In Situ Measurements of Stratospheric Reactive Trace Gases

5 In Situ Measurements of Stratospheric Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 28, 2018 | https://pubs.acs.org Publication Date: June 1, 1993 | doi: 10.1021/ba-1993-0232.ch005

Reactive Trace Gases W i l l i a m H. Brune and Richard M. Stimpfle 1

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Department of Meteorology, Pennsylvania State University, University Park, P A 16802 Department of E a r t h and Planetary Sciences, H a r v a r d University, Cambridge, MA 02138

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In situ measurements of the abundances of reactive trace gases have been essential to the understanding of stratospheric photochemistry. The measurement of those gases that directly affect the abundance of ozone—NO, NO , OH, HO , ClO, Cl, BrO, Br, and O—are of particular interest. The stratospheric environment, with its low temperatures, large range in pressure, and solar ultraviolet light, offers many measurement challenges. Simultaneous measurements of a number of trace gas species are required to develop an understanding of their distributions, and some of these measurements have been made from instruments mounted on helium-filled balloons and high-altitude aircraft. Although much has been learned about the workings of the stratosphere and, in particular, the mechanisms affecting the distribution of ozone, a truly predictive understanding has yet to be developed. 2

2

THE IMPACT OF ANTHROPOGENIC ACTIVITY

o n stratospheric p h o t o c h e m istry has b e e n an i m p o r t a n t m o t i v a t i o n for the m e a s u r e m e n t s of stratospheric trace gases for the last 20 years. A l t h o u g h the stratosphere is r e m o t e , the changes i n d u c e d i n the ozone layer b y these c h a n g i n g reactive trace gases a n d the resultant increase i n u l t r a v i o l e t radiation are of c o n c e r n for a l l l i v i n g things o n the surface o f the e a r t h . T h u s , m u c h stratospheric research has b e e n focused o n ozone d e p l e t i o n . T h o s e reactive trace gases that have d i r e c t i m p a c t o n o z o n e — N O , N 0 , O H , H 0 , C I O , C I , B r O , B r , a n d O — a r e of p a r t i c u l a r interest. T h e p h o t o c h e m i c a l systems are h i g h l y i n t e r a c t i v e , h o w 2

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0065-2393/93/0232-0133$13.80/0 © 1993 American Chemical Society

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

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MEASUREMENT C H A L L E N G E S IN ATMOSPHERIC CHEMISTRY

ever, a n d m a n y c o m p o n e n t s not d i r e c t l y r e l a t e d to o z o n e loss are nonetheless i m p o r t a n t for u n d e r s t a n d i n g what that loss is n o w a n d m i g h t be i n the f u t u r e . T h e strategy for research i n the stratosphere has b e e n to d e v e l o p c o m p u t e r simulations to p r e d i c t trends i n p h o t o c h e m i s t r y a n d o z o n e change. I n c o r p o r a t e d i n these simulations are laboratory data o n c h e m i c a l k i n e t i c s a n d p h o t o l y t i c processes a n d a theoretical u n d e r s t a n d i n g of a t m o s p h e r i c motions. A n i m p o r t a n t aspect of this approach is k n o w i n g i f the c o m p u t e r m o d e l s r e p r e s e n t the conditions of the stratosphere accurately e n o u g h that t h e i r p r e d i c t i o n s are v a l i d . T h e s e models are m a d e c r e d i b l e b y comparisons w i t h stratospheric observations. M e a s u r e m e n t s e i t h e r f r o m the g r o u n d or f r o m satellites have b e e n a major c o n t r i b u t i o n to this effort, a n d satellite i n s t r u m e n t s s u c h as L I M S ( L i m b Infrared M o n i t o r o f the Stratosphere) o n the N i m b u s 7 satellite (I) i n 1979 a n d A T M O S ( A t m o s p h e r i c Trace M o l e c u l a r Spectroscopy i n s t r u ment), a F o u r i e r transform i n f r a r e d spectrometer aboard Spacelab 3 (2) i n 1987, have p r o d u c e d v a l u a b l e data sets that still challenge o u r m o d e l s . B u t these r e m o t e techniques are not always adequate for r e s o l v i n g p h o t o c h e m istry o n the small scale, p a r t i c u l a r l y i n the l o w e r stratosphere. I n some cases, the a l t i t u d e r e s o l u t i o n p r o v i d e d b y r e m o t e t e c h n i q u e s has b e e n insufficient to p r o v i d e u n a m b i g u o u s concentrations of trace gas species at specific a l t i tudes. Insufficient altitude r e s o l u t i o n is a h a n d i c a p p a r t i c u l a r l y for those trace species w i t h large gradients i n e i t h e r a l t i t u d e or l a t i t u d e . O f t e n o n l y the most a b u n d a n t species can be m e a s u r e d . M a n y o f the reactive trace gases, the k e y species i n most c h e m i c a l transformations, have s m a l l a b u n dances that are difficult to detect accurately from r e m o t e platforms. I n situ m e a s u r e m e n t s o f stratospheric reactive trace gas abundances p r o v i d e an o p p o r t u n i t y to test the f u n d a m e n t a l p h o t o c h e m i c a l m e c h a n i s m s (3). T h e advantage of such measurements is that t h e y are l o c a l , so the s i m u l t a n e o u s measurements o f trace gases place a t r u e constraint o n the p o s sible p h o t o c h e m i c a l m e c h a n i s m s . T h e s e m e a s u r e m e n t s are also able to r e solve small-scale spatial a n d t e m p o r a l structure i n the trace constituent fields. T h e disadvantage o f i n situ measurements is that t h e y d o not c a p t u r e the global or perhaps e v e n seasonal v i e w of p h o t o c h e m i c a l transformations b e cause t h e y are s e l d o m d o n e f r e q u e n t l y e n o u g h or i n e n o u g h places to p r o v i d e that i n f o r m a t i o n . A n o t h e r disadvantage o f i n situ m e a s u r e m e n t s is that t h e y m u s t be m a d e from platforms i n the stratosphere, a n d these r e m o t e observational outposts have t h e i r liabilities. O n e o f the success stories of i n situ m e a s u r e m e n t s i n the stratosphere is the c o n f i r m a t i o n that the r a p i d loss o f o z o n e o v e r A n t a r c t i c a each O c t o b e r is i n d e e d caused b y p h o t o c h e m i s t r y r e l a t e d to the release of c h l o r o f l u o r o carbons at the surface of the earth. G r o u n d - b a s e d m e a s u r e m e n t s o f the p r i m a r y c h l o r i n e c u l p r i t , C I O , a n d 0 have g i v e n a s i m i l a r p i c t u r e (4), b u t not w i t h the fine d e t a i l possible from the i n situ t e c h n i q u e s , as s h o w n i n 3


