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 ACTA
A - N° 224 - 1980
Chemical response of the middle atmosphere to solar variations
by
P. DE BAETS, G. BRASSEUR and P.C. SIMON
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
AVANT-PROPOS
Ce texte constitue le résumé d'une communication présentée au 14ème symposium de l'ESLAB (Agence spatiale européenne) : "Physique des variations solaires" qui s'est tenu à Scheveningen ( P a y s - B a s ) du 16 au 19 septembre 1980. Il sera publié dans "Solar Physics".
VOORWOORD
Deze tekst is de samenvatting van een voordracht gehouden op het 14de ESLAB-symposium (Europese ruimte agentschap) "Physics of solar variations" die plaatsvond te Scheveningen (Nederland) van 16 tot 19 september 1980. Deze tekst zal in "Solar Physics" verschijnen.
FOREWORD
This text is the abstract of a communication presented at the 14th ESLAB-symposium (European Space Agency) "Physics of solar variations" held in Scheveningen ( T h e Netherlands) from 16 to 19 September 1980. It will be published in "Solar Physics".
VORWORT
Diese Arbeit ist die Zusammenfassung einer Vorträge vorgestellt während das 14e ESLAB-Symposium (European Space Agency) "Physics of Solar Variations" gehalten in Scheveningen (Niederlanden) vom 16 bis zum 19 September 1980. Dieser Tekst wird in "Solar Physics" heraus- gegeben werden.
CHEMICAL RESPONSE OF THE MIDDLE ATMOSPHERE TO SOLAR V A R I A T I O N S
by
P. DE B A E T S , G. B R A S S E U R ^ and P . C . SIMON
Abstract
The possible variation of the trace species concentration in the middle atmosphere related to long term solar irradiance variability is . estimated by means of a one-dimensional numerical model.
Résumé
La variation de la concentration des constituants minoritaires dans l'atmosphère moyenne en relation avec la variation à long terme de l'irradiance solaire a été estimée en moyen d'un modèle numérique à une dimension.
(*) Aspirant au Fonds National de la Recherche Scientifique.
Samenvatting
De v a r i a t i e van de d i c h t h e i d van de mtn'öTitäire komponenten in de middepatmosfeer in v e r b a n d met de v a r i a b i l i t e i t op lange t e r m i j n van de z o n n e b e s t r a l i n g s s t e r k t e w e r d door middel van een uni-dimensioneel numerisch model g e s c h a t .
Zusammenfassung
Die mögliche V a r i a t i o n der K o n z e n t r a t i o n der M i n d e r h e i t s k o m - ponenten in der m i t t l e r Atmosphäre in Zusammenhang mit der langzeitige V a r i a b i l i t ä t der Sonnen B e s t r a h l u n g s s t ä r k e w i r d abgeschätzt d u r c h eine eindimensionale Model.
T h e e f f e c t o f a v a r i a t i o n i n t h e UV s o l a r i r r a d i a n c e has been s t u d i e d in t h e r e c e n t y e a r s b y v a r i o u s a u t h o r s . F r e d e r i c k (1977) a n d R y c r o f t a n d T h e o b a l d (1978) h a v e d e t e r m i n e d t h e p o s s i b l e v a r i a t i o n in t h e c o n c e n t r a t i o n o f m e s o s p h e r i c ozone i n r e l a t i o n w i t h t h e 2 7 - d a y s o l a r r o t a t i o n p e r i o d . C a l l i s a n d Nealy (1978a a n d b ) , P e n n e r a n d C h a n g (1978) a n d C a l l i s et a l . ( 1 9 7 9 ) h a v e e s t i m a t e d t h e r e l a t i o n s b e t w e e n s t r a t o s p h e r i c t r a c e species v a r i a b i l i t y and t h e 1 1 - y e a r c y c l e of t h e s o l a r e m i s s i o n . More r e c e n t l y , P e n n e r a n d C h a n g (1980) a n d B r a s s e u r a n d Simon (1980) h a v e p e r f o r m e d s i m i l a r c a l c u l a t i o n s t a k i n g i n t o a c c o u n t t h e t e m p e r a t u r e f e e d b a c k . T h e l a t t e r w o r k has been p e r f o r m e d w i t h a t w o - d i m e n s i o n a l model in o r d e r t o e s t i m a t e t h e l a t i t u d i n a l e f f e c t o f t h e s o l a r v a r i a b i l i t y .
