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Temperature-dependent absorption cross-sections in the thermal infrared bands of SF5CF3
Journal of Quantitative Spectroscopy & Radiative Transfer 82 (2003) 483 – 490
www.elsevier.com/locate/jqsrt
Temperature-dependent absorption cross-sections in the thermal infrared bands of SF5CF3 Curtis P. Rinslanda;∗ , Steven W. Sharpeb , Robert L. Samsb a
b
NASA Langley Research Center, 21 Langley Boulevard, Mail Stop 401A, Hampton, VA 23681-2199, USA Paci)c Northwest National Laboratory (PNNL), P.O. Box 999, Mail Stop K8-88, Richland, WA 99352, USA Received 15 October 2002; received in revised form 11 November 2002; accepted 12 November 2002
Abstract Absorption cross-sections have been measured at 5ve temperatures between 213 and 323 K in the infrared bands of SF5 CF3 . The spectra were recorded at a resolution of 0:112 cm−1 using a commercial Fourier transform infrared spectrometer and a 20 cm temperature-controlled sample cell. Samples of SF5 CF3 were pressurized with high-purity nitrogen to a total pressure of 1013:3 hPa (760 Torr). Six or more spectra with varying SF5 CF3 column amounts were analyzed at each temperature. The full spectral range of the measurements was 520 –6500 cm−1 , with only weak bands observed beyond 1400 cm−1 . Absorption of thermal radiation in the 8–12 m atmospheric window region being important for climate change, we report here the integrated cross-sections of the signi5cant absorption bands in that spectral region. Our results closely match room temperature values reported previously. Only small variation of the integrated absorption cross-sections with temperature was found. Our results con5rm the accuracy of the previous measurements, which 5nd SF5 CF3 important for global climate change on a per molecule basis. Absorption cross-sections derived from a single, near Doppler-limited spectrum recorded at room temperature do not show any rotational 5ne structure in the 700 –950 cm−1 region. Published by Elsevier Ltd. Keywords: Global warming; Absorption cross-section; Infrared spectroscopy
1. Introduction Recently, the detection of SF5 CF3 (triAuoromethyl sulfur pentaAuoride, CAS: 373-80-8) in the atmosphere was reported based on its assignment as a previously unidenti5ed chromatographic peak close to the location of SF6 in stratospheric air samples [1]. Measurements of the depth pro5les of SF5 CF3 and SF6 from deep consolidated snow in eastern Antarctica showed the atmospheric growth ∗
Corresponding author. Tel.: +1-757-864-2699; fax: +1-757-867-7790. E-mail address: [email protected] (C.P. Rinsland).
0022-4073/03/$ - see front matter. Published by Elsevier Ltd. doi:10.1016/S0022-4073(03)00172-9
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C.P. Rinsland et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 82 (2003) 483 – 490
of both molecules to be remarkably similar with the emissions SF5 CF3 hypothesized as having begun in the late 1950s [1]. Analysis of air from stratospheric air samples collected during two balloon Aights resulted in an upper limit to the SF5 CF3 lifetime similar to that of SF6 (3200 years [2]). Integrated absorption cross-sections were derived for the bands in the 8–12 m atmospheric window region from room temperature infrared laboratory spectra recorded between pressures of 0.93 and 4:13 mPa [1]. A radiative forcing of 0:57 W m−2 ppb−1 was estimated, slightly higher than for SF6 [3]. The origin of atmospheric SF5 3CF3 is not entirely clear. It was not detected in samples of pure SF6 [1], nor was SF5 CF3 absorption by its strong band at 904 cm−1 observed during pollution events when SF6 tropospheric mixing ratios were elevated by up to 3 orders of magnitude [4]. As one of the dominant uses of SF6 is in gas-insulated switchgear, transformers, accelerators, and other high-voltage equipment, it was speculated that atmospheric SF5 CF3 originates as a breakdown product of SF6 in high-voltage equipment [1]. In contrast to SF6 , which has nearly all of its intensity in a single band at 947:9 cm−1 [5], the SF5 CF3 spectrum reported initially showed six bands with similar intensities in the 8–12 m atmospheric window [1]. The purpose of this paper is to present absorption cross-sections in the infrared absorption bands of SF5 CF3 as a function of temperature. The absorption cross-sections were determined from sets of laboratory spectra recorded at a spectral resolution of 0:112 cm−1 at 5ve temperatures between 213 and 323 K, covering the 520 –6500 cm−1 spectral region. Additionally, a single spectrum spanning 700 –950 cm−1 was recorded at 0:0016 cm−1 