solar uv radiation and its absorption in the mesosphere and stratosphere

17
Pageoph, Vol. 118 (1980), Birkh~iuser Verlag, Basel Solar UV Radiation and its Absorption in the Mesosphere and Stratosphere By MARCELNICOLET 1'2) Abstract - Solar radiation of A > 175 nm and of Lyman-alpha at 121.6 nm is absorbed in the mesosphere and stratosphere by molecular oxygen (A < 242 nm) and also by ozone molecules at A > 200 nm. This paper describes the photodissociation processes resulting from absorption in the Schumann-Runge bands and Herzberg continuum of molecular oxygen and also in the Hartley, Huggins and Chappuis bands of ozone. Special consideration is given to differences between the stratospheric and mesospheric problems. Key words: Ultraviolet radiation; Photodissociation; Schumann-Runge bands; Herzberg continuum; Ozone bands. L Introduction In order to study the photochemical action of solar radiation on stratospheric and mesospheric constituents, it is convenient to divide the solar spectrum in spectral ranges related to the molecular oxygen and ozone absorptions. The radiation of wavelengths less than 100 nm is absorbed by nitrogen and oxygen in the thermosphere; it leads essentially to ionization processes and is, therefore, not considered here. Only X rays of wavelengths less than 1 nm can penetrate into the atmosphere below 100km, and lead indirectly to the dissociation of molecular constituents. The radiation of wavelengths less than 242 nm is absorbed by molecular oxygen and leads to its photodissociation. The principal photodissociation continuum (Schumann-Runge continuum) at A < 175 nm corresponds to a complete absorption of the solar radiation in the thermosphere and will not be considered in this analysis. An important solar line, Lyman-~ at 121.6 nm, is situated in a so-called atmo- spheric window since the 02 absorption cross section is only of the order of 10-2o cmL Such a radiation is absorbed in the mesosphere. The second important spectral range, between 200 nm and 175 nm, is related to the 0 2 Schumann-Runge band system which includes 18 bands, (2-0) to (19-O), subject to the predissociation process. In this spectral region, the mean absorption cross sections are a function of the temperature and number of O2 absorbing molecules. 1) Ionosphere Research Laboratory, The Pennsylvania State University, University Park, Pa. 16802, USA. 2) Present address: 30 Avenue Den Doorn, B-180 Brussels, Belgium.

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Page 1: Solar UV radiation and its absorption in the mesosphere and stratosphere

Pageoph, Vol. 118 (1980), Birkh~iuser Verlag, Basel

Solar UV Radiation and its Absorption in the Mesosphere and Stratosphere

By MARCEL NICOLET 1'2)

Abstract - Solar radiation of A > 175 nm and of Lyman-alpha at 121.6 nm is absorbed in the mesosphere and stratosphere by molecular oxygen (A < 242 nm) and also by ozone molecules at A > 200 nm. This paper describes the photodissociation processes resulting from absorption in the Schumann-Runge bands and Herzberg continuum of molecular oxygen and also in the Hartley, Huggins and Chappuis bands of ozone. Special consideration is given to differences between the stratospheric and mesospheric problems.

Key words: Ultraviolet radiation; Photodissociation; Schumann-Runge bands; Herzberg continuum; Ozone bands.

L I n t r o d u c t i o n

In order to study the photochemical action of solar radiation on stratospheric

and mesospheric constituents, it is convenient to divide the solar spectrum in spectral

ranges related to the molecular oxygen and ozone absorptions.

The radiation of wavelengths less than 100 nm is absorbed by nitrogen and oxygen

in the thermosphere; it leads essentially to ionization processes and is, therefore, not

considered here. Only X rays of wavelengths less than 1 nm can penetrate into the

atmosphere below 100km, and lead indirectly to the dissociation of molecular

constituents. The radiation of wavelengths less than 242 nm is absorbed by molecular oxygen

and leads to its photodissociation. The principal photodissociation continuum

(Schumann-Runge continuum) at A < 175 nm corresponds to a complete absorption of the solar radiation in the thermosphere and will not be considered in this analysis.

An important solar line, Lyman-~ at 121.6 nm, is situated in a so-called atmo-

spheric window since the 02 absorption cross section is only of the order of 10-2o cmL

Such a radiation is absorbed in the mesosphere. The second important spectral range, between 200 nm and 175 nm, is related to

the 0 2 Schumann-Runge band system which includes 18 bands, (2-0) to (19-O),

subject to the predissociation process. In this spectral region, the mean absorption

cross sections are a function of the temperature and number of O2 absorbing molecules.

