AERONOMICA ACTA

Texte intégral

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I N S T I T U T D ' A E R 0 N 0 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° 2 7 9 - 1984

Photodissociation effects of solar UV radiation

by

P.C. SIMON and G. BRASSEUR

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 R i n g l a a n

B • 1180 BRUSSEL

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FOREWORD

This paper has been presented at the European Geophysical Society ( E G S ) , Meeting Structure of the Upper and Middle Atmosphere in Leeds, 23-27 A u g u s t , 1982, held in honour of Prof. Marcel Nicolet.

A V A N T - P R O P O S

Ce travail a été présenté à la réunion - Structure of the Upper and Middle Atmosphere - du "European Geophysical Society" ( E G S ) , organisée à Leeds, 23-27 août 1982, en l'honneur du Prof. Marcel Nicolet. Il sera publié dans "Planetary and Space Science".

VOORWOORD

Dit werk werd voorgesteld tijdens de vergadering - Structure of the Upper and Middle Atmosphere - van de "European Geophysical Society" ( E G S ) , gehouden te Leeds, 23-27 augustus 1982, ter ere van Prof. Marcel Nicolet. Het zal verschijnen in "Planetary and Space Science".

VORWORT

Diese Arbeit wurde präsentiert während der "European Geophysical Society" ( E G S ) Tagung - Structure of the Upper and Middle Atmo- sphère -organisiert in Leeds, 23-27 August 1982, zu Ehren des Professors Marcel Nicolet. Diese Arbeit wird in "Planetary and Space

Science" herausgegeben werden. /

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P H O T O D I S S O C I A T I O N E F F E C T S OF S O L A R UV R A D I A T I O N

by P . C . SIMON and G. B R A S S E U R *

Institut d'Aéronomie Spatiale de Belgique 3, Avenue C i r c u l a i r e , B-1180 B r u x e l l e s - Belgium

Abstract

T h i s paper reviews the c u r r e n t knowledge of ultraviolet solar irradiances beyond 120 nm measured during solar cycle 21. Parameters required to calculate the atmospheric attenuation of the UV radiation and the photodissociation rate of neutral constituents are discussed.

The effect of scattered sunlight is also considered.

Résumé

Ce travail fait le point sur la connaissance actuelle de l'éclairement solaire dans l'ultraviolet au-delà de 120 nm, mesuré durant le cycle so- laire 21. Les paramètres, nécessaires pour calculer l'atténuation atmo- sphérique de la radiation UV et le taux de photodissociation des éléments composants neutres, sont discutés. L'effet de la diffusion du rayonnement solaire est également pris en considération.

* Aspirant au Fonds National de la Recherche Scientifique

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;

Samenvatting

Dit werk bekijkt de huidige kennis van de zonnestraling in het ultraviolet boven 120 nm, gemeten tijdens zonnecyclus 21. De para- meters, nodig om de atmosferische verzwakking van de UV straling en de fotodissociatiegraad van de neutrale componenten te berekenen, worden besproken. Het verstrooiingseffekt van de zonnestraling wordt eveneens onderzocht.

Zusammenfassung

Diese A r b e i t erwägt die heutige Kenntnis über die Sonnenaus- strahlung im Ultraviolett über 120 nm, gemessen während des Sonnen- zykluses 21. Die Parameter, nötig um die atmosphärische Schwächung der UV Sonnenstrahlen und der Photodissoziationgrad von den neutralen Komponenten zu berechnen, werden besprochen. Der Diffusionseffekt der Sonnenaustrahlung wird auch untersucht.

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1. I N T R O D U C T I O N

T h e chemical and the thermal structure of planetary atmospheres is driven by the radiation emitted by the S u n and its absorption b y opti- cally active atmospheric constituents. A n accurate knowledge of the solar irradiance! and its variation is therefore fundamental in aeronomy and in climatology. Most of the solar e n e r g y is confined in the wave-

length range below 4000 nm while the radiation emitted by the Earth and the s u r r o u n d i n g atmosphere has a wavelength greater than

Photochemical processes of the neutral constituents in the middle atmosphere and in the lower thermosphere are initiated by ultraviolet radiation in the wavelength interval r a n g i n g from 120 nm to the visible part of the spectrum. Furthermore the HI Lyman a line is responsible for the ionization of nitric oxide (Nicolet, 1945) leading to the formation of the D region.

When penetrating in the atmospheric layers, the solar irradiance is p r o g r e s s i v e l y attenuated b y absorption and scattering. The main absorbing gases are molecular o x y g e n and ozone. A n accurate analysis of the radiative transfer in the ultraviolet requires therefore a perfect knowledge of (1) the solar irradiance at the top of the atmosphere, ( 2 ) the absorption cross sections of O2 and 03, (3) the integrated amount of these absorbing species (4) the scattering cross section and (5) the albedo.

The distribution of the trace species concentration depends direct- ly on the photodissociation rates in the atmosphere. T h e photodissocia- tion frequency of a molecule X is g i v e n by

4000 nm.

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where q ( A , z , x ) is the total photon flux into a volume element at an altitude z, for a solar zenith angle x an d f °r wavelength A.. o(X,\) is the absorption c r o s s section and e ( X , A ) the photodissociation efficiency for the given molecule. The absorption and scattering processes will therefore influence the atmospheric composition.

T h e purpose of this paper is to review the key parameters which are used as input data in photochemical models of the atmosphere. In particular, the solar irradiance, the absorption c r o s s section of mole- cular o x y g e n and ozone and the Rayleigh scattering will be d i s c u s s e d .

In order to clarify the analysis, the solar spectrum will be divided according to the various spectral regions of molecular o x y g e n corresponding to the penetration of solar radiation in different atmo- spheric layers.

2. S O L A R I R R A D I A N C E

Solar ultraviolet irradiance measurements from 135 to 400 nm will be reviewed. The solar HI Lyman a line will not be discussed here since its absolute value and its variations are analyzed by B o s s y (1983, this volume). On the other hand, measurements obtained since the first observation reported by Friedman et al. (1951) have been discussed in

several papers ( e . g . Vidal-Madjar, 1977; Simon 1978; Simon 1981).

2.1. The wavelength interval 135-175 nm

D u r i n g solar cycle 20, several rocket measurements have been made in the wavelength interval corresponding to the S c h u m a n n - R u n g e continuum of molecular o x y g e n . Heroux and Swirbalus (1976) and Samain and Simon (1976) have published the lowest irradiance values.

