Aeronomic problems of the molecular oxygen photodissociation—I. The O2 herzberg continuum

Planet. Printed

Space Sci., Vol. 34, No. in Great Bdtam

I I, pp. 1043-1059,

0032JI633/86 $3.00+0.00 Pergamon Journals Ltd.

1986

AERONOMIC PROBLEMS OF THE MOLECULAR OXYGEN PHOTODISSOCIATION-I. THE O2 HERZBERG CONTINUUM Aeronomy

MARCEL NICOLET and ROBERT KENNES Institute, 3, Avenue Circulaire, B-l 180 Brussels,

Belgium

(Receioed 3 June 1986) Abstract-An analysis of the aeronomic conditions related to oxygen photodissociation has been made by including updated absorption cross sections in the region of the Herzberg continuum at wavelengths greater than 200 nm. This analysis demonstrates that there is a decrease in the production of odd oxygen atoms compared with that obtained by using the 0, cross sections adopted before 1980. Quantitative results show that the photodissociation frequency J(0,) cannot be greater, at mesospheric and stratopause levels, than about 5 x ~O-“‘S~~ for the 200-240 nm interval. The possible errors (f 10%) in these 0, photolysis rates decrease with decreasing altitudes to reach isophotolytic levels (*O %) in the middle stratosphere, from about 25 to 30 km for increasing solar zenith angles, respectively. In the lower stratosphere, at 20 km and below, there is a rapid increase in the computed errors where the transmittance is less than lo-*. Fuller

information will be obtained after an estimation of the possible errors associated with uncertainties in the absorption cross sections of ozone molecules.

INTRODUCTION

The atmospheric ozone production depends on the dissociation of molecular oxygen. The origin of mesospheric and stratospheric ozone is directly related to the 0, photodissociation in the spectral ranges of Herzberg continuum and of the predissociated bands of the Schumann-Runge system at wavelengths less than 240 nm. At wavelengths greater than 200 nm the photodissociation depends almost exclusively on the Herzberg continuum, and from 175 to 200 nm it is associated with the Schumann-Runge bands and their underlying continua. Since the publication by Herman and Mental1 (1982) of observational results on the stratospheric transmittance in the region 200-220 nm, there is now general agreement that the laboratory cross sections used in the modelling calculations before 1982 for the Herzberg continuum absorption were too large (see, for example, Nicolet, 1983; Bucchia et al., 1985). New laboratory measurements and recent theoretical determinations (Johnston et al., 1984; Jenouvrier et al., 1986; Cheung et al., 1986; Saxon and Slanger, 1986) show that the absorption cross section of the Herzberg continuum may be determined with a good accuracy between 240 and 215 nm but that there may still be inconsistencies in the spectral region of the Schumann-Runge bands. However, since the 240-200 nm spectral interval leads to not less than 60 % of the 0, photodissociation at the stratopause level (50 km

for an overhead sun) to about 90% in the lower stratosphere (20 km), a detailed and separate study of 0, photodissociation processes at wavelengths greater than 200 nm is first required. The purpose of this paper is, therefore, to determine the various parameters of the aeronomic photodissociation of molecular oxygen in this spectral region. The analysis of the Schumann-Runge bands will be the subject of other publications and the effect of the (04) and (1-O) will not be discussed in this paper

DATA BASE DESCRIPTION

At present, it is clear that the 0, number density calculated with the available parameters in the photochemical region, i.e. in the neighbourhood of the stratosphere, does not correspond to the measured 0, concentrations. Two recent analyses by Froidevaux et al. (1985) and by Rush and Beckman (1985) establish that several aspects of the ozone problem must be clarified and that a complete understanding of the stratopause chemistry requires a detailed study of the principal aeronomic parameters. Since the principal object of this paper is the 0, absorption cross sections in the Herzberg continuum, we adopt here without any detailed discussion, for intervals of 500 cm - ‘, the spectral solar irradiances, the atmospheric molecular scattering cross sections and the ozone cross sections. Spectral irradiances accepted here are deduced (Nicolet, unpublished) from the satellite data obtained

1043

1044 TABLE

MARCEL

NICOLET and ROBERT KENNFS

1. SOLAR SPECTRAL IRRADIANCES FOR Av = 500 cm-l AND CORRESPONDING ATTENUATION SPECTRAL REGION OFIXEHERZBERG CONTINUUM OF 0, @J,)P

v(cm-‘)*

1 bm)t

qm(cm-’ s-l)$

41250 41,750 42,250 42,750 43,250 43,750 44250 44,750 45,250 45,750 46,250 46,750 47,250 47,750 48,250 48,750 49,250 49,750

0.24242 0.23952 0.23669 0.23392 0.23121 0.22857 0.22599 0.22346 0.22099 0.21855 0.21622 0.21390 0.21164 0.20942 0.20725 0.20513 0.20304 0.20100

2.04 x lOI 1.33 1.51 1.26 1.49 1.31 1.26 1.52 1.11 1.06 7.91 X 1oiz 8.19 7.12 4.37 2.54 2.07 1.69 1.49

-~$)I1

(cm’) 0.14x 0.15 0.16 0.17 0.18 0.19 0.21 0.22 0.23 0.25 0.26 0.28 0.30 0.31 0.33 0.36 0.38 0.40

9.00 x lo-‘* 7.97 6.86 5.79 4.83 4.00 4.24 2.55 1.97 1.52 1.17 8.57 x lo-l9 6.50 5.10 4.15 3.51 3.18 3.18

10-24

CROSS SECTlONS IN THE

UMS(air)T

'JMS(o,)EQU**

(cm’)

(cm21

0.14x 0.15 0.16 0.17 0.18 0.19 0.20 0.21 0.22 0.23 0.25 0.26 0.27 0.29 0.30 0.32 0.34 0.35

10-24

0.70x 0.74 0.78 0.83 0.87 0.92 0.97 1.02 1.08 1.13 1.19 1.26 1.32 1.39 1.46 1.54 1.61 1.69

10-24

*Mean wavenumber (cm-‘) for a spectral range of 5OOcm-’ between 41,250+250 and 49,750$-250cm111. tcorresponding mean wavelengths to the wavenumber in pm (vacuum) for calculation of the various mean cross sections. ISolar irradiances adopted at the top of the Earth’s atmosphere (Av = 5OOcm-‘). gOzone absorption cross section. See text. 11Molecular scattering cross section of 0,. From equation (5). See text. YMolecular scattering cross section of air. From equation (1). See text. **Equivalent scattering cross section (0,) for atmospheric applicatioh, formula (3). To be compared with the Herzberg continuum cross section in aeronomic applications.

by Heath (1981) and from rocket measurements of Mental1 et al. (198 1). The general agreement between these data considered in intervals of 500 cm- ’ (within + 3 %, except near 2 12.5 nm) indicates that a fairly accurate reference spectrum may be considered for the 0, photodissociation. However, if the results of Mount and Rottman (1983) are also taken into account, our adopted reference spectrum should be perhaps increased by about 5%. The spectral solar irradiances for intervals of 500 cm - 1are given in Table 1. It is clear that these irradiances based on observations made in 1980 are 40f 30 y0 less than those obtained in the years before 1970 (see Ackerman, 1971). The atmospheric Rayleigh scattering has been calculated recently with great accuracy by Bates (1984) and the corresponding values of the atmospheric scattering cross section are calculated using for formula (Nicolet, 1984), with an accuracy better than +I%,

The atmospheric scattering cross section was converted for use with the total number of 0, molecules so as to allow easier comparisons with 0, direct atmospheric absorption, i.e. an equivalent cross section OMS(",)EQU

= [n(M)/n(o,)]flMS(M)

= 4.8aMS(M)

t3)

where n(M) and n(0,) are the atmospheric total and oxygen concentrations molecular (cmm3), respectively. The numerical values of gMS(M) and cMS(O&QU for intervals of 500 cm-’ are given in Table 1 (columns 6 and 7, respectively). In a laboratory measurement of 0, absorption cross sections, the correction for molecular scattering (deduced from Bates’ formulas, 1984) is given by the following formula aMs(Oz) = 1.52x 10-25

cm*

(4)

where oMS = 4.02 x 1O-2s1-(4+X)

