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Remeasurement of the absolute intensities of CFC11 (CFCl3) and CFC12 (CF2Cl2)
J. Quant. Spectrosc. Radial. Transfer Vol. 39, No. 3, pp. 193-195, 1988
0022-4073/88 $3.00+0.00 Pergamon Press pie
Printed in Great Britain
REMEASUREMENT OF
CFCll
OF
THE
(CFCI3)
AND
ABSOLUTE CFC12
INTENSITIES
(CF2C12)t
P. VARANASI and S. CHUDAMANI Laboratory for Planetary Atmospheric Research, State University of New York at Stony Brook, Stony Brook, NY 11794-2300, U.S.A. (Received 28 July 1987)
Abstract--The absolute intensities of the strong absorption bands of CFCII (CFCI3) and CFCI 2 (CF2C12) have been remeasured at 300 K in view of their importance in global climatic impact and ozone depletion studies. For CFCI 1, our new values are 1718 + 17 cm -2 atm(846 cm-i band) and 671 _ 8 cm -2 atm-~ (1085 cm-t band). The values we have now obtained for the CFC 12 intensities are 1421 +__12 cm- 2atm- ~ (923 cm- ~ band), 1129 + 11 cm- 2atm(ll02cm -t band), and 717_ 14cm-~atm -j (1161cm -I band).
INTRODUCTION It has become widely known that several chlorofluorocarbons (CFCs), C F C ! 1 (CFCI 3) and CFC12 (CF2C12) being the most prominent a m o n g these, have been escaping into and settling in the atmosphere at rates that might be classified as alarming. The effect these CFCs have on the atmosphere is acknowledged to be two-fold. Besides causing depletion of ozone following the photodissociation they undergo in the stratosphere, the C F C s are predicted to perturb the climate of the planet through an enhancement of the greenhouse effect.~ To monitor the concentrations of these gases through remote sensing or to assess the climatic impact due to increases in their concentrations in the atmosphere, we must know the absolute intensities of their strong i.r. bands. Since these gases are present in amounts of a few ppb in atmospheric air, it is necessary to probe the atmosphere where their absorption is the strongest. The same strong absorption is also considered in calculations of the atmospheric greenhouse effect because it occurs in the thermal i.r. window. 2 It has been noted t'2 that the results obtained for the surface warming due to increase in the concentrations of C F C s differed significantly a m o n g the various radiative-convective models that have been proposed. The differences have been attributed to the use of different spectroscopic data a m o n g other factors. In view of this concern and the fact that our earlier measurements 3 of the intensities of the 846cm -I band of C F C l l and of the 915cm -~ band of CFC12 have differed significantly from the later measurements by K a g a n n et al, 4 we have remeasured the strengths in our laboratory. EXPERIMENTAL
DETAILS
The spectra were obtained using a 1-m Czerny-Turner grating m o n o c h r o m a t o r and two different gratings (a 75 lines/mm grating blazed at 8/~m and a 50 lines/mm grating blazed at 18 #m). Two different H g C d T e detectors maintained at liquid-N 2 temperature were employed to cover the different spectral regions with a personal, computer-controlled data-aquisition system. Signal-tonoise was typically 300:1 with the time constant of the lock-in amplifier set at lOOms. Transmittance data were recorded at up to 1000 data points for integration 5 of the spectral absorption coefficient over the sensible spectral region occupied by each band. An absorption cell with 0.53-cm spacing between two NaC1 windows, a lecture bottle of C F C 12 gas and a quart of CFC11 liquid, both obtained from a commercial supplier and purified in our laboratory, and pressures ranging from 9.07 to 90.9 torr were employed in most of our experiments. "t'Supported by the Division of Atmospheric Sciences of NSF under Grant-in-Aid No. ATM 83-17115. 193
194
P. VARANASI and S. CHUDAMANI
Table 1. Absolute intensities of CFCII and CFCI2 measured by us at 300K. CFC
Band Center cm-I
Intensity cm-2atm-I
CFCII (CFCI3)
846 1085
1718 ± 17 671 5:8
CFCI2 (CF,CI,)
923 1102 1161
1421 ± 12 1129 ± 11 717 ± 14
