Evaluation of CF2Cl2 and CFCl3 spectral data for atmospheric sensing

EVALUATION OF CFzClz AND CFC13 SPECTRAL DATA FOR ATMOSPHERIC SENSING F. CAPPELLANI, G. RESTELLIand G. MELANDRONE Applied Sciences Department Electronics Division, Joint Research Centre, Ispra, Italy (Received

24 Ocro~e~ 1978)

Abstract-High resolution infrared absorption spectra have been measured in some regions of the 10.8pm band of CF,CI, and 11.8bm band of CFCI,; the rotational fine structure has been resolved using tunable diode lasers. Broadening rates for single lines have been evaluated and the strength of few lines calculated. Considerations are given on the possibility of using in cell spectroscopy for measurement of tropospheric CF,Cl, and CFC13.

1.

INTRODUCTION

The remote sensing of atmospheric constituents and pollutants is most conveniently done by spectroscopic techniques. In the infrared, observations from gound level and from aircraft or balloon-borne spectrometers have been reported for HNO,, HF, HCl, CH4, HzO, CF,CII, CFCIJ, NH,, NO, NOz and 0, by various authors; see, for example, Refs l-4. In the case of CF,C& the sharp spectral features at 920 cm- ‘, 921.9 cm-’ and 923.2 cm-’ have been used, while for CFC& it has been found necessary to resort to the broad band centered at 847 cm - ’ .@*@ Measurements of the concentration of an atmospheric constituent using infrared techniques require a knowledge of theoretical molecular modelling with substantiating laboratory data, as well as knowledge of the atmospheric infrared background against which the constituent is to be observed. In the present study, laboratory absorption spectra of CF,Cl, and CFC13 have been analysed at selected spectral regions in correspondence with intense bands. The use of semiconductor tunable diode lasers has allowed for the first time the partial resolution of the fine structure of the spectra of these gases, in spite of the small rotational constant (co.1 cm- ‘) and the overlapping of the various chlorine isotopic bands.“’ Some considerations on the observed spectral features are presented, together with measurements of self and nitrogen broadening coefficients, and of absolute line intensities to complement the results previously reported. (*) The application of high resolution spectroscopy with tunable diode lasers in conjunction with a long path gas cell for quantitative evaluation of tropospheric CF’,C12 is discussed,

2.

EXPERIMENTAL

CF,Clz and CFCl, respectively 99.0 and 99.9 mole per cent pure, were introduced in a 12cm long gas cell. PbSnTe semiconductor diode lasers emitting, respectively, around 850 cm- ’ and 920 cm- ’ were used; extended wavelength coverage was achieved by current tuning with the diode set at different temperatures in the range 1540 K using a closed cycle refrigerator and a high precision temperature stabilizer (LAI-CTS). To achieve good reproducibility, the spectra were recorded using a slow current sweep rate (10-20 mA . min - ‘). The line separations were measured with a precision of + 7 x lo-” cm-‘, using a Ge etalon whose free spectral range in this wavelength region was calculated from the specified geometry and the refractive index. (9) Absolute line frequency assignments were made using calibration NH3 absorption lines whose wavelength values were taken from literature or measured in this laboratory.“‘) 195

F. CAPPELLANI. G. RESTELLIand

196

G.

MELANDR~NE

3. RESULTS

Different small (0.2-I cm-‘) spectral regions in the range 907-931 cm-’ for CF, CIZ and 847.5-873 cm-’ for CFC13 were examined: unfortunately, even using combined temperature and current tuning, it was impossible to continuously cover at least one of the identified branches.“’ A common characteristic of the observed spectra was the presence of many closely spaced lines ( - 200 per wavenumber) so that the rotational fine structure was barely resolved.

