Stratospheric Methane and Nitrogen Dioxide from Infrared Spectra

Pure and Applied Geophysics (PAGEOPH)

Vol. 106-108 (1973/V-VII) Birkh/iuser Verlag, Basel

Stratospheric Methane and Nitrogen Dioxide from Infrared Spectra

By M. ACKERMAN and C. MULLER 1)

Abstract - Values of the mixing ratio by volume of stratospheric NO2 and CH4 deduced from infrared spectra taken by means of balloon-borne spectrometers are presented. Possible evidence for the presence of formaldehyde in the stratosphere is also given.

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

Most of the information on minor stratospheric constituents has been deduced f r o m infrared spectra. Until recently this technique has been applied to absorbers having volume-mixing ratios of the order of 10 -7 or higher. The measurements based on absorption infrared spectrometry performed by means of balloon-borne spectrometers operating in the 30 km altitude range at solar zenith angles larger than 90 ~ are allowed to reach concentrations lower by two orders of magnitude. This method offers several advantages: changes in absorption can be related to short-term variation of the optical thickness, ensuring the absence of contamination by the instrumentation; the absorption takes place mostly at constant temperature, absorber concentration and pressure, leading to an easier quantitative interpretation. Information on the vertical distribution of minor constituents can be obtained by this method since the optical depth is m a x i m u m at the altitude of the solar grazing rays. As shown in Fig. 1, 75 ~o of the optical depth is located within a small altitude range o f the order o f 3 km.

The method has been quantitatively applied to some minor constituents. Results on stratospheric methane and nitrogen dioxide are presented here.

II. M e t h a n e

Methane was first observed in the atmosphere by MIGEOTTE [19] and is known as a permanent constituent of the atmosphere. The most significant sink is the reaction with O(1D) or O H which leads to the CH3 radical, as discussed by NICOLET and PEETERMANS

[21].

The distribution of CH4 in the stratosphere is thus essentially determined by its transport properties and precise measurements can give valuable information about the

stratospheric eddy diffusion coefficients.

1) Institut d'A6ronomie Spatiale de Belgique, 3 avenue Circulaire, B-1180--Brussels (Belgium).

1326 M. Ackerman and C. Muller (Pageoph, The tropospheric C H 4 measurements have been reviewed by FINK et al. [10]. The stratospheric mixing ratio has been shown (BAINBRIDGE and HEIDT [3]) to decrease above the tropopause; a rocket sampling near the stratopause led to a value of 3 x 10 - 7

for a vertical column ranging between 42 and 62 km (EHHALT et al. [9]).

A vertical distribution of methane can be deduced from measurements of the solar spectrum taken from a balloon-borne spectrometer for grazing altitudes of the solar rays ranging from 15.9 to 33.6 kin. The flight studied here t o o k place on 22 October 1971 between 13.10 and 18.16 G.M.T. The absorption t o o k place over south-western France and the Atlantic Ocean. The tropopause level was 13.5 km and an anticyclonic zone was located on the G u l f of Biscay,

,~ I

t - S

I I

I 2

"[ ( A r b i t r a r y u n i t s )

Figure 1

Optical depth (r) in arbitrary units versus altitude in.kilometers above the solar grazing ray smallest altitude for a constituent in mixing

Absorptions for the v3 and v2 + v4 C H 4 bands were observed and have been analysed by means of a line calculation using the data of KYLE [16]. These give the positions, line strengths, energy of the fundamental level and identification of the lines.

Confirmation of the relative line intensities has been given by MCMAHON et al, [18] and objections that can be made against the use of KYLE'S data have been summarised by TAYLOR [25] for their application to the study of the v4 band in the jovian atmosphere.

The values for the C H 4 half-width used here are those of YAMAMOTO and HIRONO [30] for Nz broadening. These are not very different from the constant value of 0.05 cm -1 s.t.p, used by KYLE and offer the advantage of giving a rotational variation. It should be noted that the influence of the line width on the integrated absorbance measured by BuRcH and WILLIAMS [7] is not very significant.