Some data were also obtained using a mixture of 3.31% (by volume) of CFC12 in N 2 and total pressures between 379 and ! 166 torr. The same mixture and a 6.30 cm cell fitted with BaF 2 windows were also used at several pressures between 5.20 and 88.6torr. Pressure measurements were performed accurately with two MKS Baratron pressure indicators designed for 0-100 torr and 0-10,000 torr ranges. The intensity data presented in Table 1 are the results of at least 12 measurements on each band. The intensity values, given with the corresponding standard deviations, were obtained using the Wilson-Wells, Penner-Weber technique) C O M P A R I S O N OF O U R E X P E R I M E N T A L DATA W I T H OTHER PUBLISHED RESULTS The present intensity data agree with the results of Kagann et al 4 better than with our previously published data. 3 The discrepancies noted by the climate-modelers ''2 in the band-intensity data as among the principal causes for the differences in various climate models have thus been partially resolved. The value 1718 cm -2 atm - ' obtained by us for the 846cm -~ band of C F C I ! at 300 K is closer to 1709cm-2atm -t reported by Goldman et al 6 than to 1789 c m - 2 a t m -~ of Kagann et al. a The other values cited in Refs. 7 and 8 are smaller than our result by a larger margin. The value o f 1421cm-2atm -~ measured by us for the 923cm -~ band of CFC12 is closer to 1427 cm -2 atm -j obtained by Kagann et al 4 than to 1471 cm -2 atm -~ derived by Goldman et al. 9 Once again, the other values for the intensities listed in Refs. 7 and 8 are much smaller. The differences in the values given in Table 1 for the intensities of the weaker bands and those reported in Refs. 3 and 4 appear to be within acceptable bounds of experimental error. Replacement of the dial-type bourdon-tube pressure gauges of lesser sensitivity with highly accurate pressure transducers of much greater sensitivity has eliminated what we consider to be the principal source of error in the measurements published previously by our laboratory) The most noticeable difference in the two data sets appears for the stronger bands. Measurements of the intensities of very strong bands required measuring such low pressures in the preparation of lean mixtures that, perhaps, the pressure gauge we had used responded inaccurately. The computerized data-acquisition procedure has also contributed appreciably to better accuracy in the present redetermination of the absolute intensity as the numerical integral of the measured spectral absorption coefficients than the older procedure involving analog records of spectra obtained on chart paper. However, lack of spectral resolution as emphasized by Kagann et al 4 is hardly the cause for the errors in our measurements or those of other laboratories employing the Wilson-Wells, Penner-Weber technique. We have not found any measurable instrumental distortion of band contours, at the pressures employed by us, when these were observed under spectral resolutions that varied between 0.2 and 1.0 c m - ' . The rotational structure of the CFC bands is so dense that it was smeared out at the pressures used in our experiments, within the limits of the sensitivity o f the instruments employed. REFERENCES
I. 2. 3. 4. 5.
Atmospheric Ozone 1985, WMO-Report No. 16, p. 821 (1986). V. Ramanathan, R. J. Cicerone, H. B. Singh, and J. T. Kiehl, J. Geophys. Res. 90, 5547 (1985). P. Varanasi, and F. K. Ko, JQSRT 17, 385 (1977). R. H. Kagann, J. W. Elkins, and R. L. Sams, J. Geophys. Res. 88, 1427 (1985). S. S. Penner, Quantitative Molecular Spectroscopy and Gas Emissivities, p. 73, Addison-Wesley, Reading, MA (1959).
Remeasuring CFC absolute intensities
195
6. A. Goldman, F. S. Bonomo, and D. G. Murcray, Appl. Opt. 15, 2305 (1976). 7. L. A. Pugh, and K. N. Rao, Molecular Spectroscopy in Modern Research, Vol. 2, p. 165, Academic Press, NY (1976). 8. M. A. H. Smith, C. P. Rinsland, B. Fridovich, and K. N. Rao, Molecular Spectroscopy in Modern Research, Vol. 3, p. 111, Academic Press, NY (1985). 9. A. Goldman, F. S. Bonomo, and D. G. Murcray, Geophys. Res. Lett. 3, 309 (1976).