(1)

where x = 0.389 i +0.09426 1-l -0.3228

(A in pm). (2)

x = 64.329 1+3.464/L-28.681

(5)

with a precision better than $0.25 % from i = 0.24pm to 0.20 pm; this formula is extended to 0.19 pm

Aeronomic

problems

of oxygen

photodissociation:

Herzberg

continuum

1045

TABLE2. ADOPTED STANDARD MODEL IN THE~OM~PHERE Altitude (km) 85

Temperature W) ._ 181

80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5

181 200 220 239 256 266 271 264 250 237 227 222 217 217 223 256

Pressure (mb) 4.12x 1.04 x 2.49 5.52 1.14x 2.25 4.28 7.98 1.49 x 2.87 5.75 1.20 x 2.55 5.33 1.21 x 2.65 5.40

10-S 10-Z

10-l

IO0

10’

102

W&!) (cm- 3, 3.46 x 8.71 1.89x 3.81 7.26 1.33 x 2.44 4.47 8.56 1.74X 3.69 8.02 1.75 x 3.87 8.48 1.80x 3.21

excluding the regions of the rotational lines of the Schumann-Runge bands. Table 1, column 5, gives also the numerical values of cr,,(O,) from 0.242 to 0.190 /irn to be recommended for correction of the Rayleigh scattering in all laboratory measurements of the 0, absorption cross sections outside of the action of the rotational lines of the bands of the 0, Schumann-Runge system. The 0, cross sections used in this calculation were taken from Ackerman (1971) between 242.4 and 213.9 nm and directly deduced from laboratory data of Inn and Tanaka (1959) at wavelengths less than 214 nm. Measurements of Molina and Molina (not yet publ~sh~; 1986) indicate that between 216 and 242 nm the differences are not greater than _t 2 %, and may reach -5% near 21Ok5 nm. The adopted 0, absorption cross sections are also given in Table 1, column 4, for intervals of 500 cm-l. Thus the total atmospheric transmittance T,, is given by TAtm

=

To,

X TMS

X To,

with the optical depths, To, = %&VW

(7)

=L$s= 4.8 %$sW%)

(8)

and to, = ao,WQ where N(0,) and N(0,) are the total number of oxygen and ozone molecules (cm-‘) corresponding to a vertical column above a certain level and deduced from the standard model (Table 2). For various solar

NO,) (cm ‘)

1013 10’4

lOif

10’6

101’

10’s

5.25X 7.00 2.21 x 7.00 2.2 1 x 7.00 2.15x 6.61 2.03 x 6.07 1.40 x 2.52 4.32 4.77 2.57 1.13 5.67 x

10’ 10% lo9 IO’O 10” 10’2

10”

NW (cm-‘) 1.79 x 4.64 1.13 x 2.51 5.20 1.02 x 1.94 3.61 6.74 1.30 x 2.59 5.39 1.15x 2.48 5.43 1.19 x 2.41

10’9 lo*0

102’

lo*2

1o*j

1oz4

NO,) (cm _.‘) 3.28 x 6.34 1.29 x 3.38 9.98 3.09 x 9.58 2.95x 9.08 2.78 x 7.65 1.73 x 3.45 5.80 7.66 8.67 9.01

lOL3 10“’

10’5 lOI 10” 10’8

zenith angles (x), equation (6) can be written: ( TAtm)f(X) = (To, x TRsx To3Jr’“)

(9)

where f(x) is the air mass corresponding for relatively low solar zenith angles to set x. In order to perform calculations of the solar flux penetration and of the various photodissociation rates, O2 absorption cross sections based on recent experimental and theoretical data must be adopted for the Herzberg continuum.

WE HERZBERGCONTINUUM The absorption cross sections of the Herzberg continuum at wavelengths greater than 200 nm based on laboratory measurements made at relatively high pressures before 1980 and extrapolated to low pressure values applicable to the stratosphere must be replaced now by experimental and theoretical results obtained recently (1984-1986). The determination of the 0, absorption cross sections by Johnston et al. (1984) for various temperatures between 206 and 327 K leads to the following values: 3.5 _tO.2 x lo- 24 cm’, 4.2 kO.2 x 1Om24 cm’, 5.2iO.2 x Wz4 cm2, 6.6+0.3x Wz4 cm-* and 7.3 +0.3 x 10Mz4cm2 at 225,220,2 15,210 and 20.5 nm, respectively, when the 0, Rayleigh scattering (see Table 1) is deduced from the total absorption cross section. Such values can be obtained to within k 5 y0 by an expression independent of the temperature of the same form as given by Johnston et

MARCEL NICOLETand ROBERTKENNES

1046

TABLE 3. MEAN ABSORPTION CROSSSECTIONSIN THE 0, HERZBERGCONTINUUMIN INTERVALSOF 500 cm-r FOR WAVELENGTHS GREATERTHAN 200 nm

Mean I (nm) Av = XKlcn-’ 242.42 239.52 236.69 233.92 231.21 228.57 225.99 223.46 220.99 218.58 2 16.22 213.90 211.64 209.42 207.25 205.13 203.04 201.00

Experimental* mean value (lo-a4cm2)

Maximum? difference (%)

1.01 1.27

rf:l _+3 +6 + 10 211 +8 1-7 +7 +6 +6 +4 +5 +4 rt4 +5

1.63 2.04 2.47 3.05 3.54 4.0 1 4.47 5.03 5.31 5.90 6.30 6.60 6.87

Adoptedl formula (10-24cmZ) 0.80 l.Gi_l 1.26 1.73 2.10 2.51 2.96 3.42 3.92 4.39 4.90 5.38 5.84 6.26 6.64 6.95 7.21 7.39

Formula/ experimentai~ (%) -1 If0 +6 +3 +2 -3 -3 -2 -2 -3 +I -1 -1 i0 +I

*The experimental mean value corresponds to the averaged cross section deduced from laboratory data obtained at Harvard Observatory, Cambridge, Mass. and at Reims Unive~ity, France. iThe nositive and negative nercentaees are for Harvard and Reims, respectiveiy. $E!.qu&on (10). See text. No scattering. §Differences between columns 4 and 2. aE. (1984), R in nm, (Tnra(OZ) = 7.5 x

10_24(

199/a)

x exp { - 50[ln (199/A)]‘} cm’.

(10)

This formula is chosen here to determine the mean absorption cross sections of the Herzberg continuum for intervals of 500 cm-’ from the threshold at 242 to 210 nm. The deduced numerical values are given in Table 3 (column 4) and are in agreement (better than f 3 %, except at 234 nm) with the experimental data based on averaging the lowest cross sections obtained by Cheung et al. (1986) with thelargest valuesdeduced by Jenouvrier et al. (1986). It should be emphasized that the differences, as shown in Table 3, column 3, between the experimental values may reach + 10% near 230 nm. Such significant differences should be resolved and, for reasons of convenience, cause us to accept the results deduced from formula (10) as given in Table 3, column 4. This formula used as a reference indicates that important differences may still exist at wavelengths less than 2 10 nm. It can be seen (in Fig. 1) that formula (10) leads to the largest values of 0, cross section below 210 nm, while the theoretical determination of Saxon and Slanger (1986) corresponds to significantly lower cross sections for the Herzberg continuum underlying the Schumann-

Runge bands. The adoption (see Fig. l), at wavelengths less than 210 nm, of cross sections corresponding to the median values of the maximum cross section deduced from formula (10) and of the minimum cross section deduced from the theoretical determination of Saxon and Slanger (1986), leads to an increasing difference reaching + 10 y0 at 201 nm (Table 3). Such differences increase up to about +20x, as illustmted in Fig. 1, if an extrapolation is made to 190 nm corresponding to the spectral range of the 213 to 6-O Schumann-Runge bands. Thus, the available sets of Herzberg continuum cross sections derived from laboratory measurements seem to reach an acceptable agreement within their experimental accuracy. But the errors of the experimental or theoretical determinations prevent a sufficiently precise extrapolation for the Herzberg continuum underlying the SchumannRunge bands between 190 and 200 nm. A special laboratory determination at low temperature (<200 K) is needed without introducing line-wing corrected cross sections. The stratospheric dete~inations of Herman and Mental1 (1982) in the region 205-220 nm are also shown in Fig. 1; they suggest that the largest Herzberg continuum cross sections considered in our analysis are not in disagreement. A more precise judgment is difficult since the quoted errors have several possible

Aeronomic problems of oxygen photodissociation:

81

+

FORMULA

0,

Herzberg continuum

HERZBERG

(1982)

JOHNSTON-PAIGE-YAO

(1984)

JENOUVRIER-COQUART-MERIENNE

(1986)

CHEUNG-VOS~INO-FARKiNSON-GUBERMAN-FREEMAN SAXON-SLANGER MEAN

VALUES

-...-LilO’_--.L.

