Infrared measurements of increased CF2CI2 (CFC- 12) absorptionabove the South Pole

Curtis P. Rinsland, Aaron Goldman, Frank J. Murcray, Frank H. Murcray, David G. Murcray, and Joel S.Levine

High-resolution ground-based solar spectra recorded at the Amundsen-Scott South Pole station in Dec. 1980and Nov. 1986 have been analyzed in the region of the CF2Cl 2 (chlorofluorocarbon 12) v8 band Q branches at1161 cm-'. An increase in the CF2 Cl2 total vertical column above the South Pole of 1.24 + 0.15 over the 6-yrperiod, corresponding to an average rate of increase of 3.6 + 2.1%, is derived. This rate of increase is lowerthan indicated by in situ measurements at the South Pole over the same time period, but there is agreementwhen the rather error bars of the spectral measurement results are considered. Spectroscopic parametersthat can successfully model CF2 C12 absorption at low temperatures are needed to improve retrieval accuraciesand could be applied to a number of pre-1980 atmospheric spectral data sets in the literature to obtain animproved record of early CF2 C12 concentration trends for comparison with estimates of historical releaserates.

1. Introduction

In Dec. 1980 and Nov.-Dec. 1986, numerous high-resolution IR solar spectra were recorded with a Mi-chelson-type Fourier transform spectrometer at theAmundsen-Scott South Pole station by F. J. M. and F.H. M. of the University of Denver Atmospheric Spec-troscopy Group. These spectra, which show a wealthof atmospheric and solar absorption features with aminimum of atmospheric water vapor interference,allow the simultaneous measurement of a large num-ber of atmospheric gases. In previous studies, infor-mation on the total column amounts and vertical dis-tributions of a number of minor and trace atmosphericgases (H2 0, C0 2 , 03, N20, CH 4, NO, NO2 , HC1, andHNO3) were presented.'- 3 In the present paper, wereport the quantitative analysis of absorption byCF2Cl2 (CFC-12), the most abundant atmosphericchlorofluorocarbon, in both the 1980 and 1986 datasets. Increased absorption by CFC-12 in 1986 com-pared to 1980 is clearly indicated by these measure-ments. These results are of interest in view of the roleincreasing levels of chlorofluorocarbons may play in

Curtis Rinsland and Joel Levine are with NASA Langley ResearchCenter, Atmospheric Sciences Division, Hampton, Virginia 23665-5225; the other authors are with University of Denver, PhysicsDepartment, Denver, Colorado 80208.

Received 1 July 1987.

the destruction of stratospheric ozone4-7 and globalclimate change7-'0 and in the formation of the australspring ozone hole over Antarctica. 11 1 2

I. Data and Analysis

The prominent CF2Cl2 Q-branch features in the921-924- and 1160-1162-cm-1 intervals are observablein both the 1980 and 1986 spectra. The features in thelower wave number region have been assigned to thevarious isotopes of the v6 band and their associated hotbands; the features in the higher wave number regionhave been assigned to the various isotopes of the v8band and their associated hot bands.13 The v6 band Qbranches are shown in an atlas that covers the 760-960-cm'1 region of the Dec. 1980 spectra.14 The nearlyconstant solar zenith angle of the South Pole observa-tions allowed the adding of several spectral scans oneach day, thereby increasing the signal-to-rms noise to-300 in both spectral regions.

Previous retrievals from the South Pole spectral-3

have been obtained using the technique of nonlinearleast-squares spectral curve fitting. Unfortunately,despite the considerable recent progress in derivingCF2C12 spectroscopic parameters for the 923-cm'1 re-gion'5-17 and utilizing these results for line-by-linesimulations,18 the quality of the resulting line parame-ters is not yet sufficient to use them for accurate spec-tral fitting atmospheric retrievals. Nearly 60,000 linesof CF2Cl2 have been used in the most recent attemptsto model this Q-branch region. Approximate absorp-tion cross sections determined from 0.02-cm-' resolu-tion room temperature laboratory spectra recorded at

1 February 1988 / Vol. 27, No. 3 / APPLIED OPTICS 627

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Fig. 1. Region of the CF2 CI2 Vs band Q branches in University of Denver South Pole spectra recorded at 0.02-cm- 1 resolution on 5 Dec. 1980(a) and 0.04-cm-1 resolution on 26 Nov. 1986 (b). The lower panels show the measured spectra (solid lines), normalized to the highestintensity in the analysis region, and the least-squares best-fit spectrum (crosses). The upper panels are plots of the residuals (measured minuscalculated). Arrows mark the CF2Cl2 v bandQ-branch absorption, which extends from '1160.7 to 1161.2 cm 1 . The solar zenith angles are

