I N S T I T U T D ' A E R O I M O M I E S P A T I A L E D E B E L G I Q U E 3 - A v e n u e C i r c u l a i r e
B • 1 1 8 0 B R U X E L L E S
A E R O N O M I C A A C T A
A - N° 257 - 1983
t
Implication for stratospheric composition of a reduced absorption cross section in the Herzberg continuum of
molecular oxygen
by
G. BRASSEUR, A. DE RUDDER and P.C. SIMON
B E L G I S C H I N S T I T U U T V 0 O R R U I M T E - A E R O N O M I E 3 - Ringlaan .
B - 1180 BRUSSEL
FOREWORD
T h i s paper will appear in Geophysical Research L e t t e r s .
A V A N T - P R O P O S
Cet article p a r a î t r a dans "Geophysical Research L e t t e r s " .
VOORWOORD
Deze t e k s t zal verschijnen in "Geophysical Research L e t t e r s " .
VORWORT
I M P L I C A T I O N FOR S T R A T O S P H E R I C C O M P O S I T I O N OF A R E D U C E D A B S O R P T I O N C R O S S S E C T I O N IN T H E H E R Z B E R G C O N T I N U U M OF
M O L E C U L A R O X Y G E N
b y
G. B R A S S E U R , A . DE R U D D E R and P . C . S I M O N
A b s t r a c t
Recent determinations of O^ absorption cross sections in the Herz- berg continuum, obtained from in situ solar irradiance measurements, have shown that the corresponding laboratory values might be o v e r - estimated by 30 to 50 percent. The change in the stratospheric com- position due to such a reduction in these cross sections has been calculated b y means of a steady state one dimensional chemical model.
A n accurate determination of the O^ absorption spectrum in the 200- 220 nm range is required since the distribution of several trace species is v e r y sensitive to the atmospheric transmissivity in this wavelength region.
Résumé
La section efficace d ' a b s o r p t i o n de l ' o x y g è n e moléculaire, dans le domaine spectral d u continuum de H e r z b e r g , a été déterminée à p a r t i r de la mesure de l'éclairement solaire, à d e u x altitudes c o n n u e s . C e s o b s e r v a t i o n s i n d i q u e n t que les v a l e u r s obtenues au laboratoire seraient de 30 à 50 p o u r c e n t t r o p élevées.
Les c o n s é q u e n c e s d ' u n e r é d u c t i o n de la v a l e u r des sections efficaces s u r le composition chimique de l'atmosphère ont été étudiées à l'aide d ' u n modèle mathématique unidimensionel. Comme la d i s t r i b u t i o n verticale des composés minoritaires est t r è s sensible à la t r a n s m i s s i o n d u rayonnement e n t r e 200 et 220 nm, une détermination p r é c i s e de la section efficace d ' a b s o r p t i o n de O2 dans ce domaine spectral est r e - q u i s e .
Samenvatting
Recente bepalingen van de werkzame absorptiedoorsneden van O^
in het Herzberg continuum bekomen door in situ metingen van de zonnestraling hebben aangetoond dat de overeenkomstige laboratorium- waarden waarschijnlijk overschat zijn met 30 tot 50 percent. De v e r - andering in de stratosferische samenstelling te wijten aan een dergelijke vermindering in deze werkzame absorptiedoorsneden werd berekend door middel van één dimensioneel chemisch model van het steady state t y p e . Een nauwkeurige bepaling van het absorptiespectrum van O2 in het 200-220 nm interval is vereist door de verdeling van verschillende spoorelementen uiterst gevoelig is aan de atmosferische transmissiviteit in dit golflengtegebied.
Zusammenfassung
Die neue Bestimmungen des O^ absorption Durchschnittmesser in das Herzberg Continuum, haben gezeigt dass die Laboratorium Messungen wahrscheinlich von 30% bis 50% zu gross sind.
Die Änderungen in der stratosphärischen Zusammensetzung einer solchen Reduktion sind abgeschäftzt worden f ü r ein Eindimensioneies chemisches Modell.
Ein genaue Bestimmung des C^ absorption Durchschittmesser zwischen 200 und 220 nm is notwendig, denn die minoritäre Bestandteile der Stratosphäre hängen merklich von die Transmittivität der, Sonnen- strählen in diesem Wellenlänge Bereich, ab.