5.

B R U N E & STIMPFLE

In Situ Measurements

of Stratospheric

Trace Gases 1 3 5

F i g u r e 1. T h i s g r a p h shows the r a p i d v a r i a t i o n o f C I O a n d 0 as the e d g e of the c h e m i c a l l y p e r t u r b e d region i n the A n t a r c t i c p o l a r vortex is p e n e t r a t e d b y the N a t i o n a l A e r o n a u t i c s a n d Space A d m i n i s t r a t i o n ( N A S A ) E R - 2 h i g h altitude aircraft o v e r the P a l m e r P e n i n s u l a o f A n t a r c t i c a o n S e p t e m b e r 16, 1987 (5). It is one of a series of 12 snapshots, o r i n d i v i d u a l flights, d u r i n g the A i r b o r n e A n t a r c t i c O z o n e E x p e r i m e n t ( A A O E ) that s h o w the d e v e l o p m e n t o f an anticorrelation b e t w e e n C I O a n d 0 that began as a c o r r e l a t i o n in m i d - A u g u s t . W h e n these t w o m e a s u r e m e n t s are c o m b i n e d w i t h a l l the others from the E R - 2 aircraft, the total data set p r o v i d e s a p r o v o c a t i v e p i c t u r e o f h o w s u c h c h e m i s t r y occurs a n d w h a t it is capable o f d o i n g to ozone. 3


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Latitude (degrees South) Figure 1. Simultaneous measurements of ClO and 0 over Antarctica on Sep tember 1 6 , 1 9 8 7 , during the AAOE mission. The boundary of the chemically perturbed region at 69°S is clearly shown by the rapid increase in the ClO mixing ratio and the rapid decrease in the 0 mixing ratio. There is an anticorrelation between ClO and 0 near the boundary. :i

3

:i

T h e measurements i n the m i d l a t i t u d e stratosphere d u r i n g the last four years have b e e n e q u a l l y successful because m o r e r e l a t e d species are b e i n g m e a s u r e d s i m u l t a n e o u s l y , a n d these data sets are p l a c i n g serious constraints o n p h o t o c h e m i c a l models (2). T h i s situation is d r a m a t i c a l l y different f r o m that i n the mid-1970s t h r o u g h the mid-1980s, w h e n c o n f i r m a t i o n that c e r t a i n trace gas species w e r e present i n the stratosphere i n a p p r o x i m a t e l y the correct abundances was still an issue (6). A n a l y s e s of m o r e recent m e a s u r e -


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


merits indicate that the c u r r e n t u n d e r s t a n d i n g is not c o m p l e t e , h o w e v e r . B u t efforts to measure the major components of a l l the trace gas families, especially d u r i n g d i u r n a l or seasonal variations, are s o l i d i f y i n g the u n d e r standing of stratospheric p h o t o c h e m i s t r y at the m i d l a t i t u d e s . T h e challenges that face the scientist s t u d y i n g the stratosphere are the same as those that face scientists i n other fields. F i r s t is the challenge of k n o w i n g w h a t needs to b e m e a s u r e d to b e t t e r u n d e r s t a n d the natural system b e i n g s t u d i e d . I n the stratosphere, this u n d e r s t a n d i n g i n c l u d e s not o n l y the trace gas abundances a n d t h e i r spatial a n d t e m p o r a l variations, b u t also the transport of those species o n b o t h short a n d l o n g t i m e scales. D i s t r i b u t i o n s of a n d correlations a m o n g l o n g - l i v e d trace gases have p r o v i d e d the best clues for h o w stratospheric transport w o r k s . S e c o n d is the challenge o f h o w to make these measurements. T h i s s k i l l i n c l u d e s k n o w i n g not o n l y h o w to measure the abundances of i n d i v i d u a l trace gas species, b u t h o w to c o m b i n e several i n s t r u m e n t s or measurements together a n d h o w to d e p l o y those i n s t r u m e n t s throughout an e x p e r i m e n t . T h e goal of this chapter is to i n s t i l l an u n d e r s t a n d i n g of i n situ m e a surements of stratospheric reactive trace gases a n d of h o w the challenges of k n o w i n g w h a t measurements to make a n d h o w to m a k e t h e m are b e i n g met. Because most of the other chapters i n this book c o n c e r n m e a s u r e m e n t s i n the troposphere, a b r i e f o v e r v i e w of the characteristics of the s t r a t o s p h e r e — its p h y s i c a l state a n d its trace gas c o m p o s i t i o n a n d p h o t o c h e m i s t r y — i s first p r e s e n t e d . T h i s discussion contains a general statement of the c u r r e n t u n d e r s t a n d i n g o f stratospheric p h o t o c h e m i s t r y a n d the areas w h e r e the lack o f k n o w l e d g e is critical. Some general g u i d e l i n e s are p r e s e n t e d o f w h a t the challenges i n stratospheric measurements are, a n d examples o f these c h a l lenges are g i v e n f r o m i n s t r u m e n t s that have b e e n f l o w n e i t h e r o n h e l i u m filled balloons or o n h i g h - a l t i t u d e aircraft.