T h e s p e c t r a l v a r i a t i o n o f t h e s o l a r i r r a d i a n c e r e l a t e d t o t h e 1 1 - y e a r c y c l e is n o t well k n o w n because o f t h e lack of d a t a a n d is t h u s s u b j e c t t o d i s c u s s i o n . Heath a n d T h e k a e k a r a ( 1 9 7 7 ) h a v e s u g g e s t e d a v a r i a b i l i t y o f a f a c t o r o f 2 at 200 nm a n d 15% at 300 nm on t h e basis of t h e i r o w n o b s e r v a t i o n s p e r f o r m e d b y s a t e l l i t e s a n d r o c k e t s b e t w e e n 1966 a n d 1975. B r a s s e u r a n d Simon (1980) h a v e q u e s t i o n e d t h e a n a l y s i s o f H e a t h a n d T h e k a e k a r a a n d h a v e s u g g e s t e d as an u p p e r l i m i t a v a r i a t i o n of 20% at 200 n m . V e r y r e c e n t l y H i n t e r e g g e r ( 1 9 8 0 ) has p r o v i d e d a s p e c t r a l d i s t r i b u t i o n o f t h e solar v a r i a b i l i t y at w a v e l e n g t h s s h o r t e r t h a n 185 nm. T h e r a t i o o f i r r a d i a n c e at h i g h a n d low solar a c t i v i t y c o n d i t i o n is o f t h e o r d e r of 2 . 8 at Lyman a ( 1 2 1 . 6 n m ) a n d d e c r e a s e s w i t h w a v e - l e n g t h t o r e a c h a b o u t 1.20 at 185 nm. F i g . 1 shows t h e r e l a t i v e v a r i a t i o n s - F . „ ) / F a d o p t e d f o r t h e c a l c u l a t i o n s d i s c u s s e d
max mm mean
in t h i s p a p e r . T h e c u r v e p r o v i d e d b y H i n t e r e g g e r has been e x t r a - p o l a t e d t o r e a c h a n u l l v a r i a b i l i t y at 205 nm ( c u r v e 2 ) a n d 340 nm ( c u r v e 1) r e s p e c t i v e l y . T h e e f f e c t of an a n t i c o r r e l a t i o n b e t w e e n solar i r r a d i a n c e and solar a c t i v i t y r e c e n t l y s u g g e s t e d b y Heath (1980) f o r w a v e l e n g t h s b e t w e e n 210 a n d 250 nm has been c o n s i d e r e d b y a d d i n g to t h e c u r v e n ° 2 a r e l a t i v e v a r i a t i o n of - 15% l a b e l l e d n ° 3 i n f i g . 1.
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RELATIVE VARIATION OF THE SOLAR IRRADIANCE ( p e r c e n t )
WAVELENGTH (nm)
. 1 . - R e l a t i v e v a r i a t i o n o f t h e s o l a r i r r a d i a n c e d u r i n g a c y c l e of s o l a r a c t i v i t y (11 y e a r s ) . S p e c t r a l d i s t r i b u t i o n s u g g e s t e d b y v a r i o u s a u t h o r s . T h e c u r v e a d o p t e d b y B r a s s e u r a n d Simon ( 1 9 8 0 ) has b e e n e x t r a p o l a t e d t o r e a c h 66% a t 125 n m . For t h e e x t r a p o l a t i o n o f t h e H i n t e r e g g e r ' s c u r v e , see t e x t .
The chemical response of the stratosphere and the mesosphere is estimated by means of a 1 - D model which extends from the g r o u n d to the altitude of 100 km. A standard chemical scheme including the odd o x y g e n , h y d r o g e n , nitrogen, and chlorine species is used but no temperature feedback is considered here. In order to avoid prohibitive computer time, the model is r u n for steady state conditions but this simplification should not considerably alter the conclusions (Penner and C h a n g , 1978).
Fig. 2 depicts the change in the ozone production rate when the solar variability s u g g e s t e d by Hinteregger is adopted. In the upper mesosphere, the enhancement is of the order of 25-30 percent from solar minimum to solar maximum activity. The results obtained below 80 km are critically dependent on the type of extrapolation adopted above 175 nm since the dissociation of O^ leading to ozone occurs essentially in the S c h u m a n n - R u n g e bands (175-200 nm) and in the Herzber.g continuum (200-242 nm). Therefore new measurements of the time dependence of the solar irradiance above 175 nm are required in order to estimate the chemical response of the middle atmosphere due to natural causes.