67.70 (a) and 68.90 (b).

the University of Denver' 9 are available' 9'2 0 for boththe 923- and 1161-cm'1 regions and are very useful forqualitative simulations, but they do not contain infor-mation on the temperature dependence, which is nec-essary for accurate atmospheric quantifications.Temperature dependence information has been re-ported from measurements of CF2Cl2 absorption from800 to 1200 cm-' at 296 and 200 K;21 however, thesespectra were recorded at too low a resolution (1.6 cm-')to be useful in analyzing the present data. More re-cently, 0.06-cm-' resolution measurements22 havebeen obtained at 297, 273, and 238 K, but these dataare not yet available for atmospheric studies.23 Forthe above-mentioned reasons, it was necessary toquantify the South Pole atmospheric CF2Cl2 measure-ments using a different approach.

The 1161-cm'1 Q branches are stronger than thosein the 923-cm'1 region, and, therefore, the higher wavenumber region has been used preferentially in theanalysis. Figure 1 shows least-squares fits to the1158.5-1162.0-cm-' region of South Pole solar spectrarecorded on 5 Dec. 1980 and 26 Nov. 1986. The fitswere obtained without CF2Cl2 lines in the calculations.Line parameters for the other gases absorbing in thisregion (primarily N20 and 03 with lesser contributionsfrom CH4 and H2 0 lines24'2 5 were adopted from the1986 HITRAN compilation 2 0 along with volume mix-ing ratio profiles retrieved from analysis of other re-gions of the same spectra. Pressures and tempera-tures for the analysis of the 1980 spectrum were takenfrom a radiosonde sounding from the South Pole onthe date of the spectral measurements (see Fig. 1 of

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Fig. 2. Residuals from Fig. 1 in the region of the CF2 C12 v8 band Qbranch.

Goldman et al.'). To our knowledge, there is no SouthPole sounding data for 26 Nov. 1986; thus we adoptedmeasurements obtained on the closest available date,25 Nov. 1986. Because of the strong CF2 CI2 absorp-tion between -1160.7 and 1161.2 cm-', not accountedfor in the calculations, the background levels of thecomputed spectra had to be adjusted manually tomatch the background levels of the measured spectra.The CF2Cl2 Q-branch absorption, indicated by arrowsin the residual plots of Fig. 1, is stronger in the 1986spectrum than in the 1980 spectrum. The increase inCF2 Cl2 absorption is shown clearly in Fig. 2, where theresiduals in the Q-branch region from both fits are

628 APPLIED OPTICS / Vol. 27, No. 3 / 1 February 1988

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plotted in a single frame. The equivalent widths of the1980 and 1986 CF2Cl2 absorptions are estimated fromthe residuals to be 0.0341 and 0.0447 cm-', respective-ly, with measurement uncertainties of 5% for bothspectra.

Because of the lack of temperature-dependentCF2Cl2 absorption coefficient data, the Q-branchequivalent width cannot be analyzed to infer reliabletotal vertical column amounts at this time. However,the radiosonde temperature profiles are similar (the1986 temperatures are lower by -5 K at the ground and5-13 K up to 80 mbar), and the absorption by CF2Cl2 inboth spectra is weak, corresponding to the linear partof the curve of growth. For these reasons, the ratio ofthe equivalent widths (after applying a 5% correctionfor the differences in the air masses of the two spectra)should be equal approximately to the ratio of the totalCF2Cl2 vertical columns on the two measurementdates. The measured equivalent width ratio indicatesthen an increase in the total CF2Cl2 vertical columnamount above the South Pole of 1.24 i 0.15 over the 6-yr period, corresponding to an average rate of increaseof 3.6 2.1%/yr. The error estimates include a 10%uncertainty in the column ratio to allow for the absenceof temperature corrections in deriving-the column ra-tio from the CF2Cl2 equivalent widths.

