AERONOMICA ACTA

Texte intégral

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I N S T I T U T D ' A E R O N O M I E S P A T I A L E D E B E L G I Q U E 3 - Avenue Circulaire

B - 1180 BRUXELLES

A E R O N O M I C A A C T A

A - N° 70 - 1970

Ozone and Hydrogen Reactions by M. NICOLET

B E L G I S C H I N S T I T U U T V O O R R U I M T E - A E R O N O M I E 3 - Ringlaan

B - 1180 BRUSSEL

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FOREWORD

"Ozone and Hydrogen Reactions" was presented at the General Scientific Assembly of the International Association of Geomagnetism and Aeronomy (IAGA), Madrid September 1-13, 1969 as a short invited paper and is a Scientific Report n° 350, March 10, 1970 of the Ionosphere

Research Laboratory, Pennsylvania State University. It will be published in Annales de Géophysique, 26,1970.

AVANT-PROPOS

"Ozone and Hydrogen Reactions" est l'exposé développé d'une courte communication donnant les résultats essentiels présentée à Madrid

lors de l'Assemblée Scientifique Générale de l'Association Internationale de Géomagnétisme et d'Aéronomie, Septembre 1-13, 1969. Cet exposé, qui est également présenté comme Sciéntific Report n° 350, March 10, 1970, Ionosphere Research Laboratory, The Pennsylvania State University, paraîtra dans le volume 26,1970 des Annales de Géophysique .

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VOORWOORD

"Ozon en waterstofreacties" is voorgedragen geworden op de Algemene Wetenschappelijke Vergadering van de Internationale Vereniging v o o r Geomagnetisme en Aeronomie, te Madrid van 1 tot 13 september 1969.

Deze uiteenzetting is reeds verschenen als het Scientific Report n° 3 5 0 , March 10, 1970, Ionosphere Research Laboratory, The Pennsylvania State University en z a l verder verschijnen in de Annales de Géophysique, band 2 6 , 1970.

VORWORT

"Ozone and Hydrogen Reactions" wurde in Madrid als eine kurze eingeladene Arbeit zur "Assemblée Scientifique Générale de l'Association Internationale de Géomagnétisme et d1Aéronomie",1-13 September, 1969 vorgestellt. Diese Arbeit ist auch als "Scientific Report n r 3 5 0 , March 10, 1970, Ionosphere Research Laboratory, The Pennsylvania State University", vorgestellt und wird in "Annales de Géophysique, 2 6 , 1970"

herausgegeben w e r d e n .

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OZONE AND HYDROGEN REACTIONS by

M. NICOLET Abstract

Solar radiation dissociates water vapor into hydrogen atoms and hydroxyl radicals in the thermosphere and upper mesosphere. In the stratosphere and lower mesosphere, dissociation of H^O is brought about mainly by a collision process with excited oxygen atoms produced by photodissociation of ozone. Such a collision process prevents the existence of normal mixing ratios for methane and molecular hydrogen at the stratopause. The rate of oxidizing processes falls off more

rapidly with increase of altitude than does that of the vertical transport by eddy diffusion.

Hydrogen molecules in the mesosphere are produced as a result of subsequent chemical reactions between hydroperoxyl radicals and hydrogen atoms. Hydrogen acts as a catalyst for the destruction of oxygen atoms and causes the ozone concentration to diminish pronouncedly in the mesosphere, from a factor of 1.5 at the stratopause to about a factor of 100 at the mesopause. The vertical distribution»of water vapor is not affected in the stratosphere and mesosphere by the dissociation process since its re-formation is rapid. In the thermosphere, water vapor diffuses into the photodissociative region by eddy diffusion which

carries it up at a sufficient rate to replace the hydrogen being lost.

Various reactions involving nitric oxide and nitrogen peroxide must be introduced in the reactions of the stratosphere. Such molecules

play an aeronomic role on the ratio of the hydroxyl and hydroperoxyl radical concentrations. In spite of a continuous production of hydrogen peroxide molecules, their concentration should be small. It is thought that reactions with nitrogen oxides lead to .nitric and nitrous acid mole- cules and are the mechanism responsible for the removal of nitric oxide which is produced in the thermosphere.

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Résumé

Si la radiation solaire ultraviolette photodissocie la vapeur d'eau surtout dans la thermosphère, cette molécule est en fait soumise à lâ dissociation dans la stratosphère et la mésosphère surtout par un processus d'oxydation très rapide avec des atomes excités résul-

tant de la photodissociation de l'ozone» Un tel processus d'oxydation conduit à la disparition partielle du méthane et de la molécule d'hydro- gène dans la stratosphère à un point tel qu'il existe une déficience, totale à la stratopause que le transport par diffusion turbulente ne peut compensero

Les molécules d'hydrogène sont en fait produites au sein de la mésosphère et en particulier à la mésopause par la réaction entre HO2 et H. L'hydrogène qui est un catalyseur dans son action de

destruction des atomes d'oxygène conduit ainsi à une diminution de la concentration d'ozone, en particulier dans la mésosphère, par rapport à sa concentration dans une atmosphère d'oxygène pur. Elle est diminuée d'un facteur 1,5 au niveau de la stratopause et est réduite à 1/100 à la mésopause.

Par suite des réactions rapides entre OH et HO2 amenant à sa reformation rapide, la vapeur d'eau maintient aisément sa distribution normale dans la stratosphère et la mésosphère. Quant à l'effet de la

photodissociation directe dans la thermosphère, il est fortement compensé par le transport de la vapeur d'eau par diffusion turbulente. En tout cas, l'hydrogène s'échappant de la thermosphère est facilement compensé par la diffusion.

Diverses réactions dans lesquelles sont impliqués l'oxyde et le peroxyde d'azote doivent être introduites pour l'étude aérono-

mique des "rapports des concentrations de OH et de H0„ dans la stratosphère.

D'ailleurs, leur effet se manifeste sur le comportement de l'eau

oxygénée. Il semble bien que le rôle de telles réactions est de produire en particulier l'acide nitrique qui par son passage dans la troposphère est ainsi la cause de la disparition de l'oxyde d'azote produit au-delà de 120 km dans la thermosphère.

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Samenvatting

In de thermósfeer en de opperste mesosfeer wordt, onder invloed van de zonnestraling, waterdamp ontbonden in waterstofatomen en hydroxyl- groepen. In de stratosfeer en de lagere mesosfeer wordt H2O voornamelijk ontbonden door een vlugge oxydatie ten gevolge van botsingen met aange- slagen zuurstofatomen die ontstaan door fotolyse van ozon. Deze botsingen verhinderen het bestaan van normale mengverhoudingen voor methaan en mole- culaire zuurstof aan de stratopauze en wel in zulke mate dat de verdwijning van deze bestanddelen de toevoer uit lagere gebieden overtreft.

In de mesosfeer en in het bijzonder aan de mesopauze ontstaan de waterstofmolecules ten gevolge van opeenvolgende scheikundige reacties tussen HC>2 en H. De waterstof treedt op als catalysator bij de verwijde- ring van de zuurstofatomen waardoor de ozonconcentratie drastisch afneemt in de mesosfeer in vergelijking met deze in een exclusieve zuurstofatmosfeer, nl. met een factor 1,5 ter hoogte van de stratopauze tot ongeveer een

factor 100 aan de mesopauze. De verticale verdeling van waterdamp in de stratosfeer en de mesosfeer wordt niet beïnvloed door dit ontbindingsproces aangezien hij vlug terug gevormd wordt.

Wervelingen in de thermosfeer zorgen er voor dat waterdamp het gebied bereikt waar fotolyse plaats heeft en wel zo dat de ontsnappende waterstof in voldoende mate aangevuld wordt.

Verschillende reacties waarbij NO en NO2 betrokken zijn moeten gevoegd worden bij de reacties in de stratosfeer. Deze moleculen spelen een aeronomische rol bij de bepaling van de verhouding tussen de concen- traties van OH en HO2. Niettegenstaande de aanhoudende vorming van H2O2 moleculen is hun concentratie eerder gering. Men onderstelt dat de

reacties, waarbij NO en NO2 betrokken zijn, door de vorming van salpeter- zuur tot de vernietiging van waterstofperoxyde leiden evenals tot de verdwijning van NO, die boven de 120 km in de thermosfeer gevormd wordt.