Vol. 106-108,1973/V-VII) Stratospheric Methane and Nitrogen Dioxide from Infrared Spectra 1327 The line strengths and half-widths were adapted in temperature and pressure using the formulae

{To~ m / E'(T o -- T))

S(T)= S~ ] exp~-l'4388 -Toot / (1)

cr = ~ o ^" ( 2 )

where E" is the rotational energy of the fundamental level and where the index 0 indicates the conditions of pressure and temperature given in the data. The exponent n was chosen equal to 1/2 which is close to the value used by VARANASI, and TEJWANI [26]

which for CH4-H2 broadening varies from 0.55 to 0.5. A value o f n close to 1 has been

,-?-= 20

<

4.0 , , , , [

10

t NO 2 -V3

0 I I [ 1 [ I

5x10 ~4 10 -3 10 -1

i I I i I}1 I I I J I [ i i I NO 2 CH 4

~1+~)3 -93 DOPPLER

NO 2

~ N T Z C H4 n=l LORENTZ

J I I t l l I I I I~J IN~

10 -2 (cm -1 ) Figure 2

Lorentz and Doppler half-width as a function of altitude for the CH4 and NO2 bands studied

derived by MCMAHON et al. [18], but Fig, 2 shows that in the narrow range of strato- spheric temperatures the difference between the two dependencies may not lead to significant errors. The exponent m corresponds to the temperature dependence of the rotational partition function.

Figure 2 shows also the Doppler half-width for the v3 band as a function of the altitude. It can be seen that the Lorentz and Doppler half-widths come to equality at an altitude of about 20 km. Above this, a Voigt profile should be used, but it can be seen that saturation effect occurs near the center of the lines even for higher altitude and that the most significant absorption for the calculation of the spectra occurs in the over- lapping Lorentzian wings. This effect, applicable to methane only because of the great optical depth, has been discussed by PLASS and FIVEL [24] and DRAYSON [8].

Another effect would be a deviation from the Lorentz line shape in the wings, as noted by TAYLOR [25] on the basis of BENEDICT'S correction for CO2, but if this effect is

1328 M. Ackerman and C. Muller (Pageoph, i m p o r t a n t in the jovian atmosphere, it must be considered as negligible in the earth's stratosphere, the value given by WINTERS et al. [27] for the beginning o f this effect being 5 cm -1 or a b o u t 70 s.t.p, half-widths.

To reduce the c o m p u t i n g time, three c o m p u t a t i o n steps are used o f which the smallest is smaller than the half-line width to obey the condition o f KYLE [17]; at each

t I ' I ' I ' I '

1(70

100

100

100

, 0 r . . . ^ 1 h

80

70 60 50 /,0 30 20 I0 0

[ ,'~ , , , . . . . ; , , , ~

N20

PV 3 CH/,

I ~ I R I ~ I i

3.7 3.6 3.5 3./~

),.(IJ.m)

Figure 3

Spectra observed from an altitude of 34 km at solar zenith angles of 90.5 ~ 91.5 ~ 92.4 ~ 93.3 ~ and 94.5 ~ corresponding to the altitudes of grazing solar rays indicated for each spectrum

step, all lines situated in an interval adjacent o f the c o m p u t i n g point are added. The absorption coefficients are then multiplied by the optical paths and the transmission can be computed. The integration net obtained is used to p e r f o r m a convolution by a triangular slit function o f 2.5 cm -1 half-width.

Figure 3 shows the spectra recorded at float altitude between 3.3 and 3.75 p m for zenith angles o f 90.5 ~ 91.5 ~ 92.4 ~ 93.3 ~ and 94.5 ~ The solar grazing rays altitudes are

Vo1.106-108,1973/V-VII) Stratospheric Methane and Nitrogen Dioxide from Infrared Spectra 1329 i n d i c a t e d for each s p e c t r u m . T h e s t r o n g v3 b a n d was used to d e t e r m i n e the m e t h a n e c o n c e n t r a t i o n on the o p t i c a l p a t h s . The w e a k lines t h a t can be o b s e r v e d between the P lines o f the v3 b a n d b e l o n g to the m e d i u m 2v2 H 2 0 b a n d at 3.17 # (HOUGHTON et al.