Q.S.R.T. 39/~-B
0022-4073/88 $3.00+0.00 Pergamon Press pie
Printed in Great Britain
REMEASUREMENT OF
CFCll
OF
THE
(CFCI3)
AND
ABSOLUTE CFC12
INTENSITIES
(CF2C12)t
P. VARANASI and S. CHUDAMANI Laboratory for Planetary Atmospheric Research, State University of New York at Stony Brook, Stony Brook, NY 11794-2300, U.S.A. (Received 28 July 1987)
Abstract--The absolute intensities of the strong absorption bands of CFCII (CFCI3) and CFCI 2 (CF2C12) have been remeasured at 300 K in view of their importance in global climatic impact and ozone depletion studies. For CFCI 1, our new values are 1718 + 17 cm -2 atm(846 cm-i band) and 671 _ 8 cm -2 atm-~ (1085 cm-t band). The values we have now obtained for the CFC 12 intensities are 1421 +__12 cm- 2atm- ~ (923 cm- ~ band), 1129 + 11 cm- 2atm(ll02cm -t band), and 717_ 14cm-~atm -j (1161cm -I band).
INTRODUCTION It has become widely known that several chlorofluorocarbons (CFCs), C F C ! 1 (CFCI 3) and CFC12 (CF2C12) being the most prominent a m o n g these, have been escaping into and settling in the atmosphere at rates that might be classified as alarming. The effect these CFCs have on the atmosphere is acknowledged to be two-fold. Besides causing depletion of ozone following the photodissociation they undergo in the stratosphere, the C F C s are predicted to perturb the climate of the planet through an enhancement of the greenhouse effect.~ To monitor the concentrations of these gases through remote sensing or to assess the climatic impact due to increases in their concentrations in the atmosphere, we must know the absolute intensities of their strong i.r. bands. Since these gases are present in amounts of a few ppb in atmospheric air, it is necessary to probe the atmosphere where their absorption is the strongest. The same strong absorption is also considered in calculations of the atmospheric greenhouse effect because it occurs in the thermal i.r. window. 2 It has been noted t'2 that the results obtained for the surface warming due to increase in the concentrations of C F C s differed significantly a m o n g the various radiative-convective models that have been proposed. The differences have been attributed to the use of different spectroscopic data a m o n g other factors. In view of this concern and the fact that our earlier measurements 3 of the intensities of the 846cm -I band of C F C l l and of the 915cm -~ band of CFC12 have differed significantly from the later measurements by K a g a n n et al, 4 we have remeasured the strengths in our laboratory. EXPERIMENTAL
DETAILS
The spectra were obtained using a 1-m Czerny-Turner grating m o n o c h r o m a t o r and two different gratings (a 75 lines/mm grating blazed at 8/~m and a 50 lines/mm grating blazed at 18 #m). Two different H g C d T e detectors maintained at liquid-N 2 temperature were employed to cover the different spectral regions with a personal, computer-controlled data-aquisition system. Signal-tonoise was typically 300:1 with the time constant of the lock-in amplifier set at lOOms. Transmittance data were recorded at up to 1000 data points for integration 5 of the spectral absorption coefficient over the sensible spectral region occupied by each band. An absorption cell with 0.53-cm spacing between two NaC1 windows, a lecture bottle of C F C 12 gas and a quart of CFC11 liquid, both obtained from a commercial supplier and purified in our laboratory, and pressures ranging from 9.07 to 90.9 torr were employed in most of our experiments. "t'Supported by the Division of Atmospheric Sciences of NSF under Grant-in-Aid No. ATM 83-17115. 193
194
P. VARANASI and S. CHUDAMANI
Table 1. Absolute intensities of CFCII and CFCI2 measured by us at 300K. CFC
Band Center cm-I
Intensity cm-2atm-I
CFCII (CFCI3)
846 1085
1718 ± 17 671 5:8
CFCI2 (CF,CI,)
923 1102 1161
1421 ± 12 1129 ± 11 717 ± 14