1) Ionosphere Research Laboratory, The Pennsylvania State University, University Park, Pa. 16802, USA.

2) Present address: 30 Avenue Den Doorn, B-180 Brussels, Belgium.

Page 2: Solar UV radiation and its absorption in the mesosphere and stratosphere

4 Marcel Nicolet (Pageoph,

The absorption, which is essentially a mesospheric process, also plays a role in various stratospheric photodissociations.

From 200 to 242 nm, the 02 absorption, which is related to the Herzberg con- tinuum with low absorption cross section (from 10 -24 to 10 -23 cm 2) occurs in the stratosphere. In addition, the ozone absorption must be introduced since this spectral region belongs to the spectral range of the 03 Hartley band. The simultaneous absorption by 02 and 08 must be considered in the stratosphere. In the mesosphere, the 03 absorption is practically negligible since the total number of ozone molecules is very small for low solar zenith angles.

At wavelengths less than 310 nm corresponding to the 03 Hartley band, the ozone molecule has its principal absorption which occurs in the stratosphere. Its limit near 310 nm must be determined with precision, since the photodissociation process in the Hartley band, which is

03 + hv(A < 310 nm)-+ O2(1Ag) + O(1D) (1)

leads to the production of O(~D) atoms responsible, particularly below 50 km, for the production of OH from H20, CH4 and H2, and also of NO from N20.

At ~ > 310 nm, the Oa Huggins bands correspond to the limit of its ultraviolet absorption. The spectral range to be considered should be between 310 n m a n d 400 nm since it corresponds also to various limits of the absorption spectrum of H202, H2CO, NO2, N20~, HNO2, HNOa, C1ONO2, HOCI . . . . .

In the visible region (410-850 nm), the Chappuis bands play an important role leading to the 03 photodissociation in the lower part of the atmosphere, troposphere

and lower stratosphere. At ~ > 300 nm, various effects such as the Rayleigh scattering and the albedo

must be introduced. In particular, the photodissociation rates of 03; ratio n(O)/n(03), of NOz, ratio n(NO2)/n(NO), and the absolute concentration of the other constituents absorbing in that spectral region are strongly affected by the Rayleigh scattering and albedo effects.

Thus, the photodissociation problem is related to a knowledge of the solar flux and its possible variations in certain spectral regions, the exact determination of the vertical distribution of the atmospheric optical depth of 02 and Oa, the measurement of the absorption cross sections and photodissociation quantum yields for each constituent, and the introduction of the atmospheric conditions related to the radia-

tion scattering and a lbedo .

H. 02 Absorption

The absorption cross section in the Herzberg continuum is known with an accuracy which is not greater than 25~ (Fig. 1). At A > 230 nm the 02 cross section is not known with sufficient precision; but since the ozone absorption is maximum in this

Page 3: Solar UV radiation and its absorption in the mesosphere and stratosphere

Vol. 118, 1980) Solar UV Radiation Absorption in Mesosphere and Stratosphere 5

15

.~ 02

~0 : . . . . . . . . . . . . . . . . . . . . . - INUUM

o i?i!!~ ..... g / "

= 10 I I , i

200 210 220 230 2/,0 WAVELENGTH ( nm )

Figure 1 Experimental data on 02 absorption cross sections in the Herzberg continuum, x DITCHBURN and

YOUNG (1962). (:30GAWA (1971). [] I-IASSON and NICHOLLS (1971). �9 SHARDANANO (1977).

Table 1 Mean value o f solar f lux (q, photons cm-2 sec-1); mean cross section (o, cm 2 for 500 cm- 1) and 02 photodissociation coefficients (j, sec- 1) at the top o f the Earth's atmosphere in the spectral range o f the 02 Herzberg continuum (A(A) = average wavelength in A o f the spectral

range, + 250 cm- 1)

A(A) q~ ao2 j~

2010 1.44 x 1012 1.50 x 10 -23 2.16 • 10 -11 2030 1.80 1.25 2.25 2051 2.08 1.00 2.08 2072 2.45 9.80 • 10 -24 2.40 2094 5.09 9.20 4.68 2116 7.12 8.50 6.05 2139 9.23 7.85 7.25 2162 8.42 7.05 5.94 2186 1.20 • 1013 6.15 7.38 2209 1.22 5.50 6.71 2234 1.77 4.75 8.4l 2259 1.60 4.05 6.48 2286 1.96 3.35 6.57 2312 1.97 2.70 5.32 2339 1.70 2.20 3.74 2367 2.00 1.65 3.30 2395 1.77 1.20 2.12 2424 1/2 (2.58) 0.75 9.80 • 10 -12