The Stratospheric Environment and Trace Gas Distribution T h e stratosphere, b e g i n n i n g at r o u g h l y 10 k m above the e a r t h a n d e x t e n d i n g u p to 50 k m , contains 1 0 % o f the air i n the atmosphere a n d 9 0 % o f the ozone. It a n d the mésosphère are the regions w h e r e m u c h o f the solar ultraviolet l i g h t deposits its e n e r g y as heat; this situation results i n a p o s i t i v e t e m p e r a t u r e gradient that maintains v e r t i c a l d y n a m i c stability. T h e stratosphere thus evolves i n relative isolation f r o m the t r o p o s p h e r e , w h e r e t u r b u l e n t m i x i n g occurs, a n d o n l y a slow exchange occurs b e t w e e n the two regions. A l t h o u g h the processes of exchange b e t w e e n the troposphere a n d stratosphere are not c o m p l e t e l y u n d e r s t o o d , it is k n o w n that air a n d a few trace constituents that are not collected a n d p r e c i p i t a t e d out of clouds e n t e r the stratosphere p r e d o m i n a n t l y i n the tropics, can b e c h e m i c a l l y transf o r m e d , a n d exit a few years later i n m i d d l e to h i g h latitudes. T h e a d m i s s i o n o f o n l y a r e l a t i v e l y few trace gases into the stratosphere, the exposure of


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gases to the ultraviolet light that is screened f r o m the troposphere b y o z o n e and to the resultant p h o t o c h e m i s t r y , the m i x i n g , a n d the p o l e w a r d transport p r o d u c e a trace gas c o m p o s i t i o n that is significantly different from that i n the troposphere. T h e temperatures i n the stratosphere range f r o m a l o w o f 200 Κ at 14 k m to 250 Κ at 50 k m for t y p i c a l m i d l a t i t u d e conditions (7). I n the p o l a r regions, p a r t i c u l a r l y o v e r the S o u t h P o l e , t e m p e r a t u r e s can fall to as l o w as 185 Κ o v e r the e n t i r e h e i g h t of the stratosphere (8). T h e average t e m p e r a t u r e profile for m i d d l e latitudes has h i g h e r temperatures t h a n those o b s e r v e d at 70°S d u r i n g the A i r b o r n e A n t a r c t i c O z o n e E x p e d i t i o n i n A u g u s t 1987 o r at 75°N d u r i n g the A i r b o r n e A r c t i c Stratospheric E x p e r i m e n t i n J a n u a r y 1989 ( F i g u r e 2). F o r comparable altitudes a n d seasons, the A r c t i c is slightly w a r m e r t h a n the A n t a r c t i c (9). I n general, the coldest regions are the l o w e r equatorial stratosphere j u s t above the tropopause a n d the w i n t e r t i m e l o w e r polar stratosphere, a n d these regions e x h i b i t temperatures at or b e l o w 200 K . T h e air pressure i n the stratosphere ranges from 100 m b a r near 15 k m to 0.1 m b a r near 50 k m , falling off e x p o n e n t i a l l y w i t h a l t i t u d e as s h o w n i n F i g u r e 2 (9). A s i n the troposphere, altitude a n d pressure are u s e d as v e r t i c a l

60

— US Std Atm (45°) — AAOE (70°S) ---AASE(75°N)

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20

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240

Temperature (K)

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10

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10

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Figure 2 . The temperature and pressure distribution of the stratosphere. The solid line is from reference 7, and the dashed lines are from measurements made by the Meteorological Measurement System (MMS) instrument on the NASA ER-2 high-altitude aircraft during the AAOE mission in 1987 (S) and the AASE mission in 1989 (9). The Arctic was colder in 1989 than usual



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MEASUREMENT CHALLENGES IN ATMOSPHERIC CHEMISTRY

coordinates t h r o u g h o u t the stratosphere, b u t p o t e n t i a l t e m p e r a t u r e b e c o m e s the most m e a n i n g f u l coordinate i n the l o w e r stratosphere, w h e r e h e a t i n g a n d c o o l i n g rates are small. P o t e n t i a l t e m p e r a t u r e is the t e m p e r a t u r e that an air p a r c e l w o u l d have i f it w e r e adiabatically c o m p r e s s e d to 1 a t m (101 kPa). A i r parcels t e n d to follow i s e n t r o p i c trajectories (constant p o t e n t i a l temperature), so the goal of measurements is often to follow constant-potential trajectory surfaces to m a p out the m e r i d i o n a l a n d z o n a l c o m p o n e n t s of a trace gas constituent field. T h i s approach i m p l i e s that b o t h p r e s s u r e a n d t e m p e r a t u r e w i l l vary along the p a t h . S u c h trajectories are m e a n i n g f u l for about a w e e k , after w h i c h diabatic effects b e c o m e i m p o r t a n t a n d m i x i n g o f air parcels from different trajectories creates a n e w air p a r c e l w i t h a n e w average trajectory. T h e v e r t i c a l d i s t r i b u t i o n s of some trace gases for some regions o f the stratosphere w o u l d be e x p e c t e d to be s m o o t h a n d slowly v a r y i n g . F o r other trace gases i n other regions, extreme v e r t i c a l stratification a n d h i g h t e m p o r a l v a r i a b i l i t y w o u l d b e m o r e l i k e l y . A n example o f this latter case is the A n t a r c t i c