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Zusammenfassung

In der Thermosphäre und in der höheren Mesosphäre dissoziert die Sonnenstrahlung Wasserdampf in Wasserstoff und in OH Radikalen. In der Stratosphäre und niedrigen Mesosphäre, ist H2O meinstens durch

Stossvorgänge mit Sauerstoff Atomen, die aus der Ozon Photodissoziation kommen, dissoziert. Durch diese Stossvorgänge kann kein normales

Mischungsverhältnis für Methan und Molekularwasserstoff zur Stratopause stattfinden. Die senkrechte Eddydiffusion ist nicht stark genug„ um ein normales Mischungsverhältnis aufzubilden»

Molekularwasserstoff wird in der Mesosphäre und besonders zur Mesopause durch Reaktionen zwischen HO 2 und H verursacht. Wasserstoff wirkt als einen Katalysator ftlr die Vernichtung der Sauerstoff Atomen und vermindert auf dieser Weise die Ozondichte, besonders in der Mesosphäre. Die Ozondichte ist bei einem Faktor 1,5 zur Stratopause und 100 zur Mesopause vermindert. Die senkrechte Wasserdampfverteilung

ist, in der Stratosphäre und in der Mesosphäre, durch die Dissoziation nicht verändert, da die Wiederbildung, sehr rasch ist. In der Thermosphäre

ist die Photodissoziation durch Eddydiffusionstransport von Wasserdampf ausgegleicht.

Verschiedene Reaktionen, worin Stickstoffoxide und Stickstoff- peroxide eine Rolle spielen, sind sehr wichtig ftfr die Feststellung der Verhältnisse von OH und HO2 in der Stratosphäre. Ihre Einwirkung zeigt sich auch beim Wasserstoffperoxide Vertragen. Solche Reaktionen ftfhren wahrscheinlich zur Bildung der Stickstoff säuren, die auf dieser Weise ftfr das Verschwinden der oberhalb 120 km gebildeten Stickstoffoxide verantwortlich sind.

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I. INTRODUCTION. DISSOCIATION OF H20 , C H4 and H2

V i b r a t i o n a l - r o t a t i o n a l bands of the h y d r o x y l r a d i c a l OH, w h i c h a p p e a r in the a i r g l o w a r o u s e d i n t e r e s t in the p h o t o c h e m i s t r y of h y d r o g e n - o x y g e n c o m p o u n d s [ B A T E S and N I C O L E T , 1950 a; H E R Z B E R G , 1 9 5 1 ] and, in p a r t i c u l a r of m e t h a n e [ B A T E S and N I C O L E T , 1950 b; B A T E S and WITHERSPOON, 1 9 5 2 ] and of w a t e r v a p o r [ B A T E S and N I C O L E T ,

1950 c ] t w e n t y y e a r s a g o .

The w a t e r v a p o r w h i c h i s quite abundant n e a r g r o u n d l e v e l w i t h a _2

f r a c t i o n a l v o l u m e c o n c e n t r a t i o n r e a c h i n g 10 , d e c r e a s e s in m i x i n g r a t i o w i t h a l t i t u d e . The w a t e r v a p o r c o n t e n t i s v e r y s m a l l in the

s t r a t o s p h e r e ; 3 x 10"^ i s not an u n r e a s o n a b l e v a l u e to adopt f o r the o r d e r of m a g n i t u d e of the f r a c t i o n a l v o l u m e c o n c e n t r a t i o n of w a t e r v a p o r

[WILLIAMSON and HOUGHTON, 1965; M A S T E N B R O O K , 1968;

SISSENWINE et al, 1 96 8] in the s t r a t o s p h e r e . M e t h a n e and m o l e c u l a r h y d r o g e n , w h i c h h a v e b e e n found a s p e r m a n e n t c o n s t i t u e n t s of the

t r o p o s p h e r e h a v e c o n t i n u o u s s o u r c e s at g r o u n d l e v e l . T h e i r total a m o u n t s by v o l u m e a r e , r e s p e c t i v e l y , 1 . 5 x 10"^ and 0 . 5 x 10"^ of the m a j o r

g a s e s N^ and O^. U n l i k e w a t e r v a p o r , h o w e v e r , t h e i r f r a c t i o n a l

c o n c e n t r a t i o n s a r e not a f f e c t e d t h r o u g h the t r o p o s p h e r e . C o n s e q u e n t l y , a b o v e the t r o p o p a u s e , the c o m b i n e d H - a t o m c o n t e n t in CH^ i s i d e n t i c a l w i t h the a do pted c o n t e n t in HzO . T h u s , the total a m o u n t of f r e e h y d r o g e n

c a n n o t be f a r f r o m 10 - 5

W a t e r vapor c a n be d i s s o c i a t e d by s u n l i g h t ( F i g u r e 1) and i t s p h o t o d i s s o c i a t i o n c o e f f i c i e n t at 100 k m i s only

Ju n ( \ > 1750 A) = 1 . 7 x 10"6 s e c "1, (1) 2

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100

LU D 3

1 0 " 10" 10

PHOTODISSOCIATION COEFFICIENT (sec'M

F I G U R E 1

Photodissociation coefficient (sec S of water vapor in the

lower thermosphere and mesosphere for various solar zenith angles.

Overhead sun, sec x = * ! sec x = 2, 4 and 6 and sun at the horizon.

r\> •

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- 3 -

and at t h e m e s o p a u s e l e v e l , 85 k m ,

J__ ~(X > 1 7 5 0 A) = 1 x 1 0 "6 s e c (2)

H^jO • T h e r a d i a t i o n ( s e e F i g u r e 2) w h i c h i s n o t a t t e n u a t e d r e a c h e s the l o w e r

m e s o s p h e r e , and a t 65 k m the p h o t o d i s s o c i a t i o n c o e f f i c i e n t i s

JH 0( X > 1 7 5 0 A ) ='5 X 1 0"7 s e c _ 1 ( 3 )

B u t , a n i m p o r t a n t p a r t of the p h o t o d i s s o c i a t i o n of ^ O i n t h e u p p e r m e s o s p h e r e and in t h e l o w e r t h e r m o s p h e r e i s due to L y m a n - a w h i c h c a n r e a c h t h e 70 k m l e v e l a s i n d i c a t e d in F i g u r e 2 . T h e p r o c e s s

HzO + h v (X = 1216 A ) - H (2S ) + OH (2S )v, < 2 ( 4 )

i s p o s s i b l e , s i n c e a n e x p e r i m e n t a l a n a l y s i s [ C A R R I N G T O N , 1 9 6 4 ] s h o w s t h a t t h e f l u o r e s c e n c e p r o c e s s of the OH b a n d s i s b e t w e e n 1 a n d 10% of t h e t o t a l p r o c e s s . T h u s , a f l u o r e s c e n c e of OH b a n d s at 3 0 6 4 A o c c u r s i n the u p p e r m e s o s p h e r e and l o w e r t h e r m o s p h e r e .

M e t h a n e c a n b e a l s o d i s s o c i a t e d by s u n l i g h t . H o w e v e r , s i n c e t h e a b s o r p t i o n c r o s s s e c t i o n i s v e r y s m a l l a t X > 1 5 0 0 A, the a e r o n o m i c p h o t o d i s s o c i a t i o n of CH^ a r i s e s f r o m L y m a n - a w i t h p h o t o d i s s o c i a t i o n c o e f f i c i e n t at z e r o o p t i c a l d e p t h

J___ (X = 1216 A) = 5 x 1 0 ~6 s e c "1. (5)

T h e p h o t o d i s s o c i a t i o n c o e f f i c i e n t i s a d e c r e a s i n g f u n c t i o n of t h e o p t i c a l d e p t h i n the m e s o s p h e r e and i s n e g l i g i b l e i n the l o w e r m e s o s p h e r e ( F i g u r e 3 ) .

) 1

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F I G U R E 2

P h o t o d i s s o c i a t i o n c o e f f i c i e n t of w a t e r v a p o r f o r r a d i a t i o n of X > 1724 A and f o r L y m a n a . The d i s s o c i a t i o n n e a r the m e s o p a u s e

l e v e l i s p r i n c i p a l l y due to L y m a n a .