Page 4: Solar UV radiation and its absorption in the mesosphere and stratosphere

6 Marcel Nicolet (Pageoph,

10

Z N ( O 2 } ~< 2 x J019 Cm , O- IO2) = 10~29cm 2 C~

~ 2 x l O 19 c m 2, l og TO2 = A l og N IO;J .B ~ )

U J

q

"T" U3 p.. O n

. o

~ 5x~0 ~

0 m <

I I i I 1 I I .r ~ I I J ~. tl r I ~ i ~ 10 7'

0 . ' 10'" t0 ~ , 10 ~~

0 2 M O L E C U L E S (crn -2)

Figure 2 Experimental variation of the Lyman-alpha absorption cross section (cm 2) with the optical depth

in molecular oxygen, N(O2)(cm2).

part of the ultraviolet spectrum, the numerical error is reduced for the value of the

total 02 photodissociation rate. Table 1 shows the variation of the mean absorption cross section based on the experimental data of DITCHBURN and YOUNG (1962),

OGAWA (1971), HASSON and NICHOLLS (1971) and SHARDANAND and RAO (1977) and increasing from about 10 -24 cm 2 at 240 nm to 1.5 • 10 -23 cm 2 at 200 nm; this last

value involves the 0-0 and 0-1 Schumann-Runge band absorption. The resulting j~(O2) from identical energy spectral ranges (500 c m - 1) shows a rapid increase from 240 nm to 230 nm (Table 1) and also a decrease at k < 210 nm. The problem of the solar flux values will be discussed in Section III.

The Lyman-a absorption occurs in the wing of an 02 band and is subject to a temperature effect (CARVER et al., 1977). Its absorption cross section is a function

of the temperature and of the 02 optical depth. Figure 2 shows how the absorption cross section varies f rom 10 -2o cm 2 to 7 x 10 -21 cm 2 at T = 190~ (mesospheric

temperature) for optical depths between 0 and 9. Lyman-c~ plays a role in the meso-

spheric photodissociation rate of several constituents, but particularly of H20, CH4 and CO2. It is responsible for the NO ionization in the mesosphere. Its variation with

solar activity will be discussed in Section III. The problem of the 02 photodissociation in the Schumann-Runge band spectral

range, and also of its atmospheric absorption has not yet reached its final solution. After various applications of the first experimental results to the atmosphere (HUDSON et al., 1969; KOCKARTS, 1971, 1976; HUDSON and MAHLE, 1972; FANG et al., 1974;

Page 5: Solar UV radiation and its absorption in the mesosphere and stratosphere

Vol. 118, 1980) Solar UV Radiation Absorption in Mesosphere and Stratosphere 7

"-,

tO

i ~ + J

0 2 and 0 3 " ~

\ 85kin 50 40 25

13 ~ 1o 2~ 10 ~' 02 M O L E C U L E S (crn 2}

Figure 3

PARK, 1974; MURAMATSU, 1975; SHIMAZAKI et al., 1977), new experimental and theoretical results (LEwis et al., 1978; FREDER]CK and HUDSON, 1979; NICOLET and PEETERMANS, 1979) show that still more attention should be given to the accuracy problem.

There is general agreement on the molecular constants used for the 02 Schumann- Runge bands (cf. FANO et al., 1974) as known from experimental data obtained by ACKERMAN and BIAUM~ (1970) and by BRIX and HERZBERG (1954), and analyzed for the fine structure of the upper level 3Z~.,> 0 by BERGEMAN and WOFSu (1972). Additional measurements with still higher resolution would be useful. But precise oscillator strengths and linewidths associated with exact line positions are an absolute necessity for accurate determination of the photodissociation processes, particularly in the mesosphere. From a comparative analysis (NICOLET and PEETERMANS, 1979) of the various parameters involved in the atmospheric 02 absorption, it can be concluded that there is no important direct effect of the Scbumann-Runge band absorption on the total J2 value in the stratosphere. If the ratio (Figure 3) of the photodissociation rate Js~B, resulting from the Schumann-Runge band spectral range of (175 nm < h < 200 nm) to the total photodissociation rate JSRB-H~RC, resulting also from the Herzberg continuum spectral region (175 nm < A < 242 nm) is greater than 80~ for a total number of 02 absorbing molecules N(O2) < 1020 cm-2; it is only between l0 and 15~ in the major part of the stratosphere. Furthermore, the essential fraction of this low percentage is due to bands corresponding to v' < 10. The (2-O), (3-0), (4-0), (5-O), (6-0) and (7-0) bands lead for N(O2) = 5 • 1021 cm -2 (stratopause neighborhood) to about 3, 3, 2.5, 2.5, 1.5 and 1.5~, respectively. Thus, the total error due to incorrect parameters in the Schumann-Runge band system cannot account for the importance of the stratospheric J2 value. On the other hand, any solar activity effect which might be introduced in the calculation of these J2 values for

