d C O does affect the strength o f the c h e m i l u m i n e s c e n t signal, N 0 c a l i b r a t i o n can b e p e r f o r m e d i n t h e i r presence. T h e l u m i n o l i n s t r u m e n t uses an aqueous, p H 12.5, 0.01 M N a S 0 , 1.0 Χ 10~ M l u m i n o l solution that is f l o w e d o v e r a disk of filter p a p e r , a n d the e m i s s i o n from the e x c i t e d 3-aminophthalate m o l e c u l e s p r o d u c e d i n the reaction is v i e w e d t h r o u g h a glass w i n d o w b y an e n d - o n p h o t o m u l t i p l i e r t u b e . L u m i n o l can be o x i d i z e d b y m o l e c u l e s o t h e r t h a n N 0 , i n c l u d i n g 0 , P A N (peroxyacetyl nitrate) (127, 128), a n d H 0 (129). T h e s e interferences are not a p r o b l e m i n the m e a s u r e m e n t of peroxy radicals because of the m o d u l a t i o n approach, w h i c h is d e s c r i b e d i n the f o l l o w i n g discussion. 2

2

2


2

2

4

3

3

2

2

O t h e r m o l e c u l e s i n the atmosphere o x i d i z e N O to N 0 aside f r o m peroxy radicals, ozone b e i n g the most abundant. T h e r e f o r e , a large b a c k g r o u n d is present from the N 0 p r o d u c e d because of the N O - o z o n e reaction as w e l l as f r o m a m b i e n t N 0 . T h i s b a c k g r o u n d m u s t be p e r i o d i c a l l y m e a s u r e d to ensure accurate d e t e r m i n a t i o n of the peroxy radical signal. T y p i c a l l y , o n e t h i r d to o n e - h a l f of the t i m e N is s u b s t i t u t e d for C O , a n d a l l those species that do not participate i n the c h a i n reaction are m e a s u r e d . T h u s , the p e r o x y radical m e a s u r e m e n t is the difference b e t w e e n this b a c k g r o u n d a n d the signal w i t h C O present. 2

2

2

2

A schematic diagram of the c h e m i c a l a m p l i f i e r system is s h o w n i n F i g u r e 11. T y p i c a l raw signals are s h o w n i n F i g u r e 12. A m b i e n t N 0 m o d u l a t i o n data from the 1988 Scotia Range f i e l d study (a r u r a l site near Scotia, P e n n sylvania) (130) are s h o w n i n F i g u r e 13. A b s o l u t e radical c a l i b r a t i o n is an area of research that w i l l h e l p b r i n g the c h e m i c a l a m p l i f i e r t e c h n i q u e to f r u i t i o n . T h e reports i n the literature have u s e d the s e c o n d - o r d e r decay of H 0 (reaction 16) i n the laboratory to d e t e r m i n e the peroxy radical s e n s i t i v i t y (119-122). A f u l l - f i e l d c a l i b r a t i o n p r o c e d u r e c u r r e n t l y u n d e r i n v e s t i g a t i o n utilizes a titration p r o c e d u r e to d e t e r m i n e the radical c o n c e n t r a t i o n i n a synthetic radical source. D e t e c t i o n l i m i t s for a 1-min averaging t i m e w i t h a stable b a c k g r o u n d signal are of the o r d e r of 1 Χ 10 m o l e c u l e s / c m w i t h the c u r r e n t system. 2

2

8

3

Missing Oxidant from Photostationary State Measurements. A s was discussed i n the i n t r o d u c t i o n , the photolysis of N 0 , the reaction of N O w i t h 0 , a n d the c o m b i n a t i o n of Ο atoms w i t h 0 f o r m the photostationary state system. If differential rate equations are w r i t t e n a n d s o l v e d a s s u m i n g 2

3

2


11.

C A N T R E L L E T AL.