resolution with a Bruker 120HR Fourier transform spectrometer using a con5guration similar to that described previously [6]. All spectra were recorded at the Paci5c Northwest National Laboratories (PNNL) in Richland, WA, USA. This paper emphasizes the quantitative analysis of the measurements in the 8–12 m atmospheric window because of their importance in gauging the climate forcing of SF5 CF3 but we brieAy discuss and illustrate portions of the higher wavenumber (cm−1 ) measurements. Previous reported laboratory studies of SF5 CF3 cross-sections include Sturges et al. [1], Nielsen et al. [7], and Newnham et al. [8]. The measurements of Nielsen et al. [7] included the absorption feature at 612:5 cm−1 , which was not measured by Sturges et al. [1]. Nielsen et al. [7] estimate the inclusion of the additional feature increases the estimated SF5 CF3 global warming forcing by 3.5%. Prior to these recent quantitative studies [1,7] both Eggers et al. [9] and GriMths [10] reported infrared and Raman band assignments based on a C4v point group for SF5 CF3 . Theoretical studies estimating infrared intensities and band positions include the work of Ball and Zhenyu et al. (see [11,12]). 2. Experimental details All spectra were obtained using a sample of SF5 CF3 purchased from Oakland Products, West Columbus, SC, USA. (The use of a speci5c vendor, by name, is not intended as an endorsement of any products cited by the authors.) The vendor was unable to specify sample purity. Multiple freeze-thaw cycles at 193 K were used to reduce contaminants by removing any air, followed by several distillations at 223 K. Finally, the sample was placed over lithium hydroxide to scavenge signi5cant amounts of CO2 . Identi5ed contaminants in the puri5ed sample consisted of no more than 0.53% SF6 , 0.003% CO2 , 0.02% CS2 and less than 0.1% H2 O per unit volume as determined from the infrared spectrum. An unidenti5ed absorption peak appeared near 1720 cm−1 . All spectra were
C.P. Rinsland et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 82 (2003) 483 – 490
485
individually corrected for the identi5able contaminants by adjusting the partial pressure of SF5 CF3 based on previously measured absorption cross-sections of the contaminants. Individual samples of SF5 CF3 were introduced at known pressure into one of the two cells, which were identical in design but with slightly diOerent lengths (19.94 or 19:95 cm). The sample cell was then pressurized with ultrahigh purity nitrogen to a total pressure of 1013:3 hPa (760 Torr). Pressures were monitored via 0 –1:3333 hPa (0 –1 Torr), 0 –13:333 hPa (0 –10 Torr) or 0 –1333:3 hPa (0 –1000 Torr) MKS 690A Baratron type pressure transducers. The speci5ed accuracy of the transducers is 0.05% of the full-scale reading. The internally mounted sample cell was placed inside the vacuum bench of the spectrometers and used to record both the low- and high-resolution data. The sample cell was constructed of stainless steel (nominal length = 20 cm × 5 cm ID) that had been electro-polished and gold plated to minimize wall reactivity. In addition, the sample cell was actively temperature stabilized by circulating heated or cooled liquid through an axial jacket surrounding the cells. Experiments requiring sample temperatures between 278 and 323 K utilized a Julabo F25-MD temperature bath circulator with a propylene glycol and water solution. Experiments requiring low temperature samples (below 173 K) utilized a Neslab ULT-80 temperature bath circulator and ethanol. The sample cell was 5tted with either Viton or Auorosilicone O-rings and wedged KCl or KRS-5 windows depending on the operational temperature, with the later combination used for temperatures below 273 K. All spectra were obtained using the Bruker Instruments supplied OPUS software that involved zero-5lling of the co-averaged interferograms by a factor of 2 and transforming the interferograms (fast Fourier transform) using boxcar apodization and Mertz phase correction. The lower resolution spectra (0:112 cm−1 ) were further processed using software supplied by Bruker instruments to correct for non-linearity associated with the HgCdTe, liquid nitrogen-cooled, photoconductive detector system. The high-resolution spectrum (0:0016 cm−1 ) did not require a correction for detector non-linearity since infrared radiation levels on the detector were extremely low.
Fig. 1. Example of procedure used to 5t multiple data points to determine absorption cross-section. Slope of 5tted line is the cross-section value at a speci5c wavenumber value (cm−1 ). This 5t corresponds to a band maximum at 692 cm−1 .