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FIGURE 3

Photodissociation coefficient of methane in the lower thermosphere and mesosphere for various solar zenith angles by solar radiation at Lyman-o.

Overhead sun, sec x = 1; s e c X = 2, 4 and 6 and sun at the horizon (x = 90°).

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Among the other hydrogen oxides which may be present in the upper atmosphere, H 0

2

and H

2

C>

2

can be subject to photodissociation. The photodissociation of H

2

0

2

is known from laboratory measurements [UREY et al, 1929; HOLT et al, 1948] . The photodissociation coefficient for zero optical depth is (see Figure 4)

The photolysis of H

2

C>

2

cannot occur in the far ultraviolet and below -5 -1 2100 A the photodissociation coefficient is less than 10 sec . There- fore the fluorescence of OH bands cannot be an important aeronomic process.

Photodissociation is not the only disruptive process of H

z

O

%

CH. and H-O-. Among the primary processes likely to be important, 3 the reaction with atomic oxygen must be considered since 0( P) atoms exist in a sunlit atmosphere where ozone is present. However, all reactions are endothermic; the oxidation of water vapor is impossible and that of methane and molecular hydrogen requires an activation energy between 8 and 10 kcal/mole.

Considering that atomic oxygen can be in the excited state, other reactions are possible in the stratosphere and mesosphere

[CADLE, 1964]. Recent experimental evidence [ DEMORE and RAPER, 1967; DEMORE, 1967a; ENGLEMAN, 1965] indicates that the rate coefficients of oxidizing reactions with 0(

l

D) atoms are of the order of 1 0 "

1 0

c m

3

sec"

1

. The following reactions occur in the stratosphere

= 1. 2 x 10 sec -4 -1 (6) J

O^D) + H 1

2

— H + OH* (v < 4) + 43. 5 kcal

(V)

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P H O T O D I S S O C I AT ION C O E F F I C I ENT (sec"

1

)

F I G U R E 4

P h o t o d i s s o c i a t i o n c o e f f i c i e n t of h y d r o g e n p e r o x i d e i n t h e m e s o s p h e r e a n d s t r a t o s p h e r e f o r v a r i o u s s o l a r z e n i t h a n g l e s .

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0(lD) + C H4 — C H3 + OH* (v < 4) + 4 3 . 5 k c a l (8)

cVd ) + HzO — OH + OH* (v < 2) + 2 8 . 8 k c a l . (9)

T h u s , the p r e s e n c e of 0(lD) a t o m s l e a d s to the p o s s i b i l i t y of the

p r o d u c t i o n of h y d r o g e n a t o m s by C H ^ H2 and H20 . With the f r a c t i o n a l v o l u m e c o n c e n t r a t i o n s w h i c h h a v e b e e n adopted, the total p r o d u c t i o n P(H)

of h y d r o g e n a t o m s i s

P ( H ) = 13 x 1 0 "1 6 n(M) n[ O ^ D ) ] c m "3 s e c '1 (10)

if n(M) is the total c o n c e n t r a t i o n and if the a v e r a g e r a t e c o e f f i c i e n t of 1 0 ~1 0 c m3 s e c "1 is adopted.

II. P R O D U C T I O N O F E X C I T E D O X Y G E N A T O M S

When an a n a l y s i s of the ozone.photodissociation i s m a d e , it is

n e c e s s a r y to d i s t i n g u i s h b e t w e e n the v a r i o u s p r o d u c t s in o r d e r to show the p r o d u c t i o n of e x c i t e d a t o m s o r m o l e c u l e s . In the s p e c t r a l r a n g e of the Huggins bands, an u n i m p o r t a n t p r o c e s s is

O- + hv (X < 4 1 2 5 A) — 02(3Z " ) + 0 (1D ) (11a)

J &

but at s h o r t e r w a v e l e n g t h s an i m p o r t a n t p r o c e s s m u s t be

O- + hv ( X < 3 1 1 0 A) — 02 (JA ) + O ^ D ) . ( l i b )

J O

An i m p o r t a n t e x p e r i m e n t a l r e s u l t [ D E M O R E and R A P E R , 1 9 6 6 ] is that at X < 3 0 0 0 A the p h o t o d i s s o c i a t i o n p r o c e s s l e a d s to a c o m p l e t e p r o - duction of O ^ D ) a t o m s and that at X = 3 1 3 0 A the O ^ D ) p r o d u c t i o n is only 0. 4 ± U. 15 of the total n u m b e r of a t o m s . T h e total n u m b e r of

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p h o t o d i s s o c i a t i o n p r o c e s s e s of O^ l e a d i n g to 0 ( D) a t o m s i s about

6 - 3 - 1

10 c m " s e c in the upper m e s o s p h e r e and r e a c h e s a m a x i m u m of 9 - 3 - 1

about 10 c m s e c f o r a n o v e r h e a d s u n b e l o w the s t r a t o p a u s e ( s e e F i g u r e 5). The v a r i a t i o n of the p r o d u c t i o n of 0 ( * D ) a t o m s w i t h the s o l a r z e n i t h a n g l e i s i m p o r t a n t and l e a d s to a r a p i d d e c r e a s e f r o m the s t r a t o p a u s e to the t r o p o p a u s e .

In o r d e r to d e t e r m i n e the c o n c e n t r a t i o n of 0 ( * D ) a t o m s in a s u n l i t a t m o s p h e r e it i s n e c e s s a r y to i n t r o d u c e the e f f e c t of the p r i n c i p a l l o s s p r o c e s s e s , n a m e l y d e a c t i v a t i o n b y N ^ and O^. A l l e x p e r i m e n t a l v a l u e s [ D E M O R E and R A P E R , 1964, S N E L L I N G and BAIR, 1967; YOUNG and B L A C K , 1967; YOUNG et al, 1969; NOXON, 1 9 6 9 ] s h o w that the - 1 1 3 - 1

quenching r a t e c o e f i c i e n t i s g r e a t e r than 10 c m s e c and m a y r e a c h 10 ^ c m3 s e c " * . The f o l l o w i n g w o r k i n g v a l u e f o r the w h o l e h o m o s p h e r e i s a d o p te d h e r e

kQ (1D , M) = 5 x 1 0 "1 1 c m3 s e c "1 (12a)

w h i c h l e a d s to

n t O ^ D ) ] = 2 x 1 01 0 n ( 03) J3 (JD ) / n(M) (12b)

w h e r e n(O^) J ^ ^ D ) i s the p r o d u c t i o n of O(^D) a t o m s i l l u s t r a t e d in F i g u r e 5. The p h o t o e q u i l i b r i u m c o n c e n t r a t i o n s f o r v a r i o u s s o l a r z e n i t h a n g l e s a r e g i v e n in T a b l e 1; the e r r o r i n v o l v e d s h o u l d be l e s s

3 - 3

than 50%. A c o n c e n t r a t i o n peak l e s s than 10 c m o c c u r s n e a r the s t r a t o p a u s e f o r an o v e r h e a d s u n . C o n c e n t r a t i o n s of Q(*D) g r e a t e r

3 - 3

than 10 c m a r e not p o s s i b l e .

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GOD) ATOM PRODUCTION ( c m - H e c -

1

)

F I G U R E 5

P r o d u c t i o n of O(^D) a t o m s by the ozone p h o t o d i s s o c i a t i o n f o r v a r i o u s s o l a r z e n i t h a n g l e s . O v e r h e a d s u n , s e c X

=

^

s e c x

=

s u n at the h o r i z o n , x

=

9 0 ° .

