Temperature effect on the ultraviolet absorption of CFCl3, CF2Cl2 and N2O

0032%0633/YO $3.00+0.00 PergamonPress plc

TEMPERATURE EFFECT ON THE ULTRAVIOLET ABSORPTION OF CFC13, CF&l, AND N,O M.F. Spectromktrie

Molkulaire

‘MERIENNE. B. COQUART and A. JENOUVRIER et Atmosphtrique, Facultk des Sciences-B.P. 347, 51062 Reims Ckdex, France

Abstract-The absorption cross-sections of chlorofluorocarbons CFCI, and CFZC1, and of nitrous oxide N,O have been measured with long absorption paths in the spectral region above 200 nm for temperatures ranging from 220 to 296 K. The spectral range for accurate measurements has thus been extended towards longer wavelengths where the cross-section values arc low. The results are compared with previous determinations and photodissociation coefficients are evaluated in consideration of the most recent data for the atmospheric transmittance.

1. INTRODUCTION

The main difficulties for accurate measurements of the absorption cross-sections of CFCI,, CFzClz and N,O are bound to the weakness of their values at longer wavelengths and to the low vapour pressure of the compounds (particularly for CFCI,) at low temperature, limiting the pressure range to be used in the experiments. Increasing the absorption pathlengths enables these problems to be overcome. The purpose of the present paper is the accurate determination of the absorption cross-sections for CFCI,, CF,C12 and N20 in the wavelength region above 200 nm at three temperatures (220,240 and 296 K) by USCof long absorption paths (20&l 60 m). From the observed data. analytical expressions give the temperature dependence of the absorption cross-sections in the range of the stratospheric temperatures. The photodissociation coefficients at different altitudes are then evaluated using the most recent data for the calculation of the atmospheric transmittance.

To evaluate the role of the atmospheric compounds involved in the destruction processes of ozone, the modelling calculations need specific parameters such as the photodissociation coefficients. In the case of chlorofluorocarbons CFCI,, CF,CI, and of nitrous oxide N:O. the concentrations calculated with the existing data are largely overestimated at altitudes higher than 20 km (see Jackman and Guthrie, 1985). This discrepancy may be explained by an erroneous estimation of the absorption cross-sections directly correlated to the photodissociation coefficients. In the past, absorption cross-sections of CFCI, and CFzClz have been extensively studied. The recommended values for atmospheric calculations (J.P.L., 1985) have been deduced from seven sets of data (Rowland and Molina, 1975; Robbins and Stolarski. 1976; Bass and Lcdford, 1976; Green and Wayne, 1976; Chou rt al., 1977; Hubrich et nl., 1977 ; Vanlaethem-Mcuree c’t ul., 1978) though large differences-sometimes up to 40%-are observed between the lower and the higher values. Moreover, for CFCI i recommended values did not exist at stratospheric temperatures. Very recently, Simon ef ul. (1988) proposed new temperature-dependent absorption cross-sections for the two compounds. For NzO, recent studies are scarce. The values determined by Selwyn et al. (1977) as a function of temperature are used as references (J.P.L.. 1985). However. differences between - 15 and + 15% at ambient temperature, and - 10 and +40% at lower temperature (T z 220 K) arc observed with the values of Hubrich and Stuhl (I 980).

2. EXPERIMENTAL

The absorption cross-sections have been mined according to Beer Lambert’s law : I(?.) = I,(;,) exp (-0(i)

. c. I).

deter-

(1)

In our experiment, I,,(A) and I(i) are the light intensities measured with the empty and tilled cell, respectively ; c is the number of molecules per unit of volume, I is the pathlength and a(?.) the absorption crosssection. The experimental set-up, designed for the study of the oxygen Herzberg continuum (Jenouvrier c>tcd.. 617