Figure 3. Abundances of gases in the stratosphere (12). The abundances of source gases marked by heavy vertical bars are the abundances at the base of the stratosphere, with the exception of H 0. The thick vertical bar indicates the range of H 0 abundances throughout the stratosphere. Shaded bars represent ranges of observed or calculated reactive or reservoir trace gases for midday midlatitude conditions. 2

2


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of Stratospheric

Trace Gases 1 3 9

ozone h o l e , w h e r e extreme gradients i n trace gas abundances are established. A s t h e p o l a r air mixes w i t h m i d l a t i t u d e air, h i g h l y v a r i a b l e t h r e e - d i m e n s i o n a l structures i n t h e trace gas d i s t r i b u t i o n s evolve (10, 11). T h e trace gases i n this h i g h l y variable p h y s i c a l e n v i r o n m e n t c a n b e g r o u p e d into c h e m i c a l families. T h e four most p r o m i n e n t i n p h o t o c h e m i s t r y are those o f oxygen, n i t r o g e n (other t h a n N ) , h y d r o g e n , a n d c h l o r i n e a n d b r o m i n e ( F i g u r e 3). Gases e m i t t e d i n the troposphere that m i g r a t e to t h e stratosphere are t h e p r i m a r y sources for these c h e m i c a l families. T h e m a i n source gases for n i t r o g e n are N O a n d to a lesser extent N O a n d N O ; for h y d r o g e n H 0 o r C H ; a n d for the halogens C H C 1 , chlorofluorocarbons, halons, a n d G H B r . T h e s e stable gases are b r o k e n d o w n i n t h e stratosphere b y e i t h e r sunlight o r c h e m i c a l products of s u n l i g h t a n d b e c o m e t h e reactive species that interact w i t h each other b u t also destroy ozone. F i g u r e 3 shows the l o n g - l i v e d source gases as s o l i d bars, a n d t h e range o f t h e r e s u l t i n g reactive g a s e s — i n c l u d i n g e v e r y t h i n g f r o m free radicals to a c i d s — i s g i v e n b y t h e shaded bars.


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3

H o w trace gases are d i s t r i b u t e d w i t h a l t i t u d e c a n b e i l l u s t r a t e d for m i d l a t i t u d e conditions. T h e abundances o f trace gases i n t h e stratosphere as a f u n c t i o n of altitude are g i v e n i n F i g u r e 4 i n terms o f v o l u m e m i x i n g ratio a n d i n F i g u r e 5 i n terms o f concentration. T h e c o n c e p t o f m i x i n g ratio is i m p o r t a n t i n t h e consideration o f the transport o f the trace gases because

Volume Mixing Ratios Figure 4. Calculated altitude distributions of the volume mixing ratios of several trace gases (12). The reactive trace gases that directly affect ozone are given by dark lines. Conditions are for the equinox at 30°N.


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C o n c e n t r a t i o n s (molecules c m " ) Figure 5. Calculated altitude distributions of the concentrations of several trace gases (12). The reactive trace gases that directly affect ozone are given by dark lines. Conditions are for the equinox at 3(PN. m i x i n g ratios are p r e s e r v e d as the air parcels d e s c e n d a n d contract o r ascend and e x p a n d . O n the other h a n d , m a n y m e a s u r e m e n t t e c h n i q u e s use a b sorption o r fluorescence, w h i c h are d e p e n d e n t u p o n the c o n c e n t r a t i o n o f the trace constituent. T h e m i x i n g ratios o f m a n y free radicals increase s u b stantially w i t h h e i g h t , b u t t h e i r concentrations are somewhat m o r e constant w i t h a l t i t u d e . N e a r the equator, the source r e g i o n for most trace gases, the abundances of the t r o p o s p h e r i c source gases are l a r g e r that those at m i d latitude for comparable v e r t i c a l coordinates. N e a r the p o l a r regions, j u s t the opposite is t r u e . A l t h o u g h this is a o n e - d i m e n s i o n a l v i e w o f trace gas d i s t r i b u t i o n s , the t h r e e - d i m e n s i o n a l v i e w is actually r e q u i r e d for careful c o m parison of m o d e l results a n d observations.

Ozone Photochemistry at Midlatitudes T h e t h r u s t of m u c h o f stratospheric research has b e e n to u n d e r s t a n d the p r o d u c t i o n a n d loss of stratospheric ozone. T h e c h e m i c a l t r a i l i n this process has b e e n m a r k e d i n F i g u r e 6 as the top four arrows. T h e p r o d u c t i o n of ozone is almost e x c l u s i v e l y b y this m e c h a n i s m : 0

2

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2

+ M

20 > 0

3

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M


(2)


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0

of Stratospheric

Trace Gases 1 4 1

2

Figure 6. Schematic of the major gas-phase cycles in the stratosphere. The Chapman mechanism (oxygen reactions) is indicated by the top four arrows. The ovals represent the odd-nitrogen, odd-hydrogen, and inorganic halogen chemical families. Arrows indicate conversion of species by reaction or pho tolysis, but reaction partners are not shown. The overlap area in the center represents heterogeneous and unknown photochemistry. T h e d e s t r u c t i o n o f ozone occurs b y a n u m b e r o f m e c h a n i s m s that r e s u l t i n 0

3

+ sunlight Ο + 0

3

>Ο + 0 »20

(3)

2

(4)

2

T h e s e four reactions constitute the C h a p m a n m e c h a n i s m for e s t a b l i s h i n g the abundance of ozone i n the stratosphere. H o w e v e r , the a b u n d a n c e of ozone is d i c t a t e d also b y o t h e r loss mechanisms that m i m i c reaction 4. T h e i n v o l v e m e n t of reactive n i t r o g e n , reactive h y d r o g e n , a n d reactive c h l o r i n e i n catalytic cycles that destroy ozone has b e e n k n o w n for about 20 years. T h e s e cycles have the f o r m 0