The latter values refer to a quiet solar full d i s k . Rottman (1981) has proposed revised values for his observations performed in 1972 and 1973. With three other flights made in 1975, 1976 and 1977, he has

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defined a so-called reference s p e c t r u m c o r r e s p o n d i n g to minimum c o n d i - tion between solar cycle 20 a n d 21, b y t a k i n g an a v e r a g e value p r o - v i d e d b y the five f l i g h t s .

T h e i r r a d i a n c e v a l u e s obtained d u r i n g cycle 21 for a h i g h solar activity level h a v e been p u b l i s h e d b y M o u n t et al. (1980) a n d M o u n t and Rottman ( 1 9 8 1 ) . T h e y are systematically h i g h e r t h a n the reference s p e c t r u m of Rottman (1981). T h e differences r e p r e s e n t l o n g - t e r m v a r i a - bility which is confirmed b y the A E - E satellite o b s e r v a t i o n s r e p o r t e d b y H i n t e r e g g e r (1981) even if the latter c o r r e s p o n d to a limited field of view on the solar d i s k of 6 arc min x 6 arc min situated near the d i s k center. New data obtained in 1982 h a v e been v e r y recently p u b l i s h e d b y M o u n t a n d Rottman (1983). T h e s e o b s e r v a t i o n s , c o r r e s p o n d i n g to an Intermediate level of solar activity are comparable to the minimum v a l u e s p r o p o s e d b y Rottman (1981). T h e y are even lower than the v a l u e s obtained in 1976 and 1977 with a similar i n s t r u m e n t . V a l u e s obtained b y Heath (1980) d u r i n g the r i s i n g p h a s e of solar cycle 21 in November 1978 are also in better agreement with the minimum v a l u e s p r o p o s e d b y Rottman (1981) rather than with those obtained d u r i n g the maximum of solar cycle 21.

O n the b a s i s of the former o b s e r v a t i o n s performed since 1978, the enhancement of solar i r r a d i a n c e with the 1 1 - y e a r solar activity seems correlated o n l y with the h i g h e s t activity levels a r o u n d the cycle m a x i - mum in 1979 a n d 1980. T h i s is in contradiction with the two-component plage model of Cook et al. (1980) which p r o p o s e s a linear relationship between the solar irradiance in the 117.5-210 nm w a v e l e n g t h interval and the Z u r i c h s u n s p o t n u m b e r . A three-component model recently p u b l i s h e d b y Lean et al. (1982) and based on spatially resolved o b s e r - vations of the Ca II K c h r o m o s p h e r e is in better agreement with the 1 1 - y e a r cycle variation d e d u c e d from the L A S P data ( R o t t m a n , 1981;

M o u n t et al., 1980; Mount and Rottman, 1981). A c c o r d i n g to this latter model, the irradiance increase in the 155-160 nm w a v e l e n g t h interval

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reaches levels higher than 25 percent after the beginning of 1978 or before mid 1971 corresponding to smooth sunspot numbers larger than 80.

T h e ratios of irradiance values integrated over 5 nm from 135 to 175 nm taking as a reference the irradiance values proposed by Brasseur and Simon (1981) are given in f i g u r e 1. T h i s graph clearly shows that the previous values of Heroux and Swirbalus (1976) are 30 percent lower than the reference spectrum. On the other hand, the divergences between the five observations of Rottman (1981) give a good idea of the reproductibility reached in this wavelength range by means of snapshot measurements which include the variability related to the 27-day solar rotational period.

Observations made by the Solar Mesosphere Explorer (SME) in- dicate a 27 day variability of the order of 12 percent in the integrated 130-175 nm wavelength range (Rottman et a l . , 1982). A detailed discussion of this question is given by Rottman (1983, this volume).

A c c u r a c y and precision of the irradiance observations need to be improved in order to quantitatively measure the solar variability d u r i n g the 11-year cycle. T h e accuracy of ± 8 percent at the two a level quoted by Mount and Rottman (1983) in the wavelength interval of 115-186.5 nm seems very promising in regard to the previous obser- vations usually made with an accuracy of ± 15 percent at the one o level. T h i s improvement has been reached by using the Synchrotron Users Radiation Facility ( S U R F ) at the National Bureau of Standards as a calibration standard.

2.2. T h e wavelength interval 175-210 nm

Solar irradiance and its variability are v e r y poorly known in the wavelength range corresponding to the Schumann-Runge bands of mole-

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I I

170 180

WAVELENGTH (nm)

Figure 1 Observed solar irradiance in the spectral range 135-210 nm expressed as ratios relative to the solar reference spectrum of Brasseur and Simon (1981). Values are integrated over 5 nm intervals.

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cular o x y g e n . F i g u r e 1 g i v e s the ratios of several o b s e r v e d i r - radiance v a l u e s integrated o v e r 5 nm t a k i n g as a reference the v a l u e s obtained b y Samain a n d Simon (1976) c o r r e s p o n d i n g to a quiet s u n a n d quoted b y B r a s s e u r and Simon ( 1 9 8 1 ) . Differences at w a v e l e n g t h s below 185 could p r o b a b l y be due to l o n g - t e r m solar v a r i a b i l i t y . D i v e r g e n c e s between individual o b s e r v a t i o n s are not uniform with w a v e l e n g t h . It s h o u l d be mentioned that, a c c o r d i n g to the two-component model of C o o k et al. (1980) a n d the t h r e e - c o m p o n e n t model of Lean et al. ( 1 9 8 2 ) , the variability d u r i n g the r i s i n g p h a s e of solar cycle 21 b e y o n d 182 nm does not exceed 30 percent. Ratios from 180 to 190 nm obtained on the b a s i s of Rottman's measurement s h o u l d be e x c l u d e d because of l a r g e r e r r o r s . In addition, the data of M o u n t et al. (1980) are not reliable b e y o n d 190 nm because of experimental problems ( M o u n t a n d Rottman, 1981). C o n s e q u e n t l y , the quantitative variation o v e r the 1 1 - y e a r cycle is still masked b y the uncertainties of all measurements in this w a v e - length r a n g e . Additional o b s e r v a t i o n s with a c c u r a c y a n d p r e c i s i o n h i g h e r than or at least comparable to those obtained b y M o u n t a n d Rottman (1983) are r e q u i r e d .