618

M. F.

MEKIENNE rt ul

19X6 : Coquart ct al., I989), was also used for the dctcrmination of the absorption cross-section of bromoform (Gillotay cl1r/l., 1989). Rricfly, it consists of an argon plasma u.v. source. a 5 m multipass absorption cell operating down to 215 K (Lux and Coquart, 1989) and a Jobin Yvon THR 1500 monochromator equipped with a Hamamatsu R955 photomultiplier. The monochromator and the data acquisition system are monitored with an Apple IIc microcomputer. The pressure in the cell is measured with an MKS Baratron capacitance manomclcr giving a precision better than 0.1% in the range 10 ‘-1000 tori (I.33 x IO ‘-1.33 x IO’ Pa). Five sensors allow the control of the temperature distribution along the inner wall of the ccl]. During the whole time ofthc measurcmerits, the temperature deviations arc always lower than 0.2 K at 220 K. lnterfcrcncc filters put at the cntrancc of the cell limit the dif7usion of the light and the photolysis of the sa~nple. For each wavclcngth that the phoregion. it was effectively controlled tolysis rate was too low to give significant drift of the signal during the scanning times. The wavclcngth calibration is ensured before each experiment by referring to tungsten atomic lines cmitted by the source and superposed to the argon continuous background. The precision is estimated to bc better than +0.02 nm. The high purity of the gas samples was checked ; the contribution of the impuritics to the determined cross-section values is always lower than 0. I l/o. For each compound. the spectral range of the scans, the absorption pathlengths and the gas pressures arc

TAB1.f:

1. ~XPFKIMENTAL

(‘OpiDlTlO~S

3. REWLTS The absorption cross-sections dctcrmincd in the present work arc listed at 1 nm intervals in Tables 3. 4 and 5 for CFCI ?. CFIClz and N20. respectively. All the quoted values arc referred to the vacuum wavclength scale; we point out that, owing to the large variation of the cross-sections as a function of wavelength. the USCof the air reference induces a correction which can be larger than 2% in the studied wavelength range. Generally, the rcferencc was not specified in the previous works. The absorption cross-sections obtained for the three compounds corroborate the qualitative conclusions of the previous authors : the tcmpcrature dcpendcnce of the cross-sections is more important at longer wavelengths for which the absorp-

FOK PKtVIOUS

Temperature (K)

Authors CFCI;&F,CI, Robbins and Stoiarski (1976) Bass and Ledford (1976) Chou <‘Itrl. (1977)

174 226 1x5 230 I86 226

Hubrich <‘/trl. (1977) Simon e/ (I/. ( 1988)

158 275 174 220 200&23XCFCI, 200 231 CF,Cl:

This work

defined in order to have the best optical density conditions required for an accurate determination of the cross-sections. In this way, the whole spectral range is covcrcd by succcssivc scans of 6-8 nm depending on the transmission conditions (15% < I(j_)/l”(i.) < 70%). For each scan. the stcpwise measurements arc integrated over 0.9 sand collected at 0.02 nm intcrvxls. Six sets of mcasuremcnts for each spectral range arc made succcssivcly with the empty ccl] (I,,) and the filled ccl] (I) to provide the data used in the lid calculation ofthc cross-section values. With the cxperimental conditions summarized in Table I and the estimated uncertainties given in Table 2. the crosssections are bclicved to be dctcrmincd with an accuracy of 2%.

(296) 296. 223 296. 252 232. 213 298. 208 295. 270 255.225 296, 240. 220 296. 240. 220

ANI> PKFSFUT

Pathlength (m)

WOKKS

Pressure (torr)

0.1 0.05

0.09 2 IO 0.10 0.1, 100 0.135. 2 20~-IO0

30 100

Bandwidth (run)

0.3

Data step

2

0.2 22 0.5, 5

<25

0.15

0.02 3 0.2-60

0.01 0.04 0.01~0.07

0.02 0.02

0.075

0.05

0.3 0.02 0.08

5 0.02

N20 Selwyn

<‘I

t/t.(1977)

Hubrich and Stuhl (1980) This work

173 240 160 250 200 240

302. 296. 263 243,225. 194 298,208 296,240. 220

0.065, 5.92 0.1, I 20 -160

0.5 130

of

U.V. absorption

CFCI,, CF,CI,

TABLI. 2. EKKOK I~“DGI:T wL..%rtl> TO THt ExPCKI\IExT,\L AIW~KI’TIOY C‘KOSS-SIX’TIONS 0.05% 0.1% 0.03- 0.

path Pressure Temperature

Optical

(~0.1 K at Absorbance Impurities Wavelength Total

296 K.

kO.2

200

I Q/co
error

-2%”

Standard deviation at each wavclcngth (six sets of measuremenls)

2%

TABLE 3. ABSORPTIONCROSS-SECTIONS FOR CFCl, 240 AND 220 K (CSITS or: 10 ” cm’)

(nm)