High resolution spectral structure analyses in the vs band have been performed by Jennings” ‘) and Restelli ef rrl.“’ near the Q-branch head at 923, 921.7 and 919cm-’ using semiconductor tunable diode lasers and by Harward”” using a COz waveguide laser continuously tunable over very narrow bandwidths (200-600 MHz) around the CO2 laser emission lines. Besides the region at 923 cm- ’ which represents a well-characterized series of transitions, only one other interesting spectral signature was in evidence in this study in the frequency interval 929.9-930Scm-‘, corresponding to the R branch of the I’~ band.“’ Strong transitions located at 929.9952, 930.0591, 930.1214, 930.1823, 930.2412, 930.3043, 930.365 1 and 930.4254 cm - ‘, regularly spaced by about 0.06 cm _ ‘, typically shown in Fig. 1, are evident. The intense lines allow maintenance of a sufficiently detailed spectral structure also at increased total pressure while the other less intense absorption lines rapidly merge into one another and form a pseudo-continuum. This is demonstrated in Fig. 2 which shows a set of CF2C12 spectra taken in the same 930 cm-’ region, N,-broadened with total pressures up to 50 torr. This result seems to be in disagreement with the observation of Harward ‘12’ that no spectral structure was detectable within the tuning range of fourteen CO* waveguide laser lines in the region from 916.58300 cm- ’ to 942.38407 cm- ‘. for total pressures greater than 20 torr (CF$% + N,). The absorption coefficient evaluated in correspondence with the pseudo-continuum appeared to be effectively nearly pressure-independenV”j with values from 35 to 50cm-’ atm-‘, in fairly good agreement with the values measured by Harward at

r

Clp, a2 1orr h 12cm c*ti SOL LAI 7l37/6

tiH3aPl6.3) 430.3061cm-I

430 3043cm- 1

S30366icm-t

G* rtabn FSR*0.04606cm-i

WAVENUMBER

Fig. I. Diode

laser spectrum

of CF,C12

km-‘)

in the 930.3cm-’

region

Evaluation

of CF2CI,

and CFCI,

spectral data

197

CFiCh 292-K I2 a

Cd1

9.

930.000

Fig.

2. Nitrogen-broadened

l,mn

fwI.‘0.04909

a-1

93Q110

WAVENUMBER km-‘) spectra of CF2CI, in the 930.05cm-’ been shifted for clear representation.

region.

The

spectra

have

and at 929.0182 cm-’ (equal to 93 1.002 17 cm - ’ (equal to 43.5 cm - * atm-‘) 45.7 cm- l atm- ‘). Single line Nz-broadening coefficients have been measured equal to 8 k 2 MHz/torr (FWHM) in good agreement with the values previously reported for the 918cm-’ region”’ and with the air-broadening coefficient of 8 MHz/torr (FWHM) estimated from an apparent air broadening of 2 MHz/torr (FWHM) for lines in the 923 cm-’ region by Jennings.” I) The value of 8 + 2 MHz/torr allows the estimation of a Lorentz halfwidth (for atmospheric pressure N,-broadened lines) of 0.1 If: 0.025 cm- ’ which must be compared with the CF2C12 Lorentz halfwidth calculated by Goldman et al.,” 3’ using a curve of growth analysis, equal to 0.0~0.08 cm-’ (at 25°C). The absolute intensity of the 930.2412 cm-’ line was measured at 295 K equal to 0.50 + 0.12 (cm-‘atm-‘). This value, which can be considered representative of the strong transitions observed in this spectral region, is larger by a factor of at least 5 than the values previously measured for the 918.621 and 918.562cm-’ lines.“’

The following regions of the absorption spectrum have been examined: 847.6-848Scm-‘; 852.6853.0cm-‘; 853.15-853.75cm-’ and 872.5-873,2cm-‘. A common characteristic of these frequency intervals is the absence of strong spectral features and the presence of many closely spaced lines which give rise to a pseudo-continuum and is responsible for rapid disappearance of the spectral structure at total gas pressures above 20 torr (CFC13 + N,). This fact inhibited the performance of an accurate evaluation of the pressure broadening coefficients. The apparent broadening in the 847 cm-’ region, as measured from the full width at the absorption level halfway between minimum and maximum absorption for each feature, is equal to 8.2 MHz/torr and 2.4 MHz/torr, respectively, for self and nitrogen broadening. The absolute intensity of the 847.8189 cm-’ line, one of the

F.

19x

CAPPELLANL

G.

RESTELLI

WAVENUMBER

Fig. 3. Diode

laser spectrum

and G.

MELANDRONE

(cm-‘)

of CFCI,

in the 847.8 cm-

’ region.

most clearly separated in the spectra examined, was measured at 295 K equal to 0.35 + 0.12 (cme2 atm- ‘). The absorption coefficient, calculated for the spectral region of Fig. 3, pressure broadened to give a pseudo-continuum, was equal to 100 f 10 cm-’ atm- ’ in very good agreement with the value of 106 cm- ’ atm- ’ calculated for the 847 cm- ’ spectral region from the curve of growth analysis.‘14’ 4.