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C.P. Rinsland et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 82 (2003) 483 – 490
The lower resolution spectra were recorded using a Bruker 66V Fourier-transform spectometer that had been modi5ed to minimize several systematic errors associated with commercial, single aperture Fourier transform infrared systems [13]. The single high-resolution spectrum was recorded using a Bruker Instruments 120HR Fourier transform spectrometer. The low-resolution absorption cross-sections (cm2 molecule−1 ) reported here were generated by 5tting 6 or more individual absorbance spectra, at each wavenumber (cm−1 ) using a weighted linear least-squares algorithm. The weights were derived by squaring the transmitance. When the transmission dropped below 0.0251, zero weight was assigned. This weighting scheme further reduced the non-linearity in the readings from the detector. The signal-to-noise was estimated to be 68,000 (root-mean-square) for the 5tted spectrum of SF5 CF3 at 298 K. That value was determined from a measurement near the peak SF5 CF3 signal at 1032:1 cm−1 relative to the noise determined from a region with few spectral features. See Fig. 1 for an example of the process used to determine absorption cross-section from the absorbance vs. the SF5 CF3 column.
3. Results and discussion We present in Fig. 2 the 5tted absorbance cross-section spectrum of SF5 CF3 at 298 K covering the 520 – 6500-cm−1 spectral region. The broad peaks of the strongest bands were located manually and are indicated in the inset panel of Fig. 2. The peak positions agree with those measured previously at 296 K [1]. It should be noted that many weaker bands were observed above 1400 cm−1 , but due to our signal-to-noise ratio no features were observed beyond 2600 cm−1 (see Fig. 3). The observed features and distribution of intensities presented here for the 8–12 m window closely match those presented by Sturges et al. [1] and Nielsen et al. [7]. We summarize in Table 1 the integrated absorption cross-sections (cm molecule−1 ) derived from our measurements at 5ve temperatures. Average values are listed in the last row with one-sigma
Fig. 2. Absorption cross-section for SF5 CF3 at 298 K from 520 to 6500 cm−1 . Inset panel is from same spectrum, but enlarged to emphasise the long-wave infrared spectral region. See text for details.
C.P. Rinsland et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 82 (2003) 483 – 490
487
Fig. 3. Absorption cross-section versus wavenumbers (cm−1 ) corresponding to 5tted SF5 CF3 spectrum. Note that many weaker features are observable up to 2600 cm−1 . See Eggers et al. [8] and GriMths [9] for tentative assignments.
Table 1 Integrated cross-section as a function of temperature, by spectral band Temperature (K)
520 –640 cm−1 (molecule−1 cm)
670 –780 cm−1 (molecule−1 cm)
840 –960 cm−1 (molecule−1 cm)
1125 –1325 cm−1 (molecule−1 cm)
213 243 278 298 323 Averagea
1:228 × 10−17 1:268 × 10−17 1:274 × 10−17 1:307 × 10−17 1:309 × 10−17 1:277(33) × 10−17
1:289 × 10−17 1:313 × 10−17 1:325 × 10−17 1:337 × 10−17 1:351 × 10−17 1:323(24) × 10−17
1:528 × 10−16 1:534 × 10−16 1:540 × 10−16 1:540 × 10−16 1:545 × 10−16 1:537(65) × 10−16
1:028 × 10−16 1:031 × 10−16 1:031 × 10−16 1:030 × 10−16 1:033 × 10−16 1:031(17) × 10−16
a
Average integrated cross-section based on the measurements at 5ve temperatures.
standard deviation in units of the last quoted digit. Table 2 presents our integrated cross-sections at 298 K and the values reported previously at 296 K [1,6]. The previous integrated cross-sections were estimated to have uncertainties of 5% [1,7]. Our measurements include the 612.5-cm−1 band which was absent in the study of Sturges et al. [1]. Our room temperature measurements indicate values of cross-sections that are all higher than the previously reported values of others with diOerences that average close to the quoted 5% uncertainty [1,7]. As indicated in Table 1 and observed in Fig. 4, the integrated cross-sections in each of the four regions in the 8–12 m window vary only slightly with temperature, between 213 and 323 K. Additionally, peak positions of the strongest features in this region change only slightly (¡ 1 cm−1 ) over the range of measured temperatures. It should be noted that no correction for sample emission has been applied, but it has been previously characterized at PNNL. Sample emission was found to contribute ∼ 1% of anomalous intensity in the longer wavelengths (10 m and longer) for the warmest samples (323 K).