3

+ X

Ο + XO

>XO + 0 >X + 0

(5)

2

(6)

2

w h e r e X = N O , O H , C I , or B r . I n each c o u p l e t of reactions, one is m u c h slower than the other a n d dictates the speed of the net reaction. F o r a l l of these species, the r a t e - l i m i t i n g step is reaction 6. C a t a l y t i c cycles of reactive h y d r o g e n are also possible; i n the l o w e r stratosphere b o t h O H a n d H 0 react w i t h 0 , a n d i n the u p p e r stratosphere b o t h O H a n d H 0 react w i t h O. T h e i m p o r t a n c e of N O , O H , a n d C I catalytic cycles for d e s t r o y i n g o z o n e is g i v e n i n F i g u r e 7, along w i t h the p u r e oxygen p r o d u c t i o n a n d losses. T h e 2

3

2



142

MEASUREMENT C H A L L E N G E S IN ATMOSPHERIC

0

CHEMISTRY

Ί

R e a c t i o n Rate (molecules c m s ) Figure 7. Calculated ozone production and loss rates for two different conditionsfrom the AER two-dimensional model. Production and loss rates above 20 km are diurnally averaged loss rates for the spnng equinox at 30°N. Midday loss rates are approximately two times larger. Production and loss rates for midday below 20 km are calculated for the chemically perturbed region over Antarctica on September 16,1987. The catalytic cycles responsible for the loss are explained in the text. Although ozone loss occurs at higher altitudes over Antarctica, in situ observations extend only to —19 km. reactive n i t r o g e n cycle dominates ozone d e s t r u c t i o n t h r o u g h o u t most o f the stratosphere, a l t h o u g h c h l o r i n e a n d h y d r o g e n are e q u a l l y i m p o r t a n t h i g h e r i n the stratosphere near 40 k m , a n d h y d r o g e n is m o r e i m p o r t a n t l o w e r i n the stratosphere near the tropopause. T h e change i n the ozone abundance at any place or t i m e can b e w r i t t e n as d0 /dt 3

= P r o d u c t i o n - Loss - ν · φ ο

(7)

3

or dO,/dt

= 2/o [0 ] - 2* [0][0 ] - 2fc [N0 ][0] 2

2

1

3

2

2

- 2 * [ H 0 ] [ 0 ] - 2* [C10][0] 3

2

4

- (other s m a l l e r terms) -

ν·φ

θ 3


(8)

5.



of Stratospheric

Trace Gases 1 4 3

w h e r e / is the photolysis rate o f 0 , the brackets i n d i c a t e c o n c e n t r a t i o n , a n d φ ο is the flux of ozone t h r o u g h the v o l u m e a n d represents the transport o f ozone (12). F o r a g i v e n location i n the stratosphere, the a b u n d a n c e o f o z o n e can b e said to b e e i t h e r u n d e r p h o t o c h e m i c a l c o n t r o l o r d y n a m i c a l c o n t r o l d e p e n d i n g o n w h e t h e r the c h e m i c a l terms or the flux t e r m i n e q u a t i o n 8 is larger. I n this v i e w , m u c h of the stratosphere above 30 k m is u n d e r p h o t o c h e m i c a l c o n t r o l , a n d m u c h b e l o w 30 k m or i n the w i n t e r t i m e p o l a r r e g i o n is u n d e r d y n a m i c a l c o n t r o l (6). H o w e v e r , this v i e w is not e n t i r e l y v a l i d for the real stratosphere, a n d it predates the u n d e r s t a n d i n g of the r a p i d c h e m i c a l loss i n the s p r i n g t i m e A n t a r c t i c stratosphere. 2


3

In a d d i t i o n to the reactive n i t r o g e n species, N O a n d N O , the o d d h y d r o g e n species, H 0 a n d O H , a n d the i n o r g a n i c c h l o r i n e species, C I O and C I , t h e r e are the other f a m i l y m e m b e r s i d e n t i f i e d i n F i g u r e 6. F o r the sake o f clarity a n d b r e v i t y the p h o t o c h e m i c a l scheme g i v e n i n F i g u r e 6 illustrates the subset o f reactions that to a large extent d e t e r m i n e s the effects of these c o m p o u n d s o n ozone. T h e i d e n t i t y o f the reactive partners (or sunlight) i n v o l v e d i n each reaction a n d reactions of lesser i m p o r t a n c e are g i v e n i n n u m e r o u s other publications (references 6 or 12, for example). £

2

T h e a d d i t i o n a l species s h o w n w i t h i n each e l l i p s e are, i n each case, r e s e r v o i r species, so-called because t h e y effectively store the active radical species i n m o l e c u l a r forms that do not catalytically destroy ozone. T h e p r e s ence o f r e s e r v o i r species d r a m a t i c a l l y affects the d i s t r i b u t i o n o f active radical species i n a g i v e n f a m i l y a n d thus the effectiveness w i t h w h i c h t h e y can destroy ozone. T h e m o l e c u l e s w i t h i n the o v e r l a p p e d b o u n d a r y areas, C 1 0 N 0 , HNO3, HOC1, a n d HC1, identify c r u c i a l r e s e r v o i r species that i n t e r l i n k the families. I n this p i c t u r e , the i n t e r l i n k i n g r e s e r v o i r species serve notice that, a l t h o u g h the d i v i s i o n of stratospheric p h o t o c h e m i s t r y into the N O , C1 ., a n d H O species is a v e r y useful construct for u n d e r s t a n d i n g stratospheric p h o t o c h e m i s t r y , i n reality the p h o t o c h e m i s t r y m u s t b e u n d e r stood as a c o m p l e t e system. A n o t h e r example o f i m p o r t a n t i n t e r f a m i l y r e actions not e x p l i c i t i n F i g u r e 6 is the reactions of C I O a n d H 0 w i t h N O to form C I and O H , respectively. 2