(SOOcm*) _-._

WAVENUMBER

FIG. ~.RECENTEXPERIMENTAL

(1986)

(19861

L_

Lo

_45_Le~

OFTHEHERZBERG

interpretations. Nevertheless, an estimation of the 0, photodissociation frequency and also of various rates can be made at wavelengths greater than 200 nm. RATES

On the basis of experimental and theoretical data available at present, we adopt (cf. Table 3 and Fig. 1) the absorption cross sections for the 0, Herzberg continuum at wavelengths greater than 200 nm as given in Table 4 with their minimum and maximum values for spectral intervals of 500 cm -‘. It is possible, therefore, to determine the changes in the rate of photodissociation (between 240 and 200 nm) of molecular oxygen in the stratosphere with the parameters of the standard atmosphere (Table 2) and the various other aeronomic parameters (Table I). Figure 2 illustrates the variation with height of the stratospheric transmittance leading to a rapid decrease of the spectral irradiances at long wavelengths. The maximum transmittance, as it is well known, occurs near 2 10 nm (see curve at 30 km). photodissociation frequencies for The 0, wavelengths greater than 200 nm are collected in Table 5 based on the absorption cross sections given in Table 4. They yield a value of 5.06 x lo-” s-l at the top of the Earth’s atmosphere with an accuracy of + 5 % corresponding to the general variation of the 0, cross sections indicated in Table 4. It can be seen (Table 5) that there is an isophotolytic level where the

.-

(cm-‘) MOLECULAROXYGEH

ANDTHEORETKALABSORPTIONCROSSSECTIONSOF

SPECTRALREGION

PHOTODISSOCIA~ON

CONTINUUM

35x162"(,0s/i)~~~~-5[,~(,~~,~)]*~

HERMAN-MENTALL

0,

1047

INTHE

CONTINUUM.

photodissociation frequency does not vary with the variation of the 0, absorption cross section. This isophotolytic level (crossing point from + to T) is between 20 and 25 km for an overhead sun. at 25 km for a solar zenith angle of 45”, between 25 and 30 km for 60” and at 30 km for 70”. Such an isophotolytic level, which corresponds to a mean transmittance related to about N(0,) = 1.5x 10z3 cm-’ and does not depend on the N(0,) = 5x lo’* cm-‘, extreme values which are adopted (& 10% intead of If 5 %, for example) for the 0, cross sections. In order to illustrate the differences that arise, depending on the solar zenith angle, various 0, photodissociation frequencies are shown between 20 and 55 km in Fig. 3. It is evident that near the stratopause level the variation is small and corresponds to the atmospheric region where photo~uilibrium conditions for owne can be applied. In the neighbourhood of the stratopause the total ozone content can be produced in less than a day. Table 6, which gives the 0, photodissociation rates (cme3 s-l) at various heights between 10 and 85 km for certain solar zenith angles (A > 200 nm), shows that, near the stratopause, the overhead 0, concentrations (lo”- 10” cme3) are produced in less than 6 h. Table 6 shows that there is also an isophotolytic production of oxygen atoms corresponding to the isophotolytic level of the 0, photodissociation frequency. These numerical results are illustrated in Fig. 4, which shows that the

1048

MARCEL NICOLET and ROBERT KENNES TABLE 4. ADOPTED ABSORPTION CROSSSECTIONS IN THE HERZBERG CONTINUUM 0, FOR INTERVALSOF 500 cm - ’ FOR THE DETERMINATIONOF THE 0, PHOTODISSOCIATION FREQUENCYAND PHOTODISSOCIATION ANGLES

Mean wavelength

OVERHEAD

RATE AT VARIOUS

OF WAVELENGTHS

AND SOLAR ZENITH

THAN

200 nm

Minimum

Maximum

(A)

0, cross section (10~24cm2)

value x

value X

2424 2395 2367 2339 2312 2285 2259 2234 2209 2186 2162 2139 2116 2094 2072 205 1 2030 2010

41,250 41,750 42,250 42,750 43,250 43,750 44,250 44,750 45,250 45,750 46,250 46,750 47,250 47,750 48,250 48,750 49250 49,750

0.80 1.00 1.26 1.73 2.10 2.51 2.96 3.42 3.92 4.39 4.90 5.38 5.84 5.94 6.25 6.47 6.63 6.72

0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.94 0.93 0.91 0.90

1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.06 1.07 1.09 1.10

SUN

II,,,, 47 500

ALTITUDES

GREATER

Mean wavenumber (cm-‘)

30

Au-soocm.' ‘“~ooo

IN THE REGION

35 II,

45000

WAVENUMBER

42500

b-fi’)

FIG. 2. SPECTRALSOLARIRRADIANCES. Number of photons cm-* SC’ in intervals of 500

cm-’ between 60 and 30 km for overhead sun between 240 and 200 nm.

production of oxygen atoms reaches its maximum in the middle stratosphere as it is already known. Since the essential features of the calculations are, in the stratosphere, very sensitive to the aeronomic parameters related to the wavelength, the spectral distributions of the solar irradiance and of the various transmittances and the associated 0, photodissoci-

ation are given in Tables 7-16 for various altitudes and overhead sun conditions. At 60 km (Table 7) where N(0,) is of the order of 10” cm-* and N(0,) of the order of 3 x 1015 cm’ for overhead sun, the optical atmospheric depth is negligible since it is generally around 0.0 1 except near 240 nm where it reaches about 0.03. The solar irradiance for spectral intervals of 500 cm-’ varies from about 2 x 1Ol3 photons cm-’ s-l at 242.4 nm to about 1.5 x 10” photonscm-2 s-l at 201 nm, but the 0, photodissociation frequency for each spectral interval is always at least 2 y0 and not more than 10 y0 of the 0, total frequency for wavelengths greater than 200 nm. Furthermore, even if the optical depth is small, it can be seen that the various percentages corresponding to the 0, absorption, the molecular scattering attenuation and the 0, absorption (in the three last columns of Table 7, respectively) give a clear indication of their relative roles; there is a dominant effect of 0, at the longest wavelengths and an increasing effect of 0, and molecular scattering towards the shortest wavelengths. There is an increasing effect on the transmittance of ozone with decreasing altitudes (Tables 8-13 and Fig. 2) down to 30 km. Below this level (Tables 14, 15 and 16) the 0, absorption takes an increasing part in the total atmospheric transmittance. At the stratopause, the optical depth between 240 and 200 nm is less than unity for all solar zenith angles

Aeronomic

problems

of oxygen

photodissociation:

Herzberg

continuum

1049

TABLE 5. PHOTODISSOCIATION FREQUENCY (s~')oF MOLECULAR OXYGEN FOR WAVELENGTHS GREATER THAN XXI nm IN THE HERZBERGCONTINUUM.VALUESATTHETOPOFTHEEARTH'SATMOSPHEREANDBETWEEN 65km AND 10km FORVARIOUSSOLAR ANGLESX.STANDARDATMOSPHERICMODELWITHATTENUATION DUETOATMOSPHERICMOLECULAR SCATTERINGANDOZONE ABSORPTION