1. I N T R O D U C T I O N
T h e photodissociation of molecular o x y g e n in t h e s t r a t o s p h e r e , which is t h e s t a r t i n g process of t h e ozone formation, is p r i m a r i l y due to t h e action of solar radiation between t h e wavelength of 200 and 240 nm.
In this spectral r a n g e , called t h e H e r z b e r g continuum, t h e absorption by Op is v e r y weak and an accurate value of t h e corresponding cross
- 2 3 2
section (< 10 cm ) is d i f f i c u l t to o b t a i n . Laboratory measurements r e q u i r e long path lengths or high p r e s s u r e in t h e absorption cell. In this l a t t e r case, t h e absorption is contaminated b y t h e presence of O4
which is formed by association of two o x y g e n molecules. Several determinations of t h e absorption cross section of 02 in t h e H e r z b e r g continuum have been performed in t h e last 20 y e a r s . T h e values measured by Hasson and Nicholls ( 1 9 7 1 ) are somewhat higher t h a n t h e data obtained b y D i t c h b u r n and Young ( 1 9 6 2 ) on which t h e tables published by Ackerman ( 1 9 7 1 ) are based. T h e absorption spectrum r e p o r t e d b y Shardanand and Prasad Rao ( 1 9 7 7 ) is c h a r a c t e r i z e d b y t h e smallest cross section at wavelength shorter t h a n 230 nm, i . e . in t h e region which contributes the most to t h e photodissociaton f r e q u e n c y in the middle and lower s t r a t o s p h e r e . O t h e r data such as t h e values b y Ogawa ( 1 9 7 1 ) or t h e tabulation b y Hudson and Reed ( 1 9 7 9 ) or b y Nicolet ( 1 9 7 8 ) are intermediate.
Recently, F r e d e r i c k and Mentall ( 1 9 8 2 ) as well as Herman and Mental! ( 1 9 8 2 ) have suggested changes in t h e O2 absorption cross section in o r d e r to explain t h e observed attenuation of the solar i r - radiance between two d i f f e r e n t heights located in t h e range of 30 to 40 km. T h e observations have been performed by a balloon borne Fastie- Ebert double monochromator f l y i n g over Palestine, T e x a s .
F r e d e r i c k and Mentall ( 1 9 8 2 ) , have deduced from t h e i r o b s e r - vations on September 15, 1980, t h a t , in t h e 200-210 nm r a n g e , t h e Hasson and Nicholls cross sections should be multiplied by factors in
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the r a n g e Of 0.52 to 0.68. a n d the S h a r d a n a n d a n d P r a s a d Rao (1977) v a l u e s b y f a c t o r s in the r a n g e of 0.85 to 0.93. It s h o u l d be noted h o w e v e r t h a t , when applied to the two s e t s of l a b o r a t o r y d a t a , t h e s e f a c t o r s do not lead to e x a c t l y the same v a l u e of t h e c o r r e c t e d c r o s s s e c t i o n s . T h i s d i s c r e p a n c y r e s u l t s p r o b a b l y from the s c a t t e r in the d a t a . T h e r e d u c t i o n f a c t o r s are calculated b y a s s u m i n g t h a t the c r o s s section of ozone is p e r f e c t l y known a n d t h a t the ozone column between the two levels of o b s e r v a t i o n is m e a s u r e d without e r r o r .
Herman a n d Mentall (1982) h a v e d e r i v e d with the same i n s t r u m e n t on A p r i l 15, 1980, a b s o r p t i o n c r o s s sections w h i c h are a b o u t 30 p e r c e n t smaller t h a n the l a b o r a t o r y r e s u l t s of S h a r d a n a n d a n d P r a s a d Rao (1977) from 200 to 210 nm a n d a b o u t 50 p e r c e n t smaller t h a n t h o s e of H a s s o n a n d Nicholls (1971). T h e s e v a l u e s d e d u c e d from in s i t u m e a s u r e - ments are t h u s somewhat smaller t h a n those s u g g e s t e d b y F r e d e r i c k a n d Mentall (1982).