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TABLE 1

Concentration of O(^D) in a Sunlit Atmosphere Equilibrium Conditions

Altitude secx = 1 secx

=

2 secx=4 secx =6 Horizon (km) ( c m "

3

) (cm ) ( c m "

3

) ( c m

- 3

) ( c m

- 3

)

20 2. 5 0. 6 5.6 6.2

-

25 1 . l x l O

1

3.2 5 . l x l O "

1

l.OxlO"

1 -

30 4.4x10

1

1.6X10

1

4. 0 1.4 2.7x10

35 1 . 5 x l 0

2

6.6X10

1

2.4X10

1

1 . l x l 0

1

4.OxlO"

4

40 4 . 4 x 1 0

2

2 . l x l 0

2

9.9x10

1

5.9x10

1

5 . 2 x l 0

_ 1

45 7 . 8 x l 0

2

4 . 3 x l 0

2

2 . l x l O

2

1.4x10

2

8. 0 50 7 . 9 x l 0

2

6.OxlO

2

3 . 8 x 1 0

2

2 . 6 x l 0

2

2. 7X10

1

55 5 . 6 x l 0

2

5 . l x l O

2

4 . 3 x l 0

2

3 .iixlO

2

5. 7x10

1

60 3 . 3 x l 0

2

3 . 2 x 1 0

2

3.OxlO

2

2. 9 x l 0

2

1.OxlO

2

65 . 2. OxlO

2

1 . 9 x l 0

2

1 . 9 x l 0

2

1 . 9 x l 0

2

1 . 3 x l 0

2

70 1. 2 x l 0

2

1 . 2 x l 0

2

1 . 2 x l 0

2

1. 2 x l 0

2

1.OxlO

2

At the mesopause, the concentration of 0( D) atoms begins to increase again due to the photodissociation of O^ in the Schumann-Runge continuum.

For an overhead sun the following concentrations are obtained:

Altitude (km) 95 100 105 110 115 120 Concentration O^D) 4 x l 0

2

10

3

2 x l 0

3

4xl0

3

5 x l 0

3

4 x l 0

3

( c m

- 3

)

As far as the O(^S) atoms are concerned, their low collisional deactivation with N

2

[STHUL and WELGE, 1969]

k (

1

S , M

2

) < 5 x 1 0 "

1 7

c m

3

s e c "

1

(13a)

(19)

- 1 2 -

and their moderate deactivation by C>

2

[STHUL and WELGE, 1969; BLACK et al, 1969]

k

O-) = 3 Q ^

to

5

X

10' c m sec" (13b) lead to equilibrium conditions written as follows:

n[0(lS)] = 1 0 1 3 n ( 0 3 ) J 3 (lS)/n(U) . ( 1 3 c )

Thus, equations (12b) and (13c) lead to

n[Q(lS)] 5 x 1 02 J 3 (1S )

(14)

n[Q(lD)] J 3 ( * D )

Numerical results for an overhead sun a r e given in the following table:

Altitude (km) 100 50 30

j (Ig, (sec"

1

) 3 . 5 x l 0 "

6

2 . 7 x l 0 "

6

3 . 0 x l 0 "

7

J j ^ D ) (sec"

1

)

8 . 6 X 1 0 "3

3 . 4 x l 0 "

3

2 . I x l O "

4

ntO^Sjl/ntO^D)] 0.20 0.40 0.71 F r o m that table in which J ^ S ) is the m a x i m u m possible value it is

clear that the concentrations of the oxygen atoms produced by the ozone photodissociation are a fraction of that of the oxygen atoms.

An oxidizing reaction with O^S)-atoms requires a.rate coefficient greater than rate coefficients withO^D) in order to be compared with (7), (8) and (9). This may happen for water vapor but not for molecular hydrogen [STHUL and WELGE, 1969]-

•HI. STRATOSPHERIC CONDITIONS

F r o m the foregoing discussion it is apparent that the dissociation

mechanism of H

2

0 , C H

4

and H

2

in the stratosphere is an oxidation process

(20)

by ^D o x y g e n a t o m s . H o w e v e r , the v e r t i c a l d i s t r i b u t i o n of H^, CH^ and H^O in the s t r a t o s p h e r e c a n be m a i n t a i n e d by a t m o s p h e r i c m i x i n g only if the o x i d a t i o n p r o c e s s i s a v e r y s l o w r e a c t i o n [ B A T E S and N I C O L E T ,

1 9 6 5 ] . .

In the p r e s e n c e of eddy d i f f u s i o n , the v e r t i c a l s p e e d w'j of c o n s t i t u e n t of s c a l e h e i g h t H j c a n be w r i t t e n a s f o l l o w s [ N I C O L E T , 1 9 68 ]

D w =

1 H

i L - 1

H1

(15)

in w h i c h Dg i s the eddy d i f f u s i o n c o e f f i c i e n t and H i s the a t m o s p h e r i c

s c a l e h e i g h t . The d i f f u s i v e u p w a r d c u r r e n t of CH^ m o l e c u l e s i s , t h e r e f o r e g i v e n by

FC H = n<C H4 > WC H4 ( 1 6 )

4 4 and m u s t c o r r e s p o n d to about

FC Ha = I O - C H . n*( l D ) nC H . HC H4 ( 1 7 )

4 4 4 4

in o r d e r to r e p l a c e the m e t h a n e w h i c h i s d e s t r o y e d c h e m i c a l l y in a sunlit' a t m o s p h e r e by r e a c t i o n w i t h e x c i t e d o x y g e n a t o m s . U s i n g n u m e r i c a l v a l u e s in (16) and (17) it i s found that eddy d i f f u s i o n i s n e c e s s a r y to m a i n t a i n CH^ m o l e c u l e s in the s t r a t o s p h e r e .

If the r a t i o of the l o c a l s c a l e h e i g h t H ( C H4) / H ( M ) = 1 / 2 it i s found 4 2 - 1

that an eddy d i f f u s i o n c o e f f i c i e n t g r e a t e r than 10 c m s e c i s r e q u i r e d n e a r the s t r a t o p a u s e ( cf F i g u r e 6). F u r t h e r m o r e , a m i x i n g d i s t r i b u t i o n of CH^, i . e . a s c a l e h e i g h t , H = 1. 1 H(CH^) i m p l i e s a p e r m a n e n t eddy 5 2 - 1 d i f f u s i o n c o e f f i c i e n t g r e a t e r than 10 c m s e c ( F i g . 6). It s e e m s , t h e r e f o r e , that it i s e x t r e m e l y d i f f i c u l t to m a i n t a i n a c o n s t a n t m i x i n g r a t i o of m e t h a n e in the s t r a t o s p h e r e .

(21)

F I G U R E 6

E d d y d i f f u s i o n c o e f f i c i e n t n e c e s s a r y t o m a i n t a i n m e t h a n e i n t h e s t r a t o s p h e r e w i t h s c a l e h e i g h t H(CH4) = 0 . 5 H(M) a n d H(CH4) = 0 . 9 H(M), r e s p e c t i v e l y ; H(M) i s t h e l o c a l a t m o s p h e r i c

s c a l e h e i g h t . M e t h a n e i s o x i d i z e d b y e x c i t e d a t o m s O ( I D ) .

(22)

By an argument s i m i l a r to that u s e d in connection with the d i s t r i - bution of methane, it m a y be found that H^ undergoes the s a m e p r o c e s s e s ( F i g u r e 7). It is c l e a r a l s o that the r e m o v a l of a single atom of H^O

through a r e a c t i o n with an excited oxygen atom l e a d s to a d i s s o c i a t i o n of the molecule at a r a t e which h a s a peak in the upper s t r a t o s p h e r e . N u m e r i c a l r e s u l t s corresponding to H = 2 H f t ^ O ) and H = 1. 1 H(H,,0) a r e i l l u s t r a t e d in F i g u r e 8 where eddy diffusion coefficients at the s t r a t o p a u s e should

4 2 -1 5 2 -1

be not l e s s than 10 c m s e c and 10 c m s e c , r e s p e c t i v e l y . In the m e s o s p h e r e , where the photodissociation of water va,por o c c u r s , it is

6 2 - 1

c l e a r that the eddy diffusion m u s t go up to 5 x 10 c m s e c to maintain a mixing distribution at the m e s o p a u s e .

Since it is e x t r e m e l y difficult to a s s u m e that p e r m a n e n t eddy 5 2 - 1 diffusion coefficients a r e m o r e than 10 cm s e c in the upper s t r a t o - s p h e r e , a d e c r e a s e of the r a t i o s n t H ^ / n f M ) and n(CH^)/n(M) m u s t be c o n s i d e r e d in the s t r a t o s p h e r e . It s e e m s , t h e r e f o r e , that one may

a s s u m e that in the region below 3 5 km, H^ and CH^ a r e oxidized and l e a d to H^O, introducing the possibility of an i n c r e a s e in the mixing ratio

n(H2,0)/n(M). The behavior of water vapor is completely different f r o m that of methane and m o l e c u l a r hydrogen for which there is no possibility of i m m e d i a t e r e - f o r m a t i o n after the attack by the oxidation p r o c e s s e s .