POINT

MONITORING

USING GAS

A MULTIREFLECTION

CELL

Recently, Reid et al. (“.16) have described a laser absorption spectrometer employing a tunable diode laser in conjunction with a multipass White cell for point monitoring of ambient concentrations of atmospheric gases. Minimum detectable concentrations at atmospheric pressure have been experimentally determined for SO,, N,O and NH3 and estimated for O,, COz, H,O, PAN. CH,, NO and CO: the sensitivities range from 0.001 ppb for CO2 to 15 ppb for H20.(r6’ The applicability of this technique is here discussed in the case of CF,Cl,. Contrary to the gases above indicated, it is impossible, in this case, to perform any measurement at atmospheric pressure since the single line structure is completely lost and the limited continuous tuning range of the diode laser does not allow the use of the entire band structure. The CF,Cl, absorption line at 930.0591 cm- ’ is selected for the measurement since the spectral structure around this line allows its observation at gas pressures (CF,Cl, + air) up to some tens of torr. In this laboratory, using a 12 cm long gas cell filled to a total pressure of 20 torr (CF,Cl, + N2), and first derivative detection (Fig. 4) a minimum detectable quantity of 5 x 10” CF2C12 molecules.cm--2 was measured. Unfortunately, the low mixing ratio of CF,Cl, in air (0.2-0.3 ppb) makes the sensitivity low for direct detection: the optical thickness of of 5 x 1014 molecules. cme2 -too CF2C12 in a 200m long path cell filled with ambient air at 20 torr pressure is in fact of the order of 2.5 x lOi molecules.cm -2. Moreover, the direct measurement would

Evaluation

of CF2C12

and CFCI,

spectral data

199

m CL2

Derivative spectra SOL LA ?137/9 Tz20.K I=l.lAmp. 12 cm cell

929.995cd

9M121 cm-’

930059cm-’

(D 0.1 Torr CF2 Cl1 Q 0.1 Torr C% CIt+lOTorr N, @ 0.1 Torr CF, CI,+ZOTorr N,

WAVENUMBER Fig. 4. First-derivative

spectra of CF,CI, N,

(cm-1 )

in the 930.06cm-’ pressures.

region

broadened

with different

be complicated by the presence of many CO2 and Hz0 absorption lines in this region of the absorption spectrum. a cryogenic collection technique In recent years, Hanst et al. (l’*‘s) have described for separation of stable gases from the air before being placed in an absorption cell for analysis by infrared (IR) Fourier transform spectroscopy. Nitrous oxide serves as an inherent calibration of the degree of enrichment; amplification factors up to 1 x lo6 have been obtained.“” If the cryogenic enrichment technique is applied to the detection of CF,C12 and the measured sensitivity at 20 torr pressure equal to 5 x lOI molecules.cm-’ is considered, it appears that an enrichment factor equal to -200 would be necessary. However, in the case of CO2 removal,“” the total pressure of the residue of the collected air in the cell will be set by NzO and then well below 0.1 torr. In this condition, as it appears from Fig. 4, the sensitivity of the derivative detection is enhanced by at least four times and then an enrichment of about 50 times would be sufficient. For a correct evaluation of the enrichment factor, any of the P and R absorption lines of the OO’l-10’0 band of NzO occurring in the 90&950 cm-’ region with absolute intensities in the range 10-22-10-23cm+2molecule-’ can be measured by the same diode laser used for the CF2C12 measurement. Simple calculations show that the peak absorption for CF2C12 and N20 lines (directly proportional to the concentration times the line strength and inversely to the linewidth) are nearly the same and that no interference must be expected since the lines are separated by frequency intervals which are large with respect to the line widths. The same measurement technique using cryogenic enrichment can be demonstrated feasible for CFC13 using, for example, the line at 847.8189 cm-‘. 5. CONCLUSION The importance of high resolution spectral studies for the application of remote sensing infrared techniques has been stressed many times. The laser systems, in particular, offer potential as highly selective sensors, but accurate spectral data at matching resolu-