313


Luminol =

D.l.hLO

Air

N0 Source 2

waste

m

Luminol :Air

H 0 Source 2

Liquid Pump

Scrubber Coil

2


Inlet Luminol N 0

2

Detector

H—h NO=j

PC waste

CO

N

b= waste 2

Figure 11. Schematic diagram of chemical amplifier system of Cantrell et al. (130).

a steady-state concentration for either N O or N 0 , a relation is derived that 2

relates the concentrations of N O , N 0 , 0 , and two kinetic parameters, 2

3

namely j o and k : N

2

u

JNO,[N0 ] 2

(42)

= [o ]

U N O ]

3

W h e n atmospheric measurements are p e r f o r m e d for [ N O ] , [ N 0 ] , [ 0 ] , j o , a n d t e m p e r a t u r e (to d e t e r m i n e fc ), the ratio o n the left o f e q u a t i o n 42 is t y p i c a l l y larger than the ozone concentration. I n other w o r d s , the [ N 0 ] / [ N O ] ratio is larger than w o u l d be expected o n the basis of the o t h e r values i n the e q u a t i o n . T h i s finding can be i n t e r p r e t e d i n terms of reaction 16a, w h i c h is the oxidation of N O to N 0 b y a peroxy radical. I f reaction 16a is i n c l u d e d i n the photostationary state system, the five m e a s u r e m e n t s m e n t i o n e d y i e l d an estimate of the total peroxy r a d i c a l concentration (weighted i n v e r s e l y b y the rate coefficient for the reaction w i t h N O ) . 2

N

2

3

17

2

2

*16a V

M N O J


(43)



314

Figure 12. Sample modulation data of chemical amplifier of Cantrell et al. (130), showing stability over a 1-h time period (top) and the time dependence on a shorter time scale (3 min).


11.

CANTRELL ET AL.

315


200


150 h >

α α

100

h

Ο

2

Figure 13. Six days ofN0 modulation data from the chemical amplifier system from the Scotia Range, Pennsylvania, study, summer 1988 (130). 2

T h e t e r m i n large brackets y i e l d s the so-called " m i s s i n g o x i d a n t " i n units of ozone c o n c e n t r a t i o n , a n d the rate constant ratio (k /k ) converts to units of peroxy radical c o n c e n t r a t i o n . N e a r r o o m t e m p e r a t u r e , k /k is about 1/500 for H 0 a n d C H 0 , a fact i n d i c a t i n g the m u c h greater efficiency of peroxy radicals i n o x i d i z i n g N O , p e r u n i t c o n c e n t r a t i o n , as c o m p a r e d to 0 . Because the calculation may i n v o l v e a r e l a t i v e l y small difference b e t w e e n two larger values (equation 43), h i g h l y precise a n d accurate c o n c e n t r a t i o n a n d p h o t o l y t i c rate data are r e q u i r e d . S e v e r a l photostationary state m e a surements have b e e n r e p o r t e d (131-133). A s an e x a m p l e , m i s s i n g oxidant concentrations calculated b y u s i n g data f r o m the Scotia R a n g e f i e l d study of 1988 (J30) are s h o w n i n F i g u r e 14. T h e y can b e c o m p a r e d w i t h R 0 m e a surements m a d e b y u s i n g the c h e m i c a l a m p l i f i e r (discussed i n a p r e c e d i n g section) d u r i n g the same t i m e p e r i o d ( F i g u r e 15). W i t h i n the scatter o f the m e a s u r e m e n t s , the agreement is good. T h i s result demonstrates that the value of peroxy radical measurements is e n h a n c e d b y c o m p a r i s o n w i t h o t h e r p e r t i n e n t species, a l t h o u g h clearly the m i s s i n g oxidant m e t h o d for the d e t e r m i n a t i o n of peroxy radical concentrations is rather i n d i r e c t a n d o n l y usable i n d a y l i g h t hours. T h e d e t e c t i o n l i m i t for this m e t h o d d e p e n d s o n a v a r i e t y l7

m3

l7

2

3

m

2

3

2


316


90

D 0)

JD Q. Q.

60

X


Ο

30h

25

26

27

1

28

29

30

July

Figure 14. Six days of missing oxidant calculations from measurements at the Scotia Range, Pennsylvania, study, summer 1988 (130).

150

50 100 "Missing" oxidant

150

Figure 15. Comparison of radical signals (see Figure 13) and missing oxidant values from the Scotia Range, Pennsylvania, study, summer 1988 (130).