T h e variability of the solar irradiance with the 27 d a y rotation period has been o b s e r v e d b y Nimbus 7 ( H e a t h , 1980) a n d b y S M E (Rottman et a l . , 1982). Heath (1980) o b s e r v e s a maximum v a r i a b i l i t y of 5 percent a r o u n d 180 nm while Rottman et al. report a v a r i a b i l i t y of 10 percent for the whole spectral interval of 175 to 190 nm.

2 . 3 . T h e spectral region b e y o n d 210 nm

F i g u r e 2a p r e s e n t s the ratio of irradiance v a l u e s integrated o v e r 5 nm from 205 to 270 nm t a k i n g as a reference the v a l u e s p r o p o s e d b y B r a s s e u r and Simon (1981). Systematic d i v e r g e n c e s clearly appear between most measurements in the 205-240 wavelength interval. T h e y are p r o b a b l y due to experimental e r r o r s . Differences between all the f l i g h t s do not exceed 20 percent. T h e y are within 12 percent when

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c o n s i d e r i n g o n l y the balloon o b s e r v a t i o n s reported b y Simon et al.

( 1 9 8 2 a , b ) a n d within 10 percent when keeping o n l y the 1980 a n d 1982 rocket f l i g h t r e s u l t s p u b l i s h e d b y M o u n t a n d Rottman (1981, 1983). All these measurements confirm that the v a l u e s of B r o a d f o o t (1972) h a v e to be lowered b y at least 25 p e r c e n t below 260 nm. T h e lowest v a l u e s of M o u n t et al. (1980) are not reliable in this w a v e l e g n t h r a n g e because of experimental problems encountered d u r i n g this f l i g h t ( M o u n t a n d

Rottman, 1981). A b o v e 240 nm, the d i v e r g e n c e s are within 15 p e r c e n t . Data reported b y M o u n t a n d Rottman (1981) h a v e been r e v i s e d on the b a s i s of improved calibration methods obtained with the S y n c h r o t r o n U s e r s Radiation Facility ( S U R F ) at the National . B u r e a u of S t a n d a r d ( s e e M o u n t and Rottman, 1983).

F i g u r e 2b r e p r e s e n t s the same comparison for integrated i r r a d i a n c e values o v e r 5 nm between 270 and 350 nm. T h e agreement between all the o b s e r v a t i o n s is good (± 10 p e r c e n t ) up to 295 nm. B e y o n d t h i s w a v e l e n g t h , the data obtained in 1980 and r e v i s e d b y M o u n t a n d

Rottman ( 1 9 8 3 ) , as well as those of Broadfoot ( 1 9 7 2 ) , become s i g n i f i - cantly lower than the o t h e r s . Balloon o b s e r v a t i o n s also e x h i b i t l a r g e r d i s c r e p a n c i e s up to 15 percent, at l a r g e r w a v e l e n g t h s . It s h o u l d be noted that the q u a n t u m yield of the solar blind detectors u s e d for rocket and balloon o b s e r v a t i o n s decreases r a p i d l y b e y o n d 250 nm a n d becomes v e r y small for w a v e l e n g t h s g r e a t e r than 300 nm. N e v e r t h e l e s s , the agreement with the v a l u e s obtained b y Heath (1980) is within 5 percent for the 1979 balloon o b s e r v a t i o n (Simon et a l . , 1982b).

B e y o n d 330 nm, new irradiance v a l u e s h a v e been p u b l i s h e d b y Neckel and L a b s (1981). T h e s e latter v a l u e s which s u p e r s e d e those p r e v i o u s l y p u b l i s h e d b y Labs a n d Neckel (1970) are p r o b a b l y the most reliable in the near ultraviolet and in the v i s i b l e . A c c o r d i n g to Table 3 in Simon ( 1 9 8 1 ) , they are in good agreement with the o b s e r v a t i o n reported b y Heath (1980). T h e measurements of A r v e s e n et al. (1969) have to be carefully adjusted because of e r r o r s in their w a v e l e n g t h

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WAVELENGTH (nm)

270 280 290 300 31C . 320 330 340

SOLAR I R R A D I A N C E I N T E R C O M P A R I SON

Broadfoot (1972) Mount and Rottman (1981) Simon (1975) Simon et al. (1982a) Heath (1980) Simon et al. (1982 b)

» Mentall et al. (1981) Mount and Rottman (1983)

_l i I i 1 i I i ' » i • ' 210 220 230 2AO 250 260 270

WAVELENGTH (nm)

F i g u r e 2 Observed solar i r r a d i a n c e in t h e spectral, ranges 205-275 nm ( b o t t o m ) and 270-340 nm ( t o p ) expressed as ratios r e l a t i v e to the solar reference spectrum of Brasseur and Simon (1981).

Values are i n t e g r a t e d o v e r 5 nm i n t e r v a l s . T h e labeling is as follows : B 70 : Broadfoot (1972); S 76 : Simon et al. (1982a) f l i g h t dated J u l y 1, 1976.

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scale below 400 nm a n d of uncertainties due to c h a n g e s in the spectral irradiance scale of the National B u r e a u of S t a n d a r d s in 1973.

L o n g - t e r m variability in this spectral r a n g e is g e n e r a l l y c o n s i d e r e d as being negligible from the aeronomic point of view a n d is also masked b y the uncertainties in available measurements. V a r i a t i o n s less than 2 percent related with the 2 7 - d a y rotational period of the s u n h a v e been reported b y Heath (1980) from N i m b u s 7 o b s e r v a t i o n s . T h e y are c o n - firmed b y the recent S M E o b s e r v a t i o n s ( R o t t m a n , 1983).

2 . 4 . S u m m a r y

In c o n c l u s i o n , solar irradiance v a l u e s are p r o b a b l y k n o w n with an u n c e r t a i n t y of ± 20 percent from 135 to 175 nm for solar minimum c o n - dition. T h e long term v a r i a t i o n s are certain, r e a c h i n g a factor of two a r o u n d 150 nm between the maximum and the minimum of activity d u r i n g the r i s i n g p h a s e of solar cycle 21. In the wavelength interval of 175- 200 nm, the u n c e r t a i n t y is not smaller (± 20 p e r c e n t ) a n d the l o n g - t e r m variations are still masked b y the a c c u r a c y and the precision of a v a i - lable o b s e r v a t i o n s . Solar i r r a d i a n c e s can be estimated within ± 15 percent of a c c u r a c y a r o u n d 210 nm and with ± 5 percent at 300 nm.