Page 6: Solar UV radiation and its absorption in the mesosphere and stratosphere

8 Marcel Nicolet (Pageoph,

Table 2 Parameters for determination of 02 optical depth

Temperat~e:190~ Temperature:230~ Temperature:270~ B~d ~,o ~.1 ~,o ~ . 1 ~.o ~.1

19-0 -22.2647 0.4938 -20.511 0.4552 -20.5433 0.4546 18-0 -26.2477 0.5914 -27.0032 0.6070 -26.8968 0.6036 17-0 -21.5859 0.4877 -21.5692 0.4874 -21.5642 0.4871 16-0 -23.1643 0.5184 -22.9975 0.5160 -23.0408 0.5176 15-0 -20.5035 0.4512 -21.2109 0.4684 -20.0262 0.4449 14-0 -22.2214 0.4747 -22.3230 0.4808 --22.1773 0.4807 13-0 -25.9576 0.5466 -26.4881 0.5608 -26.1471 0.5566 12-0 -25.7030 0.5480 -25.4172 0.5442 -25.9842 0.5575 11-0 -21.2361 0.4553 -21.9458 0.4739 -22.4196 0.4873 10-0 -25.0795 0.5159 -24.1133 0.5009 -22.6346 0.4751 9-0 -24.1337 0.4957 -23.2161 0.4803 -23.6780 0.4923 8-0 -25.3090 0.4979 -23.4269 0.4848 -24.0370 0.5015 7-0 -24.0797 0.4845 -24.7619 0.5012 -24.3381 0.4959 6-0 -28.8867 0.5697 -25.3433 0.5042 -24.9085 0.4989 5-0 -31.0862 0.6078 -30.5188 0.6000 -26.9407 0.5346 4-0 - 34.4926 0.6683 -33.6523 0.6549 - 33.1046 0.6474 3-0 -41.5449 0.7963 -40.7366 0.7827 - 39.1489 0.7546 2-0 -49.2427 0.9377 -48.8488 0.9304 -46.8581 0.8944

A < 200 nm could play only a minor role corresponding to its 1070 contr ibution to the stratospheric photodissociation o f molecular oxygen.

In the mesosphere, a l l Schumann-Runge bands between (2-0) and (19-O) must be considered. As an example, for N ( O 2 ) = 1020 cm -2, at and above 75 km, the

(7-0) band corresponds to the maximum of the order o f 107o while (2-0) and (15-O)

have a less significant role, only between 1 and 2~ . It is permissible (see references above), therefore, to use simplified formulas in

order to compute the J2 factors of various constituents for their stratospheric absorp-

tion. In their analysis, NICOLET and PEETERMANS (1979) have introduced, for the 02

mean optical depth, a polynomial function, N(O2) being the total number of 02

molecules (cm - 2).

In r(O2)b~nd = ~ d~ [In N(O2)] ~ (2) ~ = 0

which can be applied to almost all atmospheric problems with only two terms

In r(O2)b~nd = do + d l In N(O2). (3)

A formula with six terms has also been deduced, and leads to a complete agreement with the results of the detailed calculation. The numerical values do and dl are given

in Table 2. They can be applied to stratospheric problems without any restriction, but

they may require several improvements for mesospheric applications when the physical

parameters are known with better accuracy. Nitric oxide is an exception since it

Page 7: Solar UV radiation and its absorption in the mesosphere and stratosphere

Vol. 118, 1980) Solar UV Radiation Absorption in Mesosphere and Stratosphere 9

requires a specific analysis (CIESLm and NICOLET, 1973; CIESLIK, 1977, 1978; FREDERICK and HUDSON, 1979; NICOLET and CmSLIK, 1979).

I lL The solar flux

In the wavelength range of h > 175 nm, which is involved in mesospheric, strato- spheric and tropospheric photodissociation processes, it has not yet been possible to identify solar flux variations associated with specific solar activity phenomena.

