- Email: [email protected]
Quantitative comparisons of absorption cross-section spectra and integrated intensities of HFC-143a
Journal of Quantitative Spectroscopy & Radiative Transfer 151 (2015) 13–17
Contents lists available at ScienceDirect
Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt
Quantitative comparisons of absorption cross-section spectra and integrated intensities of HFC-143a Karine Le Bris n, Laura Graham Department of Physics, St. Francis Xavier University, P.O. Box 5000, Antigonish, NS, Canada B2G 2W5
a r t i c l e i n f o
abstract
Article history: Received 27 June 2014 Received in revised form 1 September 2014 Accepted 5 September 2014 Available online 17 September 2014
The integrated absorption cross-sections of HFC-143a (CH3CF3) differ substantially in the literature. This leads to an important uncertainty on the value of the radiative efficiency of this molecule. The ambiguity on the absorption cross-sections of HFC-143a is highlighted by the existence of two significantly different datasets in the HITRAN database. To solve the issue, we performed high-resolution Fourier transform infrared laboratory measurements of HFC-13a and compared the spectra with the two HITRAN datasets and with the data from the Pacific Northwest National Laboratory (PNNL). The experimental methods and data analysis techniques are examined and typical sources of errors are discussed. The integrated intensities of the main bands are compared to other literature values. It was found that the integrated absorption cross-section values in the highest range – around 1 in the 570–1500 cm 1 spectral band – show the most con13:8 10 17 cm:molecule sistency between authors. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Hydrofluorocarbon HFC-143a Cross-section Temperature-dependence Integrated band intensity Laboratory measurements
1. Introduction HFC-143a (1,1,1-trifluoroethane) is used as a substitute for CFC-12, HCFC-22 and R-502 in refrigeration. Because this molecule is not an ozone-depleting substance, its emission is not controlled by the Montreal protocol and its amendments. However, its atmospheric lifetime of 47.1 year [1] coupled with its strong absorbance in the midinfrared spectral region makes HFC-143a a substantial greenhouse gas with a radiative forcing of 1.9 mWm 2 in 2011 [2]. This molecule is currently the third most abundant HFC in the atmosphere with a global average of 12.04 ppt in 2011 increasing by 6.39 ppt since 2005 [2]. Previous studies have given HFC-143a a radiative efficiency ranging from 0.13 to 0.22 Wm 2ppb 1 [3]. The Fifth Assessment Report (AR5) of the IPCC has chosen a mean value of 0.159 Wm 2ppb 1. The uncertainty on the radiative efficiency results from the inconsistency between
n
Corresponding author. Tel.: þ1 902 867 2392; fax: þ1 902 867 2414. E-mail address: [email protected] (K. Le Bris).
http://dx.doi.org/10.1016/j.jqsrt.2014.09.005 0022-4073/& 2014 Elsevier Ltd. All rights reserved.
the integrated absorption cross-section spectra available in the literature. The HITRAN 2012 database [4] contains, at this time, two sets of cross-section spectra for HFC-143a – one from Smith et al. [5] in the main directory and the other one from Di Lonardo and Masciarelli [6] in the supplemental directory. The integrated band intensities vary significantly between the two sets. The discrepancy is particularly obvious near the Q-branch of the 1281.2 cm 1 band, an important region for atmospheric remote sensing. To resolve these disparities, we recorded high-resolution absorption cross-section spectra of a pure vapour of HFC-143a at the University of Toronto and compared our data with the HITRAN datasets and with the broadened dataset from the Pacific Northwest National Laboratory (PNNL) [7]. 2. Dataset summary Smith et al. [5] presented pure vapour infrared crosssections at six different temperatures (203, 213, 233, 253, 273, and 297 K) and 37.5-, 150-, and 750.1-Torr air-broadened cross-section spectra at 297 K. The data are separated in
14
K. Le Bris, L. Graham / Journal of Quantitative Spectroscopy & Radiative Transfer 151 (2015) 13–17