F u r t h e r o b s e r v a t i o n s with an a c c u r a c y of ± 5 percent a n d a p r e c i s i o n better than ± 2 percent are still needed in o r d e r to p r o v i d e useful irradiance v a l u e s a n d to determine the possible l o n g - t e r m v a r i a b i l i t y b e y o n d 175 nm. S u c h goals are only achievable b y r e f e r r i n g the cali- bration to a common stable s o u r c e as the s y n c h r o t r o n to i n s u r e the continuity of f u t u r e o b s e r v a t i o n s . It is also r e q u i r e d to compare the different instrument performances when o b s e r v i n g the s u n from the same platform. Intercomparison of solar spectrometers s h o u l d make f u t u r e measurements on board balloon, rocket and space shuttle more comparable and s h o u l d help to the validation of satellite o b s e r v a t i o n s , p a r t i c u l a r l y for the instrument d r i f t in o r b i t .

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3. 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 OF M O L E C U L A R O X Y G E N

3.1. The Lyman a window region

Below 130 nm, the absorption c r o s s section of molecular o x y g e n varies rapidly with wavelength. Near the HI Lyman a solar line, the minimum values of the o x y g e n cross-section allows the solar radiation to reach the mesosphere, down to the altitude of 70 km. Since this solar line is responsible for the dissociation of water vapor, methane, carbon dioxide and the ionization of nitric oxide, an accurate knowledge of the optical depth in this wavelength region is required.

Many laboratory measurements have been made ( e . g . Watanabe, 1958; Ogawa, 1968; Dose et al., 1975) leading to the working value of 1 x 10 cm at 121.6 nm. Hall (1972) has pointed out that the variation in the molecular o x y g e n cross section across the solar Lyman a line which has the appearance of a doublet (Purcell and T o u s e y , 1960) requires the calculation of an effective cross section dependent of the o x y g e n content above a given altitude. On the other hand, Smith and Miller (1974) and Weeks (1975) have pointed out the temperature de- pendance of the molecular o x y g e n absorption cross section near Lyman a. C a r v e r et al. (1977) have measured the o x y g e n absorption cross sections at 0.02 nm intervals between 121.4 and 121.86 nm at three temperatures of 294, 195 and 82 K. The effective c r o s s sections have been calculated as a function of the integrated Lyman a extinction for the three temperatures. Results are significantly lower than the effective cross sections published by Hall (1972) and based on the Ogawa's measurements.

Nicolet and Peetermans (1980) using the data of C a r v e r et al.

(1977), have deduced simple expressions, adopting an average tempera- ture of 190 K, for the photodissociation rate calculations.

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The optical depth is given by the expression :

XLy a(° 2} = 4'1 7 X 1 0~1 9 N( V ° '9 1 7 (2 a) 19 2

for N(C>2) > 1 x 10 molecules/cm corresponding to an effective cross section given b y :

aL y a( 02) = 4-1 7 x 1 0~1 9 N ( 02) "0-0 8 3 (2b)

19 2 -?0

For N ( 0 ~ ) ^ 1 x 10 molecules/cm , a cross section value of 1 x 10 cm can be adopted. 2

3,2. The S c h u m a n n - R u n g e continuum

The heating in the lower thermosphere as well as the formation of the atomic o x y g e n due to the photodissociation of molecular o x y g e n in this atmospheric layer has to be attributed to the absorption of solar radiation in the 135-175 nm wavelength interval.

The absorption cross section of molecular o x y g e n in this range has been reviewed several times. The first analysis was made by Nicolet and Mange (1954) and followed by those of Watanabe (1958), Ackerman (1971) and Hudson (1971). The latter author has also extensively discussed the sources of e r r o r s in the ultraviolet absorption cross section measurements. He claimed that the total uncertainty with the laboratory techniques available at that time, is larger than 5 percent.

Furthermore, errors on the cross section values are larger when photo- graphic emulsions are used instead of photoelectric techniques de- tection. Actually, the first investigations of molecular o x y g e n absorption cross section have been conducted using photographic techniques and yielded values 20-25 percent higher than those obtained by photoelectric measurements. Later on, the data obtained by Goldstein and Mastrup (1966) did not confirm the higher cross section values obtained by

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photographic emulsions. Figure 3 compares several photoelectric mea- surements between 135 and 160 nm obtained since 1953 at room tempera- ture with those of Goldstein and Mastrup (1966). The divergences between the photoelectric observations do not exceed 10 percent and are therefore within the ± 10 percent of accuracy quoted by most in- vestigators. Specific measurements have been performed only around the peak of the absorption spectrum in order to obtain reliable cross section

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values of the order of 1.5 x 10 cm in this region ( e . g . , Watanabe and Marno, 1956; Huffman et al., 1964). Measurements performed between 160 and 180 nm are displayed in figure 4. The temperature de- pendence of the absorption cross section is illustrated by the data obtained around 300 and 600 K by Hudson et al. (1966) and by Lean and Blake (1981). The positive temperature effect becomes significant for wavelengths greater than 160 nm and should be taken into account in photodissociation calculations in the thermosphere. According to Blake et al. (1980), the temperature coefficient becomes negative at shorter wavelengths but the change is limited to less than - 4 percent at 149 nm instead the - 10 percent at 160 nm as measured by Hudson et al. (1966). A semi-empirical two terms model of the temperature dependence of the molecular o x y g e n absorption cross section is p r o - posed by Lean and Blake (1981).

The S c h u m a n n - R u n g e continuum adjoins the S c h u m a n n - R u n g e band system at 175 nm and underlies the bands below about 183 nm. T h e r e - fore, this continuum contributes to the absorption cross section minima in the band system and should be taken into account in cross section calculations discussed in the following section.

3.3. The S c h u m a n n - R u n g e band system

The attenuation of the solar radiation in the wavelength range of 175-200 nm is driven by the absorption of the molecular o x y g e n in the S c h u m a n n - R u n g e band system. The absorption takes place mainly in the

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MOLECULAR OXYGEN ABSORPTION CROSS SECTION- SCHUMANN-RUNGE CONTINUUM

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. Watanabe et al. (1953)

• Watanabe & M a r m o (1956) . H u f f m a n et al. (1964)

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• O g a w a & O g a w a (1975)

• Lean & Blake (1981)

J L J I I L J I I L J I I I

135 u o K 5 150

WAVELENGTH (nm)

155 160

Figure 3 Absorption cross section of molecular o x y g e n between 135 and 160 nm.