All products of the oxidation and d i s s o c i a t i o n of H^, CH^ and H^O lead to the f o r m a t i o n of water vapor through atomic hydrogen, hydroxyl and hydroperoxyl r a d i c a l s . When a uniform mixing is accepted for I ^ O in the s t r a t o s p h e r e , the hypothesis is made that water vapor is involved in a sufficiently rapid catalytic action so that an anomalously high eddy 5 2 -1 diffusion coefficient (>10 c m s e c at the s t r a t o p a u s e , cf F i g u r e 8) is not r e q u i r e d . At the m e s o p a u s e and in the lower t h e r m o s p h e r e ,

(23)

EDDY D I F F U S I O N COEFFICI ENT ( c m

2

s e c -

!

)

F I G U R E 7

E d d y d i f f u s i o n c o e f f i c i e n t n e c e s s a r y to m a i n t a i n m o l e c u l a r h y d r o g e n i n t h e s t r a t o s p h e r e w i t h a s c a l e h e i g h t H(H2) = 0 . 5 H(M) a n d H(CH2) = 0 . 9 H(M), r e s p e c t i v e l y ; H(M) i s t h e l o c a l a t m o s p h e r i c

s c a l e h e i g h t . H2 i s d e s t r o y e d b y O ( l D ) .

(24)

EDDY DIFFUSION COEFFI CIENT (cm

2

sec

-1

)

F I G U R E 8

E d d y d i f f u s i o n c o e f f i c i e n t w h i c h w o u l d b e n e c e s s a r y to m a i n t a i n w a t e r v a p o r f r o m t h e s t r a t o s p h e r e to l o w e r t h e r m o s p h e r e w i t h n o r e - f o r m a t i o n p r o c e s s of H2O; H(M) = 2 H ( H20 ) a n d H(M) = 1 . 1 H ( H 2 0 ) c o r r e s p o n d to t h e l o c a l a t m o s p h e r i c s c a l e h e i g h t a n d t h e a s s u m e d w a t e r v a p o r s c a l e h e i g h t .

(25)

- 1 8 -

w h e r e t h e H ^ O r e - f o r m a t i o n i s s l o w , t h e e d d y d i f f u s i o n l e a d s t o a v e r t i c a l t r a n s p o r t of H ^ O m o l e c u l e s f r o m t h e m e s o s p h e r e .

I V . H Y D R O G E N - O X Y G E N A T M O S P H E R E

I n o r d e r t o d e s c r i b e a h y d r o g e n - o x y g e n a t m o s p h e r e i n a s i m p l e w a y , i t i s n e c e s s a r y t o e l i m i n a t e t h e l e s s p r o b a b l e r e a c t i o n s , [ N I C O L E T , 1 9 6 6 ] .

T h e p r i n c i p a l r e a c t i o n s b e t w e e n a t o m i c h y d r o g e n , h y d r o x y l a n d h y d r o p e r o x y l r a d i c a l s i n a n o z o n e - a t o m i c o x y g e n a t m o s p h e r e a r e s h o w n i n F i g u r e 9 . It i s c l e a r t h a t t h e a t o m i c h y d r o g e n c o n c e n t r a t i o n m u s t i n c r e a s e w i t h t h e a t o m i c o x y g e n c o n c e n t r a t i o n a n d t h a t t h e h y d r o p e r o x y l r a d i c a l c o n c e n t r a t i o n i n c r e a s e s w i t h t h e o z o n e c o n c e n t r a t i o n .

A s i m p l e e x p r e s s i o n f o r t h e r a t i o n ( O H ) / n ( H ) i n t h e w h o l e h o m o s p h e r e w h e r e a n a d e q u a t e n u m b e r of o x y g e n a t o m s a r e p r e s e n t i s

n ( O H ) a . n ( M ) n(O) + a , n ( O ^ )

= - J ^ . (18)

n(H) a5 n ( O )

In t h e s a m e w a y t h e e x p r e s s i o n f o r t h e e q u i l i b r i u m r a t i o n ( H 02) / n ( H ) i s o b t a i n e d

n ( H 02) a j n ( M ) n ( Oz) a& n ( 03) n ( M ) n ( Oz) + a2 n ( 03)

_ + x #

n(H) a? n ( O ) a5 n(O) a? n ( O )

(19)

E x p r e s s i o n s (18) a n d ( 1 9 ) , a s s u m i n g a ^ = a ^ , l e a d t o

n ( H O ) a n ( M ) n ( 0?) a , n ( 0 )

= 1 £ + _ b ^ ( 2 Q )

n ( O H ) 3il n ( M ) n ( Oz) + a2 n ( 03) a? n ( O )

i

(26)

0 OH H H0 2 - 0 3

nO

I

F I G U R E 9

P r i n c i p a l r e a c t i o n s in w h i c h H, OH and HC>2

a r e involved w i t h O and O^.

(27)

- 2 0 -

In equations (18), (19) and (20), the three-body reaction of atomic hydrogen with molecular oxygen is of greater importance;

(a

x

) ; H + 0

2

+ M — HC>

2

+ M + 46 kcal (21a) hag a rate coefficient [CLYNE and THRUSH; 1963b] which is taken for

aeronomic purposes as

a

2

n(M) = 3.3 x 1 0 ~

3 3

e

8 0 0 / T

n(N

2

, CL,) c m

3

s e c "

1

. (21b) The other equally important loss process for atomic hydrogen is

(a

2

) ; H + 0

3

— 0

2

+ OH*

v < 9

+ 77 kcal (22a) for which a rate coefficient [PHILIPS and SCHIFF, 1962; KAUFMAN,

1964, 1969] with practically no activation energy is

a

2

= 1. 5 x 1 0 "

1 2

T

1 / 2

c m

3

s e c "

1

. (22b) Detailed studies have been made of the visible and infrared emission

of this reaction in the laboratory [McKinley et al, 1955 ; GARVIN and McKINLEY, 1956; CAWTHON and McKINLEY, 1956; GARVIN, 1959;

GARVIN et al, I960 and 1962; BASS and GARVIN, 1962; ANLAUF et al, 1968].

The infrared chemiluminescence arising f r o m this reaction

[ ANLAUF et al, 1968] indicates a vibrational distribution of OH with increasing populations up to v

M

= 9 the highest possible level according to (22). The total emission rate in the night airglow must correspond

1 2 - 2

to 5 x 10 photons cm

[WALLACE, 1962] in the lower thermosphere

and perhaps also in the upper m e s o s p h e r e [PACKER, 1961] .

(28)

- 21 -

The p r o d u c t i o n of h y d r o p e r o x y l r a d i c a l s by

H + 03 — O + H Oz + 22 k c a l (23)

w i t h a r a t e c o e f f i c i e n t a^ w h i c h h a s not y e t b e e n m e a s u r e d , and by a t h r e e - b o d y a s s o c i a t i o n

OH + O + M M + H Oz + 63 k c a l (24)

w i t h a c o n v e n t i o n a l v a l u e of the t h r e e - b o d y r a t e c o e f f i c i e n t a^, a r e n e g l e c t e d c o m p a r e d w i t h r e a c t i o n (21).

The b i m o l e c u l a r p r o c e s s

( a5) ; OH + O — H + 02 + 1 6 . 6 k c a l (25a)

i s an i m p o r t a n t r e a c t i o n [ C L Y N E and THRUSH, 1963 a; K A U F M A N , 1964, 1 96 9] w i t h a r a t e c o e f f i c i e n t

a = 3 x 1 0 '1 2 T1 / 2 c m3 s e c "1 (25b)

D

1 1 3 1 l e a d i n g to the tfalue 5 x 10 c m s e c at 0 C.