296 K

200

618 547 482 424 372 325 2x2 244 210 180 153 I29 109 90.6 75.2 62.1 51.2 42.4 35.1 29.0 23.x 19.6 16.1 13.2 10.9 X.96 7.38 6.07 4.9X 4.07 3.32 2.70 2.20 I .X0 I .47 1.20 0.978 0.795 0.654

201 202 203 204 205 206 207 ‘OX 200 2 I0

21I 212 113 214 215 216 217 218 219 320 321 212 223 224 225 226 227 22X 229 230 231 232 233 234 235 236 237 23x

(nm)

240aK 592 524 460 401 347 299 255 216 IX? IS? 127 105 X6.4 70.9 5X.0 47.4 3X.6 31.3 25.2 2il.3 16.3 13.1 10.6 X.60 6.99 5.65 1.57 3.69 2.98 2.40 I .93 1.55 1.25

I .oo 0.806 0.649 0.524 0.421 0.330

NZO

619

AHsoKPnoI\. (‘Koss-sEcTIoNs ITOKCF,CI, 240 mu 220 K (IJNITS OF IO- ” cm’) 296

K

AT 296.

24:K

22;

63.6 49.5 3X.3 29.5 22.6 17.3 13.2 IO.1 7.6X 5.85 4.46 3.42 2.64 2.02 1.55 I.18 0.904 0.69 I 0.530 0.407 0.314 0.242 0.187 0.144 0. I IO 0.085 0.066 0.052 0.040 0.031 0.025 0.020

57.5 44.5 34.2 26.2 20.0 15.2 I I.5 x.7x 6.66 5.06 3.84 2.93 2.24 I.71 I .30 0.986 0.746 0.565 0.429 0.330 0.25 I 0.192 0.147 0.1 I2 0.087 0.067 0.052 0.04 I 0.032 0.025 0.020 0.016

K

I %I

K at 220 K)

m the sample cahbrntion

TAHLE 4.

and

AT 296,

22iK 575 50x 445 3x7 335 2X7 244 206 173 I44 II9 9x.0 80.2 65.3 53.0 43.0 34.X 28.1 22.X IX.1 14.5 I I.6 9.33 7.50 6.03 4.x4 3.90 3.14 2.53 2.03 I.64 I.31 I .05 0.843 0.675 0.541 0.435 0.349 0.273

20 I 202 203 204 205 206 707 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 22X 229 230 231

x5.3 67.6 53.3 42.0 33.0 75.9 20.3 15.9 12.5 9.78 7.65 5.97 4.62 3.57 2.76 2.14 I .6X 1.34 I .05 0.x23 0.647 0.509 0.40 I 0.315 0.246 0. I92 0.149 0.1 I6 0.090 0.072 0.057 0.045

tion is lower. This temperature effect is more nificant for CF&I, and N,O than for CFCI;.

sig-

The deviations (in pcrcentagc) of the most recent determinations or of the recommended values (J.P.L., 1985) compared with our values arc shown in Figs 1, 2 and 3 for CFCI ?, CF,Cl 2and N,O. respectively. On account of the diversity of the temperatures used in the previous studies (see Table I for the experimental conditions). the comparison is rcferrcd to the tempcraturcs (296, 240 and 220 K) of the present work in adjusting the values by interpolation between two nearby temperatures or by calculation with analytical formulae deduced by the previous authors. For chlorofluorocarbons, the values of Rebbert and Ausloos (1975) given at a few wavelengths arc not represented, neither are the values proposed by Doucet et al. (1973), Huebncr et ul. (1975), Rowland and Molina ( 197.5). and Greene and Wayne

620

M.

F. MERIENNE et al.

TABI.E 5. ABSORPTION TROSS-SECTIONSFOR NzO AT 296, 240 AND 220 K (UNITS OF IO-” cm*)

i. (v2ic.i

0

(nm)

296 K

24; K

39.9 35.0 30.5 26.3 22.5 19.1 16.0 13.5 11.2 9.30 7.70 6.34 5.23 4.30 3.52 2.81 2.32 I .86 1.50 1.20 0.956 0.762 0.608 0.486 0.388 0.307 0.244 0.194 0.153 0.123 0.0973 0.0772 0.0609 0.0480 0.0380 0.0303 0.0242 0.0193 0.0155 0.0124 0.0101

32.4 28.0 24.0 20.3 17.1 14.3 11.X 9.70 7.93 6.44 5.22 4.23 3.40 2.74 2.19 1.75 I .3x I .07 0.854 0.665 0.519 0.406 0.318 0.249 0.194 0.151 0.117 0.0903 0.0697 0.0543 0.0426 0.0333 0.0260 0.0204 0.0161 0.0128 0.0103 0.00827 0.00670 0.00548 0.00456

200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 22x 229 230 231 232 233 234 235 236 237 23X 239 240

(I 976) because they values (discrepancies Vanlaethen-Meuree revised by Simon et here.