v

A

t

2

A n example of the c o m p l e x i t y i m p l i e d b y the i n t e r l i n k i n g of the c h e m i c a l families i n F i g u r e 6 is the effect of i n c r e a s i n g C l i n the l o w e r stratosphere, b e l o w 30 k m (13,14). P r e s e n t l y i n this r e g i o n the NO , C l , a n d H O . species account for —70, 20, a n d 1 0 % of ozone loss, r e s p e c t i v e l y , as seen i n F i g u r e 7. I f C\ w e r e to increase significantly, t h e n the natural b u f f e r i n g effect o f N O o n C l b y C l O N O formation w o u l d b e d e p l e t e d as N O is m o r e or less c o m p l e t e l y titrated to C l O N 0 . T h e n the excess C l c o u l d destroy signifi cantly m o r e ozone t h a n the NO system it replaces. I n a d d i t i o n , the loss of H O t h r o u g h the formation of H N 0 w o u l d decrease w i t h the N 0 decrease, thus raising the O H concentration a n d l i b e r a t i n g m o r e C i , f r o m the H C 1 r e s e r v o i r because of the reaction o f H C 1 w i t h O H that forms C I . T h i s t y p e of n o n l i n e a r a t m o s p h e r i c response to increases i n one o f the p h o t o c h e m i c a l t

x

X

A

x

x

x

£

v

2

t

x

x

3

2


144


families makes it i m p e r a t i v e that the c u r r e n t u n d e r s t a n d i n g of stratospheric p h o t o c h e m i s t r y b e tested.


The High-Latitude Lower Stratosphere in Winter and Spring A l t h o u g h m a n y measurements of trace species have b e e n made i n the m i d d l e altitudes over the last 20 years, r e l a t i v e l y few m e a s u r e m e n t s w e r e m a d e i n the polar regions u n t i l about 1986, p a r t i c u l a r l y i n the coldest t i m e s of the year i n w i n t e r a n d s p r i n g . A s a result, a l t h o u g h researchers t h o u g h t t h e y h a d a good u n d e r s t a n d i n g of the p h o t o c h e m i s t r y at m i d d l e latitudes (15,16), little was k n o w n about the polar regions. It is really no surprise that no one r e c o g n i z e d the possible consequences of the f o r m a t i o n of p o l a r stratospheric clouds, w h i c h h a d b e e n o b s e r v e d b y satellites since 1980 (17), a n d that the first report that r a p i d ozone loss was o c c u r r i n g came i n 1985 f r o m F a r m a n et a l . at the B r i t i s h A n t a r c t i c S u r v e y (18). T h i s loss, apparent i n the early 1980s, began i n S e p t e m b e r a n d b e c a m e greatest i n O c t o b e r . I n 1986, 3 5 % of the total c o l u m n of ozone was c h e m i c a l l y d e s t r o y e d over an area the size of A n t a r c t i c a . I n 1987, the loss for O c t o b e r was 5 0 % of the 1979 c o l u m n a m o u n t (19). A l t h o u g h the ozone loss i n 1988 was substantially less, the losses i n 1989, 1990, a n d 1991 have b e e n e q u a l to that i n 1987 (20, 21). E s s e n t i a l l y a l l of the ozone b e t w e e n the altitudes of 13 a n d 23 k m is r e m o v e d i n an air mass that remains located r o u g h l y o v e r the A n t a r c t i c c o n t i n e n t (22) i n a v o l u m e of air inside the c i r c u m p o l a r vortex that is c a l l e d the " c h e m i c a l l y perturbed region". W h e n the ozone hole was a n n o u n c e d i n 1985, several theories sprang u p i m m e d i a t e l y to e x p l a i n the loss. O n l y the t h e o r y that c h l o r i n e f r o m c h l o r o f l u o r o c a r b o n s — a l o n e (23) a n d i n c o m b i n a t i o n w i t h b r o m i n e (24) a n d to a lesser extent H 0 (25)—was responsible has s u r v i v e d the process of scientific investigation. W h a t is r e q u i r e d for ozone b e t w e e n the altitudes of 13 a n d 22 k m a n d south of ~ 6 5 ° S latitude to be d e s t r o y e d at the o b s e r v e d rate of 2 % p e r day appears i n retrospect to b e a c h e m i c a l conspiracy. F i r s t , the polar stratospheric clouds (PSCs) f o r m at t e m p e r a t u r e s b e l o w 195 K , w h i c h is 4 to 7 Κ above the frost p o i n t of water vapor (26, 27). R e a c t i v e n i t r o g e n a n d water cocondense on b a c k g r o u n d sulfuric a c i d aerosols, p r e s u m a b l y as n i t r i c a c i d t r i h y d r a t e ( H N 0 · 3 H 0 ) , a l t h o u g h o t h e r forms also appear l i k e l y . R e a c t i v e n i t r o g e n , c a l l e d NO (NO = N O 4- N O + N 0 + N 0 + HONO + HN0 + H0 N0 + C l O N 0 + o t h e r reactive nitrogens) is thus r e m o v e d f r o m the gas phase a n d c o n v e r t e d i n t o H N 0 . 2

3

2

y

2

5

3

2

2

y

£

3

2

3

T h i s reactive n i t r o g e n m a y t h e n b e actually r e m o v e d from the air p a r c e l w h e n the particles c o n t a i n i n g it g r o w large e n o u g h , at least several m i c r o meters i n d i a m e t e r , to gravitationally settle out i n less than a day (26). T h e m e c h a n i s m s for this particle g r o w t h are not c o m p l e t e l y u n d e r s t o o d , b u t one p o s s i b i l i t y is that water vapor condenses o n the n i t r i c a c i d - w a t e r core as the t e m p e r a t u r e decreases b e l o w the frost point. T h e s e particles t h e n b e -