three bands (580.00–630.00, 749.99–1049.90, and 1099.90– 1499.90 cm 1) in the HITRAN database. Di Lonardo and Masciarelli [6] dataset contains pure vapour, 150-Torr and 600-Torr air-broadened spectra at six different temperatures (203, 213, 233, 253, 273, and 293 K). The spectral acquisition was divided into three regions 790–870 cm 1, 920–1030 cm 1 and 1120–1470 cm 1. Each region was recorded at different pressures to account for the diversity of band strengths. However, the data appears in the HITRAN database as a single continuous spectral band from 694.249 to 1504.192 cm 1. The division lines between the spectral regions, apparent by a change in the signal-to-noise ratio, are visible at 885 cm 1 and 1050 cm 1. The PNNL dataset [7] is composed of N2-broadened spectra for a total pressure of 760 Torr at three different temperatures (277, 298 and 323 K) over the 600–6500 cm 1 spectral range. Our data for a pure vapour were recorded at 263, 273 and 283 K between 550 and 3500 cm 1. The experimental setup was described in a previous article [8]. A survey of the molecule spectrum is presented in Fig. 1. HCFC-143a has five separated main bands in the 570-1500 cm 1 spectral region. The assignment of the bands was done using Nivellini et al. [9] results. The bands corresponding to the harmonic frequencies ν9 and ν3 are overlapping and are therefore analysed as a single band in the 1100–1350 cm 1 range. The same applies to the band corresponding to the harmonic frequency ν2 and the combination ν4 þ ν5 in the 1350–1480 cm 1 range. Because of our larger spectral range, we were also able to observe a very weak band, centered around 2975 cm 1, corresponding to the ν1 vibrational frequency and another band, centered around 3034 cm 1, corresponding to the ν7 vibrational frequency. 3. Experimental setup and method All the datasets had been acquired using a Fourier transform spectrometer with a globar source, a KBr beamsplitter and a liquid-nitrogen cooled mercury–cadmium– telluride (MCT) detector. Pressures were read through a Baratron gauge. Table 1 summarizes the main characteristics for each dataset. Acquisitions were performed using samples of stated purity equal or better than 99%. Samples were purified by several freeze-pump-thaw cycles and injected inside the
cell either as pure vapours or were previously mixed with dry air or nitrogen. The advantage of air- or N2-broadened vapour spectra is that they give spectral shapes similar to those found in atmospheric measurements. Pure vapours offer a better precision on pressure, and thus a better integrated intensity accuracy. The cross-section spectra, however, need to be processed through a pseudo-line method [10] before being used for atmospheric retrieval. Resolution plays an important role in resolving the fine structures in a pure vapour. Time is often the main constraint because higher resolutions require more co-added scans to keep a suitable signal-to-noise ratio. To determine if there are no resolution-induced spectral effects, our pure vapour spectral acquisitions have been performed at a resolution slightly higher than the 0.03 cm 1 used by references [5,6]. Mixed vapours have broader spectral bands and therefore a lower resolution can be used without losing information. Molecules containing C–F bonds have high absorption strengths in the mid-IR. For pure vapours, the values of the cross-section in the Q-branches are often in the 10 18 to 10 17 cm2 molecule 1 order of magnitude. A relatively short cell length is therefore suitable to prevent nonlinearity on the strongest bands while allowing pressures large enough for good precision readout. This point is particularly important for pure vapour spectral acquisitions where the cross-sections in the Q-branches can dramatically increase as the temperature drops. The accuracy of the cross-section depends strongly on the precision of the full-scale and zero photometric values. Variations of the source intensity during acquisitions are one of the most common sources of errors. The simplest way to improve accuracy is to take baseline spectra before and after each sample acquisition. To further reduce the full-scale error on our data, we also adjusted, by a polynomial regression, the baseline to several zero-absorption spectral bands on the sample spectrum. Imprecision on the zero of the photodetector is often responsible for non-linear effects on the strong bands. In our experiment, the zero was obtained by subtracting the residual baseline offset observed outside the limit of the detector and on saturated absorption bands. All groups used a second aperture at the beam focal point to prevent most of the secondary mid-IR radiation from reaching the detector. Errors on the 100% and 0% line location had been accounted for in the error calculation of Di Lonardo and Masciarelli [6]. Smith et al. [5] recorded HFC-143a spectra
Fig. 1. Survey spectrum of a pure vapour of HFC-143a at 283 K in the mid-IR at a resolution of 0.02 cm 1.