CFCI,

22;K 30.3 26. I 22.2 18.8 15.7 13.0 10.7 X.77 7.13 5.76 4.64 3.74 3.00 2.41 I .92 1.52 1.19 0.909 0.736 0.568 0.432 0.336 0.262 0.204 0.158 0.123 0.0943 0.072 1 0.0552 0.0427 0.0334 0.0260 0.0203 0.0160 0.0128 0.0103 0.00840 0.00685 0.00556 0.00455 0.00388

deviate too much from the mean larger than 30%). The values of et a/. (1978) have recently been al. (1988) and are not discussed

(a) CFCl, anu’ CFzC12. In using long absorption paths. the spectral range for the measurements has been extended towards the longer wavelengths. It can be shown (Figs 1 and 2) that the agreement, at ambient temperature, is generally good between the different determinations in the wavelength region

220 w

.

I i

FIG. 1. DEVIATIONS (IN X) OF THE MOST RECENT DPTERMINATIONSCOMPAREDWITH THE PRESENT WOKK FOR CFCI,. (A) : Bass and Ledford, 1976; (0) : Robbins and Stolarski, 1976; (u): Chou CI al., 1977; (*): Hubrich et rrl., 1977; (e--e): J.P.L., 1985: (x-x): Simon L’I al., 1988; (I): standard deviation (&2%).

(200 < i. < 215 nm) where the cross-sections are the highest (a > 10~ ” cm’). For i > 215 nm, the spread of the values increases as their magnitude decreases. Over the whole region, the agreement between the values of Bass and Ledford (1976) and ours is good but the differences between the recent values of Simon et a/. (1988) and ours are quickly increasing (up to + 15% for CFCI,, - 15% for CFzCIZ) above 215 nm. At lower temperatures, the determinations are scarce ; particularly, no recommended values exist for CFCI,. The remarks about the values at ambient temperature are also valuable at lower temperature : there is a relatively good agreement for i, < 2 15 nm ; at longer wavelengths the present values are very consistent with those of Bass and Ledford (1986) but large discrepancies (- 15%) are observed with the values of Simon et al. (1988). This is clearly shown in Figs 4 and 5 for which our reduction factors (ar/nlgJ at T = 240 and 220 K are plotted against wavelength. The comparison at 220 K with the other determinations shows that Simon et cd. (1988) have a temperature dependence more important than ours while

U.V. absorption Z9‘K

of CFCl,,

CF,CI,

621

and NzO

CF,CI, .

20

t

i-

1

mo

210 wa”clmglh

220

0

230

(“In,

FIG. 2. DEVIATIONS (IN %) OF THY. MOST RECENT DETERM,NAT,ONSCOMpAREU WITH THE PRFSENTWORK FOR CF,Cl,. (A) : Bass and Ledford, 1976; (0) : Robbins and Stolarski.

1976; (m): Chou ef u/.. 1977: (*): Hubrich et al.. 1977; (@--0): J.P.L., 1985; (x-x): Simon et (II., 1988; (I): standard deviation (+2%).

are consistent with those of Bass and Ledford (1976) and Chou et ul. (1977). For instance, at 230 nm the ratio obtained in the present work for CFCI, induces a 50% reduction of the cross-section at 220 K; the same reduction factor is already obtained at 240 K by Simon et ul. (1988). The absorption cross-sections determined by Chou et ul. (1977) at ambient and lower temperatures are always larger than ours but their temperature dependence is quite similar. Our reduction factors for CFzClz give a temperature effect less important (-5%) than that deduced from the recommended values (J.P.L., 1985). Finally, it can be seen that the temperature effect on the absorption cross-sections of CFCI,, not taken into account in the modelling calculations, is weak at 200 nm (4% at 240 K, 7% at 220 K) but becomes substantial as the wavelength increases (32% at 240 K. 40% at 220 K for j” = 220 nm).

FIG. 3. DEVIATIONS (IN %)

OF THE MOST RELENT DETERMINATIONS COMPAKEII WlTH THE PKESENT WORK FOR N20.