T h i s r e a c t i o n (25) in c o n j u n c t i o n w i t h (22) f o r m s an i m p o r t a n t c h a i n l e a d i n g to the f o r m a t i o n of o x y g e n m o l e c u l e s . In o t h e r w o r d s , a t o m i c h y d r o g e n a c t s a s a c a t a l y s t f o r the d e s t r u c t i o n . o f odd o x y g e n a t o m s , and the a t o m i c o x y g e n and o z o n e d i s t r i b u t i o n s a r e a f f e c t e d . The c a t a l y t i c a c t i o n of h y d r o g e n i s a l s o of i n t e r e s t , in that it i n f l u e n c e s the n o c t u r n a l c o n c e n t r a t i o n s .

Other r e a c t i o n s i n v o l v e h y d r o x y l r a d i c a l s and o z o n e . The r e a c t i o n

( a6) ; OH + 03 — H Oz + Oz + 39 k c a l (26a)

(29)

- 2 2 -

for which no direct m e a s u r e m e n t has been reported should have an upper -13 3 -1

limit of about 5 x 10 cm sec at r o o m temperature [KAUFMAN, 1964, 1969] . With an activation energy of 4 kcal [BATES and NICOLET, 1950c], the following rate coefficient

. , ,„-11 „ 1 / 2 -2000/T 3 -1

a. = 1.5 x 10 T e . c m sec (26b) 6

13 3 1 o 15 leads to 1. 5 x 10 cm sec at the stratopause (270 K) and 5 x 1 0

3 1 o

cm sec at the tropopause (190 K) which is less than the value which has been used in various aeronomic calculations [HUNT, 1966 ; HESSTVEDT, 1968]. With a steric hindrance factor, it becomes

. ,

i n

- 1 2 1/2 -1500/T 3 -1 , , .

a^ = 1 . 5 x 1 0 T e cm sec (26c)

- 1 3 3 - 1 - 1 5 3 which leads to 10 cm sec at the stratopause and 7. 5 x 10 cm

sec * at the tropopause ; (26c) should be a m a x i m u m value. Neverless, such a reaction should be considered as a loss process of hydroxyl

radicals; it has been introduced to explain the photolysis of ozone in the presence of water vapor. However, the following reaction between ozone and hydroperoxyl radicals has been also introduced in the OH-catalyzed chain decomposition of ozone

(a, ); HO '+ O- — OH + 20

DC u J d. o

+ 31 kcal . (26) There is no experimental evidence for such a reaction [DEMORE, 1967b] ;

in any case its rate coefficient must be very small since such a reaction requires the simultaneous breakage of. the bonds OH-O and O^-O. The fact that OH radicals are involved in the catalyzed chain decomposition of O^ in laboratory investigation is explained by the presence of vibration- aliy excited OH molecules which are a major reaction product [KAUFMAN,

1964; DEMORE, 1967b]. In stratospheric conditions, such OH' excited

(30)

- 23 -

molecules cannot play an active role and HO^ as a chain c a r r i e r [HAMPSON, 1965; HUNT, 1966; HESSTVEDT, 1968; CRUTZEN, 1969] is therefore

doubtful. In fact, the principal reaction leading to OH involved atomic oxygen [KAUFMAN, 1964].

(a

?

); O + H O

z

— O

z

+ OH*

v

<

6

+ 55 kcal (27a) for which a rate coefficient

a

?

= 3 x 1 0 '

1 2

T

1 / 2

c m

3

s e c "

1

(27b) identical to a

5

is adopted in order to simplify the aeronomic analysis

(cf equation 20), even if it is perhaps a factor of two too high [KAUFMAN, 1964].

It may be noted that reaction (27) may lead to excited OH molecules up to the level of vibrational quantum numbers v < 6 while reaction (22) can provide an energy to levels 7, 8 and 9 f r o m which emission bands a r e observed in the airglow. Nevertheless, since atomic oxygen is an i m p o r - tant minor conslituent in the mesosphere, reaction (27) may lead to an additional emission of OH bands originating f r o m levels of vibrational quantum number 6 or less, in the dayglow when atomic oxygen is

present. It is expected, however, that deactivation p r o c e s s e s of excited OH molecules are important in the middle and lower m e s o s p h e r e .

Reaction (22) leads to a peak in the lower themosphere where the night time emission is observed while (27)can play a role only below the mesopause.

Expressions (18) and (19) a r e obtained since it is a s s u m e d that

a

?

n(O) > J

H Q

(28)

(31)

- 24 -

i . e . that the photodissociation coefficient of the hydroperoxyl radical is less important than the loss coefficient by reaction with atomic oxygen.

Nothing is known about the photodissociation of the hydroperoxyl radical.

Due to the similarity of the structure of HC^ and that of hydrogen peroxide, -4 -1

for which J-, ~ = 1 . 4 x 10 sec and also to the high photodissociation

H

? 2

L

- 3 - 1

coefficient of nitrogen peroxide J-J^Q = 3 . 5 X 10 sec , possible values 2 such as

1 0 "

3

< J

H Q

< 1 0 "

4

s e c

_ 1

(29) can be suggested. Thus, the photodissociation could play a role on the

ratio nfOHj/nlHO^) in the lower stratosphere.

V. AERONOMIC PROCESSES OF WATER VAPOR

After the photodissociation of water vapor in the m e s o s p h e r e and lower thermosphere and the oxidizing reaction with O(^D) atoms

(equation 9), the reactions between two hydroxyl radicals lead to its r e - f o r m a t i o n

( a

l 6

) ; OH + OH — H

z

O + O + 17 kcal (30a) with an activation energy not greater than 2 kcal [ KAUFMAN. 1964, 1969;

WESTENBERG and DE HAAS, 1965; WILSON and O'DONOVAN, 1967].

A minimum rate coefficient in the stratosphere and m e s o s p h e r e will be given by

_ . . - 1 2 „ 1 / 2 -1000/ T 3 -1

a ^ = 5 x 1 0 T e cm sec . (30b)

In the same way, the reaction between hydroxyl and perhydroxyl radicals

(32)

- 25 -

l e a d s to HzO [ F O N E R and HUDSON, 1962; KAUFMAN, 1964]

( a1 ?) ; OH + H 02— HzO + C>2 + 72 kcal (31a)

with p e r h a p s a s m a l l activation energy ( ~ 1 kcal) to l e a d to a r a t e coefficient such a s

a t a x i c " 1 3 T 1 / 2e - 5 0 0 / T . (31b)

Other r e a c t i o n s can l e a d to the f o r m a t i o n of H2Q , but they a r e not important in the m e s o s p h e r e and s t r a t o s p h e r e c o m p a r e d with other r e a c t i o n s leading to its r e - f o r m a t i o n .

Another two-body p r o c e s s yielding hydrogen peroxide m u s t be c o n s i d e r e d

( a2 ?) ; H 02 + HOz — H202 + Oz + 42 kcal (32a) for which we adopt [ N I C O L E T , 1964] a r a t e coefficient which is s i m i l a r to al 6

- , n- 1 2 _ l / 2 - 1 0 0 0 / T 3 -1

a2 7 = 5 x 10 T e c m s e c . (32b)

Inside the m e s o s p h e r e and in the whole s t r a t o s p h e r e , the following equation m a y be u s e d

dn(OH) dh(HO,) 7 2

— + d t L + 2 al 6 n (OH) + 2 a1 ? n(OH) n(HOz) + 2 a2 ? n (HC>2) = 2 P(H) (33)

where P(H) is the d i r e c t or indirect production of hydrogen a t o m s , and in order to s i m p l i f y for an a p p r o x i m a t e calculation a ^ = a2 7 = a ^ / 2 ,

(33)

- 2 6 -

dn(OH) d n ( H Oz)

+ 2 al 6 [n(OH) + n(HC>2)]2 = 2 P ( H )

dt + dt 16

(34a)

F o r n i g h t - t i m e c o n d i t i o n s (34a) b e c o m e s

dn(OH) d n ( H Oz)