K. Le Bris, L. Graham / Journal of Quantitative Spectroscopy & Radiative Transfer 151 (2015) 13–17
15
Table 1 Main characteristics of the laboratory dataset of Smith et al. [5], Di Lonardo and Masciarelli [6], the PNNL [7] and our data. Experimental setup
Ref. [5]
Ref. [6]
PNNL
This work
Sample Range (cm 1) Resolution (cm 1) Type Cell length (cm) Spectrometer Purity Average error
Pure and air-broad. at 297 K 560–1900 0.03 Averaged 5.12 Bruker IFS 120HR 99.3% 5%
Pure and air-broad. 694.2–1504.2 0.03 Averaged 10.50 Bomem DA8 99% 2.68%
N2-broad. 500–6500 0.112 Composite 19.96 Bruker IFS 66v/S 99% o 3% ð2sÞ
Pure 550–3500 0.02 Composite 2.93 Bomem DA8 99% o 5% ð2sÞ
Table 2 Wavenumber shifts of Smith et al. [5] and Di Lonardo and Masciarelli [6] spectra relatively to our experimental data for pure vapours and the PNNL data for broadened vapours. Reference
Band (cm 1)
Red shift (cm 1) Pure vapour
Broadened vapour
[5]
580.00–630.00 749.99–1049.90 1099.90–1499.90
0.01 0.03 0.09
0.00 0.07 0.09
[6]
694.25–885.00 885.00–1050.00 1050.00–1504.19
0.02 0.02 0.03
0.02 0.02 0.04
with the globar on and off to remove secondary source artifacts. 4. Data processing The cross-sections from Smith et al. [5] and Di Lonardo and Masciarelli [6] come from averaged measurements. Series of co-added scans are taken in a closely related range of pressures, which are all chosen to give a good signal-tonoise ratio without obvious saturation. The final crosssection at each wavenumber is obtained by averaging the data. While this technique is efficient for small spectral windows where absorption bands can be of the same intensity range, it becomes troublesome for spectral windows where weak and strong absorption bands co-exist. Small spectral features in low-pressure acquisitions can be buried in the background noise while high-pressure acquisitions often trigger non-linear effects on the strongest bands. An alternative technique consists in acquiring several spectra in a large range of pressures for each given temperature. The optical depth χ is then calculated as a function of the pressure for each wavenumber ν [7]. To prevent nonlinear effects in an optically thick medium while keeping a good signal-to-noise ratio at every wavenumber, the data corresponding to optically thick χ ðνÞ 4 χ max or optically thin χ ðνÞ o χ min conditions are eliminated. χmax and χmin values are empirically determined. A linear behavior is obtained for strong absorption bands from the lowpressure measurements while weak absorption features are represented by the high-pressure measurements. For broadened vapours, a linear fit of the optical depth versus the pressure gives the cross-section at each wavenumber [7]. When a pure vapour is used, the spectral lines
Fig. 2. Wavenumber bias for (top) a pure vapour at 273 K and (bottom) a broadened vapour at 297 K (293 K for [6]). The sharpest lines from our work come from the higher resolution and the removal of self-broadening effects. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
are sharper and self-broadening effects may be visible. In this case, the optical depth does not increase linearly with the pressure. However, it was observed [11] that the crosssection can still be retrieved through a quadratic regression. The resulting composite spectra are at a theoretical zeroTorr limit removed from self-broadening effects. Those spectra present good signal-to-noise ratios everywhere and no saturation effects. The inconvenience of this technique is obviously the time of acquisitions. All our data as well as the PNNL dataset are composite cross-section spectra. 5. Dataset comparison 5.1. Wavenumber shift The wavenumber accuracy of our instrument had been tested prior to the acquisitions by comparing several lines of CO2 at high resolution with the HITRAN database. We therefore used our data as reference for pure vapour spectra. For broadened vapours, the reference was the PNNL database wavenumbers. The results are reported in Table 2. Red shifts appear in both [5] and [6] with Smith
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et al. (1998) data having the largest relative difference (see Fig. 2). Because the biases remain constant within each experimental spectral window, they may be due to rounding errors in the HITRAN files. In the rest of the article, [5,6] data are handled with their corrected wavenumbers. 5.2. Integrated intensity The ν5 band is at the limit of the MCT detector and the ν4 band is of very low intensity. Therefore, uncertainties due to noise do not allow a fair comparison between the datasets in this spectral range. The next three bands are of higher intensities. The comparison of their integrated intensities is presented in Fig. 3. The maximum deviation of the integrated intensities of Smith et al. [5] data with temperature are always well within their error estimate. This is not the case for the values of Di Lonardo and Masciarelli [6], which present variations between 8 and 10% as the temperature decreases. Ideally, integrated band intensities should be independent of resolution and pressure broadening effects. A comparison of the integrated intensities of Smith et al. [5]
Fig. 4. Absorption cross-section spectra of Smith et al. [5] and Di Lonardo and Masciarelli [6] around the ν3 Q-branch. Non-linearity behaviours are visible on the 600-Torr broadened vapour spectrum at 253 K and all Ref. [6] spectra at 203 K. The artificially enlarged Q-branches of Di Lonardo and Masciarelli [6] spectra explain partially the raise of their integrated crosssections observed in Fig. 3.