(V): Selwyn et crl., 1977: (*): Hubrich (I):

standard

deviation

and Stuhl, (*2%).

1980;

our results

(b) NZO. As said previously, only two recent determrnattons ot- the absorption cross-sections have been

no

120

210 Wavolrnglh

0

(nml

FIG. 4. RATIO OF ABSORPTION CROSS-SECTIONS CJ~/U?~)~~ vs WAVELENGTHFOK CFCl,. (0) : This work (T = 240, 220 K) ; (A) : Bass and Ledford, 1976:(m):Chouetal., 1977(T=220K);(x):Simon~r trl.. 1988 (7-z 220 K)

M. F.

MERIENNE

er

(11

N,O

0.4

0.3

(XT-

200

230

210 Wavelength

5.

RATIO

OF AHSOKPTION

CXOSS-SECTIOUS

u~/u~,,(,~

I

240

(nm)

vs

WAVELENGTH FOK CFzC12. This work (T = 240, 220 K) ; (A) : Bass and Ledford, (7-z 220 K): (m-mm):Chou c/ rrl.. 1977 (7-= 220 K); -/I: J.P.L., 1985 (T= 220 K): (x ): Simon CI t/l.. 1988 (7-z 220 K).

performed for N,O (Selwyn ct al.. 1977; Hubrich and Stuhl. 1980) ; presently the recommended values (J.P.L., 1985) arc deduced from the data of Selwyn ct cd. (1977) at different temperatures. At ambient tcmpcrature, our values agree very well with the results given by these authors in the whole region 200-240 nm. The differences with the values proposed by Hubrich and Sluhl (1980) arc within + 15% (Fig. 3). At lower temperatures, the agrccmcnt between our values and those of Selwyn rt al. (1977) remains acceptable in the region 200&215 nm but diffcrcnccs increase towards Ihe longer wavelengths as the cross-sections bccomc smaller ((r < IO 22cm’). The discrepancies with the values of Hubrich and Stuhl (1980) at the lowest temperature arc even larger (> 35% at 230 nm) than those obtained in the case of chlorofluorocarbons. Since there is a sniall deviation between the present results and those of Selwyn et rd. (1977) at wavelengths longer than 215 nm, the temperature cffcct is slightly less important (-5%) than that deduced from the rccommcnded values (see Fig. 6). To sum up. for the three compounds. the spread of the different results is weak in the spectral regions (0 > 2 I5 nm) for which the absorption cross-sections are large enough ((T > IO “I cm’) to bc dctcrmincd without experimental difficulties. For lower crosssections (0 < 10 ” cm’), acceptable agreement exists only between authors who have used sufficiently long absorption paths and low sample pressures lead-

(0) : This work (T = 240, 220 K); ( x ) Selwyn et ul., 1977 (T = 240, 220 K).

ing to optimal experimental conditions for an accurate determination [Bass and Ledford (1976) for CFCl? and CFzC12, Selwyn ct cd. (1977) for NzO ; see Table I]. Though the results obtained by Simon et cd. (1988) are in agreement with ours for the higher absorption cross-sections (this was also obscrvcd in a common work on bromoform: Gillotay et cd.. l989), the discrepancies found in the prcscnt work for the weak values of CFCI 1and CFICl 2 can only be explained by the large difference between the pathlcngths used for the measurements. WC again point out the agreernent bctwccn our results and those of Selwyn et al. (1977) for N?O. A similar agreement has already been observed between the two groups (Johnston et cd., 1984; Jcnouvrier et rd., 1986) for the weak values of the absorption cross-sections of oxygen (IO- ‘a cm’ < 0 < IO ml’ cm’); these new values are considered at the present time for the revision of the previous recommended ones which are up to 40% larger. For these reasons, we think that the present absorption cross-sections of CFCI,, CFzCIL and N,O above 200 nm are the most accurate and should be considered in further modelling calculations. 3.3. A tnmphrk

qpliccrtiorz

In view of atmospheric purposes, we have considcrcd an expression titling our experimental values in the region 200-230 nm (or 200-240 nm according to the compound) as a function of tcmpcrature : Ino(i,. T) = A(l.)+B(i)(T-296)+C(i)(T-296)‘, (2)

U.V. absorption where the cocfficicnts written in the form

A(i),

of CFCI,,

B(i,) and C(j.) can bc

1-1, X(j.) = 1 .\-,(&2(X))’ /m I

’