The relative variation of the ozone concentration below 100 km as computed in our model is given in figure 3 for different solar varia bilities. If the Heath and Thekaekara curve (fig. 1) is adopted, the ozone concentration is increased by more than 40 percent in the upper stratosphere. T h i s number is much higher than long term variations ever recorded. Even if the temperature feedback is taken into account, the large variation given b y Heath and Thekaekara should be questioned ( B r a s s e u r and Simon, 1980).
The variability suggested by B r a s s e u r and Simon (1980) leads to a maximum ozone enhancement of the order of 15 percent. T h i s value is still high but as shown by our 2 - D model calculations ( B r a s s e u r and Simon, 1980) this value should be reduced to about 10 percent when
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RELATIVE VARIATION (percent)
Fig. 2.- Relative variation of the ozone production rate d u r i n g a solar cycle of 11 years when the solar irradiance variability suggested by Hinteregger is adopted. Curves 1, 2 and 3 refer to the different extrapolations suggested on f i g . 1.
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Fig. 3.- Relative variation of the ozone concentration computed for the solar variability suggested by Heath and Thekaekara (1977), Brasseur and Simon (1980) and Hinteregger (1980; extrapolation n° 2 on f i g . 1).
Ground albedo : 30 percent.
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the temperature feedback is taken into account. Finally, the H i n t e r e g g e r ' s irradiance variability (extrapolated to 205 nm - curve 2 on fig. 1) leads to an increase of the ozone concentration from minimum to maximum activity conditions which is of the order of 5 percent.
Above 65-75 km, the ozone concentration is expected to decrease when the solar irradiance is enhanced. In fact, the ozone production increase is counterbalanced b y the change in the concentration of h y d r o x y l , hydroperoxyl radicals, and of atomic h y d r o g e n . At these levels, H, O H , and HO^ are produced by the water vapor photo- dissociation in the spectral range of the Lyman a emission line. Since, in the model the intensity of this line is assumed to v a r y by 100 percent d u r i n g a solar cycle, the response of the odd h y d r o g e n species Is rather impressive. At 80 km, the concentration of OH and H 02 is increased from the minimum to the maximum period of solar activity by about 50 percent and that of H by 25 percent when the curve by Brasseu.r and Simon is adopted.
3 1 The concentration of o x y g e n atoms in their p and D states is reduced as does the ozone in the upper mesosphere ( - 35% at 80 km), whereas below 75 km it is enhanced (+ 15% between 45 and 65 km). In the stratosphere the expected increase in the atomic o x y g e n concen- tration due to the simultaneous enhancement of the ozone amount and solar irradiance at the top of the atmosphere is reduced by the presence of an increased ozone column which absorbs the UV radiation.
The relative variation of the amount of 0 ( D ) becomes even negative in our model below 35 km when the solar variability suggested by B r a s s e u r and Simon (1980) is used for the calculation.
The concentration of carbon monoxide is enhanced especially above the stratopause where the variability of the Lyman a line which d i s - sociates carbon dioxide becomes sensitive. At 80 km the corresponding increase could reach about 80 percent from solar minimum to solar
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maximum (fig. 4). Finally, the behavior of nitric oxide is also interesting since its distribution is sensitive to the value of its photo- dissociation rate. In the mesosphere the loss of NO is essentially com- pensated by its transport from the thermosphere and a high sensitivity to the solar activity is observed in fig. 4.
It should be noted that our results assume constant thermal and dynamical averaged conditions and that the introduction of temperature and transport feedbacks could modify considerably the values presented here. Therefore simplified l - D models provide only crude estimates of the possible response of the trace species to the solar variations.
RELATIVE VARIATION (percent)
F i g . 4 . - R e l a t i v e v a r i a t i o n in the c o n c e n t r a t i o n of c a r b o n m o n o x i d e a n d n i t r i c o x i d e d u r i n g a s o l a r c y c l e of 1 1 - y e a r s . T h e c a l c u l a t i o n s r e f e r to the s o l a r v a r i a b i l i t y s u g g e s t e d b y B r a s s e u r a n d S i m o n ( 1 9 8 0 ) .
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