shows that their values also remain consistent with our higher resolution composite data and the lower resolution N2-broadened composite data of the PNNL. The integrated intensities of Di Lonardo and Masciarelli [6], on the other hand, are systematically lower than the other groups with variations as large as 15.5% for the 920–1020 cm 1 and 1100–1350 cm 1 bands. Only in a few occurrences in the 1100–1350 cm 1 band do the reference [6] integrated intensities reach the values of the other authors. However, a close look shows that this apparent similarity is only due to important non-linear effects on the ν3 Q-branch of Di Lonardo and Masciarelli [6] cross-sections (see Fig. 4). Non-linear shapes appear at 203 K and 233 K for a pure vapour, 203 K, 213 K, and 273 K for a 150-Torr broadened vapour and 203, 253 and 273 K for a 600-Torr broadened vapour. Table 3 presents the averaged band integrated intensities of the four mentioned datasets. They are compared with the literature values of Sihra et al. [12] at 296 K, Highwood and Shine [13] at 253 K, Pinnock et al. [14] at 296 K, Oliff and Fisher [15] and Gehring [16] at “room temperature”. The total integrated intensity values in the highest range (this work, references [5,7,16]) are very consistent with each other with a maximum percentage difference of less than 2%. The values of the total integrated intensities in the lowest range, however, spread between 12.01 and 1 13:09 10 17 cm:molecule . 6. Conclusion
Fig. 3. Comparison of the integrated intensities of the three strongest bands of HFC-143a as a function of temperature. For clarity, only Smith et al. [5] pure vapour and Di Lonardo and Masciarelli [6] uncertainty ranges have been inserted.
We presented three high-resolution composite crosssection spectra of a pure vapour of HFC-143a in the 263–283 K temperature range. The two datasets in the HITRAN database have been evaluated in light of those new spectra and the PNNL data. The integrated absorption
K. Le Bris, L. Graham / Journal of Quantitative Spectroscopy & Radiative Transfer 151 (2015) 13–17
17
Table 3 Comparison of the integrated intensities of HFC-143a bands. Bold text indicates a value averaged over the available spectra. Reference
This work PNNL Sihra et al. Di Lonardo and Masciarelli Smith et al. Highwood and Shine Pinnock et al. Oliff and Fisher Gehring
Integrated cross section ( 10 17 cm.molecule 1) ν5
ν4
ν10
ν9 , ν3
ν2, ν4 þ ν5
Totala
0.39 0.37
0.06 0.08
1.69 1.67
10.11 9.91
1.60 1.55
0.37
0.07 0.09
1.48 1.69
9.12c 10.1
1.52 1.62
0.31 0.35
0.08 0.16
1.65 1.75
9.38 9.90
1.43 1.59
13.85 13.58 12.71 (13.07)b 12.19 (12.55)b 13.87 12.01 12.30 (12.66)b 12.84 13.72
a The two weak bands in the 3000 cm 1 area observed by the PNNL and by us, have not been included in the total integrated intensity. For information, 1 1 for the ν1 bands and around 1:12 10 18 cm:molecule for the ν7 bands we found an integrated intensity lower than 3 10 20 cm:molecule 1 (2:80 10 20 and 1:18 10 18 cm:molecule respectively for the PNNL database). 1 b The values in parenthesis represent the total integrated cross-section with the addition of an average value of 0:36 10 17 cm:molecule for the missing ν5 band. c The bands with nonlinear effects have been removed from the calculation.
cross-sections of the datasets have also been compared to other available literature values. Our data are very consistent with the values of Smith et al. [5] and the PNNL database. For lower temperatures, the pure vapour spectra of Smith et al. [5] remain the first choice until a more complete dataset becomes available. The red shifts should be corrected prior use. Di Lonardo and Masciarelli data [6] include broadened vapour crosssection spectra in a range of temperature down to 203 K. However, because of the lower values of the integrated intensities relative to recent other sources and the presence of strong non-linearity effects on the ν3 Q-branch at 1281.2 cm 1, the data should be used with caution. It also appears from this study that the radiative efficiency of HFC-143a should be revised upward due to the higher value of the total integrated cross-sections. Acknowledgements This work was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). We thank Prof. Kimberly Strong for the use of the Bomem DA8 Fourier transform spectrometer at the University of Toronto. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j. jqsrt.2014.09.005.