In f a c t , the l a r g e scatter in the balloon data does not allow to d e r i v e a c c u r a t e v a l u e s f o r the O2 a b s o r p t i o n c r o s s s e c t i o n s . T h e o b s e r - vation of the solar i r r a d i a n c e at two d i f f e r e n t a t m o s p h e r i c levels o n l y s u g g e s t s t h a t a reduction h a s to be applied to the l a b o r a t o r y d a t a . T h e r e f o r e , the absolute v a l u e s of the C>2 c r o s s section determined b y Herman a n d Mentall are characterized b y an e r r o r of ± 10 p e r c e n t from 120 to 215 mm, ± 20 p e r c e n t at 220 nm a n d ± 40 p e r c e n t at 225 nm.
More l a b o r a t o r y as well as in s i t u measurements are t h u s r e q u i r e d .
F i g u r e 1 s h o w s that the d i s p e r s i o n in the available data is quite important a n d leads to s i g n i f i c a n t u n c e r t a i n t i e s in the p h o t o d i s s o c i a t i o n rate of several trace s p e c i e s . T h e reduction of the O2 c r o s s section s u g g e s t e d b y the o b s e r v a t i o n s in the s t r a t o s p h e r e s h o u l d o b v i o u s l y modify the s t r a t o s p h e r i c composition.
10 ,-22
E u 10' 23
Z o u UJ to
l/l I/)
o cr u
CL
§ '0 t/i (D <
,-24
MOLECULAR OXYGEN HERZBERG CONTINUUM
10 .-25
M e a s
• SHARDANAND & PRASAD RAO (1977)
• HASSON & NICHOLLS (1971) o HERMAN & MENTALL (1982)
- ACKERMAN (1971) NICOLET (1977) EXTRAPOLATION
190 200 210 220
WAVELFNGTH 'nm)
230 240 250
Fig. 1 . - Absorption cross section of molecular oxygen in t h e H e r z b e r g con- t i n u u m . T h e d a s h - d o t t e d line r e p r e s e n t s the adopted extrapolation for the Herman and Mentall values.
- 7 -
2. REDUCED O , CROSS SECTION AND ATMOSPHERIC OPACITY
In order to analyze the implication of a reduction in the absorption coefficients, different model calculations have been performed. The widely used cross sections of O2 compiled by Ackerman (1971) have been used as the reference case. The values derived by Herman and Mentall (1982), with an extrapolation towards the long wavelengths have been considered for comparison calculations. The change in the t r a n s - mission factor beyond 200 nm can be seen in figure 2. It clearly appears that the modification introduced by the in situ measurements appears essentially at wavelength shorter than 220 nm with a most pronounced effect at the lowest altitudes. Beyond 230 nm, the absorption is due essentially to ozone and an accurate value of the O^
cross section is not anymore required. This allows us to use an e x t r a - polation in the spectral range where no in situ measurements are available.
Since the irradiance in the stratosphere is increased with the new values of the absorption cross section, the photodissociation frequency of the species whose absorption spectrum is located around 200 nm ( e . g . CFCIg, CFgClg, NgO, HNOg, ) , is enhanced especially at the lower levels ( f i g u r e 3 ) . In the case of molecular oxygen, the photo- dissociation coefficient and consequently the ozone production rate is increased below 28 km but is reduced in the upper part of the strato- sphere. The photolysis frequency of O^ is indeed the product of the absorption cross section and the transmission factor which v a r y in the opposite direction.
Since the reduction in the Og cross section modifies the ozone distribution and consequently the opacity between 200 and 300 nm, the predicted change in the trace species photodissociation rate and con- centration arises not only from the reduced O^ cross section but also from the consequent change in the calculated ozone profile.
Fig. 2.- Transmittivity of the atmosphere between 200 and 230 nm calculated with the cross section values derived by Herman and Mentall (1982) (full lines) and compiled by Ackerman (1971) (dashed lines) - overhead sun.
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3U
• • i ^ v i i i i 111 i i r
1 l l 1 1 | 1 1 1 1 11 1 1
•
45
\ % CF2Cl ^
PHOTODISSOCIATION
FREQUENCY
—
40
X. • X. X. »X. x. • V X. • Tk\ • vi
\ \
-
35
-\ \CvF C l3
\ A \ A.