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MOLECULAR OXYGEN ABSORPTION CROSS SECTION SCHUMANN-RUNGE CONTINUUM

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Hudson et al. (1966)

Lean & Blake (1981)

!0 160 -20 J I I L J I I L 1 J I I L 1 J I I L J I L

165 170 175 WAVELENGTH ( n m )

1 8 0 185

u r e 4 A b s o r p t i o n c r o s s s e c t i o n o f m o l e c u l a r o x y g e n b e t w e e n 160 a n d 185 n m .

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m e s o s p h e r e , between the altitudes of 60 a n d 90 km. However the p e n e - tration is large e n o u g h so that the radiation in this spectral r a n g e c o n - t r i b u t e s s i g n i f i c a n t l y to the photodissociation of several c o n s t i t u e n t s in the s t r a t o s p h e r e .

S e v e r a l s t u d i e s of the photodissociation of molecular o x y g e n in the middle atmosphere h a v e been performed b y many a u t h o r s s u c h as K o c k a r t s ( 1 9 7 1 ) , H u d s o n and Mahle ( 1 9 7 2 ) , F a n g et al. ( 1 9 7 4 ) , P a r k (1974), K o c k a r t s ( 1 9 7 6 ) , B l a k e ( 1 9 7 9 ) , Nicolet and Peetermans (1980) and F r e d e r i c k a n d H u d s o n (1979).

In situ o b s e r v a t i o n s at 40 km altitude of solar i r r a d i a n c e between 184 and 202 nm h a v e been reported b y F r e d e r i c k et al. (1981) a n d h a v e p r o v i d e d a check on the a b s o r p t i o n c r o s s sections of molecular o x y g e n . T h e differences with detailed attenuation calculations are of the o r d e r of 10 percent above 194 nm, 20 percent between 189 a n d 194 nm and as large as 40 percent below 189 nm. A c c o r d i n g to F r e d e r i c k et al. ( 1 9 8 1 ) , s u c h disagreements could be attributed to uncertainties in s p e c t r o s c o p i c data but e r r o r s associated with the o b s e r v a t i o n s s h o u l d not be neglected.

T h e c r o s s section v a l u e s in the S c h u m a n n - R u n g e b a n d s y s t e m which v a r y b y about 5 o r d e r s of magnitude between 175 and 205 nm are calculated on the b a s i s of experimental s p e c t r o s c o p i c parameters for the S c h u m a n n - R u n g e rotational lines, a d d i n g the c o n t r i b u t i o n of u n d e r l y i n g continua which become important near both e n d s of the b a n d s y s t e m . Nicolet and Peetermans (1980) emphasize the difficulties of o b t a i n i n g accurate experimental v a l u e s of the oscillator s t r e n g t h and of the p r e - dissociation linewidth. F i g u r e 5 illustrates the linewidth measured since the w o r k of A c k e r m a n and Biaume ( 1 9 7 0 ) , i n c l u d i n g the new v a l u e s recently obtained b y Gies et al. (1981). Theoretical v a l u e s d e d u c e d from models are not reported in this f i g u r e . T h e large d i f f e r e n c e s between the available data s h o u l d play an important role on the p h o t o -

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0

+

+

• • o

o

a

I 1 _L

T T

+ Ackerman & Biaume (1970)

• Hudson & Mahle (1972) x Lewis et al. (1979)

a Frederick & Hudson (1979) o Gies et al. (1981)

+

8 S

g +

* a

o * + + • +

1

o o . • °

±

± J

U 6 8 10 12 K VIBRATIONAL NUMBER V

16 18 20

- 1

F i g u r e 5 Experimental values of t h e l i n e w i d t h (cm ) in t h e Schumann- Runge bands of molecular o x y g e n .

- 1 8 -

(21)

dissociation rates of several trace species. I n d e e d , if smaller linewidths are u s e d , the a b s o r p t i o n b y molecular o x y g e n is more concentrated in the line c e n t e r s . C o n s e q u e n t l y thie solar radiation in the w a v e l e n g t h interval of 175-200 nm penetrates more deeply in the s t r a t o s p h e r e a n d e n h a n c e s the photodissociation rates at lower altitudes.

Recent s t u d i e s on the photodissociation of the molecular o x y g e n in the S c h u m a n n - R u n g e b a n d s ( K o c k a r t s , 1976; B l a k e , 1979; Nicolet a n d Peetermans, 1980; F r e d e r i c k a n d H u d s o n , 1979) are based on detailed computation of the rotational s t r u c t u r e of the b a n d s y s t e m . T h e y r e - q u i r e lengthly computation a n d , in o r d e r to reduce the computer time and cost, these a u t h o r s have also developped parameterization methods which can be applied either to calculate the photodissociation rate of molecular o x y g e n itself or the penetration of the solar radiation in the wavelength r a n g e of 175-200 nm. T h e s e approximations are only valid when the a b s o r p t i o n c r o s s section of the trace species can be a v e r a g e d o v e r each g i v e n wavelength intervals (I nm, 500 cm , i n t e r v a l s between s u c c e s s i v e b a n d h e a d s ) . O n the c o n t r a r y , when c o n s t i t u e n t s s u c h as nitric oxide are characterized b y rotational s t r u c t u r e s in their c r o s s sections, special treatments are r e q u i r e d (Nicolet, 1979; Allen and F r e d e r i c k , 1982).

K o c k a r t s (1971) has introduced reduction factors defined b y the e x p r e s s i o n s :

n

and

Rb( M ) = I e x p ( - T .b) (3)

i = l

nb

W = r 1 £b ai b e xp("Ti b) ( 4 )

i = l

where b denotes each b a n d in the S c h u m a n n - R u n g e b a n d s y s t e m a., the O0 c r o s s section calculated e v e r y 0 . 5 cm - 1

ib d.

the predissociation probability c o r r e s p o n d i n g to b a n d b the optical depth e v e r y 0 . 5 cm - 1

' b l b

n. the number of intervals in b a n d b.

b

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(22)

T h e r e d u c t i o n s f a c t o r s are calculated u s i n g the4 molecular parameters taken from A c k e r m a n et al. (1970). A p p r o x i m a t i o n s of these reduction factors t a k i n g into account the atmospheric temperature profile h a v e been g i v e n b y K o c k a r t s (1976) :

6

£n Rb (x) = - a exp [ I c . f x - x ^1 ] (5) i = l

where x is the natural logarithm of the total content of molecular

- 2

o x y g e n in cm . T h e same e x p r e s s i o n s can also be applied for i n t e r v a l s of 500 cm and 1 nm. T h e coefficients x , a and c . t h r o u g h c~ are - 1 o 1 53 6 g i v e n in table 2, 3, 5, 6, 8 and 9 of K o c k a r t s (1976) for each type of wavelength i n t e r v a l s . T h e calculations are o n l y valid for altitudes above 30 km. T h e e r r o r s > u s i n g these a p p r o x i m a t i o n s , are g e n e r a l l y less t h a n 5 percent for most of the i n t e r v a l s . T h e same set of coefficients can be u s e d for different solar zenith a n g l e s a n d the choice of the w a v e l e n g t h interval is g u i d e d b y the aeronomic problem to be s t u d i e d .