[15]), even in t r o p o s p h e r i c c o n d i t i o n s , t h e y d o n o t c o n t a m i n a t e the high J lines t h a t were used for the m e a s u r e m e n t s . T h e w e a k v2 + v4 CH4 b a n d a b s o r b s t o o w e a k l y to be r e s p o n s i b l e for all the a b s o r p t i o n o b s e r v e d between 3.5 a n d 3.75 #. P a r t o f this a b s o r p - t i o n can be a t t r i b u t e d to the v2 + v3 N 2 0 b a n d , to the v2 + 2v3 a n d vl + v2 + va b a n d s o f

I I I I

/

2o 1/

I k m

~ ^{2 o}

I I I I

3.7 3.6 ~S 3.~. 3.3

Ism

Figure 4

Spectrum taken at a solar zenith angle of 94.5 ~ with laboratory spectra of N20 and CH20. The atmos- pheric absorption observed from 3.55 to 3.70/~m could be explained by CH20

o z o n e a n d to the v, f u n d a m e n t a l o f h e a v y w a t e r ; b u t the m a i n a b s o r p t i o n p o s s i b l y r e s p o n s i b l e for the s t r u c t u r e o b s e r v e d on the lowest a l t i t u d e s p e c t r u m is f o r m a l d e h y d e , as shown in Fig. 4, where l a b o r a t o r y s p e c t r a o f H C H O a n d N 2 0 are shown with s t r a t o s p h e r i c spectra. T h e two r e s o n a n t vl a n d v5 b a n d s c o n s t i t u t e one o f the s t r o n g e s t a b s o r p t i o n s o b s e r v e d on the f o r m a l d e h y d e s p e c t r u m (PIzRSON et al. [23]). A value o f the m i x i n g r a t i o can h a r d l y be e s t a b l i s h e d now because o f the l a c k o f d a t a c o n c e r n i n g

Table 1

Stratospheric CH4 mixing ratios

Z (km) Mixing ratios 15.9 (2.2 • 0.7) x 10 .6 24.6 (2.2 i 0.3) x 10 -6 29.6 (1.3 • 0.3) x 10 -a 31.9 (1.0 • 0.3) x 10 .6 33.6 (7.5 • 4) x 10 -7

1330 M. Ackerman and C. Muller (Pageoph, the line strengths. High resolution spectrum and rotational constants of C H 2 0 are given by YAMADA et al. [28]. An upper limit of 10 -8 can be deduced from the spectrum given by PIERSON et al. [23]. Formaldehyde absorption seems to be present in all spectra.

Z,0

-g

LLI 123 :--" 30

<

20

' ' ' ' ' ' ' 1

EHHALT etl _9 [ I (1972)

CH/..

>

( )

( ),

( )

o

O B A I N B R I D G E a n d H E I D T (19561

T H I S WORK

,( ),

O

o

0 [ i i i i i i i i N i i l i I i i

1 0 -7 1 0 - 6 1 0 -5

M I X I N G RATIO

Figure 5

Measurements of CH4 mixing ratios versus altitude

The values found for the methane mixing ratios are indicated in Table I. Figure 5 compares these values with the balloon sampling results of BAINBRIDGE and H~IDa" [3]

for a stratopause located at 14.5 km, the results of the rocket sampling of ]~HHALT et al.

[9] is also indicated. A methane scale height of 3.4 • 0.5 km above the altitude of 25 km can be deduced from our measurements.

III. Nitrogen dioxide

Some forty years ago, GOETZ [12] was searching experimental evidence for the presence of nitrogen dioxide in the atmosphere. Stratospheric absorption due to NO2 has been identified (ACKERMAN and FRIMOUT [2], GOLDMAN et al. [11 ], ACKERMAN [1 ]) in infrared spectra of the solar radiation recorded from balloon-borne gondolas floating

Vol. 106-108,1973/V-VII) Stratospheric Methane and Nitrogen Dioxide from Infrared Spectra 1331 at mid-latitudes in the altitude range of 30 km when the solar zenith angle was larger- than 91 ~ .

A quantitative interpretation of the available data leading to stratospheric mixing ratios of NO2 from the edge of the tropopause at 12 km up to 28 km is reported here.