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

— — — — —

X

X s

9

10* 10 10

1 2

1-30 min 1-30 min

— —

8 1 2.5

0.1-1.0 h

—

— —

Χ

X

—

x

x

x

— —

s

9

10 10

9

6

T

10 10 10

s

9

fi

minutes 0.5-2.0 h

T

—

7

6

0.05 X 10 -1 X 10 0.6 χ io -4 X 10 2 X 10 1 x 10

4 2 1

Detection Limit"

0.6-0.8 h

— — — — —

minutes minutes minutes

Averaging Time

A A LA L LA A

A A L LA A L

— — — — —

L L L

h

L or A d

ST

τ τ

— —

τ

S

— —

—

S S

— — — — — — —

S or Τ

2

2

2

rf

2

higher values than spectral promising not tested in atmosphere for R 0 sensitivity questions total R 0 , calibration problems daytime only, total R 0

Ο} interference cross section uncertain diode laser possible not tested not tested not possible—photodissociation not tested not tested for peroxy radicals proven for upper atmosphere proven for upper atmosphere large magnet NO, interference, H 0 and R 0 used for H O only possible interference problems

Comments

'This number describes minimum detectable concentration in molecules per cubic centimeter; conditions are discussed in the text. ''('ode describes whether the technique has been used in the laboratory (L), the atmosphere (A), or both (LA). '('ode describes whether atmospheric measurements were performed in the stratosphere (S), troposphere (T), or both (ST). A dash indicates that insufficient information is known about a system to provide an entry in this column or that known problems preclude its use.

Spectroscopic methods UV absorption Near IR absorption Middle IR absorption Far IR absorption Millimeter-wave absorption UV fluorescence Near IR fluorescence Middle IR emission Far IR emission Millimeter-wave emission Laser magnetic resonance Matrix isolation-ESR Spin trapping-ESR Mass spectroscopy Chemical conversion methods H O resonance fluoresence H O laser-induced fluorescence Photofragment spectroscopy Near IR chemiluminescence Chemical amplification Missing oxidant calculation

Technique

Table I. Summary of Peroxy Radical Measurement Methods


2

318


of factors, i n c l u d i n g the stability of the photolysis rate a n d species c o n c e n trations, b u t an estimated detection l i m i t of 2 Χ 10 m o l e c u l e s / c m is r e a sonable. 8

3


Summary S e v e r a l p o t e n t i a l peroxy radical m e a s u r e m e n t techniques exist i n the r e a l m of a t m o s p h e r i c c h e m i s t r y studies, although most have b e e n u s e d o n l y i n the laboratory. T h e techniques are s u m m a r i z e d i n T a b l e I. P o s s i b l y , some l a b oratory methods c o u l d be a p p l i e d to a t m o s p h e r i c m e a s u r e m e n t s . T h e d a tabase for a m b i e n t peroxy radical concentrations i n the t r o p o s p h e r e a n d stratosphere is meager. M u c h of the available stratospheric data y i e l d c o n centrations of H 0 h i g h e r than those calculated w i t h c o m p u t e r m o d e l s . T h e reasons for this systematic difference are not k n o w n . I n the t r o p o s p h e r e , m o r e measurements are c a l l e d for i n c o n j u n c t i o n w i t h o t h e r r e l a t e d species such as ozone, N O ^ , NO , j , a n d jo « It w i l l also b e appropriate to d e v e l o p m u l t i p l e m e t h o d s , a n d , w h e n they have reached m a t u r i t y , to p e r f o r m i n t e r c o m p a r i s o n studies. 2

y

N 0 2

3

Acknowledgments T h a n k s to L e n n y N e w m a n for the i n v i t a t i o n to participate i n the M e a s u r e m e n t C h a l l e n g e s i n A t m o s p h e r i c C h e m i s t r y s y m p o s i u m at the A m e r i c a n C h e m i c a l Society m e e t i n g i n B o s t o n , Massachusetts, A p r i l 1990. T h a n k s also to F r e d F e h s e n f e l d , D a v e P a r r i s h , a n d M a r t y B u h r of the N a t i o n a l O c e a n i c a n d A v i a t i o n A d m i n i s t r a t i o n A e r o n o m y L a b for r e n e w i n g o u r interest i n the c h e m i c a l a m p l i f i e r t e c h n i q u e i n the late 1980s. F i n a l l y , a b i g thank y o u to Geoffrey T y n d a l l , J o h n L i n d , J o h n O r l a n d o , S e l e n a S l y t e r , a n d Steve M a s s i e for assistance i n the p r e p a r a t i o n of this m a n u s c r i p t .

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for review January 29, 1991.

ACCEPTED

revised manuscript June 22, 1992.


Measurement Methods for Peroxy Radicals in the Atmosphere

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