In his model, B l a k e (1979) has adjusted the linewidth v a l u e s , u s e d as a parameter, in o r d e r to improve the agreement between the cal- culated and the measured c r o s s section value. T h e best fit is obtained with a linewidth in good agreement with those o b s e r v e d b y Lewis et al.

(1978) and the c r o s s section model is in good agreement with the mea- s u r e m e n t s of A c k e r m a n et al. (1970) except at a few w a v e l e n g t h s . B l a k e (1979) p r o p o s e d an e x p r e s s i o n g i v i n g the mean t r a n s m i s s i o n T in the middle atmosphere for two wavelength i n t e r v a l s , namely 1.0 a n d 0.2 nm :

T = exp - [ ^ — + Y] N ( 02) (6)

u + p x N ( o2) r

where N d e s i g n a t e s the o x y g e n column- d e n s i t y . X , Y and 0 are coeffi- cients g i v e n b y Blake (1979) in his tables 3 and 4.

- 2 0 -

(23)

T h e effective local cross section ag is obtained with a polynomial function :

£n(cr ) = exp [ I a Xn ] (7)

, 6 n=0 n

where

X = { log [N(02)] - 20}/3.5 (8)

T h e an coefficients are given by Blake (1979) in his tables 5 and 6.

The standard deviation over all points is 6 percent for the 1.0 nm model and 7 percent for the 0 . 2 nm model.

Nicolet and Peetermans (1980) have calculated an effective cross section following the expression :

• = I VJ

1 / < 1 + e P )

»>

where a . , and a are the maximum and the minimum photodissociation M m cross section defined by :

1 n

aM( T ) = n 1 ai( T ) ( 1 0 )

i = l

when N(C>2) 0, and by :

a (T) = [ a . ( T ) ] (11) m l m

which corresponds to the smallest values of o in the adopted range when N ( 02) -*• » . |n these expressions, cr^T) is the absorption cross section corresponding to a narrow interval labelled i and n is the number of intervals. Values of CT^ and om are given for three tempera- t u r e s , 190, 230 and 270 K in table 7 in Nicolet and Peetermans (1980).

Parameter p is a function given by the polynomial expression :

- 2 1 -

(24)

n

p = Z p. [An N ( 02) ]1 (12)

n=0

Values using only two terms (n = 1) in expression (12) are given b y Nicolet and Peetermans (1980) in their table 8 for the aforementioned temperatures. T h e transmission related to the mean optical depth 1(0^) is defined by the expression :

exp [ - t ( 02) ] = exp ( - ed) (13)

where d is given by a second polynomial function :

d = I d. [£n N ( 02) ]x

i=0

for which coefficient values using only two terms (n table 9 of Nicolet and Peetermans (1980).

T h e temperature effect cannot be neglected when accurate values are r e q u i r e d . Moreover precise calculations should take into account the specific effect of the solar zenith angle since, due to the temperature dependence of the cross section values, optical paths with the same O2

column content but d i f f e r e n t zenith angles can lead to s i g n i f i c a n t l y d i f f e r e n t solar attenuation and dissociation rates. When the calculation with a 10 percent precision is s u f f i c i e n t , the expressions with two terms can be used with an average temperature of 230 K and for a maximum

23 2 total content of molecular oxygen less than 5 x 10 molecules/cm ,

c o r r e s p o n d i n g to stratospheric levels above 30 km.

Allen and F r e d e r i c k (1982) have also recently published a simple polynomial representation of the effective photodissociation cross section, taking into account the effect of the altitude z and of the zenith angle

log CT^ (z,

x

(14)

= 1)are given in

0°) = I a . . X . ( z )J~

J J

(15)

-22-

(25)

where X . ( z ) is either the temperature T ( z ) or the logarithm of the pressure log P ( z ) . The coefficients a., are presented by Allen and

'J

_ 1

Frederick (1982) in table 2, where indice i refers to the 500 cm wave-

length intervals in the spectral region of the Schumann-Runge bands.

The error averaged over all altitudes is 9 percent with a maximum value of 22 percent at 50 km. Allen and Frederick (1982) stated that such an error is within the uncertainties of the detailed calculations made by

Frederick and Hudson (1979), namely ± 40 percent.

The effective cross section variation with the zenith angle x is given by :

a .

e

( e , x ) / a

i e

( z , 0 ) - (Sec

X

) "

C ( z )

0 * ) where C j ( z ) is given by :

log C . ( z ) = I b . . [log N ( z ) ]

j _ 1

(17)

1

' J

1 J

where N ( z ) is the zenith oxygen column density above the altitude z.

Coefficients bj. are presented by Allen and Frederick (1982) in table 3.

3.4 The Herzberg continuum

The penetration of the solar radiation in the wavelength range of 200-240 nm is driven by the absorption of the molecular oxygen in the Herzberg continuum but also by the absorption of ozone which becomes predominant beyond 220 nm. In addition, extinction by Rayleigh scattering contributes for at least 10 percent of the total attenuation. It contributes also to the minimum values in the Schumann-Runge band system as the underlying Herzberg continuum. The absorption cross sections of molecular oxygen are very weak beyond 200 nm (lower than 10"

2 3

cm

2

) and accurate values are difficult to obtain. Laboratory measurements require long path lengths or high pressure in the ab-

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sorption èell. In this latter case, the absorption ife contaminated by the presence ôf O^ which is formed by association of two oxygen molecules.