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Table 2 Integrated cross-sections from this work and from previously reported studies Spectral band (cm−1 )
Integrated cross-sectiona (molecule−1 cm)
Integrated cross-section (molecule−1 cm)
520 – 640 670 –780 840 –960 1125 –1325 Total
1:277(33) × 10−17 1:323(24) × 10−17 1:537(65) × 10−16 1:031(17) × 10−16 2:828 × 10−16
1:23 × 10−17b 1:24(6) × 10−17c 1:45(7) × 10−16c 9:63(48) × 10−17c 2:66 × 10−17b;c
a
This work, average value of measurements at 5ve temperatures. Nielsen et al. [7]. c Sturges et al. [1]. b
Fig. 4. EOect of temperature on absorption cross-sections by spectral band. Note that the integrated cross-sections are conserved, while there are signi5cant changes in the band shape due to shifts in rotational populations.
The absorption cross-section at the 5ve temperatures derived from the 0:112 cm−1 resolution spectra will be made available for the full measured spectral range for inclusion in updates to HITRAN [14]. The single 0:0016 cm−1 resolution spectrum recorded at near Doppler-limited resolution in Fig. 5 does not show rotational 5ne structure in the 700 –950 cm−1 spectral region. Our results con5rm the reported accuracy of the cross-sections of the bands measured previously at 296 K [1,7], show only small changes in the integrated band intensities and peak positions with temperature in the 8–12 m window, and extend measured SF5 CF3 cross sections to 520 –6500 cm−1 .
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Fig. 5. High resolution (0:0016 cm−1 ), absorption cross-section spectrum corresponding to SF5 CF3 . Neat sample consisting of approximately 33 P a SF5 CF3 at 296 K, in a 20 cm cell.
Acknowledgements Work at NASA Langley Research Center was supported by the National Aeronautics and Space Administration through the Atmospheric Chemistry Modeling and Analysis Program and the Network for the Detection of Stratospheric Change. The laboratory measurements were supported by the US Department of Energy and performed and analyzed at the W.R. Wiley Environmental Molecular Sciences Laboratory, a national scienti5c user facility sponsored by the Department of Energy’s OMce of Biological and Environmental Research located at the Paci5c Northwest National Laboratory (PNNL). The Battelle Memorial Institute under contract DE-AC06-76RL-1830 operates the PNNL facility for the US Department of Energy. We thank V. Malathy Devi of the College of William and Mary, Williamsburg, VA, USA, for reviewing the manuscript prior to its submission. We also appreciate helpful comments from the two anonymous referees. References [1] Sturges WT, Wallington TJ, Hurley MD, Shine KP, Sihra K, Engel A, Oram DE, Penkett SA, Mulvancy R. A potent greenhouse gas identi5ed in the atmosphere: SF5 CF3 . Science 2000;289:611–3. [2] Ravishankara AR, Solomon S, Turnipeed RF, Warren RF. Atmospheric lifetimes of long-lived halogenated species. Science 1993;259:194–9. [3] Scienti5c assessment of ozone depletion, 1988. Global ozone research and monitoring project, report 44, World Meteorological Organization, Geneva, 1999. [4] Rinsland CP, Mahieu E, Zander R, Goldman A, Demoulin P, Chiou LS. SF6 ground-based infrared solar absorption measurements: long-term trend, pollution events, and a search for SF5 CF3 . JQSRT 2003;78:41–53. [5] Rinsland CP, Brown LR, Farmer CB. Infrared spectroscopic detection of sulfur hexaAuoride. J Geophys Res 1990;95:5577–85. [6] Malathy Devi V, Benner DC, Brown LR, Smith MAH, Rinsland CP, Sams RL, Sharpe SW. Multispectrum analysis of self- and N2 -broadening, shifting and line mixing coeMcients in the 6 band of 12 CH3 D. JQSRT 2002;72:139–91.