First, fluctuations of the solar constant S = 137 + 1 mW cm-2 have not been established by direct measurements; only a few of atmospheric phenomena have

been associated with the bi-annual variation of 6.6~o in flux due to the variations of the Earth-to-Sun distance variation. Only Lyman-~ at 121.6 nm shows a clear evidence of an association of its intensity with solar activity controlled by identified chromo- spheric plages. It is not possible to examine here all rocket or satellite measurements made since 1949 (see FRIEDMAN, 1960; WEEKS, 1967; PRINZ, 1974; VIDAL-MADJAR, 1975, 1977). It is clear that a more precise absolute calibration of the total Lyman-~ line flux is still required; on the other hand, the profile measurements (TousEY, 1963; BRUNER and PARKER, 1969; BRUNER and RENSE, 1969; LEMAIRE et al., 1978; ARTZNER, 1978) indicate that the global profile is a variable average profile whose form is determined by various solar features between the center and the limb which are influenced by solar activity.

Thus, a mean value of 3 x 1011 photons cm -2 sec -1 (see, for example, THOMAS

and ANDERSON, 1976) for the Lyman-~ solar flux at the mean Earth-to-Sun distance

is almost a conventional value, since the accuracy cannot be better than + 25~. I f we write

q~(Lyman-~) = (3 + 1) x 101~ photons cm -2 sec -~ (4)

we may consider 2 x 10 ~ photons cm -2 sec -1 as a minimum working value and 4 x 1011 photons cm -2 sec -1 as an acceptable maximum value corresponding to quiet Sun and active solar conditions, respectively. There is, therefore, a variation

of a factor of the order of 2 over an average solar cycle. As far as the variations of the Lyman-~ intensity associated with short-term fluctuations (27 days, for example)

are concerned, it seems reasonable to use as an indication the preliminary empirical relations established by VIDAL-MADJAR (1975).

We consider that the Lyman-~ flux variations with solar activity represent the maximum possible differences that can occur in the solar flux at ,~ > 100 nm. Further- more, there is no astrophysical result leading to such possibilities at ~ > 175 nm, except for a few emission lines of relatively low intensity related to solar plages.

However, a recent review by HEATH and THEKAEKARA (1977) describes various observational results obtained between 1964 and 1975 which would indicate a variation of a factor of the order of 2 at 200 nm and not less than 4 +__ 10~ at 300 nm. On the

Page 8: Solar UV radiation and its absorption in the mesosphere and stratosphere

10 Marcel Nicolet (Pageoph,

Table 3 Solar f lux (q| photons cm -2 sec-a), mean absorption cross section (~MAX cm 2) at T = 230~ and 02 photodissociation coefficients (j, sec- 1) at the top o f the Earth's atmosphere in the spectral range o f

the 02 Schumann-Runge bands

02 band q~ ouAx ]=

19-0 1.58 x 101~ 7.52 x 10 -20 1.19 • 10 -9 18-0 2.23 1.41 x 10 -19 3.14 17-0 2.80 1.71 4.79 16-0 3.66 1.64 6.00 15-0 5.18 1.44 7.46 14-0 6.60 1.28 8.45 13-0 8.79 9.91 • 10 -2o 8.71 12-0 1.03 x 1011 7.14 7.35 11-0 1.43 4.91 7.02 10-0 2.07 3.17 6.56 9-0 2.09 2.02 4.22 8-0 2.56 1.16 2.97 7-0 3.96 6.06 x 10 -21 2.40 6-0 4.63 2.86 1.32 5-0 6.38 1.16 7.40 • 10 -1~ 4-0 7.16 4.05 x 10 -22 2.90 3-0 1.14 • 1012 1.29 1.47 2-0 1.54 3.88 x 10 -2a 5.98 • 10 -11

other hand, SIMON (1978) and DELABOUDINII~RE et aL (1978) do not reach the con-

clusion tha t there is any such var iabi l i ty dur ing the 11 year cycle. In fact, if we examine

the observa t iona l results ob ta ined in the ul t raviole t region f rom the first rocket

measurements to the last observa t iona l results, we can see tha t there is a systematic

decrease in the observed values of this solar ul t raviolet flux. Fu r the rmore , if we

consider only the more recent observa t iona l da t a in the spectra l range o f the 02

S c h u m a n n - R u n g e band system (ROTTMAN, 1974; SIMON, 1975; SAMA1N and SIMON,