Synthetic spectra have been computed on the basis of the spectroscopic constants given for NOz by OLMAN and HAUSE [22] and BLANK et al. [5] using a Lorentz line full width of 0.16 cm -~ in s.t.p, conditions. The choice of this value is based on the work of YAMAMOTO and AOK~, [29]. Figure 2 shows that a Lorentz line profile could be adopted for all observations. The line intensity was determined by comparing the integrated

100

?0

o z 6 o

- 60

z ~0

30

10

0 1605

H20 O

H20

15110 1515

WAVENUMBER(cm -1 )

Figure 6

1620

Synthetic absorption spectrum of a portion of the v3 band of NO2 computed on the basis of the available spectroscopic data and calibrated in absolute values by means of the laboratory measurements of GUTTMAN. Optical thickness, 0.014 arm cm; total pressure, 1,18 atm ; temperature, 298 ~ The position of water vapor lines is indicated, one of them overlapping the NO2 absorption marked 0. Absorptions 1

and 2 are unaffected by water vapor and appear clearly in the atmospheric spectra

synthetic spectra with the integrated absorption measured in the laboratory by GUTTMAN [13]. Figure 6 shows the transmission versus wave number of a portion of the v3 band computed for the laboratory conditions used by GUTTMAN [13]. The positions of water vapor lines are shown at the wave numbers given by BENEDICT and CALFEE [4]. One of these overlap the NO2 absorption feature numbered,0 on the figure. Absorp- tions numbered 1 and 2 are very well isolated. This part of the spectrum, particularly well marked in the atmospheric recordings, has mainly been used for the quantitative interpretation. Observed and computed spectra are shown in Fig. 7. it has to be pointed out that the water vapor line overlapping the NO2 feature marked 0 in Fig. 6 increased with solar depression angle while the NO2 absorption remains more or less constant.

The vl + v3 band (ACKERMAN and FRIMOUT [2], ACKERMAN [1]) underlies very clearly the P branch of the v a CH4 band as it appears in Fig. 8, showing spectra recorded at

1332 M. Ackerman and C. Muller (Pageoph, sunset from an altitude of 29 km on 8 October, 1970, 5y means of a balloon-borne instrument launched from Aire sur l'Adour (Landes, France). The synthetic spectrum degraded by a triangular slit function of 6 cm -~ half-width, and matching the spectrum for an altitude of the solar grazing rays of 12.5 km, is compared with this one in Fig. 9.

100 90 >

8 0 - a ~

7 0 - z o 8 0 - u) co 8 0 -

~E 4 0 - u~

z 3 0 - 2 0 . i,-

1 0 . 1610

C 0

138

1619 1610 1619 C 100,

80 701- -.I

601- H

501- H

401- '-'1

301- H

201- "-I 101- "-I

0 1610

1 3 9

1619 1610 1619 W A V E N

0 c o C 0

100,

701- -4 -

601- ~ -

50 1- ~ ~-

/ 4 0 ~- 4 -

301- -4 -

I i140

201- -I

10P '-I -

0 I ,

1610 1619 1610 1619 U M B E R I c m -11

100 9o

8 0 - -

70 - -

6 0 - - 5 0 - -

40 - -

30 - 20 -

10- -

0 1610

141

1619 1610 1619

F i g u r e 7

O b s e r v e d ( O ) a n d c o m p u t e d ( C ) N O 2 s p e c t r a . T h e w a t e r v a p o r l i n e o v e r l a p p i n g t h e N O 2 a b s o r p t i o n f e a t u r e n u m b e r e d 0 i n F i g . 6 a p p e a r s c l e a r l y i n t h e s t r a t o s p h e r i c s p e c t r a . T h e n u m b e r s r e f e r t o t h e n u m b e r i n g g i v e n b y GOLDMAN et al. [11 ]. A t r i a n g u l a r slit f u n c t i o n o f 0 . 5 c m -1 h a s b e e n u s e d t o d e g r a d e

t h e s y n t h e t i c s p e c t r a

100 r ~ I

80 70

50

~0

I 0 ~ N o I'3' ' I'0 . . . . 5 ~ ' ' ' 2

0 L_______._~._._ L PV 3 C

36 3.6 3~ 3,3

XllJ.rnl F i g u r e 8

S t r a t o s p h e r i c s p e c t r a o f t h e N O 2 v~ + v3 b a n d u n d e r l y i n g t h e va C H 4 b a n d f o r t w o m i n i m u m a l t i t u d e s o f t h e g r a z i n g s o l a r r a y i n d i c a t e d i n k i l o m e t e r s