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[7] Nielsen OJ, Nicolaisen FM, Bacher C, Hurley MD, Wallington TJ, Shine KP. Infrared spectrum and global warming potential of SF5 CF3 . Atmos Environ 2002;36:1237–40. [8] Newnham DA. New cross-sections and line parameters from the RAL molecular spectroscopy facility. Seventh HITRAN database conference, session 3, spectroscopic parameters. Cambridge, MA USA: Harvard-Smithsonian Center for Astrophysics, 12–14 June 2002. [9] Eggers Jr. DF, Wright HE, Robinson DW. Infrared spectrum of SF5 CF3 . J Chem Phys 1961;35:1045–50. [10] GriMths JE. Infrared and raman spectra of ClSF5 and CF3 SF5 . Spectrochim Acta 1967;28A:2145–57. [11] Li Z, Yang JY, Hou JG, Zhu Q. Hybrid density-functional study of SF5 CF3 . Chem Phys Lett 2002;359:321–5. [12] Ball DW. Formation and vibrational spectrum of triAuormethyl sulfur pentaAuoride, CF3 SF3 , a new greenhouse gas. Gaussian-2 and Gaussian-3 calculations. J Mol Struct 2002;578:29–34. [13] Rothman LS, Barbe A, Benner DC, Brown LR, Camy-Peyret C, Carleer MR, Chance K, Clerbaux C, Dana V, Devi VM, Fayt A, Flaud J-M, Gamache RR, Goldman A, Jacquemart D, Jucks KW, LaOerty WJ, Mandin J-Y, Massie ST, Nemtchinov V, Newnham DA, Perrin A, Rinsland CP, Schroeder J, Smith MAH, Tang K, Toth RA, Vander Auwera J, Varanasi P, Yoshino K, The HITRAN molecular spectroscopic database: Edition of 2000 including updates of 2001, JQSRT, doi: 10.1016/S0022-4073(03)146-8. [14] Rothman LS, Rinsland CP, Goldman A, Massie ST, Edwards DP, Flaud J-M, Perrin A, Camy-Peyret C, Dana V, V Mandin J-Y, Schroeder J, McCann A, Gamache RR, Wattson RB, Yoshino K, Chance KV, Jucks KW, Brown LR, Nemtchinov V, Varanasi P. The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation), 1996 ed. JQSRT 1998;60:665 –710.
www.elsevier.com/locate/jqsrt
Temperature-dependent absorption cross-sections in the thermal infrared bands of SF5CF3 Curtis P. Rinslanda;∗ , Steven W. Sharpeb , Robert L. Samsb a
b
NASA Langley Research Center, 21 Langley Boulevard, Mail Stop 401A, Hampton, VA 23681-2199, USA Paci)c Northwest National Laboratory (PNNL), P.O. Box 999, Mail Stop K8-88, Richland, WA 99352, USA Received 15 October 2002; received in revised form 11 November 2002; accepted 12 November 2002
Abstract Absorption cross-sections have been measured at 5ve temperatures between 213 and 323 K in the infrared bands of SF5 CF3 . The spectra were recorded at a resolution of 0:112 cm−1 using a commercial Fourier transform infrared spectrometer and a 20 cm temperature-controlled sample cell. Samples of SF5 CF3 were pressurized with high-purity nitrogen to a total pressure of 1013:3 hPa (760 Torr). Six or more spectra with varying SF5 CF3 column amounts were analyzed at each temperature. The full spectral range of the measurements was 520 –6500 cm−1 , with only weak bands observed beyond 1400 cm−1 . Absorption of thermal radiation in the 8–12 m atmospheric window region being important for climate change, we report here the integrated cross-sections of the signi5cant absorption bands in that spectral region. Our results closely match room temperature values reported previously. Only small variation of the integrated absorption cross-sections with temperature was found. Our results con5rm the accuracy of the previous measurements, which 5nd SF5 CF3 important for global climate change on a per molecule basis. Absorption cross-sections derived from a single, near Doppler-limited spectrum recorded at room temperature do not show any rotational 5ne structure in the 700 –950 cm−1 region. Published by Elsevier Ltd. Keywords: Global warming; Absorption cross-section; Infrared spectroscopy
1. Introduction Recently, the detection of SF5 CF3 (triAuoromethyl sulfur pentaAuoride, CAS: 373-80-8) in the atmosphere was reported based on its assignment as a previously unidenti5ed chromatographic peak close to the location of SF6 in stratospheric air samples [1]. Measurements of the depth pro5les of SF5 CF3 and SF6 from deep consolidated snow in eastern Antarctica showed the atmospheric growth ∗
Corresponding author. Tel.: +1-757-864-2699; fax: +1-757-867-7790. E-mail address: [email protected] (C.P. Rinsland).