Altitude (km)

6”5 60 55 50 45 40 35 30 25 20 15 10

secx = 1.414 J(O,)HER

secx = 1 J(O,)HER 5.06x 5.03 5.00 4.89 4.62 3.95 2.69 1.32 4.92x 1.14x 1.32x 5.97~ 2.70x

lo-“(&5%*)

10-“(*4T) (k2%) lo-“(T4%) lo-“‘(T18%) 10W16(f60%)

5.06x 5.02 4.91 4.82 4.45 3.60 2.18 9.11 x 2.65x 3.95x 2.15x 2.82x 1.30x

lo-“‘(+5%)

10-10 lo-“(*3%) lo-‘*(*o%) lo-13(T9%) lo-15(930%) 1om1s

secx=3 J(O,)HER

secx=2 J(O,)HER 5.06x 5.01 4.93 4.73 4.23 3.17 1.68 5.73 x 1.20x 9.71 x 1.81x 4.11x 7.88 x

lO_“(f5%)

5.06x 4.98 4.87 4.58 3.89 2.59 1.13 2.85x 3.45x 1.10x 4.16x 3.39x 2.35 x

lo-” lo-“(k2%) 10_‘3( F3%) 10m’4(f18%) W”(f50%) lo-=

lO~“(~S%)

lo-” lo-‘Z(fo%) 10_13(T9%) 10-16(T30%) lo-~~(TlOO%) lO_”

* (_+x‘A) or ( T x ‘A) corresponds to the variation of + y % in the 0, absorption cross section as indicated in Table 4 of 0, cross sections. tDecreasesofJ(O,)HER to approximately lo-‘, lo-‘, 10V3 and 10m4correspond to about _+4%, kO%,less than T 10% and less than F20 %, respectively, with the variation of the 0, absorption cross sections as indicated in Table 4 (+ 5 y0 to * 10%).

70” (Tables 9a and 9b), but at 45 km (Tables 10a and lob) unit optical depths are reached at the longest wavelengths, particularly at large solar zenith angles. It should be stressed that the transmittance for a certain zenith angle corresponds to the transmittance for a vertical column T,,, to a certain power, i.e. T& for set x = 2 and T&,, for set x = 3,. . ., and that the atmospheric optical depth varies accordingly. less than

Unit optical depth is reached at 30 km for overhead sun (Table 13) at wavelengths less than 210 nm, between one and two in the spectral interval 200-2 15 nm. The spectral region 2 10 f 5 nm corresponds to a peak in the solar flux reaching the lower atmosphere. Furthermore, the spectral region of the wavelengths greater than 220 nm is unimportant in the photodissociation of molecular oxygen. Thus, if the accuracy in the absorption cross section

J

16

0,

PHOTODISSOCIATION

FREQUENCY

(d)

FIG. 3. PHOTODISSOCIATION FREQUENCYOFMOLECULAROXYGENJ(O~),(S-~),ATSTRATOSPHERIC LEVELS,FOR THE SPECTRAL REGION OF WAVELENGTHS GREATER THAN 200nm AND VARIOUS SOLAR ZENITH ANGLES,

MARCEL NICOLET and ROBERT KENNES

1050

TABLE ~.PHOTODISQCIATI~N RATES (cm -3 S-l) OF MOLECULAR OXYGEN FOR WAVELENGTHS GREATER THAN 200IlItl IN THE HERZBERG CONTINUUM.VALUES BETWEEN 85 km AND 10km FOR VARIOUS SOLAR ZF~VITHANGLES. STANDARD ATMOSPHERIC MODEL WITHATTENUATION DUETOATMOSPHERIC MOLECULAR SCATTERINGANDOZONEABSORPTION

Altitude (km) 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10

sec~=2 (cm -3s-1I

secx = 1.414 (cm m3s-r)

seCx=l (cm -3s-1)

1.75x 4.40 9.50 1.92x 3.64 6.61 1.18x 1.99 3.06 3.80 3.35 2.12 6.88 x 8.31 x 2.39 x 2.34

1.75x 104(*5%)* 4.40 9.50 1.92x lOs(f5%) 3.65 6.64 1,19x106(+5%) 2.01 3.38 4.68 4.87 3.98 (k4%) 1.98 (k2%)

5.11 x 105(i4%) 5.06 x 104(i 18%) 4.86 x 102(f60%)

loy+s%) 105(+5%)

106(45%)

(Yb3%) lOQO%) 104(F9%) 103(T 30%)

* ( k x %) or ( T x %) in the values of the 0, photodissociation absorption cross section as indicated in Table 4.

of the Herzberg continuum at wavelengths greater than 200 nm is better than f 5 % between 200 and 240 nm in the neighbourhood of the stratopause (50 f 10 km), it is only required between 200 and 230 nm in the middle stratosphere (30 + 5 km) and only needed in the lower stratosphere (20 km) at waveiengths less than 215 nm (Table 15). It can be also said that the solar irradiance corresponding to the number of photons cmm2 s-l in

HERZBERG -1 s

1.75x 4.40 9.50 1.92x 3.63 6.56 l.I5x 1.89 2.71 2.92 2.11 9.35x

104(c.5%) lO’(+S%)

106(&5%)

(r-+5%)

105(52%) 1.74 (T3%) 7.02x 103(v18%) 3.48x lO'(TSO%)
secx=3 (cm -3s-1) 1.75x lO’(k5%) 4.40 9.50 1.92x lO’(iS%) 3.61 6.48 1.12x 106(+5%) 1.74 2.22 1.91 1.05 (&3%) 2.76x 105(+0%) 1.75x lO‘yT9%) 1.17x 10Z(T30%)
rates corresponds to the variation (5-5 % to t_ 10%) of the 0,

spectral intervals of 500 cm-’ depending on atmospheric transmittance less than 10e2, i.e. optical depths greater than 4.5, can be neglected in the determination of the total effect on the O2 photodissociation when the optical depth reaches unity near 205 nm (see Table 13). At 2.5km, where the optical depth is of the order of two near 205 nm for overhead sun, the solar irradiances with transmittance less than 2.5 x 10m3,

CONWNUUM

200nm

SOLAR ZENITH ANGLE

70% 1'

/,

(cm-3s-‘) INTHESTRATOSPHEREATVARlOUSSGLAR ZENITH ANGLES FOR THE SPECTRAL REGION OF WAVELENGTHS BETWEEN 200 AND 240nm.

FIG.4. PHOTODISSOCIATIONRATESOFMOLECULAROXYGEN

Aeronomic TABLE 7. SPECTRAL THE 0,

ITS DIVISIONS (7;)

(A)

FREQUENCY,

INTO THE 0,

0.97 0.97 0.98 0.98 0.98 0.98 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99

2 3 4 4 6 6 7 10 9 9 8 9 8 5 3 3 2 2

= 1.01 x 10” ctt~-~; tN(O,)

PHOTODlSSOCIATION

ITS DIVISIONS (“A)

FREQUENCY,

INTO THE 0,

2424 2395 2367 2339 2312 2285 2259 2234 2209 2186 2162 2139 2116 2094 2072 2051 2030 20 10 *N(O,)

1.87 x 1013 1.23 1.41 1.19 1.41 1.25 1.21 1.47 1.08 1.03 7.73 x 10’2 8.02 6.98 4.28 2.40 2.03 1.66 1.46 = 1.93 x lo*’ cm-*;

tN(O,)

OF THE SOLAR IRRADIAN~E,

(2422200 nm)

OF

AND OF THE ATMOSPHERIC OPTICAL DEPTH WITH ATTENUATION

AND THE 0,

0, absorption (%) 3 4 6 8 11 16 21 28 35 43 51 58 64 68 71 73 74 74

0.03 0.03 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

HERZBERG

CONTINIJIIM

OF THE ATMOSPHERIC TRANSMITTANCE

ABSORPTION*,

Solar irradiance (Photons cm-’ s-l) (Av = 5OOcm-‘f

105 1

continuum

Molecular scattering (%) 2 3 3 4 5 6 7 8 10 11 12 14 15 15 16 16 16 17

ABSORPTIONt

0, absorption (%) 95 93 91 88 84 78 72 64 55 46 37 28 21 17 13 11 10 10

= 3.08 x 1015 cm.-2.