1976; HEROUX and SWIRBALUS, 1976; BRUECKNER et al., 1976), it is c lear tha t the

es t imated precision in the flux measurement is of the o rde r o f +_20~. Thus, any

mean value adop ted for this spectral region reflects the lack of accuracy resul t ing

f rom the dubious charac te r of absolute cal ibrat ions , and also f rom the l imited preci-

sion due to various uncertaint ies in l abo ra to ry and a tmospher ic measurements in this

spectral region. Consequent ly , it must be said that any solar act ivi ty effect leading to

a possible var ia t ion o f the solar flux canno t yet be dis t inguished f rom differences

between var ious observat ions , even if they have been analyzed after discussion between

observers. There is no clear indicat ion leading to a perfect choice for mean or specific

values o f the solar flux in this spectral region leading to the 02 photodissoc ia t io n. The

adop ted numerica l values given in Table 3 should be accepted as provis ional values,

since the accuracy cannot be given and the precision cannot be better than _+ 25~ .

They indicate, however, an increase in the solar flux f rom 175 nm to 200 nm of the

o rde r o f a factor of l0 for identical spectral ranges (Av = 500 c m - 1) and o f abou t a

Page 9: Solar UV radiation and its absorption in the mesosphere and stratosphere

Vol. 118, 1980) Solar UV Radiation Absorption in Mesosphere and Stratosphere 11

o ~

e~ t

0

9

g

l I I I

X X X X

& &

X X

X X X X

~ ~ l ~ ~ ~ ( ' ~ l ~ ~ l ~ ~,

X X X X

i k

X X

X X

Page 10: Solar UV radiation and its absorption in the mesosphere and stratosphere

12 Marcel Nicolet (Pageoph,

fac tor o f 100 f rom (19-O) to (2 -0) band ranges. Nevertheless, it is not excluded that

a systematic e r ror o f 50700 could exist in a par t icu la r domain of this spectral region.

In any case, reference must be made to the observat ional results (BRUECKNER et aL,

1976) on the var ia t ion o f the solar flux in the spectral range o f the S c h u m a n n - R u n g e

bands due to solar act ivi ty (about 5700).

In the region o f the 02 Herzberg cont inuum, 200 nm < A < 242 nm, several

a tmospher ic measurements have been made (see SIMON, 1978). The values adop ted

here are based essential ly on da ta publ i shed by BROADrOOT (1972) and SIMON (1975)

and have been given in Table 1 with the related 02 absorp t ion cross sections for 500

c m - 1 spectral ranges. M o r e observat ions are needed to improve the accuracy o f the

a dop t ed values, even i f it seems tha t the precis ion is bet ter than at h < 200 nm.

In the spectral region covered by the Har t l ey band f rom 242 nm to 310 nm, we have

also adop ted rocke t da t a by BROAOFOOT (1972) with the values ob ta ined by ba l loon

measurements (SIMON, 1975) at ,~ > 284 nm in order to avoid cer ta in discrepancies

between var ious observa t iona l da t a par t icu la r ly in the spectra l region 200--400 nm (see

DE LuIsI, 1975; SIMOr~, 1978). The adop t ed results (NICOLET, 1975) a re given in

Table 4 with the co r respond ing O3 cross sections and pho tod i s soc ia t ion rates.

Besides, in the ul t raviolet region, co r respond ing to the Huggins bands, it is

necessary to in t roduce in the numer ica l values a smooth t rans i t ion f rom abou t 300 nm,

Table 5 Solar flux (q, photons cm -2 sec -1) with average cross section (o, cm 2 for AA = 5 rim) and 03 photodissociation coefficient (j, sec- 1) in the spectral range of the 03 Huggins bands, at the top

o f the Earth's atmosphere

A(A) q ~ ~ros j~

3100 4.95 • 1014 1.05 x 10 -1~ 5.20 • 10 -5 3150 5.83 5.23 • 10 -20 3.05 3200 6.22 2.91 1.81 3250 6.96 1.50 1.04 3300 8.61 7.78 x 10 -21 6.70 x 10 -6 3350 8.15 3.72 3.03 3400 8.94 1.71 1.53 3450 8.44 7.46 x 10 -22 6.30 x 10 -v 3500 8.69 2.66 2.31 3550 9.14 1.09 9.96 • 10 -s 3600 8,23 5.49 • 10 -23 4.52 3650 1.07 • I0 is 3700 1.08 3750 9.72 3800 1.11 3850 8.98 3900 1.18 3950 9.34 4000 1.69 4050 1.70

Page 11: Solar UV radiation and its absorption in the mesosphere and stratosphere