Some data were also obtained using a mixture of 3.31% (by volume) of CFC12 in N 2 and total pressures between 379 and ! 166 torr. The same mixture and a 6.30 cm cell fitted with BaF 2 windows were also used at several pressures between 5.20 and 88.6torr. Pressure measurements were performed accurately with two MKS Baratron pressure indicators designed for 0-100 torr and 0-10,000 torr ranges. The intensity data presented in Table 1 are the results of at least 12 measurements on each band. The intensity values, given with the corresponding standard deviations, were obtained using the Wilson-Wells, Penner-Weber technique) C O M P A R I S O N OF O U R E X P E R I M E N T A L DATA W I T H OTHER PUBLISHED RESULTS The present intensity data agree with the results of Kagann et al 4 better than with our previously published data. 3 The discrepancies noted by the climate-modelers ''2 in the band-intensity data as among the principal causes for the differences in various climate models have thus been partially resolved. The value 1718 cm -2 atm - ' obtained by us for the 846cm -~ band of C F C I ! at 300 K is closer to 1709cm-2atm -t reported by Goldman et al 6 than to 1789 c m - 2 a t m -~ of Kagann et al. a The other values cited in Refs. 7 and 8 are smaller than our result by a larger margin. The value o f 1421cm-2atm -~ measured by us for the 923cm -~ band of CFC12 is closer to 1427 cm -2 atm -j obtained by Kagann et al 4 than to 1471 cm -2 atm -~ derived by Goldman et al. 9 Once again, the other values for the intensities listed in Refs. 7 and 8 are much smaller. The differences in the values given in Table 1 for the intensities of the weaker bands and those reported in Refs. 3 and 4 appear to be within acceptable bounds of experimental error. Replacement of the dial-type bourdon-tube pressure gauges of lesser sensitivity with highly accurate pressure transducers of much greater sensitivity has eliminated what we consider to be the principal source of error in the measurements published previously by our laboratory) The most noticeable difference in the two data sets appears for the stronger bands. Measurements of the intensities of very strong bands required measuring such low pressures in the preparation of lean mixtures that, perhaps, the pressure gauge we had used responded inaccurately. The computerized data-acquisition procedure has also contributed appreciably to better accuracy in the present redetermination of the absolute intensity as the numerical integral of the measured spectral absorption coefficients than the older procedure involving analog records of spectra obtained on chart paper. However, lack of spectral resolution as emphasized by Kagann et al 4 is hardly the cause for the errors in our measurements or those of other laboratories employing the Wilson-Wells, Penner-Weber technique. We have not found any measurable instrumental distortion of band contours, at the pressures employed by us, when these were observed under spectral resolutions that varied between 0.2 and 1.0 c m - ' . The rotational structure of the CFC bands is so dense that it was smeared out at the pressures used in our experiments, within the limits of the sensitivity o f the instruments employed. REFERENCES
I. 2. 3. 4. 5.
Atmospheric Ozone 1985, WMO-Report No. 16, p. 821 (1986). V. Ramanathan, R. J. Cicerone, H. B. Singh, and J. T. Kiehl, J. Geophys. Res. 90, 5547 (1985). P. Varanasi, and F. K. Ko, JQSRT 17, 385 (1977). R. H. Kagann, J. W. Elkins, and R. L. Sams, J. Geophys. Res. 88, 1427 (1985). S. S. Penner, Quantitative Molecular Spectroscopy and Gas Emissivities, p. 73, Addison-Wesley, Reading, MA (1959).
Remeasuring CFC absolute intensities
195
6. A. Goldman, F. S. Bonomo, and D. G. Murcray, Appl. Opt. 15, 2305 (1976). 7. L. A. Pugh, and K. N. Rao, Molecular Spectroscopy in Modern Research, Vol. 2, p. 165, Academic Press, NY (1976). 8. M. A. H. Smith, C. P. Rinsland, B. Fridovich, and K. N. Rao, Molecular Spectroscopy in Modern Research, Vol. 3, p. 111, Academic Press, NY (1985). 9. A. Goldman, F. S. Bonomo, and D. G. Murcray, Geophys. Res. Lett. 3, 309 (1976).
Q.S.R.T. 39/~-B