-30
- 1ƒ 1
/ /
-
25
— HNO,/ J
/
y /
-
20
w
1 1 1 1 1 1 1 1 1 / I I I
—i—L I I 1 1 1 I . l 1 1 1
101 102 103
RELATIVE VARIATION ( p e r c e n t )
3 . - Relative variation in the 24 hours average photodissociation coefficient when the absorption cross sections are taken from Herman and Mentall
3. REDUCED 02 CROSS SECTION AND T R A C E S P E C I E S CONCEN T R A T I O N
The modification of the photodissociation rates arising from the reduction of the 02 absorption in the Herzberg continuum leads to changes in the distribution of most minor constituents. This effect which has been recently considered by Froidevaux and Yung (1982) is investigated by means of the one-dimensional model which has been described by Brasseur et al. (1982). This steady state model derives the concentration of the most important middle atmosphere species belonging to the oxygen, hydrogen, nitrogen and chlorine families. The reaction rates recommended by WMO (1982) are adopted. The photo- dissociation frequencies are calculated assuming a 24 hour average solar illumination at the equinox. The solar irradiance is taken from Brasseur and Simon (1981) and is very similar to the values recommended by WMO (1982). The net vertical transport is parametrized by means of an exchange coefficient whose value at all altitudes is taken from Massie and Hunten (1982).
The temperature profile is prescribed and kept constant. Since the temperature/ozone coupling introduces a negative feedback, the present calculation with a fixed temperature profile provides upper limits in the calculated variations.
Species such as CFCIg, CF^Ci^ and N20 which are produced at ground level and destroyed in the stratosphere by photodissociation, will be affected by the reduction in the O^ absorption cross section. As shown by figure 4, the concentration at 25 km altitude is reduced by 36 percent in the case of CFCIg, by 8 percent in the case of CF^Cl^ and NgO. At 30 km the mixing ratios computed with the cross sections values suggested by Ackerman should be multiplied by 0.86, 0.86 and 0.45 in the case of NgO, C F2C I2 and CFCIg respectively. The corresponding factors determined by Froidevaux and Yung (1982) are
- 1 1 -
0 . 7 0 , 0 . 6 2 and 0 . 1 9 . T h e d i f f e r e n c e should be a t t r i b u t e d mainly to t h e f a c t t h a t t h e reduced O2 cross sections have been applied only above 200 nm in t h e p r e s e n t paper and down to 196.1 nm in t h e s t u d y b y Froidevaux and Y u n g ( 1 9 8 2 ) . M o r e o v e r , t h e value of t h e eddy d i f f u s i o n coefficient adopted b y Froidevaux and Y u n g ( 1 9 8 2 ) (Model Kg) is some- what smaller t h a n t h e value adopted in t h i s w o r k , which makes t h e t r a c e species d i s t r i b u t i o n more sensitive to t h e photochemical conditions.
These changes b r i n g closer t o g e t h e r t h e observed and calculated values b u t nevertheless t h e disagreement which appears in all s t r a t o s p h e r i c models is not completely suppressed. For example, t h e observed mixing ratio of CFCIg at 25 km at m i d - l a t i t u d e lies between 3 and 5 p p t v (Schmidt et a l , 1980) while t h e model gives 31 p p t v f o r Ackerman's cross sections and 1 9 . 7 p p t v when t h e data b y Herman and Mentall a r e adopted. Similarly, t h e calculated mixing ratios of CFgClg at 30 km a r e respectively 5 3 . 1 p p t v and 4 5 . 6 p p t v f o r t h e two absorption coefficients of Og while t h e observation suggests only a relative concentration of 9 to 30 p p t v (Schmidt et a l . , 1980). T h e concentration of total odd chlorine in t h e u p p e r s t r a t o s p h e r e ( 2 . 4 6 p p t v ) is u n c h a n g e d . A f u r t h e r reduction of t h e calculated concentration of chlorofluorocarbons could be obtained with an increased penetration of t h e solar i r r a d i a n c e in t h e S c h u m a n n - R u n g e bands. Herman and Mentall ( 1 9 8 2 ) claim t h a t t h e transmissivity d i r e c t l y observed in t h e atmosphere between 195 and 200 nm is l a r g e r than the one which is calculated with t h e cross sections given b y Allen and F r e d e r i c k ( 1 9 8 2 ) . T h i s discrepancy should be a t t r i b u t e d to a reduction in t h e H e r z b e r g continuum which lies beneath t h e bands between 180 et 200 nm ( F r e d e r i c k et a l . , 1981) and to a possible reduction in t h e widths of t h e d i f f e r e n t spectral lines.