200

F. CAPI*I:LLANI,G. RESTELLIand G. MELANDRONE

tion for both the molecules of interest and for all the interfering species must be available; this is mandatory for a correct interpretation of the measurements. The scope of this study has been to obtain accurate spectral data on CFzClz and CFCI, molecules for which high resolution observation of the absorption spectrum has been only very recently undertaken. The spectral data observed have been compared to data obtained using low resolution studies showing a satisfactory agreement, at least for regions where no strong spectral features exist. The observation of strong spectral signatures like that in evidence at pressures (<4&50 torr), about 930 cm- ’ which maintain a line structure at stratospheric are of definite interest for the application of remote measurements of concentrations of these pollutants using high resolution spectroscopic techniques. The same spectral region has been considered for a feasibility study of quantitative detection of tropospheric levels of CFzCll or CFCI, using long path absorption cells and derivative detection with tunable diode lasers: it is shown that this application could be successful, resorting to a cryogenic enrichment technique. The use of a high resolution technique which operates on a single absorption line of the pollutant obviously assures a better specificity and accuracy to the measurement with respect to potential interference by other known or unknown species having absorption features in the same spectral region. REFERENCES I. HARRIS. J. E.. J. R. BIRCH. J. W. FLEMMINC~.N. W. B. STONE, D. G. Moss. N. R. W. SWANN & G. F. NEILL. Proc. 3rd Cor$ m r/w Clirntrric Assessn~vr~~ Progrton (edited by A. J. BRO~ERICK and T. M. HARD), pp. 197-212. Dept. of Transp. DOT-TSC-OST 74-l S (1974). 2. FONTANELLA.J. C.. A. GIRARD. L. GRAMONT & N. LUISNARD. Appl. Opt. 14, 825 (1975). 3. BRADFORD.C. M.. F. H. MURCRAY. J. W. VAN ALLEN, J. N. BROOKS. D. G. MURCRAY & A. Gor DMAN. Gc~oph,v. Rcs. Leff. 3. 387 (1976). 4. BARKER. D. B.. J. N. BROOKS, A. GOLDMAN. J. J. KOSTERS, D. G. MLJRCRAY.F. H. MURCRAY, J. W. VAN ALLEN & W. J. WILLIAMS.U.S.A. Aj~ntrls No 7.5CH/O04-I. 16-6 (1976). 5. MCIRCRAY.D. G. F. S. BONOMO.J. N. BROOKS.F. H. MURCRAY & W. J. WILLIAMS,Geoph~s. Re.7. Lcrr. 2, 109 (1975). 6. WILLIAMS.W. J.. J. J. KOSTERS.A. GOLDMAN & D. G. MURCRAY. Geop/r!x Res. Lerr. 3, 379 (1975). 7 GEORGIANNI.S.. A. GAMBI. L. FRANCO & S. GHERSETTI.J. ndet~. Sprcrrosc. To be published. 8. RESTEI.LI. G.. F. CAPPELLANI& G. MELANDRONE.Pure crppl. Geophyx 117 (1979). In press, 9. ICENOC~LE. H. W.. B.C. PLATT & W. L. WOLFE.Appl. Opt. 15, 2348 (1976). IO. CAPPELLANI. F. & RESTELLI.J. m/et. Sprcrrnsc. To be published. II. JFNNINC;S.D. E. Grop/~~s. Rcs. Lerr. 4. 241 (1978). 12. HARWARD. C. N. Appl. Opt. 17. 1018 (1978). 13. GOLDMAN.A.. F. S. BONOMO& D. G. MURTRAY, Geoph~x Res. Lerr. 3, 309 (1976). 14. GOLDMAN. A.. F. S. B~NOMO & D. G. MURCRAY, Appl. Opt. 15, 2305 (1976). 15. REID. J.. J. SHEWCHUN.B. K. GARSIDE& E. A. BALLIK, Opt. hg. 17, 56 (1978). 16. REID. J.. J. SHEWCHUN.B. K. GARSIDE& E. A. BALLIK. Appl. Opr. 17, 1806 (1978). Il. HANST. P. L.. L. L. SPILLER.D. M. WATTS. J. W. SPENCE& M. F. MILLER, J. Air Pdut. Conrrol A.wx. 25, 1220 (1975). 18. HANST. P. L.. Appl. Opt. 17, 1360 (1978).