(3)

with i. in nanomctrcs and n = 3 or 4. For chlorofluorocarbons, A(i) and B(i) COcfficients arc sufficient to describe the variation of In a(;. T) while for N :O, the variation is no longer linear but is a quadratic function of temperature. Concerning this point. let us recall that the formulation given hcrc has no physical foundation but is useful for atmospheric calculations. The computed values for the parameters .y, arc collcctcd in Table 6. The absorption cross-sections calculated with formula (2) represent the cxpcrimcntal data with differences lowel than *2X. Using the present results for >. > 200 nm. those of Simon c’t trl. ( 1988) for CFCI i and CF&I, and those of Selwyn ct rd. (1977) for NZO in the wavclcngth region i < 200 nm. the photodissociation cocficicnts have been evaluated at diRerent altitudes : according to the relations : .I_ = a,q,(z) (/,

(1) =

cl.. ( x

CFICI,

623

and N?O

The photodissociation coefficients calculdted I~I the three compounds with an overhead Sun (set x = 1) arc given in Table 7 for the altitude range 2060 km. Figure 7 represents their spectral distribution for 500 cm ’ intervals at the altitude of 30 km, indicating the predominant contribution in the spectral region of the -‘atmospheric window” (I 90-220 nm). For CFCI i and CFZC12. a comparison between the present absolute J values and those of Simon c’f trl. (1988) cannot be made directly owing to the diffcrewes in the input data (solar irradiance and transmittance in the Schumann Runge bands region). Howcvcr. the relative values J,,, = J71:JZ4h can bc cfficicntly compared. These values agree quite well in the cast of CF&I, (at each altitude, differences < 10%). For CFCl,. our values are close to unity in contrast to the results of Simon cf cl/. (1988). Figure 7 shows that the main contribution to the temperature dependence of J arises from the spectral region above I95 nm. Indeed. the tcmperaturc effect dctermincd in this work for the absorption cross-sections of CFCI i in the rang 200-240 nm is weaker than that of Simon (jr (I/. (1988) and leads to ;I small variation of the photodissociation cocfficicnt for this compound.

(4)

(5)

1T,,(z)

r----Z=3Oklll

Ill T,(z) = -

I

[~,(O,)r1(0,)+rr,(01)12(0~) +rr,,(air)n(air)]

secxd:.

S.CX

=1

(6)

qi,(:) is the solar irradiancc, T,(Z) the transmittance factor resulting from oxygen. ozone and atmospheric Rayleigh scattering absorptions; II is the number of particles per unit volume. For the calculation, the absorption cross-section values of ozone are those of Molina and Molina (1986) and the Raylcigh scattering absorption is calculated according to Nicolet (1984). The absorption cross-sections of oxygen arc those of.Ienouvrier et cd. (1986) and Yoshino ct cd. (1988) in the spectral region of the Herzbcrg continuum alone. namely at cncrgies lawcr than 49,000 cm ‘. In the spectral region of the Schumann Rungc bands. the transmittance has rcccntly been expressed in a convcnicnt form (500 by Nicolet and Kcnncs cm ’ spectral intervals) (1989). Finally. oxygen and ozone concentrations at a given altitude and the corresponding temperature are taken from the standard model of atmosphere. The solar irradiances q,(z) arc from S’pn~lah 2 observations (Van Hoosier and Brueckncr, 1987).

FIG. ANI)

7. PHOTOIMSSMYATION COLL‘I,IC‘ILNTS ok CFCI ,. CF,C12 N ?O AGAINST WAVM~GTH AT Z = 30 km WITH OVEKHtAU (p)

J,- (0 =

SLlk (SK

l’(T)):

(

X =

1).

) J2q,> (0 at 296

K).

M. F. MERIENNE et al.

624

TABLE 6. PARAMETERSA, B AND C FOK POLYNOMIALFIJNCTION[SEEFORMULA(2)] CFCI 1 -41.935548 -I.l42857x 10 -3.12034x IO-’ 3.6699 x IO ’

aj

a:

a, u‘i

CF,C12

f

3.58977 x IO 4 3.02973 x IO--” -1.I3x IO h

b, b? h, hcl

-43.8954569 -2403597x 10-l -4.2619x IO-’ 9.8743 x IO h

-44.658943 -1.3451IIx10-’ -3.40472x IO--i 3.948 x IO- ’