0022-4073/03/$ - see front matter. Published by Elsevier Ltd. doi:10.1016/S0022-4073(03)00172-9
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of both molecules to be remarkably similar with the emissions SF5 CF3 hypothesized as having begun in the late 1950s [1]. Analysis of air from stratospheric air samples collected during two balloon Aights resulted in an upper limit to the SF5 CF3 lifetime similar to that of SF6 (3200 years [2]). Integrated absorption cross-sections were derived for the bands in the 8–12 m atmospheric window region from room temperature infrared laboratory spectra recorded between pressures of 0.93 and 4:13 mPa [1]. A radiative forcing of 0:57 W m−2 ppb−1 was estimated, slightly higher than for SF6 [3]. The origin of atmospheric SF5 3CF3 is not entirely clear. It was not detected in samples of pure SF6 [1], nor was SF5 CF3 absorption by its strong band at 904 cm−1 observed during pollution events when SF6 tropospheric mixing ratios were elevated by up to 3 orders of magnitude [4]. As one of the dominant uses of SF6 is in gas-insulated switchgear, transformers, accelerators, and other high-voltage equipment, it was speculated that atmospheric SF5 CF3 originates as a breakdown product of SF6 in high-voltage equipment [1]. In contrast to SF6 , which has nearly all of its intensity in a single band at 947:9 cm−1 [5], the SF5 CF3 spectrum reported initially showed six bands with similar intensities in the 8–12 m atmospheric window [1]. The purpose of this paper is to present absorption cross-sections in the infrared absorption bands of SF5 CF3 as a function of temperature. The absorption cross-sections were determined from sets of laboratory spectra recorded at a spectral resolution of 0:112 cm−1 at 5ve temperatures between 213 and 323 K, covering the 520 –6500 cm−1 spectral region. Additionally, a single spectrum spanning 700 –950 cm−1 was recorded at 0:0016 cm−1 resolution with a Bruker 120HR Fourier transform spectrometer using a con5guration similar to that described previously [6]. All spectra were recorded at the Paci5c Northwest National Laboratories (PNNL) in Richland, WA, USA. This paper emphasizes the quantitative analysis of the measurements in the 8–12 m atmospheric window because of their importance in gauging the climate forcing of SF5 CF3 but we brieAy discuss and illustrate portions of the higher wavenumber (cm−1 ) measurements. Previous reported laboratory studies of SF5 CF3 cross-sections include Sturges et al. [1], Nielsen et al. [7], and Newnham et al. [8]. The measurements of Nielsen et al. [7] included the absorption feature at 612:5 cm−1 , which was not measured by Sturges et al. [1]. Nielsen et al. [7] estimate the inclusion of the additional feature increases the estimated SF5 CF3 global warming forcing by 3.5%. Prior to these recent quantitative studies [1,7] both Eggers et al. [9] and GriMths [10] reported infrared and Raman band assignments based on a C4v point group for SF5 CF3 . Theoretical studies estimating infrared intensities and band positions include the work of Ball and Zhenyu et al. (see [11,12]). 2. Experimental details All spectra were obtained using a sample of SF5 CF3 purchased from Oakland Products, West Columbus, SC, USA. (The use of a speci5c vendor, by name, is not intended as an endorsement of any products cited by the authors.) The vendor was unable to specify sample purity. Multiple freeze-thaw cycles at 193 K were used to reduce contaminants by removing any air, followed by several distillations at 223 K. Finally, the sample was placed over lithium hydroxide to scavenge signi5cant amounts of CO2 . Identi5ed contaminants in the puri5ed sample consisted of no more than 0.53% SF6 , 0.003% CO2 , 0.02% CS2 and less than 0.1% H2 O per unit volume as determined from the infrared spectrum. An unidenti5ed absorption peak appeared near 1720 cm−1 . All spectra were
C.P. Rinsland et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 82 (2003) 483 – 490
485
individually corrected for the identi5able contaminants by adjusting the partial pressure of SF5 CF3 based on previously measured absorption cross-sections of the contaminants. Individual samples of SF5 CF3 were introduced at known pressure into one of the two cells, which were identical in design but with slightly diOerent lengths (19.94 or 19:95 cm). The sample cell was then pressurized with ultrahigh purity nitrogen to a total pressure of 1013:3 hPa (760 Torr). Pressures were monitored via 0 –1:3333 hPa (0 –1 Torr), 0 –13:333 hPa (0 –10 Torr) or 0 –1333:3 hPa (0 –1000 Torr) MKS 690A Baratron type pressure transducers. The speci5ed accuracy of the transducers is 0.05% of the full-scale reading. The internally mounted sample cell was placed inside the vacuum bench of the spectrometers and used to record both the low- and high-resolution data. The sample cell was constructed of stainless steel (nominal length = 20 cm × 5 cm ID) that had been electro-polished and gold plated to minimize wall reactivity. In addition, the sample cell was actively temperature stabilized by circulating heated or cooled liquid through an axial jacket surrounding the cells. Experiments requiring sample temperatures between 278 and 323 K utilized a Julabo F25-MD temperature bath circulator with a propylene glycol and water solution. Experiments requiring low temperature samples (below 173 K) utilized a Neslab ULT-80 temperature bath circulator and ethanol. The sample cell was 5tted with either Viton or Auorosilicone O-rings and wedged KCl or KRS-5 windows depending on the operational temperature, with the later combination used for temperatures below 273 K. All spectra were obtained using the Bruker Instruments supplied OPUS software that involved zero-5lling of the co-averaged interferograms by a factor of 2 and transforming the interferograms (fast Fourier transform) using boxcar apodization and Mertz phase correction. The lower resolution spectra (0:112 cm−1 ) were further processed using software supplied by Bruker instruments to correct for non-linearity associated with the HgCdTe, liquid nitrogen-cooled, photoconductive detector system. The high-resolution spectrum (0:0016 cm−1 ) did not require a correction for detector non-linearity since infrared radiation levels on the detector were extremely low.