0, dissociation (%)

(242-200

ATTENUATION

55 km and overhead sun**?. Atmospheric Total atmospheric optical transmittance depth

2 3 4 4 6 6 7 10 9 9 8 9 8 5 3 3 2 2 = 9.57 x 1Oz5 cms2.

0.92 0.92 0.93 0.94 0.95 0.96 0.96 0.97 0.97 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98

nm)

OF THE SOLAR IRRADIANCE,

OF

AND OF THE ATMOSPHERIC OPTICAL DEPTH WITH

THE ATMOSPHERIC SCATTERING

Altitude Mean wavelength (A)

CONTINUUM

60 km and overhead sun**? Total Atmospheric atmospheric optical transmittance depth

TABLE 8. SPECTRAL DISTRIBUTION IN THE RANGE OF THE 0, THE 0,

HERZBERG

Herzberg

THE ATMOSPHERIC SCATTERING

Altitude 0, dissociation (%)

1.98 x lOi 1.30 1.48 1.23 1.46 1.29 1.24 1.50 1.10 1.05 7.83 x 10” 8.11 7.05 4.33 2.43 2.05 1.67 1.47

0,

photodissociation:

OF THE ATMOSPHERIC TRANSMITTANCE

ABSORPTION*,

Solar irradiance (Photons cm-2s-1) (Av = 5OOcm’)

2424 2395 2367 2339 2312 2285 2259 2234 2209 2186 2162 2139 2116 2094 2072 2051 2030 2010 *N(O,)

of oxygen

DISTRIBUTION IN THE RANGE OF THE

PHOTODISSOCIATION

Mean wavelength

problems

0.090.08 0.07 0.06 0.05 0.04 0.04 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

0, absorption (%) 2 2 4 5 8 11 15 20 27 34 41 49 57 61 66 68 69 69

AND THE 0,

Molecular scattering (%) 1 2 2 :: 4 5 6 I 9 10 12 13 14 14 15 16 16

ABSORPTIONt

0, absorption (%) 97 96 94 92 89 85 80 74 66 58 49 39 30 25 20 17 15 1s

1052

and ROBERT KENNES

MARCELNICOLET

TABLE~~.SPECTRALDISTRIBUTIONINTHERANGEOFTHEO~HERZBERGCONTINUUM (242-200 nm) OF THESOLARIRRADIANCE, OFTHE~~PHOTODlSSOCIATIONFREQUENCY,OFTHEATMOSPHERICTRANSMITTANCEANDOFTHEATMOSPHERICOPTlCALDEPTH ANDTHEo ABSORPTlONt WITH ITSDIVISIONS (%)lNTOTHE& ABSORPTION*, THEATMOSPHERlcSCATTERINGATTENUATION ztitude 50 km and overhead sun*.?

Mean wavelength

Solar irradiance (TV = 5oo cm_S-* ‘) ) (Photons cm-*

diss;l*

(A) 2424 2395 2367 2339 2312 2285 2259 2234 2209 2186 2162 2139 2116 2094 2072 205 1 2030 2010 *N(O,)

1.56x 1Ol3 1.04 1.22 1.05

: 3 4 6 6 7 10 9 9 8 9 9 6 3 3 3 2

1.28 1.15 1.13 1.39 1.03 9.94 x lOZZ 7.48 7.80 6.8 1 4.19 2.35 1.99

1.62 1.43 = 3.61 x 10” cm-2;

tN(0,)

atz$!eric transmittance 0.76 0.79 0.81 0.84 0.86 0.88 0.90 0.91 0.93 0.94 0.94 0.95 0.96 0.96 0.96 0.96 0.96 0.96