+ 2 a1 6 [n(OH) + n(HC>2)]2 = 0 (34b)

dt dt 16

w h i c h shows that t h e r e i s a n o c t u r n a l d e c a y [ N I C O L E T , 1964] of the O H and H 02 c o n c e n t r a t i o n s i n the s t r a t o s p h e r e a n d m e s o s p h e r e . A t the m e s o p a u s e l e v e l , the p h o t o d i s s o c i a t i o n of H20 by Lyman-or is the p r i n c i p a l p r o d u c t i o n p r o c e s s . A t l o w e r a l t i t u d e s the r e a c t i o n of O ( ^ D ) a t o m s w i t h H ^ O and other h y d r o g e n c o m p o ù n d s is the s o u r c e . Table II i n d i c a t e s the v a r i a t i o n of the p h o t o d i s s o c i a t i o n c o e f f i c i e n t of H20 w i t h a l t i t u d e and the s o l a r z e n i t h a n g l e . A c o m p l e t e k n o w l e d g e (see F i g u r e 8) of the v e r t i c a l eddy d i f f u s i o n c o e f f i c i e n t is needed i n o r d e r to d e t e r m i n e

5 2 the exact v e r t i c a l d i s t r i b u t i o n of H20 . V a l u e s v a r y i n g f r o m 5 x 1 0 c m s e c " * and 2 x 10^ c m2 sec * at 65 k m to 7 x 10^ c m2 sec * h a v e been a s s u m e d [ H E S S T V E D T , 1968]. If the v e r t i c a l eddy d i f f u s i o n c o e f f i c i e n t 5 2 -1 is g r e a t e r than 10 c m sec , it is c e r t a i n ( F i g u r e 8) that the s c a l e h e i g h t of w a t e r v a p o r H ( H20 ) > ^ H the a t m o s p h e r i c s c a l e h e i g h t .

6 7 2 -1

E d d y d i f f u s i o n c o e f f i c i e n t s between 10 and 10 c m sec i n d i c a t e that n ( H _ 0 ) / n ( M ) w i l l not f a l l off s h a r p l y t h r o u g h the l o w e r t h e r m o s p h e r e .

(34)

- 27 - s

TABLE II

Photodissociation Coefficient of Water Vapor by Lyman- a

Altitude secx-1 Ratio Ratio Ratio

(km) (sec secx

=

4 / s e c x = 1 secx=6/secx=l H o r i z o n / s e c x

=

1 90 4. OxlO"

6

0. 76 0. 63 2 . 2 4 x l 0 "

2

85 3.5x10 0. 52 0.31 1. -1 OxlO"

4

80 2. 6 x l 0 "

6

0. 21 7 . 3 x l 0 "

2

6 . 8 x l 0 "

1 0

75 1 . 3 x l 0 "

6

2 . 7 x l 0 "

2

2 . 5 x l 0 "

3 -

70 3.OxlO"

7

3 . 3 x l 0

_ 1

1.6x10"®

-

65 1. 5 x l 0 "

8 _ _ _

In the lower mesosphere and stratosphere it is possible to deduce the photochemical conditions with (34). The essential result (see Table III) is that the time to reach the equilibrium (50% of the equilibrium concen- tration or f r o m 50% to 80%) is less than three hours in the whole

stratosphere and the mesosphere up to 70 km. Thus, H O can be kept 2 in mixing conditions since loss and production processes of water vapor are sufficiently rapid. On the other hand, equilibrium conditions can be used to compute (see Table III) the total concentration n(OH) + nfHC^)

7 -3 8 -3

which is not less than 10 cm and reaches 10 cm in the upper

stratosphere. Numerical values which are given in Table III correspond to overhead sun conditions and different conditions are illustrated in Figure 10 where the concentrations correspond to various solar

zenith angles. Variations in the stratosphere are particularly important

in its lower part. Above 3 5 km the total concentration n(OH) + nfHC^) does

not change very much except when the sun is near the horizon.

(35)

- 2 8 -

( 0 H + H 0

2

) C O N C E N T R A T I O N S (cm-

3

)

FIGURE 10

Total concentration of hydroxyl and hydroperoxyl radicals in the mesosphere and stratosphere for photoequilibrium conditions

corresponding to various solar zenith angles. Overhead sun,

sec x

=

1;

8e

c X

=

4 and 6; sun at the horizon, x

=

90°.

(36)

- 29 - T A B L E HI

P r o d u c t i o n of OH a n d HC>2 by O x i d a t i o n of HzO , ( P ) , T o t a l C o n c e n t r a t i o n n(OH) + nlHO^) f o r a n O v e r h e a d S u n , and the T i m e to R e a c h 50% of t h e E q u i l i b r i u m V a l u e .

A l t i t u d e (km)

P

( c m s e c )

n(QH) + n ( H Oz) ( c m )

E q u i l i b r i u m T i m e ( s e c )

15 7 . 2 x l 02 3 . 3 x l 07 1 . 0 6 x l 04

20 2 . 6 x l 03 5 . 8 x l 07 6 . 2 x l 03

25 5 . l x l O3 7 . 4 x l 07 4 . O x l O3

30 9 . 3 x l 03 9 . 2 x l 07 2 . 7 x l 03

35 1 . 5 x l 04 1 . O x l O8 1 . 8 x l 03

40 2 . l x l O4 1 . l x l O8 1 . 3 x l 03

4 5 2 . 0 x l 04 9 . 6 x l 07 1 . 3 x l 03

50 1 . l x l O4 7 . l x l O7 1 . 8 x l 03

55 4 . 2 x l 04 4 . 4 x l 07 2 . 9 x l 03

60 1 . 4 x l 03 3 . l x l O7 5 . 9 x l 03

65 4 . 6 x l 02 2 . l x l 07 1 . 3 x l 04

70 2 . 8 x l 02 1 . 6 x l 0? 2 . 8 x l 04

T h u s , n e a r 70 k m , i n t h e m i d d l e of the m e s o s p h e r e w h e r e t h e f o l l o w i n g c o n c e n t r a t i o n s c a n b e c o n s i d e r e d f o r a n o v e r h e a d s u n : n(O) = 4 x 1 09 c m "3; n ( 03) = 1 07; n(H) = 1 07; n(OH) = 1 0?; n ( H Oz) = 7 x 1 06,

p h o t o c h e m i c a l c o n d i t i o n s s e e m to be w e l l e s t a b l i s h e d . B e l o w 4 0 k m , h o w e v e r , it i s d i f f i c u l t to d e t e r m i n e the e x a c t v a l u e of the r a t i o

n ( H 02) / n ( 0 H ) s i n c e t h e r e a c t i o n of HO^ w i t h O^ i s n o t w e l l k n o w n . F u r t h e r m o r e , the e f f e c t of n i t r o g e n o x i d e s m u s t b e i n t r o d u c e d .

(37)

- 30 - VI. E F F E C T OF NITROGEN OXIDES

The just described reaction chains which contribute to the dissocia- tion and r e - f o r m a t i o n of water vapor are not yet adequate since the

formation and destruction of H, OH and H 0

2

may depend on reactions with nitric oxide.

A rapid reaction [PHILLIPS and SCHIFF, 1962] such as

( a

2 5

) ; H + N O

z

- OH + NO + 30 kcal (35a) with a rate coefficient

a

o c

= 2 x 1 0 "

1 2

T

1 / 2

c m

3

s e c "

1

(35b) is not an important aeronomic process compared with another rapid

reaction [ TYLER, 1962]

( a

2 6

) ; H O

z

+ NO — OH + N O

a

+ 9 kcal (36a) No data a r e available on the rate coefficient. Assuming two extreme

values

a

2 6

= 3 x 1 0 '

1 2

T

1 / 2

c m

3

s e c "

1

(36b) in order to reach not less than 5 x 10

1 1

c m

3

sec

1

at 273°K, and

a

2 6 =

3 x l 0 "

1 2

T

1 / 2

e "

1 2 5 0 / T

c m

3

s e c "

1

(36c)

leading to about 1 0 "

1 1

c m

3

s e c

- 1

at 500°K, it is possible to see that a 8 -3

leading role could be played by (36a) if n(NO)> 10 cm in the strato-

sphere. Thus equation (20) must be replaced in the lower stratosphere

by

(38)

- 31 -

n ( H Oz) r a j n(M) n(C>2) a n(O)

= 4 + a , n(CL) n(OH) Vaj n(M) n ( 02) + a2 n ( 03)

} / { a 7 n(O) + a2 6n ( N O ) |

(37) w h i c h i s a n a p p r o x i m a t i o n of t h e m o r e g e n e r a l e q u a t i o n

n ( H Oz) r n(M) n ( 02) n(O)

- 1 : — + a , n ( 0 )

n(OH) ^ ax n(M) n ( Oz) + a2 n ( 03) ° •

{JH Oz + a7 n ( 0 ) + a1 7 n ( O H ) + a2 6 n ( N O ) + 2 a2 7 n (H°2 ) }

(3 8) w h e r e t h e v a r i o u s l o s s p r o c e s s e s of H O ^ a r e c o n s i d e r e d , n a m e l y p h o t o d i s s o c i a t i o n , t h e r e a c t i o n w i t h OH a n d t h e p r o d u c t i o n of h y d r o g e n p e r o x i d e .