Fig. 1. Example of procedure used to 5t multiple data points to determine absorption cross-section. Slope of 5tted line is the cross-section value at a speci5c wavenumber value (cm−1 ). This 5t corresponds to a band maximum at 692 cm−1 .
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C.P. Rinsland et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 82 (2003) 483 – 490
The lower resolution spectra were recorded using a Bruker 66V Fourier-transform spectometer that had been modi5ed to minimize several systematic errors associated with commercial, single aperture Fourier transform infrared systems [13]. The single high-resolution spectrum was recorded using a Bruker Instruments 120HR Fourier transform spectrometer. The low-resolution absorption cross-sections (cm2 molecule−1 ) reported here were generated by 5tting 6 or more individual absorbance spectra, at each wavenumber (cm−1 ) using a weighted linear least-squares algorithm. The weights were derived by squaring the transmitance. When the transmission dropped below 0.0251, zero weight was assigned. This weighting scheme further reduced the non-linearity in the readings from the detector. The signal-to-noise was estimated to be 68,000 (root-mean-square) for the 5tted spectrum of SF5 CF3 at 298 K. That value was determined from a measurement near the peak SF5 CF3 signal at 1032:1 cm−1 relative to the noise determined from a region with few spectral features. See Fig. 1 for an example of the process used to determine absorption cross-section from the absorbance vs. the SF5 CF3 column.
3. Results and discussion We present in Fig. 2 the 5tted absorbance cross-section spectrum of SF5 CF3 at 298 K covering the 520 – 6500-cm−1 spectral region. The broad peaks of the strongest bands were located manually and are indicated in the inset panel of Fig. 2. The peak positions agree with those measured previously at 296 K [1]. It should be noted that many weaker bands were observed above 1400 cm−1 , but due to our signal-to-noise ratio no features were observed beyond 2600 cm−1 (see Fig. 3). The observed features and distribution of intensities presented here for the 8–12 m window closely match those presented by Sturges et al. [1] and Nielsen et al. [7]. We summarize in Table 1 the integrated absorption cross-sections (cm molecule−1 ) derived from our measurements at 5ve temperatures. Average values are listed in the last row with one-sigma
Fig. 2. Absorption cross-section for SF5 CF3 at 298 K from 520 to 6500 cm−1 . Inset panel is from same spectrum, but enlarged to emphasise the long-wave infrared spectral region. See text for details.
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487
Fig. 3. Absorption cross-section versus wavenumbers (cm−1 ) corresponding to 5tted SF5 CF3 spectrum. Note that many weaker features are observable up to 2600 cm−1 . See Eggers et al. [8] and GriMths [9] for tentative assignments.
Table 1 Integrated cross-section as a function of temperature, by spectral band Temperature (K)
520 –640 cm−1 (molecule−1 cm)
670 –780 cm−1 (molecule−1 cm)
840 –960 cm−1 (molecule−1 cm)
1125 –1325 cm−1 (molecule−1 cm)
213 243 278 298 323 Averagea
1:228 × 10−17 1:268 × 10−17 1:274 × 10−17 1:307 × 10−17 1:309 × 10−17 1:277(33) × 10−17
1:289 × 10−17 1:313 × 10−17 1:325 × 10−17 1:337 × 10−17 1:351 × 10−17 1:323(24) × 10−17
1:528 × 10−16 1:534 × 10−16 1:540 × 10−16 1:540 × 10−16 1:545 × 10−16 1:537(65) × 10−16
1:028 × 10−16 1:031 × 10−16 1:031 × 10−16 1:030 × 10−16 1:033 × 10−16 1:031(17) × 10−16
a
Average integrated cross-section based on the measurements at 5ve temperatures.
standard deviation in units of the last quoted digit. Table 2 presents our integrated cross-sections at 298 K and the values reported previously at 296 K [1,6]. The previous integrated cross-sections were estimated to have uncertainties of 5% [1,7]. Our measurements include the 612.5-cm−1 band which was absent in the study of Sturges et al. [1]. Our room temperature measurements indicate values of cross-sections that are all higher than the previously reported values of others with diOerences that average close to the quoted 5% uncertainty [1,7]. As indicated in Table 1 and observed in Fig. 4, the integrated cross-sections in each of the four regions in the 8–12 m window vary only slightly with temperature, between 213 and 323 K. Additionally, peak positions of the strongest features in this region change only slightly (¡ 1 cm−1 ) over the range of measured temperatures. It should be noted that no correction for sample emission has been applied, but it has been previously characterized at PNNL. Sample emission was found to contribute ∼ 1% of anomalous intensity in the longer wavelengths (10 m and longer) for the warmest samples (323 K).