4.8438 x 10 A 4.96145 x 1o-J -5.6953 x IO-h

4.1336 x IO-’ 2.6594 x lo-.” 1.2013 x IO-” -2.3x IO IT 5.94 x lo-” 1.87069 x IO-’ - 1.043799 x lo.-’ 2.61939 x IO -’

(‘I Cl

C?

c1 i. range (nm) T rltnge (K)

:

: 200--2X 120-296

200 231 220~~296

TABLE 7. PHOTOUISSOCIATIOKC‘OF.FFICIFXTS (s '1OF CFCI?, CF,Cl, AND N,O vs ALTITULX (setx = 1) [f&a =.I’(?-)). JZ’!f>((rY>h).J,,! = Jr!./%1

z (km) CFCI,

CFCI,

20 2s 30 35 40 45 50 55 60 20 25 30

N?O

35 40 45 so 5s 60 20 25 30 35 40 45 SO 55 60

17=

N,O

J, f(n

6.46x IO x 5.75 x 1om7 2.21 x 10mh 4.85x 10~” 7.69x 10 ’ 9.93 x IO_ h l.l5xlo~’ 1.27 x IO-’ 1.37x 1o-5 4.34x IO-~’ 4.17 x lomx 1.75x IO_’ 4.36x IO 7 7.96x 10 7 1.20x10 (’ 1.58 x IO -O 1.87x lo-” 2.14x IO oh 3.08 x IO-’ 2.76 x IO-” 1.07x IO_’ 2.38 x IO ’ 3.87 x lo-’ S.19xlO ’ 6.15 x IO-’ 6.69 x lo-’ 7.09x10 ’

J Ki 6.90x10 ’ 6.14 x 1O-7 2.38 x lWh 5.21 x rn- h 8.18x 10 6 1.04x10 5 1.19x 10--s 1.31x10 5 1.42x 10 ’ 8.05x10-” 7.3.5x IO 8 2.92 x lo-’ 6.63 x IO- ’ 1.09x 10 ci 1.48x10 6 1.84x 10 fi 2.22 x 10 G 2.66 x IO- b 4.23 x IO-’ 3.73x Io-s 1.42 x lo-’ 3.07x10-’ 4.76 x IO-’ 6.00x IO ’ 6.87 x lo-’ 7.60 x IO-’ 8.29 x IO ’

0.939 0.936 0.929 0.93 I 0.940 0.955 0.966 0.969 0.965 0.539 0.567 0.599 0.658 0.730 0.811 0.859 0.842 0.805 0.728 0.740 0.754 0.778 0.813 0.865 0.895 0.880 0.855

ilclino~~/rc!yc~~len/.s---This work was supported by the Minis&e de 1‘Envilonnement under contract 86.094. We should like to thank J. P. Lux for his technical assistance. It is a pleasure to acknowledge Mr M. Verhille for providing us with the circulating Ruid of our cooling system.

200-240 220-296

REFERENCES Bass, A. M. and Ledford, A. E., Jr. (1976) Ultraviolet photoabsorption cross sections of CFCII and CFCI, as a function of temperature. 12th lnt. Conf. on Photochemistry, Gaithersburg (U.S.A.). Chou, C. C.. Smith, W. S., Vera Ruiz, H.. Moe, K., Crescentini, G.. Molina. M. J. and Rowland, F. S. (1977) The tempcraturedependences ofthe ultraviolet absorption cross sections ofCCl,F, and CCf,F and their stratospheric SiJ@iCWC~. J. ,&‘.s. Chf’m. 81, 286. Coquart, B., Merienne, M. F. and Jenouvrier, A. (t990) 0, Herzberg continuum absorption cross sections in the region 196-205 nm of the Schumann Runge bands Planet. Space Sci. 38, 287. Doucct, J.. Sauvageau, P. and Sandorfy, C. (1973) Vacuum ultraviolet and photoelectron spectra of lluoro-chloro derivatives of methane. J. c/rem. PIrJx. 58, 3708. Gillotay, D.. Jenouvrier, A., Coquart, B.. Merienne, M. F. and Simon. P. C. (1989) Ultraviolet absorption cross sections of bromoform in the temperature range 295 240 K. Plutw~. Spew Sci. 37, I 127. Green, R. G. and Wayne, R. P. (1976) Vacuum ultraviolet absorption spectra of halogenated methanes and ethanes. J. Photo&m. 6. 375. Hubrich,C. and Stuhl, F. (1980) The ultraviolet absorption ofsome halogct~dted methanes and ethanes ofatmosph~ric interest. J. P/iotoche~rr. 12, 93. Hubrich. C., Zetzsch. C. and Stuhl. F. (1977) Absorptionsspectren von Halogenierten Methanen in bereich van 275 bis 160 nm bei Temperaturen von 298 und 208 K. Ber. Eusenges. phJ)s. Chum. 81, 431. Huebner, R. X-I.,Bushnell, D. L., Celotta, R. J., Mielczarek, S. R. and Kuyatt, C. E. (1975) Ultra violet photoabsorption by halocarbons 11 and 12 from electron impact measurements. Natwe 257, 376. Jackman, C. H. and Guthric, P. D. (1985) Sensitivity of N,O, CFCI j and CF,CI, two dimensional distributions to O2 absorption cross sections. J. geoplz~,s. Rex. 90, 3919. Jenouvrier, A., Coquart, B. and Merienne. M. F. (1986) Long pathlength measurements of oxygen absorption