Vol. 118, 1980) Solar UV Radiat ion Absorption in Mesosphere and Stratosphere

Table 6

Solar f lux (q photons cm-2sec -1) with average cross section (%cm 2 for A)t = 5 rim) and 03 photodissociation coefficient (j, sec -1) in the spectral range

o f the Chappuis bands

13

)t(nm) q~ j~(03) )t(nm) q| j~o(03)

400 1.69 630 2.62 8.99 x 10 -6 405 1.70 635 2.62 8.31 410 1.84 5.35 x 10 -8 640 2.63 7.21 415 1.97 6.19 645 2.60 6.79 420 1.95 7.78 650 2.55 6.17 425 1.81 1.18 x 10 -7 655 2.48 5.46 430 1.67 1.14 660 2.57 5.19 435 1.98 1.71 665 2.61 4.83 440 2.02 2.25 670 2.61 4.36 445 2.18 3.25 675 2.62 4.03 450 2.36 4.04 680 2.62 3.72 455 2.31 4.90 685 2.57 3.21 460 2.39 8.53 690 2.52 2.82 465 2.38 8.76 695 2.60 2.65 470 2.39 9.70 700 2.58 2137 475 2.44 1.19 x 10 -6 705 2.52 2.12 480 2.51 1.78 710 2.51 1.93 485 2.30 1.94 715 2.48 1.73 490 2.39 1.98 720 2.45 1.54 495 2.48 2.25 725 2.48 1.41 500 2.40 2.93 730 2.45 1.29 505 2.46 3.99 735 2.44 1.16 510 2.49 3.93 740 2.39 1.07 515 2.32 3.71 745 2.40 1.01 520 2.39 4.25 750 2.41 9.04 x 10 -7 525 2.42 5.01 755 2.40 7.80 530 2.55 6.50 760 2.38 6.95 535 2.51 6.88 765 2.34 6.46 540 2.49 7.17 770 2.32 6.26 545 2.55 7.83 775 2.30 6.44 550 2.53 8.02 780 2.33 6.64 555 2.54 8.53 785 2.34 2.90 560 2.50 9.70 790 2.29 5.04 565 2.57 1.11 x 10 -5 795 2.29 4.17 570 2.58 1.20 800 2.27 3.70 575 2.67 1.27 805 2.27 3.97 580 2.67 1.21 810 2.20 4.18 585 2.70 1.17 815 2.22 4.11 590 2.62 1.16 820 2.18 3.71 595 2.69 1.24 825 2.20 3.34 600 2.63 1.29 830 2.14 3.04 605 2.68 1.30 835 2.14 3.00 610 2.66 1.21 840 2.13 2.98 615 2.59 1.10 845 2.09 2.97 620 2.69 1.05 850 2.05 2.97 625 2.61 9.40 x 10 -6

Page 12: Solar UV radiation and its absorption in the mesosphere and stratosphere

14 Marcel Nicolet (Pageoph

in the range of the spectral limit of observational data obtained by BROADFOOT (1972), to 400 nm where the solar flux data obtained by ARVES~N et al. (1969) can be accepted if the published values are reduced to a lower solar constant value (NICOLET, 1975). Such data for AA = 5 nm are given in Table 5. New measurements of the solar flux between 300 nm and 400 nm would be useful in helping to improve the accuracy of absolute values.

Finally, the solar flux which is adopted in the visible region corresponds to the numerical values adjusted to the present value of the solar constant (NICOLET, 1975) and deduced also from observational data published by ARVESEN et al. (1969). They are given in Table 6 with the corresponding values of the 03 photodissociation rates corresponding to the Chappuis bands. Since the experimental cross sections of the 08 visible band are certainly good and are not subject to a temperature effect, the total photodissociation rate J3 (Chappuis) = 3.4 • 10-4 sec- 1 seems to be accurate; the total error (< + 107o) corresponds to the accuracies of the solar flux observations and of the experimental 03 absorption cross sections. In the region of the 08 Huggins bands, where the temperature effect on the absorption cross section value is significant, new solar observations and laboratory measurements are certainly needed.

IV. Photodissociation rates

The spectral distribution of the 02 photodissociation has been discussed in its general form corresponding essentially to two spectral ranges for the mesosphere and stratosphere: the Schumann-Runge band system and Herzberg continuum. The action of Lyman-% with an average value of about 3 • 10 -9 sec- t for its photodissociation coefficient at the mesopause, must be introduced in the upper mesosphere.

As far as the ozone is concerned the whole spectrum at 2, > 200 nm must be considered, and at low levels the Rayleigh scattering, atbedo and aerosol effects must be considered particularly on account of their action on the spectral regions of wave- lengths greater than 300 nm.