T h e p h o t o d i s s o c i a t i o n p r o c e s s (6)

H202 + h v ^ OH + OH (3 9)

d o e s n o t s e e m to b e a n i m p o r t a n t m e c h a n i s m e v e n if t h e p h o t o d i s s o c i a t i o n - 4 - 1

c o e f f i c i e n t f o r z e r o o p t i c a l d e p t h i s n o t l e s s t h a n 10 s e c . In t h e s t r a t o s p h e r e i t r e a c h e s l e s s t h a n 5 x 1 0 ^ s e c ^ a t 20 k m . A m o n g t h e s e v e r a l l o s s p r o c e s s e s t h e r e a c t i o n w i t h a h y d r o x y l r a d i c a l

( a3 Q) ; OH + H202 — HzO + H Oz + 30 k c a l (40)

s e e m s to b e t h e m o s t i m p o r t a n t m e c h a n i s m s i n c e [ G R E N I E R , 1968]

. , „ - 1 3 „ 1 / 2 - 6 0 0 / T 3 - 1

a 3 0 = 4 x 10 T e c m s e c . (41)

(39)

( - 3 2 -

If n i t r o g e n o x i d e s a r e i n v o l v e d [ N I C O L E T , 1 9 6 5 ] , h y d r o g e n p e r o x i d e d e c o m p o s e s a c c o r d i n g t o o v e r a l l r e a c t i o n s w h i c h m a y c o m e , f o r e x a m p l e , f r o m

( a3 2) ; N O + H2 O z ^ H N 02 + O H + 11 k c a l ( 4 2 )

( a3 3) ; N Oz + H202 — H N O j + O H + 4 k c a l ( 4 3 )

f o r w h i c h t h e r a t e c o e f f i c i e n t s a r e n o t k n o w n . I t i s c l e a r t h a t of t h e v a r i o u s r e a c t i o n s w h i c h t h e o x i d e s of n i t r o g e n c a n u n d e r g o w i t h h y d r o g e n p e r o x i d e t h o s e w h i c h m a y g i v e n i t r o u s a c i d a n d n i t r i c a c i d r e s p e c t i v e l y

- 1 2 3 - 1

m u s t b e c o n s i d e r e d . I n o r d e r t o r e a c h a b o u t 10 c m s e c at 3 0 k m , a r a t e c o e f f i c i e n t

_ . _ - 1 2 1 / 2 - 1 0 0 0 / T 3 - 1

a3 2 = a3 3 = X 6 c m s e c ^ ^

w i t h a n a c t i v a t i o n e n e r g y of n o t m o r e t h a n 2 k c a l i s r e q u i r e d . W i t h s u c h 8 - 3

a v a l u e , N O a n d N O ^ c o n c e n t r a t i o n s g r e a t e r t h a n 10 c m l e a d t o l o s s c o e f f i c i e n t s of H202 g r e a t e r t h a n t h e p h o t o d i s s o c i a t i o n c o e f f i c i e n t . I t s e e m s p o s s i b l e , t h e r e f o r e , t h a t n i t r o g e n o x i d e s c a n a f f e c t t h e c o n c e n - t r a t i o n of H202 i n t h e s t r a t o s p h e r e a n d t h a t t h e r a t i o n ( 0 H ) / n ( H 02) d e p e n d s o n t h e a c t i o n of n i t r i c o x i d e .

H e n c e , t h e f o l l o w i n g e q u a t i o n m u s t b e u s e d d n ( H O )

+ n ( H70?)

d t L L JH202 + a3 2 n<N O> + a3 3 n ( N CV = a2 7 n 2 ( H °2)

( 4 5 ) i n o r d e r t o d e t e r m i n e t h e b e h a v i o r of h y d r o g e n p e r o x i d e i n t h e s t r a t o - s p h e r e a n d m e s o s p h e r e .

(40)

- 33 -

The f a c t that nitric acid has been recently o b s e r v e d in the s t r a t o s p h e r e [MURCRAY et al, 1968; 1969] is an indication that the nitrogen oxides play a role in a hydrogen-oxygen a t m o s p h e r e . This shows that the introduction of nitrogen oxides d e c r e a s e s the concentration of hydro- peroxyl r a d i c a l s and hydrogen peroxide produced in an ozone-atomic oxygen a t m o s p h e r e . In place of the s i m p l e s c h e m e p r e s e n t e d in Figure 9 it is n e c e s s a r y to consider the whole s y s t e m ( F i g u r e 11) not only with the d i s s o c i a t i o n of H^O into OH and H and the oxidation of H^O leading to two OH r a d i c a l s , but a l s o with r e s p e c t to the r e - f o r m a t i o n of water vapor by reaction between OH and HO^ with concentrations ( F i g u r e 12) depending on r e a c t i o n s of HO^ and H^O^ with nitrogen oxides.

VII. M O L E C U L A R HYDROGEN

The principal sink of m o l e c u l a r hydrogen in the s t r a t o s p h e r e and m e s o s p h e r e is (7) which leads to the following values of the oxidation coefficient for an overhead sun:

Altitude(km) 20 30 40 50 60 70 ( s e c "1) 2 . 5 x l 0 "1 0 4 . 4 x 1 0 4 . 4 x l 0 - 8 7 . 9 x l 0- 8 3 . 3 x l 0- 8 1 . 2 x l 0 "8 The m o s t important s o u r c e of m o l e c u l a r hydrogen [ B A T E S and N I C O L E T ,

1965]

( a2 3) ; H + HOz — H2 + Oz + 57 kcal (46a)

with a rate coefficient which can be given by

^ i n- 1 2 „ 1 / 2 - 1 0 0 0 / T . . .

a2 3 = 5 x 1 0 T e (46b)

(41)

•26

• OH a 33 a 32

'16

•NO 2 *NO

FIGURE 11

Fundamental H2O reaction scheme. Initiating and terminating

reactions depend on the production and destruction of hydroxyl radicals.

Nitrogen oxides are involved and nitric acid must be formed.

(42)

C O N C E N T R A T I O N ( c m "

3

)

F I G U R E 12

C o n c e n t r a t i o n s of h y d r o x y ! a n d h y d r o p e r o x y l r a d i c a l s d e p e n d i n g on r e a c t i o n s w i t h n i t r o g e n o x i d e s . E x t r e m e m i n i m u m of OH a n d e x t r e m e

m a x i m u m of HO2 w i t h o u t n i t r o g e n o x i d e s e f f e c t o n H 0 2 a n d H 2 O 2 , r e s p e c t i v e l y .

(43)

- 36 -

leads to an important production of hydrogen molecules in the upper m e s o s p h e r e (cf Table IV for H and HO^ concentrations). The produc- tion is

80 85 3 . 5 x l 0

3

4x10*

Thus m o r e molecular hydrogen is produced below the mesopause than is destroyed there; molecular hydrogen f r o m this layer flows down- wards into the upper stratosphere where process (7) converts it into

atomic hydrogen and hydroxyl radicals. It also flows upwards into the thermosphere. However, the exact distribution depends on the eddy diffusion coefficient which is introduced into the continuity equation

9n(H_) 9 [ n ( H

?

) w ]

i - + ^ = n(H) n(OH) (47)

at dz "

where w is the eddy diffusion velocity given by (15).

A reliable estimate of the concentration of molecular hydrogen could be made only with appropriate values of the eddy diffusion

coefficient which unfortunately are not known for the whole mesosphere, f r o m the stratopause to the mesopause.

Altitude (km) 50 60 70 75

( c m

- 3

s e c "

1

) 7x10

1

8x10* 4.5x10* 3 x l 0

2

Figure

Updating...

Références

Updating...

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