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Table 2 Integrated cross-sections from this work and from previously reported studies Spectral band (cm−1 )
Integrated cross-sectiona (molecule−1 cm)
Integrated cross-section (molecule−1 cm)
520 – 640 670 –780 840 –960 1125 –1325 Total
1:277(33) × 10−17 1:323(24) × 10−17 1:537(65) × 10−16 1:031(17) × 10−16 2:828 × 10−16
1:23 × 10−17b 1:24(6) × 10−17c 1:45(7) × 10−16c 9:63(48) × 10−17c 2:66 × 10−17b;c
a
This work, average value of measurements at 5ve temperatures. Nielsen et al. [7]. c Sturges et al. [1]. b
Fig. 4. EOect of temperature on absorption cross-sections by spectral band. Note that the integrated cross-sections are conserved, while there are signi5cant changes in the band shape due to shifts in rotational populations.
The absorption cross-section at the 5ve temperatures derived from the 0:112 cm−1 resolution spectra will be made available for the full measured spectral range for inclusion in updates to HITRAN [14]. The single 0:0016 cm−1 resolution spectrum recorded at near Doppler-limited resolution in Fig. 5 does not show rotational 5ne structure in the 700 –950 cm−1 spectral region. Our results con5rm the reported accuracy of the cross-sections of the bands measured previously at 296 K [1,7], show only small changes in the integrated band intensities and peak positions with temperature in the 8–12 m window, and extend measured SF5 CF3 cross sections to 520 –6500 cm−1 .
C.P. Rinsland et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 82 (2003) 483 – 490
489
Fig. 5. High resolution (0:0016 cm−1 ), absorption cross-section spectrum corresponding to SF5 CF3 . Neat sample consisting of approximately 33 P a SF5 CF3 at 296 K, in a 20 cm cell.
Acknowledgements Work at NASA Langley Research Center was supported by the National Aeronautics and Space Administration through the Atmospheric Chemistry Modeling and Analysis Program and the Network for the Detection of Stratospheric Change. The laboratory measurements were supported by the US Department of Energy and performed and analyzed at the W.R. Wiley Environmental Molecular Sciences Laboratory, a national scienti5c user facility sponsored by the Department of Energy’s OMce of Biological and Environmental Research located at the Paci5c Northwest National Laboratory (PNNL). The Battelle Memorial Institute under contract DE-AC06-76RL-1830 operates the PNNL facility for the US Department of Energy. We thank V. Malathy Devi of the College of William and Mary, Williamsburg, VA, USA, for reviewing the manuscript prior to its submission. We also appreciate helpful comments from the two anonymous referees. References [1] Sturges WT, Wallington TJ, Hurley MD, Shine KP, Sihra K, Engel A, Oram DE, Penkett SA, Mulvancy R. A potent greenhouse gas identi5ed in the atmosphere: SF5 CF3 . Science 2000;289:611–3. [2] Ravishankara AR, Solomon S, Turnipeed RF, Warren RF. Atmospheric lifetimes of long-lived halogenated species. Science 1993;259:194–9. [3] Scienti5c assessment of ozone depletion, 1988. Global ozone research and monitoring project, report 44, World Meteorological Organization, Geneva, 1999. [4] Rinsland CP, Mahieu E, Zander R, Goldman A, Demoulin P, Chiou LS. SF6 ground-based infrared solar absorption measurements: long-term trend, pollution events, and a search for SF5 CF3 . JQSRT 2003;78:41–53. [5] Rinsland CP, Brown LR, Farmer CB. Infrared spectroscopic detection of sulfur hexaAuoride. J Geophys Res 1990;95:5577–85. [6] Malathy Devi V, Benner DC, Brown LR, Smith MAH, Rinsland CP, Sams RL, Sharpe SW. Multispectrum analysis of self- and N2 -broadening, shifting and line mixing coeMcients in the 6 band of 12 CH3 D. JQSRT 2002;72:139–91.
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