In t h e p r e s e n t model, because of t h e lack of d a t a , no change has been applied to t h e absorption coefficients below 200 nm. I t should be emphasized t h a t the calculated concentration values are v e r y sensitive to t h e adopted eddy diffusion profile and t h a t a recalibration of t h i s coefficient should change considerably t h e vertical d i s t r i b u t i o n of all l o n g - l i v e d species.
T h e d i s t r i b u t i o n »of n i t r i c acid is also modified with t h e lower values of t h e H e r z b e r g continuum cross sections. T h e model suggests a 13 p e r c e n t reduction at 35 km (see f i g . 4 ) b u t again a much l a r g e r decrease ( a b o u t a f a c t o r of 2 ) is r e q u i r e d to reproduce most of t h e observations.
T h e r e l a t i v e change in t h e ozone d i s t r i b u t i o n Is shown in f i g u r e 5.
T h e adoption of smaller absorption cross sections leads, in t h e u p p e r atmosphere, to less ozone production and consequently to a decrease in t h e concentration of about 18 p e r c e n t between 40 and 50 km. T h e new
10 - 3
ozone concentration at 50 km Is now equal to 4 . 7 x 10 cm instead of 1 0 - 3
5 . 7 x 10 cm . T h e number obtained with t h e Herman and Mentall cross sections Is lower t h a n t h e US S t a n d a r d Atmosphere ( 1 9 7 6 ) mid-
1 0 - 3
latitude value ( ( 6 . 6 ± 1 . 1 ) x 10 cm ) . T h e observations of t h e C>3
concentration in t h e v i c i n i t y of t h e stratopause are however still sparse and t h e r e f o r e r a t h e r u n c e r t a i n . Nevertheless t h e newly calculated concentration seems smaller t h a n most available data and t h e discrepancy has probably to be a t t r i b u t e d to an incorrect determination of t h e solar penetration in t h e Schumann-Runge bands. As stated b y Clesllk ( 1 9 8 2 ) , t h e equivalent cross section f o r a large solar penetration is somewhat smaller if t h e linewidths recently measured b y Lewis et al. (1978; 1979) a r e adopted instead of t h e values determined p r e v i o u s l y . In t h a t case the contribution of t h e S c h u m a n n - R u n g e bands In t h e photodissoclation process should be enhanced near t h e s t r a t o - pause. More laboratory data are r e q u i r e d before a final statement can be made. In t h e lower stratosphere ( 2 0 k m ) , where most of t h e ozone is confined, and where t h e I n t e n s i t y of t h e solar radiation is enhanced, t h e O - concentration is Increased by about 20 p e r c e n t . These changes in t h e vertical d i s t r i b u t i o n which are in good agreement with t h e results of Froidevaux and Y u n g ( 1 9 8 2 ) , lead to an enhancement in t h e ozone column of 7 . 6 p e r c e n t .
- 1 3 -
F i g . 4 . - Relative v a r i a t i o n öf thé côrscentrâtiô.fî öf C f C l g , C F g C l g , N20 arid HNOg when t h e a b s o r p t i o n c r e s s sections are t a k e n from Herman a n d Mental! (1982) instead of A c k e r m a n (1971),
RELATIVE VARIATION (percent)
Fig. 5 . - Same as f i g . 4 but for ozone.
4. CONCLUSIONS AND SUMMARY
A reduction of the absorption cross sections in of the Herzberg continuum which has been suggested from direct observation in the atmosphere, leads to modifications in the calculated distributions. of stratospheric minor constituents. The results show the necessity of an accurate determination of the absorption spectrum of O^. However, thé discrepancies between observed and computed concentrations for several species which are photolysed around 200 nm are only partly reduced.
Additional measurements of stratospheric solar irradiance between 30 and 40 km are required to obtain unambiguous values of the absorption cross section in the 200-220 nm range and of the transmission factor in the Schumann Runge bands.
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H A S S O N , V . and R . W . N I C H O L L S , Absolute spectral absorption measurements on molecular o x y g e n from 2460-1920 Â : II continuum measurements 2430-1920 Â , J. P h y s . B . : Atom. Molec. P h y s . , 4 , 1789-1797, 1971.
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