AtTr

abs$ion

0.21 0.24 0.2 I 0.18 0.15 0.13 0.11 0.09 0.08 0.06 0.06 0.05 0.04 0.04 0.04 0.04 0.04 0.04

1 2

~~~~

abs$$ion 98 97 92 95 93 90 87 82 76 69 60 50 41 34 29 25 22 22

1 1

2

1

3 5 7 10 14 19 25 32 41 49 54 59 62 64 64

2 2 3 3 4 5 6 8 9 10 12 12 13 14 14

= 2.95 x 10*6cm-Z.

TABLE 9b. SPECTRAL DISTRIBUTIONIN THE RANGE OF THE 0, HERZBERG CONTINUUM (242-200 nm) OF THE SOLAR IRRADIANCE, OF

Mean wavelength (A) 2424 2395 2367 2339 23 12 2285 2259 2234 2209 2186 2162 2139 21I6 2094 2072 2051 2030 20 10

Solar ,rrad,aZ 45’ Atm. Opt. trans. depth (Photons cm -‘s-l) (Av = 5OOcm-‘) (tot.) (tot.) 1.39x 9.45 x 1.12x 9.76 x 1.20x

1OlJ 10’2 10’3 f0’2 10’3

1.a9 1.08 1.33 9.95 x lOI 9.65 7.28 1.62 6.66 4.10 2.30 1.95 1.59 1.40

0.68 0.71 0.74 0.77 0.81 0.83 0.86 0.88 0.90 0.91 0.92 0.93 0.94 0.94 0.94 0.94 0.94 0.94

0.38 0.34 0.30 0.25 0.22 0.18 0.16 0.13 0.11 0.09 0.08 0.07 0.06 0.06 0.06 0.06 0.06 0.06

Altitude 50 km x=60” Solar irradiance Atm. Opt. (Photonscm -‘sml) trans. depth (AI. = SOOcm-‘) (tot.) (tot.) 1.19x 8.21 x 9.93 8.80 1.10 x 1.01 1.01 I .21 9.53 x 9.31 7.06 7.42 6.51 4.01 2.25 1.91 1.56 1.37

i.e. optical depths greater than six, can be neglected (Table 14) in the calculation of J(O,),,,. At 20 km, where the optical depth is not less than four near 205 nm overhead sun (Table 15), the accuracy of the total solar irradiances at wavelengths greater than 200 nm depends only on the accuracy of

10’3 10’2

10’3

10’2

0.58 0.62 0.66 0.70 0.73 0.77 0.80 0.83 0.86 0.88 0.89 0.91 0.91 0.92 0.92 0.92 0.92 0.92

0.54 0.48 0.42 0.36 0.3 I 0.26 0.22 0.18 0.15 0.13 0.11 0.10 0.09 0.08 0.0x 0.08 0.08 0.08

x = 70‘5 Solar irradiance Atm. Opt. (Photonscm-2s~-‘) trans. depth (Av = 5OOcm-‘) (tot.) (tot.) 9.05 x 10’2 6.45 8.05 7.35 9.42 8.86 9.06 1.16x 1OlJ 8.83x 10” 8.73 6.68 7.07 6.22 3.85 2.16 1.83 1.49 1.31

0.44 0.48 0.53 0.58 0.63 0.68 0.72 0.76 0.80 0.82 0.84 0.87 0.88 0.88 0.88 0.88 0.88 0.88

0.81 0.72 0.63 0.54 0.46 0.39 0.33 0.27 0.23 0.19 0.17 0.15 0.13 0.13 ‘0.12 0.12 0.12 0.13

the observational and experimental data between 200 and 2 15 nm, i.e. a very short spectral interval. At 15 km (Table 16) in the same conditions, the transmittance is less than 10e3 corresponding to atmospheric optical depths greater than seven. A more detailed analysis of the various tables may

Aeronomic

problems

of oxygen

photodissociation:

Herzberg

1053

continuum

TABLEIOI.SPECTRAL DISTRIBUTION r~ THERANGEOFTHE 0, HERZBERG CONTINUUM (242-2OOnm) OFTHESOLARIRRADIANCE, OF THE 0, PHOTODISSOCIATION FREQUENCY,OFTHE ATMOSPHERIC TRANSMITTANCE AND OF THE ATMOSPHERICOPTICAL DEPTH ANDTHE03ABSORPTIONt WITH ITS DIVISIONS('A) INTO THE 0, ABSORPTION*, THEATMOSPHERICSCATTERINGATTENUATION

A$tude Mean wavelength (A) 2424 2395 2367 2339 2312 2285 2259 2234 2209 2186 2162 2139 2116 2094 2072 205 1 2030 2010 *NjO,)

Solar irradiance (TV = 500cm_,) (Photons cm-* s-l )

diss;&ion

8.92x lOI* 6.37 7.98 7.33 9.42 8.90 9.14 1.17 x 10’3 8.97x 10” 8.90 6.83 7.25 6.40 3.96 2.23


2 2 3 6 10 9 10 8 10 9 6 4

1.89 1.55 1.36

3

= 6.74 x 10” cm- ‘; $N(O,)

45 km and overhead

atzgeric transmittance

Atzi;‘i’

0.44 0.48 0.53 0.58 0.63 0.39 0.73 0.77 0.81 0.84 0.86 0.88 0.90 0.91 0.91 0.91 0.91 0.91

(A) 2424 239s 2367 2339 2312 2285 2259 2234 2209 2186 2162 2139 2116 2094 2072 205 1 2030 20 10

(Photonscm-“s-l) (Av = 5COcm“) 6.33 x 10” 4.70 6.14 5.85 7.80 7.59 8.01 1.05 x 10’3 8.29 x 10” 8.27 6.42 6.89 6.12 3.81 2.15 1.82 1.49 1.31

zzi$!i

0.83 0.74

abs$$on 99 98 98 97 96 94 92 88 84 79 77 64 54 47 40 3s 33 32


0.64

0.54 0.46 0.39 0.73 0.26 0.21 0.18 0.15 0.12 0.10 0.10 0.09 0.09 0.09 0.09

2 2 3 4 4 5 7 8 10 11 12 12 13

12 17 22 30 38 43 49 53 55 55

0, HERZBERGCONTINUUM (242-2OOnm) OFTHESOLARIRRADIANCE, ATMOSPHERICOPTICAL DEPTH ATVARIOUSSOLAR ZENITH ANGLESX Altitude

Solar irradiatz4“

abs$ion

= 9.08 x 1OX6cm- *.

TABLE lob. SPECTRALDISTRIBUTIONINTHERANGEOFTHE OFTHE ATMOSPHERICTRANSMITTANCEANDOFTHE

MeaIl wavelength

sun**t

Atm.

Opt.

trans.

depth

(tot.)

(tot.)

0.3 1 0.35 0.41 0.46 0.52 0.58 0.63 0.69 0.74 0.78 0.8 1 0.84 0.86 0.87 0.88 0.88 0.88 0.88

1.17 1.04 0.90 0.77 0.65 0.55 0.45 0.37 0.30 0.25 0.2 1 0.17 0.15 0.14 0.13 0.13 0.12 0.13

45 km

Solar irradia::

(Photonscm-’

60” Atm.

SK’) trans.

(Av = 5OOcm-‘) 3.90 x 10’2 3.06 4.23 4.26 5.96 6.05 6.63 9.01 7.26 7.47 5.89 6.41 5.75 3.59 2.04 1.73 1.42 1.24

lead to additionai conclusions on the various roles of the photolysis parameters related to the spectral region of wavelengths greater than 200 nm related to the 0, Herzberg continuum. Finally, it may be also useful to consider the sensitivity of the atmospheric transmittance to

Opt.

depth

(tot.)

(tot.)

0.19 0.23 0.28 0.34 0.40 0.46 0.53 0.59 0.65 0.70 0.75 0.78 0.8 1 0.82 0.83 0.84 0.84 0.84

1.65 I .47 1.27 1.08 0.92 0.77 0.64 0.52 0.42 0.35 0.29 0.24 0.2 I 0.20 0.18 0.18 0.17 0.17

x=70? Solarirradiance

(Photonscm-‘s-l) (Ae = SOOcm-‘)

-.--_

1.71 x 10’2 1.46 2.24 2.48 3.77 4.11 4.81 6.94 5.87 6.27 5.08 5.67 5.16 3.26 1.86 1.58 1.30 1.14

Atm.

Opt.

trans. (tot.)

depth (tot.)

0.08 0.11 0.15 0.20 0.25 0.3 1 0.38 0.46 0.53 0.59 0.64 0.69 0.72 0.75 0.76 0.76 0.77 0.76

2.48 2.21 1.91 I .63 1.37 1.16 O.Y6 0.78 0.64 0.53 0.44 0.37 0.3 1 0.29 0.28 0.27 0.26 0.27

uncertainties in the O2 cross sections corresponding to the Herzberg continuum at various stratospheric altitudes. After fixing the values of the molecular scattering attenuations and of the O3 absorption cross sections, a variation of f 10% is adopted for the 0, absorption cross sections and introduced in the

1054

MARCEL

NICOLET and ROBERT KENNEL

TABLEI~.SPECTRALDISTRIBUTIONIN~ERANGEOFTHEO~HERZBERGCONTINUUM(~~~-~~~ nm) OFTHESOLARIRRADIANCE,OF THE& PHOTODISSOCIATIONFREQUENCY,OFTHEATMOSPHERICTaANSMITTANCEANDOFTHEATMOSPHERICOPTICALDEPTHWITH ITSDIVISIONS(%) INTO THE 0, ABSORPTION*, THE ATMOSPHERIC SCATTERING ATTENUATION AND THEO, ABSORPTION?

Mean

Solar irradiance

wavelength (A)

(Photons cm-’ SC’) (Av = 500 cm-‘)

2424 2395 2367 2339 2312 2285 2259 2234 2209 2186 2162 2139 2116 2094 2072 205 1 2030 2010

1.65 x 10” 1.43 2.20 2.46 3.17 4.14 4.88 7.08 6.03 6.48 5.29 5.93 5.42 3.44 1.97 1.68

1.38 1.21

*N(O,)

= 1.29 x 10” cm-‘;

tiV(O,)

Altitude40 km and overhead sun**? Atmospheric Total 0,

dissociition (%)

optical depth

atmospheric transmittance


2.51 2.23 1.93 1.64 1.38 1.15 0.95 0.76 0.6 1 0.49 0.40 0.32 0.21 0.24 0.22 0.21 0.20 0.20