5.


c o m e large a n d heavy e n o u g h that t h e y can fall to l o w e r a l t i t u d e s , perhaps out o f the stratosphere. F r o m m e a s u r e m e n t s of NO,,, N O , a n d c o l u m n a b u n dances of H N 0 a n d N 0 , it is k n o w n that m u c h of the total NO is actually r e m o v e d f r o m the stratosphere b y the s e d i m e n t a t i o n of large particles c o n t a i n i n g the reactive n i t r o g e n . O v e r A n t a r c t i c a , the stratospheric a i r is b o t h d e n i t r i f i e d a n d d e h y d r a t e d , a n d these observations s u p p o r t the m e c h a n i s m of s e d i m e n t a t i o n p r e s e n t e d (for examples of these m e a s u r e m e n t s , see m a n y papers i n reference 28). H o w e v e r , a n u m b e r of o t h e r possible m e c h a n i s m s can a c c o m p l i s h the same effect (29). A l t h o u g h most o f the reactive n i t r o g e n is r e m o v e d , some r e m a i n s , p r e s u m a b l y i n the f o r m o f H N 0 , w h i c h is p h o t o l y z e d o n l y s l o w l y back into N 0 i n the weak u l t r a v i o l e t s u n l i g h t o f the s p r i n g t i m e polar r e g i o n . A t the same t i m e , the P S C s are excellent sites for the c o n v e r s i o n o f c h l o r i n e c o m p o u n d s f r o m the r e l a t i v e l y inactive r e s e r v o i r forms of H C l a n d C 1 0 N 0 , w h i c h m a k e u p 9 9 % of the c h l o r i n e b u d g e t i n the l o w e r strato sphere, to p h o t o l y t i c a l l y l a b i l e species such as C l , H O C 1 , a n d C l O N O (30-32): 3


Trace Gases 1 4 5

In Situ Measurements of Stratospheric

y

2

3

2

2

2

C10N0

2

+ HCl

> C l (gas) + H N O 2

ClONO, + H 0

> H O C 1 (gas) + H N 0

2

N 0 2

5

(solid)

( 3

+ HCl

3

> C l O N O (gas) + H N 0

(9)

(solid) 3

(10)

(solid)

(11)

In the weak s u n l i g h t of polar s p r i n g , these gas-phase c h l o r i n e species release t h e i r c h l o r i n e atoms, w h i c h attack ozone almost e x c l u s i v e l y . T h e catalytic cycle that r e q u i r e s a reaction b e t w e e n C l O a n d Ο is not v e r y effective, because few oxygen atoms exist i n these c o l d , r e l a t i v e l y dark regions. I n stead, C I O reacts w i t h another C I O m o l e c u l e , f o r m i n g C 1 0 , w h i c h can t h e n b e easily p h o t o l y z e d b y the weak v i s i b l e s u n l i g h t that penetrates the atmosphere. 2

CIO

+ CIO + M

C1 0 2

2

+ sunlight ClOO + M Cl

+ 0

3

> C1 0 2

2

2

+ M

(12)

» CI + C l O O

(13)

> Cl + 0

(14)

2

» CIO + 0

+ M 2

(15)

A second catalytic cycle involves the C l O a n d B r O radicals, w h i c h react; some f o r m C I a n d B r atoms, w h i c h can t h e n react w i t h o z o n e to f o r m C l O and B r O again. CIO

+ BrO

> Br + ClOO

(16a)

CIO

+ BrO

» Br + OCIO

(16b)

CIO

+ BrO

> B r C l + O.

(16c)


146

MEASUREMENT CHALLENGES IN ATMOSPHERIC CHEMISTRY

ClOO +

M

B r C l + sunlight CI + 0


Br +

3

0

3

CI + 0

+

2

(17)

M

» B r + CI

(18)

> CIO +

0

2

BrO +

0

2

(19) (20)

A p p r o x i m a t e l y o n e - h a l f of the total reaction of C I O a n d B r O results i n the d e s t r u c t i o n of ozone. O t h e r m e c h a n i s m s exist, such as a catalytic cycles that are r a t e - l i m i t e d b y the reaction b e t w e e n C I O a n d Ο a n d b e t w e e n C l O a n d H 0 (25), b u t the c o n t r i b u t i o n from these reactions is s m a l l i n the polar regions. 2

A large n u m b e r of observations, b o t h r e m o t e a n d i n s i t u , c o n f i r m this qualitative p i c t u r e of the loss of ozone over A n t a r c t i c a . T h e i n situ data have c o m e f r o m i n s t r u m e n t s c a r r i e d o n s m a l l balloons a n d the N A S A E R - 2 h i g h altitude aircraft. S m a l l - b a l l o o n measurements are of particle d i s t r i b u t i o n s a n d sizes, ozone, a n d water vapor (23, 33). E R - 2 m e a s u r e m e n t s , l i s t e d i n T a b l e I, are of particle size a n d c o m p o s i t i o n ; a t m o s p h e r i c parameters such as t e m p e r a t u r e , pressure, lapse rate, a n d w i n d s ; a n d trace gas abundances of 0 , N 0 , N O or N O , C l O a n d B r O , a n d stable gases, i n c l u d i n g C H , chlorofluorocarbons, halons, a n d others (34-45). 3