Vol. 106-108,1973/v-vii) Stratospheric Methane and Nitrogen Dioxide from Infrared Spectra 1333 The m i x i n g ratios, b y v o l u m e , s h o w n in T a b l e II and in Fig. 10 refer to the altitudes o f the solar g r a z i n g rays a n d have been d e d u c e d f r o m the NO2 c o n c e n t r a t i o n s d e t e r m i n e d f r o m the s p e c t r a a n d f r o m the t o t a l c o n c e n t r a t i o n given in the M i d - L a t i t u d e , S p r i n g - F a l l m o d e l o f U.S. S t a n d a r d A t m o s p h e r e S u p p l e m e n t s , 1966. The q u o t e d errors c o r r e s p o n d

100 ' ' ' I '

9 0

8 0

70 Z O 60

I E 5 0 o3 z ,<

m: 4 0

30

20

\

A/

0 i i i i ] i i

2 8 5 0 2 9 0 0

"~ (crn-b Figure 9

Comparison between observed and computed spectra of the vl + v3 NO2 band Table II

NO2 mixing ratios

Balloon Solar zenith Grazing solar rays NO2 mixing ratio Observed

altitude angle altitude (kin) (by volume) band

(km)

Reference to spectra

29 94.1 12.5 (3.4 • 1) • 10 -9 v~ + v3 [1, 2]

34 94.3 16.1 <2.3 • 10 -9 vl + v3 [1, 21

29.7 92.8 21.9 (8.3 + 1.5) • 10 -1~ v3 [11]

29.7 92.4 23.9 (1.7 • .5) x 10 -9 v3 [11 ]

29.7 92.0 25.7 (3.8 :t: 1.3) x 10 -9 v3 [111

29.7 91.5 27.5 (7.8 • 1.5) x 10 -9 v3 [ l l l

29.7 91.2 28.3 (4.0 • 1.5) x 10 9 va [11]

to lower a n d u p p e r values o f t h e a m o u n t o f a b s o r b e r in the o p t i c a l p a t h l e a d i n g to c o m p u t e r - g e n e r a t e d s p e c t r a t h a t c o u l d be d i s t i n g u i s h e d f r o m the o b s e r v e d a t m o s p h e r i c spectra. T h e relatively high value o b s e r v e d at the lowest a l t i t u d e is t h o u g h t to be due to a t r o s p o s p h e r i c influence. U s i n g the e x p e r i m e n t a l d a t a , it has to be k e p t in m i n d t h a t t h e y were o b t a i n e d in s e p a r a t e e x p e r i m e n t s p e r f o r m e d at two different l o c a t i o n s a n d at

1334 M. Ackerman and C. Muller (Pageoph, different moments. The mixing ratio appears to increase from 20 to 28 km, showing the presence of a stratospheric layer of NO2. This peculiarity might be the reason why no evidence for NO2 has been found up to now in submillimetre stratospheric emission spectra (HARRIES et al. [14]) taken from an altitude of 12 km at a zenith angle of 75 ~ Following the theoretical investigation made by NICOLET [20] on the role of NO2 on the stratospheric ozone, the values listed in the table indicate an influence of natural

30

2 ~.

20

i l l I i i i i i i i

t J

i D

N O 2

i i i i i i i i ' 1 i i

10 -9 1~ 8

M I X I N G RATIO

Figure 10

NO2 mixing ratio by volume versus altitude

nitrogen dioxide on O3. In sunset conditions, as is the case for these determinations, NO2 mixing ratios can be taken, within experimental error, equal to NOx.

IV. Conclusion

The data presented in this article appear to b e useful to gain information on stratospheric vertical transport (NIcoLET and PEETERMANS [21]) and on the strato- spheric nitrogen oxygen compounds (BRASSEUR and CIESLIK [6]). More measurements are, of course, needed to improve the picture now available, not only in the atmosphere but also in the laboratory.

Acknowledgement

We express our thanks to Dr. M. NICOLET for his help and suggestions throughout this work.

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[2] M. ACKERMAN and D. FRIMOUT, Mesure de l'absorption stratosphdrique du rayonnement solaire de 3,05 ~ 3,7 microns, Bull. Acad. Roy. Belgique, C1. Sc. 55 (1969), 948-954.