0.08 0.11 0.15 0.19 0.25 0.32 0.39 0.47 0.54 0.61 0.67 0.72 0.76 0.19 0.80 0.81 0.82 0.81

1 1 3 4 5 9 9 10 10 12 12 8 5 4 4 3

0, absorption (%)
Molecular scattering (%)
1 1 1 1 1 2 2 3 4 5 I I 9 10 10 11

03 absorption (%) 99 99 98 98 97 96 95 92 90 86 80 74 65 59 52 47 44 43

= 2.77 x 10” cm-*

THE~,PHOTODISSoCIATIONFREQUENCY,OFTHEATMOSPHERICTRANSMITTANCEANDOFTHEATMOSPHERICOPTICALDEPTHWITH ITS DIVISIONS(%) INTO THE 0, ABSORPTION*, THE ATMOSPHERICSCATTERING ATTENUATION AND THE 0, ABSORPTIONt

Altitude Mean wavelength

Solar irradiance (Av = 5oo (Photons cm-cm_ * s-‘) 1)

dissgion

(4 2424 2395 2367 2339 2312 2285 2259 2234 2209 2186 2162 2139 2116 2094 2072 2051 2030 20 10 *N(O,)

2.03 x 2.88 7.58 1.42 x 3.45 3.64 9.51 1.93 x 2.16 2.88 2.16 3.58 3.60 2.43 1.45 1.21 1.05 9.24 x = 2.59x

1022cm-2;

10”’

10”

10’2

10” tN(0,)


35 km and overhead atz$eric transmittance 9.9 x W4 2.2 x 1o-3 5.0 1.1 x 1om2 2.3 4.3 7.6 0.13 0.19 0.27 0.35 0.44 0.51 0.56 0.59 0.61 0.62 0.62

sun*,?

“27

abs$tion

6.91 6.13 5.29 4.49 3.11 3.14 2.58 2.06 1.63 1.30 1.05 0.83 0.66 0.59 0.52 0.49 0.47 0.48


zii?

abs$ion


1 2 2 3 4 5 6 7 8 9 9

99 99 99 99 98 97 96 95 92 89 85 79 12 66 60 55 51 51

Aeronomic

problems

of oxygen

photodissociation:

Herzberg

105.5

continuum

TABLE ~~.SPECTRALDISTRIBUTIONINTHERANGEOFTHEO~HERZBERGCONTINUUM(~~~-~~~ nm) OFTHESOLAR~RRAD~ANCE,OF THE~2PHOTODISSOCIATIONFREQUENCY,OFTHEATMOSPHERICTRANSMITTANCEANDOFTHEATMOSSSPHERICOPTICALDEPTHWITH ITS DIVISIONS(%) INTOTHE 0, ABSORPTION*,THE ATMOSPHERIC SCATTERING ATTENUATION AND THE 0, ABSORPTlONt

Mean wavelength (A)

Solar irradiance (Photons cm-’ s-t) (Av = SOOcm~‘)

2424 2395 2367 2339 2312 2285 2259 2234 2209 2186 2162 2139 2116 2094 2072 2051 2030 2010 *N(O,)

3.56 x 1.35x 1.62x 5.22 3.14x 1.12 x 3.88 1.49 x 2.86 5.11 7.62 1.31 x 1.58 1.20 7.76 x 7.17 6.09 5.29 = 5.38x

10z2cm-*;

Altitude 0, dissociation C%)

lo6 10’ lOa lo9 1oro

1.8 x 1.0 X 6.7 x 4.1 x 2.1 X 8.6 x 3.1 x 9.8 x 2.6x 5.4 x 9.6~ 0.16 0.22 0.28 0.32 0.35 0.36 0.36

$1 <1

10”

lo’*

10”

tN(O,)

30 km and overhead sun**? Atmospheric Total optical atmospheric depth transmittance

1 2 5 7 14 19 15 10 10 9 8

lo.-’ 10-G 10+ 10-S lo-4 1o-4 10-a 10-l lo-* 10-z 1O--2

> 10 > 10 > 10 10.10 8.46 7.06 5.78 4.62 3.66 2.91 2.34 1.83 1.46 1.29 1.14 1.06 1.02 1.04

0, absorption (%)
t 1 1 2 3 4 5 8 11 16 21 26 31 35 38 38

Molecular scattering (%)
0, absorption (%) loo 99 99 99 98 97 96 95 93 90 86 80 74 68 62 57 53 53

= 1.72x 10’8cm-2.

TABLE ~~.~PECTRALDISTRIBUTIONINTHERANGEOFTHEO~HERZBERGCONT~NUUM(~~~-~~ nm) OFTHESOLARIRRADIANCE,OF THE~2PHOTODISSOCIATIONFREQUENCY,OFTHEATMOSPHERICTRANSMITTANCEANDOFTHEATMOSPHERICOPTICALDEPTHWITH ITS DIVISIONS(%) INTOTHE 0, ABSORPTION*,THE ATMOSPHERIC SCATTERING ATTENUATION AND THE 0, ABSORPTION,'

Altitude Mean wavelength (A)

Solar irradiance (Photons cm-’ s-r) (AI* = SC@cm-‘)
2424 2395 2367 2339 2312 2285 2259 2234 2209 2186 2162 2139 2116 2094 2072 2051 2030 2010 *N(O,)

= 2.48x

1023cm-2;

10’ lo4 10’ 106 lo8 lo9

0, dissociation (%)

61 41

1oro 10”

tN(O,)

3 9 18 17 14 14 13 11 = 5.80x

25 km and overhead sun*st Atmospheric Total optical atmospheric transmittance depth

1.0 x10-= 4.5 x lo-.-” 1.7 x 10-g 4.4 x 10-B 7.2 x 10-7 9.2 x 1O-6 9.3 x 10-S 6.5x 1O-4 2.9x 10-j 8.9x 1O--3 2.5 x IO-’ 4.7 x 10-Z 7.2 x 1O-2 9.5 x 10-Z 0.11 0.12 0.12 10’8cm~2.

> 10 > 10 > 10 > 10 > 10 > 10 > 10 9.28 7.35 5.86 4.72 3.71 2.97 2.63 2.34 2.18 2.10 2.13

0, absorption (%)
Molecular scattering (%)
0, abortion i%) 100 99 99 99 98 97 96 95 92 89 85 79 73 67 61 56 52 51

MARCEL

1056 TABLE 15.SPECTRALDISTRIBUTION THE 0,

PHOTODISSOCIATIoN

ITS DIVISIONS

(%)

IN

NICOLET

INTO THE 0,

THE RANGEOF THE 0, HERZBBRC CONTINUUM (242-200 nm) OF THE SOLAR IRRADIANCE,OF OF THE ATMOSPHERIC TRANSMITTANCE AND OFTHEATMOSPHERIC OPTICALDEPTHWITH

ABSORPTION*,

Solar irradiance (Photons cm-as-‘) (Av = 5OOcm-‘)

2312 2285 2259 2234 2209 2186 2162 2139 2116 2094 2072 2051 2030 2010

4.95 4.70 x 3,28x 1.91 x 3.50x 4.00x 1.97 x 1.10x 2.18 3.40 2.96 3.29 3.00 2.48

*iV(O,) = 2.48 x 1O23 cm-‘;

TABLE THE 0,

KENNES

FREQUENCY,

THE ATMOSPHERIC

Altitude Mean wavelength (A)

ROBERT

and

0, dissociation (%)

3.3 x 3.6 x 2.6 x 1.3 x 3.2 x 3.8 x 2.5 x 1.3 x 3.9 x 7.8 x 1.2 x 1.6x 1.8x 1.7 x

1 41
tN(0,)

20 km and overhead Total atmospheric transmittance

-

Ior IO4 IO6 10’ 10s lo9 lOI0

SCATTERING

ATTENUATION

0, absorption (%)

> 10 > 10 > 10 3> 10 > 10 > 10 8.30 6.62 5.41 4.86 4.39 4.14 4.03 4.10

Molecular scattering (%)

ITS DIVISIONS

(%)

FREQUENCY,

INTO THE 0,

OF THE ATMOSPHERIC

ABSORPTION*,

Solar irradiance (Photons cm-zs-l) (Av = 5OOcm~‘)

(A) 2259 2234 2209 2186 2162 2139 2116 2094 2072 2051 2030 2010

2.49 4.50 x 2.06x 4.67 x 3.74x 3.16 x 1.01 x 1.39 1.26 1.41 1.24 9.51 x

*N(C),) = 2.48 x 10z3cm-*;

calculation

1 2 2 3 4 5 6 7 8 9 10 10

virtually

3 14 17 17 19 17 13

lo8

2.0 x 3.0x 1.9x 4.4 x 4.7 x 3.9x 1.4x 3.2x 5.2 x 6.8 x 7.4 x 6.4 x

ATTENUATION

AND THE 0,

0, absorption (%)

> 10 > 10 > 10 > 10 > 10 10.2 8.68 8.05 7.54 1.29 7.22 7.36

OPTICAL

6 8 12 16 22 29 31 42 48 52 54 54

DEPTH

OF

WITH

ABSORPTIONt

Molecular scattering (%)

03 absorption (%)

2 3 3 4 5 7 8 9 10 11 12 13

92 89 85 80 73 64 55 49 42 37 34 33

= 5.80~ 10’8cm~Z.

transmittance the atmospheric to 500 cm - ’ intervals. The results given

17 show that, for overhead no difference

SCATTERING

lo--‘2 lo-‘0 10-s lo-’ 10-e 10”s W4 1O-4 10 -’ 10-j 1o-4 10.. 4

OF THE SOLAR IRRADIANCE,

AND OF THE ATMOSPHERIC

15 km and overhead sun*,? Atmospheric Total atmospheric optical transmittance depth

of

corresponding in Table

Gl $1 $1 *i tl

lo3 10s lo6 10’ 10’ lo9

TRANSMITTANCE

THE ATMOSPHERIC

0, dissociation (%)

tN(O,)

98 96 95 93 90 81 82 75 67 61 54 49 46 45

1

= 5.80 x 10” cm-‘.

Altitude Mean wavelength

0, absorption (%)

I

1 3 4 5 8 10 14 20 21 32 38 42 44 45

16. SPECTRALDISTRIRUTION IN THERANGEOFTHE0, HERZBERGCONTINUUM (242300 nm) PHOTODISSOCIATION

ABSORPTlONt

sun*.t

Atmospheric optical depth

10-l” lo-” 10-s lo-.’ 10-s lo-’ 1O-4 10-J 10--s 1O-3 10-Z IO-’ 10-r 10-r

AND THE 0,

sun, there is

in the transmittances

at the

larger percent changes in the transmittance at short wavelengths

observed

are associated with the relative

increasing role of the 0,

absorption

from 240 to 200

nm compared with that of 0,. At 25 km the differences

stratopause level and in the upper stratosphere. In the

increase