Several determinations of the absorption cross section of molecular oxygen in the Herzberg continuum have been performed in the last 20 years. T h e values measured by Hasson and Nicholls (1971) are some- what higher than the data obtained by Ditchburn and Young (1962) on which the tables published by Ackerman (1971) are based. T h e absorption spectrum reported by Shardanand and Prasad Rao (1977) is characterized by the smallest cross section at wavelengths shorter than 230 nm, i.e. in the region which contributes the most to the photo- dissociation frequency in the middle and lower stratosphere. Other data such as the values by Ogawa (1971) or the tabulation by Hudson and Reed (1979) or by Nicolet (1978) are intermediatR.

Recéntly, Frederick and Mentall (1982) as well as Herman and Mentall (1982a) have derived the molecular oxygen absorption cross section spectrum which is required to explain the observed attenuation of the solar irradiance between two different heights located in the range of 30 to 40 km. The observations have been performed by means of the same balloon borne double monochromator.

Frederick and Mentall (1982), have deduced from their observa- tions on September 15, 1980, that, in the 200-210 nm range, the Hasson and Nicholls (1971) cross sections should be multiplied by factors in the range of 0.52 to 0.68 and the Shardanand and Prasad Rao (1977) values by factors in the range of 0.85 to 0.93. It should be noted however that, when applied to the two sets of laboratory data, these factors do not lead to the same value of the corrected cross sections. In principle, they are calculated assuming that the cross section of ozone is perfectly known and that the ozone column between the two levels of observation is measured without error.

Herman and Mentall (1982a) have derived with the same instrument on April 15, 1980 absorption cross sections which are about 30 percent

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smaller than the laboratory 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 about 50 p e r c e n t smaller t h a n those of H a s s o n and Nicholls (1971). T h e s e v a l u e s d e d u c e d from in situ mea- s u r e m e n t s are t h u s somewhat smaller than 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 ( 1 9 8 2 ) .

F i g u r e 6 s h o w s that the d i s p e r s i o n in the available data is quite important and leads to s i g n i f i c a n t uncertainties in the photodissociation rate of several trace species. O n e dimensional models have been u s e d v e r y recently b y F r o i d e v a u x and Y u n g (1982) a n d b y B r a s s e u r et al.

(1983) in o r d e r to estimate the importance of the s t r a t o s p h e r i c com- position c h a n g e s due to a b s o r p t i o n c r o s s section v a l u e s 30 to 50 p e r c e n t lower than those c u r r e n t l y u s e d in photodissociation calculation,

4. 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 O F O Z O N E

T h e ozone a b s o r p t i o n c r o s s section has been measured in the ultraviolet up to 350 nm and in the visible r a n g e . O n l y one measurement with an estimated u n c e r t a i n t y of ± 10 percent is available below 200 nm ( T a n a k a et a l . , 1953). Numerical v a l u e s have been d e d u c e d from t h i s w o r k for modelling p u r p o s e s ( A c k e r m a n , 1971).

B e y o n d 200 nm, the Hartley b a n d s peak a r o u n d 250 nm with a -17 2

value of 1.146 x 10 cm for the a b s o r p t i o n c r o s s section ( H e a r n ,

- 2 2 2

1961) which decreases below to 10 cm t h r o u g h to the H u g g i n s b a n d s b e y o n d 300 nm. T h e values measured b y I n n and T a n a k a (1953) are widely u s e d in the photodissociation calculation from 200 to 300 nm.

B e y o n d this w a v e l e n g t h , the temperature dependence of the a b s o r p t i o n c r o s s section has to be taken into account. T h e w o r k of V i g r o u x (1953) has long been the only available reference.

S i n c e the review p u b l i s h e d b y A c k e r m a n (1971), new measurements are now in p r o g r e s s at the National B u r e a u of S t a n d a r d s ( U S A ) and at

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10 -22

o Z 10 •23

<_>

UJ (ƒ)

If) in o tr u

| I f f * i/i CD <

10 •25 190

MOLECULAR OXYGEN

H E R Z B E R G CONTINUUM

C* SHARDANAND & PRASAD RAO (1977) Meas.< • HA5S0N & NICHOLLS (1971)

[o HERMAN & MENTALL (1982) '---, ACKERMAN (1971)

NICOLET (1978)

200 210 220 230

WAVELENGTH (nm)

240 250

F i g u r e 6 A b s o r p t i o n c r o s s s e c t i o n of m o l e c u l a r o x y g e n b e t w e e n , 200 a n d 250 nm.

- 2 6 -

i

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the University of Reims (France). Preliminary results for the wave- length range of 245 to 345 nm at 295 K and 243 K have been reported by Bass and Paur (1981). These measurements are relative to the peak absorption value measured by Hearn (1961) at 254 nm. Absolute measurements performed by Daumont et al. (1983) at 293 and 223 K between 300 and 350 nm are systematically 12 percent lower than those reported by Bass and Paur (1981). The uncertainty in the new values of the cross section is estimated to be 2 percent or less. The latter measurements resolve with a better precision the features of the absorption spectrum (the instrumental width is about 0.02 nm). Conse- quently, these data with resolved structure, especially beyond 300 nm, can be used to analyze the solar backscattered radiation measured by satellites (Mc Peters and Bass, 1982),

The absorption of solar radiation by ozone takes place in the mesosphere and in the stratosphere for wavelengths greater than 180 nm. At higher altitude, the ozone density is too small to play any role in the penetration of the ultraviolet solar radiation. The photodis- sociation of ozone leading to the formation of the excited state of atomic oxygen has been investigated for a long time, particularly by Nicolet (1939).

5. S C A T T E R I N G

When penetrating in the atmosphere, the solar radiation is not only absorbed by optically active constituents but also scattered by the gas molecules and the aerosols. The irradiance q at a given altitude z, wavelength K and solar zenith angle x is given by :

q ( M , X ) = e "

( T a + T s )

+ J

Q

K A . z . f i ) dQ (18) where l ( \ , z , Q ) is the intensity of the light which is scattered in the

solid angle fi.

(30)

T h e optical depth corresponding to the absorption (t ) can be

3

expressed as :

ta( A , z , x ) = sec x [aa(02;A) / n(02) dz' + aa( 03; \ ) / n(03) d z ' ]

z w z (19)

since the main absorbing gases are molecular oxygen and ozone. T h e attenuation due to scattering processes appears through a second con- tribution of the optical depth ( ig) which can be written as :

oo oo

^ dz' + CTm / r^ dz*] (20)

z J z

where a and n are respectively the scattering cross section and the concentration of the Rayleigh ( R ) and Mie (M) scatters.