2

y

4

D a t a f r o m the E R - 2 w e r e c o l l e c t e d d u r i n g the A A O E that was based i n P u n t a A r e n a s , C h i l e , i n A u g u s t a n d S e p t e m b e r 1987 (46). T w e l v e flights w e r e made from P u n t a A r e n a s (54°S) to the base of the P a l m e r P e n i n s u l a , A n t a r c t i c a (72°S), a n d back d u r i n g the s i x - w e e k - l o n g m i s s i o n . F l i g h t paths w e r e r e s t r i c t e d to a n a r r o w range of longitudes, a n d altitudes w e r e chosen so that the p o t e n t i a l t e m p e r a t u r e r e m a i n e d constant d u r i n g each l e g of the flight. P o t e n t i a l t e m p e r a t u r e surfaces b e t w e e n 420 a n d 470 Κ w e r e f l o w n . M e a s u r e m e n t s over a large range of p o t e n t i a l temperatures w e r e t a k e n w h e n the p i l o t executed a d i v e to 340 Κ before r e c o v e r i n g to a h i g h e r p o t e n t i a l t e m p e r a t u r e surface a n d f l y i n g n o r t h . S u c h a p r o g r a m a l l o w e d s a m p l i n g of air parcels from 470 to 340 Κ i n s i d e the c h e m i c a l l y p e r t u r b e d r e g i o n a n d also p e r m i t t e d the observation of strong gradients i n trace gas abundances o n a g i v e n p o t e n t i a l t e m p e r a t u r e surface. A n o t h e r goal o f the m i s s i o n was to sample as d e e p l y into the c h e m i c a l l y p e r t u r b e d r e g i o n as possible, a n d measurements as d e e p as 15° latitude i n s i d e the vortex w e r e a c h i e v e d o n some flights. T h e s e measurements give a p i c t u r e of the extent a n d e v o l u t i o n of the ozone loss over A n t a r c t i c a . T h e presence of large amounts of reactive c h l o r i n e , 500 t i m e s the c o n c e n t r a t i o n at m i d l a t i t u d e s , a n d the absence of reactive n i t r o g e n , 5 t i m e s less than that at m i d l a t i t u d e s , is b u t one result of these m e a s u r e m e n t s . A n o t h e r result is the e v o l v i n g anticorrelation b e t w e e n C l O a n d 0 , of w h i c h one snapshot is g i v e n i n F i g u r e 1 (5). To establish q u a n t i t a t i v e l y that the o b s e r v e d abundances of C l O a n d B r O can result i n the o b s e r v e d d e c l i n e i n o z o n e is somewhat m o r e difficult. W i t h the a s s u m p t i o n that the abundances of C l O 3


5.


In Situ Measurements of Stratosphenc Trace Gases

147

Table I. In Situ Measurements on the E R - 2 Aircraft Measuring

Device

Method


M MS (pressure, temperature, and wind vector) Microwave temperature profiler Aerosol and cloud spectrometer: 0.1- to 3.0-μπι particles 0.3- to 20.0-μπι particles Condensation nuclei counter Particle chemistry impactor Multifilter sampler (total nitrate, sulfate, and acidic chloride and fluoride) Whole-air sampler ( C 0 , C H , N 0 , C O , C F C s , and halons) Airborne tunable laser absorption spectrometer (ATLAS) (N 0) L y a hygrometer ( H 0 vapor) 2

4

2

temperature and pressure sensors; inertial navigation system passive microwave radiometry of 0 thermal emission laser scattering: inside a cavity in the free air stream growth in alcohol-saturated chamber; optical particle counting particle impaction; postflight X-ray analysis filter collection; postflight aqueous extraction and ion chromatography

34

pressurized canisters; postflight analysis with gas chromatography infrared absorption by tunable diode laser spectroscopy

40

photodissociation by hydrogen (Lya) emission at 121.6 n m ; detection of O H ( Α Σ -> Χ Π) emission U V absorption at 254 nm; comparison of signals from scrubbed and unscrubbed airstreams N O : chemiluminescence reaction of N O + 0 and N 0 * detection NO,,: catalytic conversion to N O and chemiluminescence N O detection chemical conversion with reagent N O to CI or B r ; resonance fluorescence detection of atoms

42

2

35 36

37 38 39

41

2

2

2

Dual-beam U V absorption ozone photometer N O and N O

tf

detector

+

3

C l O - B r O detector

2

43

44

2

45

and B r O are z o n a l l y u n i f o r m a n d that air parcels are n e i t h e r h e a t e d n o r c o o l e d v e r y r a p i d l y d u r i n g the six weeks o f observations, t h e n calculations of ozone loss, from the o b s e r v e d C l O a n d B r O amounts a n d the m e c h a n i s m s g i v e n , can be c o m p a r e d to the o b s e r v e d ozone loss. T h e c a l c u l a t e d o z o n e loss rates are s h o w n i n F i g u r e 7 i n c o m p a r i s o n w i t h the m i d l a t i t u d e ozone loss rates, a n d the t i m e e v o l u t i o n o f ozone loss d u r i n g A u g u s t a n d S e p t e m b e r for t h r e e p o t e n t i a l t e m p e r a t u r e surfaces is g i v e n i n F i g u r e 8 (47). W i t h i n the c o m b i n e d uncertainties of the measurements a n d calculations, the c a l c u l a t e d loss matches the o b s e r v e d loss. I m p r o v i n g this c o m p a r i s o n w i l l r e q u i r e a c o m p l e t e u n d e r s t a n d i n g o f the air p a r c e l trajectories a n d the v a r i ability of the trace gas abundances along those trajectories (48), b u t the large i n v o l v e m e n t of halogen p h o t o c h e m i s t r y has b e e n v e r i f i e d . T h e studies o v e r A n t a r c t i c a have s h o w n that the motions o f the strat ospheric a i r parcels m u s t b e w e l l k n o w n a n d that tracers o f some sort m u s t

American Ctemical Society Library 16th inSUHW. Newman; Measurement1155 Challenges Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993. Washington. OC. 200»

148

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

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•

0

— — — •

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In Situ Measurements of Stratospheric Reactive Trace Gases

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