The present results are the first Antarctic measure-ments of CF2Cl2 obtained using remote sounding tech-niques. Gas chromatograph measurements indicateincreasing CF2Cl2 concentrations in the ground levelair at the South Pole26 27 and at Palmer Station, Ant-arctica.2 8 The rate of increase of CF2 Cl2 during 1980to 1986 reported in these studies is higher (-5-/ 2 /yr)than our measured rate of increase of the total CF2Cl2column over this time period, but there is agreementwhen the rather large error bars of the present resultsare considered.

III. Pre-1980 Spectroscopic Data Sets

It is interesting to note that there are a number ofpre-1980 high-resolution solar spectra that cover theCF2Cl2 Q-branch absorption regions. The Migeotte etal. 2 9 infrared solar atlas shows no evidence of absorp-tion by either the 923- or 1161-cm-' Q branches fromabove Jungfraujoch observatory (latitude 46.50 N,8.00 E, elevation 3.58 km) in 1950-51. A comparison ofthe 1161-cm-' Q-branch region in frame 64a of theJungfraujoch atlas with solar spectra recorded fromKitt Peak in 1981 at a similar air mass30 indicates thatCF2Cl2 absorption must have been at least 3 times lessintense in 1951 than 1981. The oldest published solarspectrum showing absorption features of CF2 Cl2 wasrecorded in Aug. 1968 with a balloon-borne gratingspectrometer and was used to estimate CF2Cl2 andCFCl3 volume mixing ratios in the lower strato-sphere.3' Ground-based solar spectra obtained in1972 at 0.08-cm-1 resolution3 2 show absorption byboth the 923- and 1161-cm-' CF2 Cl2 Q branches, as dosimilar resolution spectra recorded during the mid-1970s from the ground.2 4 33-35 Hanst et al.3 6 applied acryogenic procedure for concentrating trace gases from

ambient ground-level air and analyzed absorption fea-tues of CFC-12 and other trace gases detected in longpath IR spectra recorded in 1975. Since the trend ofCF2Cl2 concentration with time is quite uncertain be-fore 1979, given the scatter in the reported measure-ments (see Fig. 25 of Logan et al.

3 7), analysis of such ca.

1975 spectra with accurate CF2Cl2 line parametersmay prove to be an important way to obtain improvedestimates of early CF2Cl2 concentration trends forcomparison with estimates of historical releaserates.38 ' 39

Research at the University of Denver was supportedby the National Science Foundation (NSF) undergrant DPP-8118005 and by the National Aeronauticsand Space Administration. Acknowledgment is madeto the National Center for Atmospheric Research,which is supported by NSF, for computer time used inthis research. The authors thank John Lynch of thePolar Atmospheric Sciences Program of the NationalScience Foundation for sending the 1986 South Polesounding data. We also thank Joy Garnett, SusanEdwards, and Pamela Rinsland of NASA Langley forhelp in preparing this manuscript.

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G. Murcray, "Spectral Least Squares Quantification of SeveralAtmospheric Gases from High Resolution Infrared Solar Spec-tra Obtained at the South Pole," J. Quant. Spectrosc. Radiat.Transfer 29, 189 (1983).

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3. F. J. Murcray, F. H. Murcray, A. Goldman, D. G. Murcray, andC. P. Rinsland, "Infrared Measurements of Several NitrogenSpecies Above the South Pole in December 1980 and Novem-ber-December 1986," J. Geophys. Res. 92, 13,373 (1987).

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13. M. Morillon-Chapey, A. 0. Diallo, and J.-C. Deroche, "Isotopicand Vibrational Assignments of Fluorocarbon-12 in the 8.6-,umand 10.8-um Regions," J. Mol. Spectrosc. 88, 424 (1981).

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35C12 (Freon 12), Studied by Infrared-Microwave DoubleResonance," J. Mol. Spectrosc. 91, 87 (1982).

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18. A. Goldman and C. Deroche, "Line Parameters for F12 in the920 cm-' Region," U. Denver, Physics Department (July 1986).

19. S. T. Massie, A. Goldman, D. G. Murcray, and J. C. Gille,"Approximate Absorption Cross Sections of F12, F11, ClONO2 ,N 20 5 , HNO3 , CCl4 , CF4 , F21, F113, F114, and HNO 4 ," Appl.Opt. 24, 3426 (1985).

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Elizabeth Thorpe Davis of SUNY-Schurmacher Institute for VisionResearch, a participant in AMOSA 1987 in Rochester last October.

Photo: F. S. Harris, Jr.

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