U.V. absorption

of CFCl,,

cross sections in the wavelength region 2055240 nmn. J. quclnt. Spec~rosc. rudirrt. Trunsfkr. 36, 349. Johnston. H. S., Paige, M. and Yao, F. (1984) Oxygen absorption cross sections in the Herzberg continuum and between 206 and 327 K. J. groph,,.c. Rex. 89, I 1661. J.P.L. (1985) Chemical Kinetics and Photochemical Data for use in Stratospheric Modeling. J.P.L. Publication 8537. Pasadena. California. Lux, J. P. and Coquart, B. (1989) A cooled multipass cell for the absorption study of atmospheric compounds. J. Phys. E: xi. krutn. 2i, 961. Mohna, L. T. and Molina, M. J. (1986) Absolute absorption cross sections of ozone in the 185 to 350 nm wavelength range. .I. geopll~.s. Res. 91, 14501. Nicolet, M. (1984) On the molecular scattering in the terrestrial atmosphere : an einpirical formula for calculation in the homosphere. d’hnrt. SpaceSri. 32. 1467. Nicolet. M. and Kennes, R. (1989) Acronomic problems of molecular oxygen photodissociation-VI. Photodissociation frequency and transmittance in the spectral range of the SchumannRunge bands. Aeronomica Acta A, No. 342, I. Rebbert, R. E. and Ausloos, P. J. (1975) Photodecomposition of CFCI, and CF,CI,. J. Photochem. 4,419. Robbins, D. E. and Stolarski, R. S. (1976) Comparison of stratospheric ozone destruction by fluorocarbons I I, 12. 21 and 22. Geophys. Re.s. Left. 3, 213.

CF,Cl,

and N,O

625

Rowland, F. S. and Molina, M. J. (1975) Chlorofluoromethanes in the environment. Ret,. Geophys. Space Phys. 13. I. Selwyn, G., Podolske, J. and Johnston, H. S. (1977) Nitrous oxide ultraviolet absorption spectrum at stratospheric temperatures. Geophys. Res. Left. 4, 427. Simon. P. C., Gillotay, D., Vanlaethem-Meuree. N. and Wisemberg, J. (1988) Ultraviolet absorption cross sections of chloro and chlorofluoromethanes at stratospheric temperatures. J. utmos. Chem. 7, 107. Van Hoosier, M. E. and Brueckner, G. E. (1987) Solar ultraviolet spectral irradiance monitor (SUSIM) : calibration and results from Spucrluh 2. in Proceedings of the 8th Workshop on the Vcrcuum Ultruriolet Calihrrrtion of Space E.xperiments, 18-19 March. Joint Institute for Laboratory Astrophysics, Boulder, Colorado. Vanlaethem-Meuree. N.. Wisemberg, J. and Simon, P. C. (1978) Influence de la temperature sur les sections efficaces d’absorption des chlorofluoromtthanes dans l’ultraviolet. Bull. Acud. R. Bd9. CI. Sci. 64. 42. Yoshino, K.. Cheung, A. S. C., Esmond, J. R., Parkinson, W. H., Freeman, D. E.. Guberman. S. L., Jenouvrier, A., Coquart. B. and Merienne, M. F. (1988) Improved absorption cross sections of oxygen in the wavelength region 205 240 nm of the Herzberg continuum. Plrrr~et. SI)LI(,~Sci. 36, 1469.