[3] A. E. BAINBRIDGE and L. E. HEIDT, Measurements of methane in the troposphere and lower strato- sphere, Tellus 18 (1966), 221-225.

Vol. 106-108,1973/V-VII) Stratospheric Methane and Nitrogen Dioxide from Infrared Spectra 1335 [4] W. S. BENEDICT and R. F. CALFEE, Line Parameters for the 1.9 and 6.3 Micron Water Vapor Bands, ESSA Professional Paper 2, U.S. Department of Commerce, Washington, D.C. (1967).

[5] R. E. BLANK, M. D. OLMAN and C. D. HAUSE, Upper state molecular constants for the (0, O, 3) and (1, O, 3) vibration-rotation bands o f nitrogen dioxide, J. Mol. Spect. 33 (1970), 109-118.

[6] G. BRASSEVR and S. CIESLIK, On the behavior o f nitrogen oxides in the stratosphere, Pure and Appl. Geophys. 106-108 (1973), 1431.

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[8] R. S. DRAYSON, Atmospheric transmission in the C02 bands between 12 and 18 ~t, App. Opt. 5 (1966), 385-391.

[9] D. H. ENHALT, L. E. HEIDT and E. A. MARTELL, The concentration o f atmospheric methane between 44 and 62 kilometers altitude, J. Geophys. Res. 77 (1972), 2193-2196.

[10] U. FINK, R. H. RANK and T. A. WIGGINS, Abundance o f methane in the earth's atmosphere, J. Opt.

Soc. Amer. 54 (1964), 472-474.

[11 ] A. GOLDMAN, D. G. MURCRAY, F. H. MURCRAY, W. WILLIAMS and F. S. BONOMO, Identification o f the va NOz band in the solar spectrum by means o f a balloon-borne spectrometer, Nature 225 (1970), 443-444.

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[13] A. GUTTMAN• Abs•hlte in•.ared intensity measurements •n nitr•gen di•xide and dinitr•gen tetr•xide•

J.Q.S.R.T. 2 (1962), 1-15.

[14] J. E. HARRIES, N. R. W. SWANN, J. E. BECKMAN and P. A. R. ADE, High resolution observations o f the submillimetre stratospheric emission spectrum, Nature 236 (1972), 159-161.

[15] J. T. HOUGHTON, N. D. P. HUGHES, T. S. Moss and J. S. SEELEY, An atlas o f the infrared solar spectrum from 1 to 6.5 r observed from a high-altitude aircraft, Phil. Trans. Roy. Soc. (London), Ser. A 254 (1961), 47-123.

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[17] T. G. KYLE, Net interval for calculating absorption spectra, J. Opt. Soc. Amer. 58 (1968), 192-195.

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[19] M. MIGEOTTE, Spectroscopic evidence o f methane in the earth's atmosphere, Phys. Rev. 73 (t948), 519.

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[22] M. D. OLIVtAN and C. D. HAUSE, Molecular constants o f nitrogen dioxide from the near infi'ared spectrum, J. Mol. Spect. 26 (1968), 241-253.

[23] R. H. PIERSON, A. N. FLETCHER and E. ST. CLAIR GANTZ, Catalogue o f infrared spectra for qualitative analysis o f gases, Anal. Chem. 28 (1956), 1218-1239.

[24] G. N. PLASS and D. I. FIVEL, Influence o f Doppler effect on line-absorption coefficient and atmos- pheric radiation transfer, Ap. J. 117 (1953), 225-233.

[25] F. W. TAYLOR, Methods and approximations for the computation o f transmission profiles in the v4 band o f methane in the atmosphere o f Jupiter, J.Q.S.R.T. 12 (1972), 1151-1156.

[26] P. VARANASI and G. D. T. TEJWANI, Experimental and theoretical studies on collision broadened lines in the vgfundamental o f methane, J.Q.S.R.T. 12 (1972), 849 855.

[27] B. H. WINTERS, S. SILVERMAN and W. S. BENEDICT, Line shape in the wings beyondthe b a n d h e a d o f the 4.3 p band o f COz, J.Q.S.R.T. 4 (1964), 527-537.

[28] K. YAMADA, T. NAKAGAWA and Y. MORINO, Band contom" analysis o f the v1 and v5 fundamentals o f formaldehyde, J. Moh Spect. 38 (1971), 70-83.

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