We cannot discuss here the behavior of all photodissociation rates, but we may consider only typical examples. A few tables which give the respective percentages

Table 7

Photodissociation o f water vapor

Lyman-~ SRC SRB N(O2) (Line) (Continuum) (Bands) (cm -2) 70 % 70

1 x 10 I~ 58 31 11 1 x 10 TM 87 0 13 2.5 88 12 5 88 12 1 • 1020 86 14 2.5 70 30 5 27 73 1 • 1021 1 95

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Vo1.118,1980) Solar UV Radiat ion Absorpt ion in Mesosphere and Stratosphere

Table 8

Photodissociation of carbon dioxide

15

N ( O 2 ) (cm- 2)

Lyman-a SRC SRB (Line) (02 continuum) (02 bands) % % %

1 x 1016 20 79 1 1 x 1019 96 0 4

2.5 96 4 5 96 4 1 x 1020 95 5 2.5 88 12 5 53 47 1 x 1021 2 98 2.5 0 100

Table 9

Photodissociation of nitrous oxide

N(O~) (cm -2)

SRC SRB Herzberg (02 continuum) (02 bands) (02 continuum) % % %

1 x 1016 1 • 1019

1 x 102~ 1 • 1021 ] • 1022

2.5 5 1 • 1022 2.5 5 1 • 1024

59 0

28 77 63 57 45 40 36 33 25 14 3

13 33 37 43 55 60 64 67 75 86 97

Table 10

Photodissociation of nitric acid

N ( O 2 ) (cm- 2)

SRB (02 bands) %

Herzberg (02 continuum) %

Hartley (03 band) %

Huggins (03 bands) %

1 • 1016 1 • 1019 1 • 1020 1 • IO 2~ 1 • 1022 2.5 5 1 • 10 za 2.5 5 1 • 1024

49 48 45 40 34 31 28 23

6 0 0

36 37 39 43 54 59 59 53 19

1 0

14 14 15 16 11 8 9

13 28 29 26

1 1 1 1 1 2 4

11 47 69 74

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16 Marcel Nicolet (Pageoph,

T a b l e 11

Photodissociation o f carbon tetrachloride

N(Os) (cm -2)

SRB Herzberg (02 bands) (02 continuum) % %

1 x 1016 33 67 1 x 1019 30 70 1 x 1020 24 76 1 x 102t 17 83

2.5 14 86 5 12 88 1 x 1022 11 89 2.5 10 90 5 10 90 1 x 102a 10 90 2.5 8 92 5 4 96 1 x 1024 1 99

N(02) (cm -2)

Table 12

Photodissociation of methyl chloride

SRB Herzberg (02 bands) (02 continuum) % %

1 • 1018 92 8 1 x 1019 90 10 1 x 1020 88 12 2.5 86 14 5 84 16 I x 1021 82 19 1 • l022 67 33

2.5 59 41 5 52 48 1 x 102a 44 56 2.5 35 65 5 21 79 1 x 102~ 5 94

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Vol. 118, 1980) Solar UV Radiation Absorption in Mesosphere and Stratosphere

T:~ble I3

Trichlorofluoromethane photodissociation

SRB Herzberg N(O2) (02 bands) (O~ continuum) (cm -2) ~o %

1 x 1016 65 35 1 x 10 TM 63 37 1 x 1020 58 42 1 x 1021 50 50 2.5 46 54 5 42 58 1 x 1022. 38 62 2.5 32 68 5 29 71 1 x 1023 27 73 2.5 20 80 5 11 89 1 x 102~ 2 98

17

Table 14

Dichlorodifluoromethane photodissociation

SRB Herzberg N(02) (02 bands) (02 continuum) (cm -~) ~ %

1 x 10 I6 93 7 1 x 1019 93 7 1 x 1020 90 10 1 x 1021 87 13 2.5 83 17 5 79 21 1 x 1022 75 25 2.5 67 33 5 61 39 1 x 1023 53 47 2.5 43 57 5 28 72 1 x 1024 8 92

related to the various spectral ranges lead to clear indications about their specific

roles.

It is part icularly impor tan t to consider the respective percentages related to the

spectral ranges of the 02 S c h u m a n n - R u n g e band system (A < 200 nm) and of the 02

Herzberg con t inuum (~ > 200 nm). The variat ion of the solar flux with solar activity

may be different in these two spectral regions. It is also impor tan t to consider the

variat ion with altitude, i.e. with the total number of 02 molecules. Each const i tuent

has a different behavior.

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18 Marcel Nicolet (Pageoph,

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