middle stratosphere

reach

absorption

plays a more important

less than 2 15 nm. The

absorption

and where the total optical depth is greater

only

at 30 km, the differences

& 3-4 % at wavelengths

up to

59 %. At

20 km,

where

the 0,

role than the O3

0.76&O%* 0.79 0.81 0.84

0.86 0.88 0.90 0.91 0.93

0.94 0.945 0.95

0.96 0.96 0.96 0.96

0.96 0.96

2424 2395 2367 2339

2312 2285 2259 2234 2209

2186 2162 2139

2116 2094 2072 2051

2030 2010

45 km

0.915 0.915

0.90 0.9If<0.5% 0.91 0.915

0.84 0.86 0.89

0.63 0.68 0.73 0.11 0.81

0.44&O% 0.48 0.53 0.58

-

0.82 0.81

0.62 0.62

0.51 0.56 0.59 0.61

0.76 0.79 0.80 0.81 *I%

0.27 0.35 0.44

0.61 0.67 0.72 +I%

2.32 4.31 7.60 9.80 0.19

0.25 0.32 0.39 0.47 0.54

35 km

9.93 x lo-“,O% 2.17 x lo-l3 5.02 1.13x 10-Z

-. 8.11x lo-afo% 0.11 0.15 0.19

40 km

0.36 0.35

0.22 0.28 0.32 0.35

5.44 9.63 0.16

+4%

+6%

2.11x 10-4+1% 8.55 3.08 x lo-3&2% 9.80 2.58 x 10-a

4.14x 10_5fO%

30 km

0.12 0.12

+9%

1.77 1.66

+20% +20%:

3.90 +f6% 7.79 Ibl7% 1.21 18% 1.59 x 10-2+_ +19%

4.73 7.23 9.53 0.11

&7% +_8%

3.77x lo-s& 12% 2.49x 10-4+ 13% 1.34x 10_3f14%

20 km

2.86x 1O-3 +5% 8.92 4 f6% 2.46 x lo-’

9.34x 10_5,4% 6.45 x W4

25 km

ANDOVERHEAD SUN

*The percentages (hx%) shown in the Table correspond to a variation of T 10% in the 0, absorption cross sections and no variation in the O3 absorption cross sections. TTransmittances less than lo-’ are not shown in the Table. fvalues of +x% at various solar zenith angles x compared with the values for overhead sun increase by a factor of at least set x.

50 km

Wavelength (A)

Altitudes 50-20 km

TABLE 17. SPECTRALTRANSMITTANCES IN THEREGIONS OF TNEOx HERZBERGCONTNJUMFORINTERVALS OF 5OOcm-’

Y

5

8J i I: g 0;: 8 2. 2 E

409 B +o % a a I 5. e,

‘CI a 9 E : z

; i;

a

R

1058

MARCEL NICOLETand ROBERTKENNES

than five, the differences in the spectral transmittance (lower than lo- 3, may reach f 20 % for a variation of & 10% in the 0, absorption cross section. CONCLUSIONS

The data presented here represent the parameters that have a significant role in the photolysis of oxygen at wavelengths greater than 200 nm. Further refinement in the calibration of cross sections in the Herzberg continuum cannot change the mean absolute values of the 0, photodissociation frequency and of the photodissociation rate for wavelengths longer than 200 nm by more than + 10 % at altitudes above 20 km. The fact that there are isophotolytic levels in the middle stratosphere, between about 25 and 30 km for various solar zenith angles, leads to the conclusion that much smaller differences are found for atmospheric optical depths greater than unity. Much larger differences than the differences in the 0, cross sections occur only when the atmospheric optical depths are greater than 4 in the 205i 5 mn spectral region. In any case, the global behaviour will remain above the stratopause level, i.e. a low value of JfO,) of about 5x lo-‘* s-r with a spread corresponding exactly to the differences in the 0, absorption cross sections. Thus the uncertainties of this input parameter cannot modify the conclusion that the production of oxygen atoms in the neighbourhood of the stratopause is less than was considered a few years ago. Other photodissociation processes such as the photodissociation of 160180 molecules proposed by Cicerone and McCrumb (1980) or the photodissociation from the excited state OJrAJ suggested by Frederick and Cicerone (1985) cannot constitute signi~cant sources of atomic oxygen (Blake et al., 1984, and Simonaitis and Leu, 1985, respectively). The of odd oxygen atoms from the production photodissociation of 1601s0 molecules cannot be more than 3 + 1 y0 of the production by ’ “0, of oxygen atoms at mesospheric levels near 701t 10 km. On the other hand, the photodissociation of metastable 0, would require an absorption cross section which should be several thousand times greater than the actual cross sections of extremely weak transitions. A further inference resulting from the present analysis is that the spectral transmittances in the stratosphere down to at least 25 km, as shown in Table 17, are obtained with good accuracy and, therefore, that the uncertainties are small in the upper and middle stratosphere. This is important since it must be considered in the analysis of the uncertainties in the photodissociation rates of nitrous oxide and various chiorofluorocarbons.

Nevertheless, an uncertainty remains in the analysis of the treatment of ultraviolet radiation between 200 and 240 nm. For proper interpretation, in this spectral region, of the attenuated solar radiation field at stratospheric altitudes, a special series of calculations must be done to determine the importance of uncertainties which may result from possible differences occurring in the ozone absorption cross sections at wavelengths less than 240 nm. This will be the subject of another paper. In addition, since the molecular scattering cross section is known with high accuracy, it will be also possible to determine more precisely the amplification factor due to the additional solar flux available near 200 nm for photodissociation processes at stratospheric levels. Acknowledgements-Marcel Nicolet wishes to thank Sir David Bates for stimulating discussions and comment at the various stages of this work and for his assistance in the preparation of the final version of the manuscript. He is most grateful to the various scientific teams which facilitated the development of this work, not only by useful discussions but also by preliminary copies of manuscripts. In this respect, he thanks especially D. Freeman, W.H. Parkinson and K. Yoshino of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass. and A. Jenouvrier with B. Coquart and M. F. Merienne of the Facuiti des Sciences de I’Universitt de Reims. He appreciates also having received from T. Slanger the detailed results of calculations with the copy of the manuscript. This work was partly supported by the Fonds National de la Recherche Scientifique de Belgique and by the Commission des Communaut~ Europeennes, Programme de Recherche Environnement, Direction gitnerale de la Science, Recherche et Dbveloppement.

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problems

of oxygen

photodissociation:

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continuum

1059

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