T h e f i r s t effect is governed by air molecules while the second contribution is due to aerosol particles. As indicated by Meier et al.

(1982), test calculations have shown that, for standard atmospheric load, Mie scattering has a small effect on the value of the solar i r r a - diance. T h e cross section related to Rayleigh scattering is given by

(Frohlich and Shaw, 1980) :

= 32 7T (H - 1) f ( 6 ) ( 2 1 )

T 2 A4

3 nR A.

where (j is the refractive index of the air which depends on the pres- sure, the temperature, the relative humidity and varies with wave- length. T h e function :

f(6) = (6 + 3 6)/(6 - 7 6) (22)

where 6 is the depolarization factor, accounts for the anisotropy of the molecules. For a standard mixture of air it has a value close to 1.016 (Frohlich and Shaw, 1980). When calculating explicitely the Rayleigh scattering cross section can be expressed as :

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3.93 x 1 0 "

R ~ — — ; — (23) [X(nm)] + X

where x is g i v e n by ( F r ö h l i c h and S h a w , 1980) :

x = 74 \ + 5 x 1 0 "5M " 0 . 0 8 4 ( 2 4 )

K being e x p r e s s e d in nm.

T h e effect of scattering on the ultraviolet and visible radiation field in the t r o p o s p h e r e and stratosphere has been recently analyzed b y Meier et al. (1982) and Nicolet et al. (1982). T h e i r results show that multiple scattering and g r o u n d albedo can lead to large amplifications of the solar irradiance in the photochemically active spectral region. T h e s e a u t h o r s have calculated the c o r r e s p o n d i n g enhancement factors as a function of height, solar zenith angle and wavelength. Numerical tables of these factors are g i v e n in convenient format for computional u s e .

From their solar ultraviolet o b s e r v a t i o n s performed at the altitude of 40 km, Herman and Mentall (1982b) have determined the relative impor- tance of scattering v e r s u s absorption. T h e i r analysis shows that the scattered flux in the upper stratosphere is of the o r d e r of 1 percent of the direct flux between 220 and 300 nm and becomes larger than 10 percent beyond 310 nm.

5. C O N C L U S I O N S

Significant improvements are required in solar irradiance measure- ments. A c c u r a c y of the o r d e r of 5 percent is needed for the calculation of photodissociation frequencies. S u c h an objective is achievable b y appropriate calibration and measurement strategies. In particular, the magnitude of the solar variability related to the rotation period of the S u n and the 11-year cycle should be f u r t h e r investigated.

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New in situ measurements of the atmospheric attenuation of the solar irradiance have questionned the accepted values of the absorption cross section of molecular oxygen in the Herzberg continuum. T h e low absorption recently observed in this wavelength range needs to be con- firmed .

T h e penetration of the solar radiation in the Schumann-Runge bands is strongly dependent on spectrometric parameters which are not sufficiently well known for an accurate determination of the photo- dissociation frequencies in this part of the spectrum.

T h e s optical depth around 210 nm is also driven by the ozone absorption. New values of the corresponding cross section in the vicini- ty of this wavelength are required in order to confirm the presently available data. Determination of the solar penetration should also consider the effect of multiple scattering which plays an important role above 300 nm and cannot be neglected in the calculation of the attenua- tion around 200 nm.

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R E F E R E N C E S

A C K E R M A N , M. (1971). Ultraviolet solar radiation related to meso- s p h e r i c p r o c e s s e s , in M e s o s p h e r i c Models a n d Related E x p e r i m e n t s ( E d . G. Fiocco), p. 149, Reidel, D o r d r e c h t , Holland.

A C K E R M A N , M. a n d B I A U M E , F. ( 1 9 7 0 ) . S t r u c t u r e of the S c h u m a n n - R u n g e b a n d s from the ( 0 - 0 ) to the ( 1 3 - 0 ) b a n d . J. Molec.

S p e c t r o s c . 35, 73.

A C K E R M A N , M . , B I A U M E , F. a n d K O C K A R T S , G. ( 1 9 7 0 ) , A b s o r p t i o n c r o s s sections of the S c h u m a n n - R u n g e b a n d s of molecular o x y g e n . Planet. Space S e i . 18, 1639.

A L L E N , M. and F R E D E R I C K , J . E . ( 1 9 8 2 ) . Effective photodissociation c r o s s sections for molecular o x y g e n a n d nitric oxide in the S c h u m a n n - R u n g e b a n d s . J . Atmos. S e i . , 39, 2066.

A R V E S E N , J . C . , G R I F F I N , R . N . , J r . a n d P E A R S O N , B . D . , J r . ( 1 9 6 9 ) . Determination of e x t r a t e r r e s t r i a l solar spectral i r r a d i a n c e from a r e s e a r c h aircraft. A p p l . O p t . 8, 2215.

B A S S , A . M . a n d P A U R , R . J . ( 1 9 8 1 ) . U V a b s o r p t i o n c r o s s - s e c t i o n s for ozone : the temperature d e p e n d e n c e . J. Photochem. 17, 141.

B L A K E , A . J . (1979). A n A t m o s p h e r i c a b s o r p t i o n model for the S c h u m a n n - R u n g e b a n d s of o x y g e n . J. G e o p h y s . R e s . , 84, 3272.

B L A K E , A . J . , M c C O Y , D . G . , G I E S , H . P . F . a n d G I B S O N , S . T . ( 1 9 8 0 ) . Photoabsorption in molecular o x y g e n . Interim Sei. R e p . Department of P h y s i c s , U n i v . of Adelaide ( A u s t r a l i a ) .

B O S S Y , L. (1983) same i s s u e .

B R A S S E U R , G. and S I M O N , P . C . (1981). S t r a t o s p h e r i c chemical a n d thermal r e s p o n s e to long-term variability in solar U V i r r a d i a n c e , J.

G e o p h y s . R e s . 86, 7343.

B R A S S E U R , G . , D E R U D D E R , A . and S I M O N , P . C . ( 1 9 8 3 ) . Implication for s t r a t o s p h e r i c composition of a reduced a b s o r p t i o n c r o s s section in the H e r z b e r g continuum of molecular o x y g e n , G e o p h y s . R e s . Lett., 10, 20.

B R O A D F O O T , A . L. (1972). T h e solar spectrum 2100-3200 Ä , A s t r o p h y s . J. 173, 681.

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