References [1] Montzka SA, Reimann S, Engel A, Krueger K, ODoherty S, Sturges WT, et al. Ozone-Depleting Substances (ODSs) and related chemicals. Chapter 1 in scientific assessment of ozone depletion: 2010, Global Ozone Research and Monitoring Project-Report No. 52, World Meteorological Organization, Geneva, Switzerland; 2011. p. 516.
[2] IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Stocker TF, Qin D, Plattner G-K, Tignor M, Allen SK, Boschung J, et al. editors. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA; 2013. p. 1535. ohttp://dx.doi.org/10.1017/CBO97811074153244. [3] Hodnebrog Ø, Etminan M, Fuglestvedt JS, Marston G, Myhre G, Nielsen CJ, et al. Global warming potentials and radiative efficiencies of halocarbons and related compounds: a comprehensive review. Rev. Geophys. 2013;51:300–78. [4] Rothman LS, Gordon IE, Babikov Y, Barbe A, Benner DC, Bernath PF, et al. The HITRAN 2012 molecular spectroscopic database. J Quant Spectrosc Radiat Transf 2013;130:4–50. [5] Smith K, Newnham D, Page M, Ballard J, Duxbury G. Infrared absorption cross-sections and integrated absorption intensities of HFC-134 and HFC- 143a vapour. J Quant Spectrosc Radiat Transf 1998;59:43751. [6] Di Lonardo G, Masciarelli G. Infrared absorption cross-sections and integrated absorption intensities of HFC-125 and HFC-143a. J Quant Spectrosc Radiat Transf 2000;66:12942. [7] Sharpe SW, Johnson TJ, Sams RL, Chu PM, Rhoderick GC, Johnson PA. Gas-phase databases for quantitative infrared spectroscopy. Appl Opt 2004;58:1452–61. [8] Le Bris K, Strong K. Temperature-dependent absorption crosssections of HCFC-142b. J Quant Spectrosc Radiat Transf 2010;111: 364–71. [9] Nivellini G, Tullini F, Celli A, Becucci M. Vibrational spectrum of 1,1,1triuoroethane. J Chem Soc Faraday Trans 1998;94:2909–12. [10] Toon GC, Sen B, Kleinboehl A. Derivation of pseudo-lines from laboratory cross-sections. Jet Propulsion Laboratory n.d. Web 01 August 2014 URL: 〈http://mark4sun.jpl.nasa.gov/data/spec/Pseudo/ Readme〉. [11] Le Bris K, McDowell J, Strong K. Measurements of the infrared absorption cross-sections of HCFC-141b (CH3CFCl2). J Quant Spectrosc Radiat Transf 2012;113:1913–9. [12] Sihra K, Hurley MD, Shine KP, Wallington TJ. Updated radiative forcing estimates of 65 halocarbons and nonmethane hydrocarbons. J Geophys Res 2001;106:20493–505. [13] Highwood EJ, Shine KP. Radiative forcing and global warming potentials of 11 halogenated compounds. J Quant Spectrosc Radiat Transf 2000;66:169–83. [14] Pinnock S, Hurley MD, Shine KP, Wallington TJ, Smyth TJ. Radiative forcing of climate by hydrochlorofluorocarbons and hydrofluorocarbons. J Geophys Res 1995;100:23227–38. [15] Olliff MP, Fischer G. Integrated absorption intensities of haloethanes and halopropanes. Spectrochim Acta Part A—Mol Biomol Spectrosc 1994;50:2223–37. [16] Gehring DG. Personal communication reported in Nature 1990;344: 513–516.
Contents lists available at ScienceDirect
Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt
Quantitative comparisons of absorption cross-section spectra and integrated intensities of HFC-143a Karine Le Bris n, Laura Graham Department of Physics, St. Francis Xavier University, P.O. Box 5000, Antigonish, NS, Canada B2G 2W5
a r t i c l e i n f o
abstract
Article history: Received 27 June 2014 Received in revised form 1 September 2014 Accepted 5 September 2014 Available online 17 September 2014
The integrated absorption cross-sections of HFC-143a (CH3CF3) differ substantially in the literature. This leads to an important uncertainty on the value of the radiative efficiency of this molecule. The ambiguity on the absorption cross-sections of HFC-143a is highlighted by the existence of two significantly different datasets in the HITRAN database. To solve the issue, we performed high-resolution Fourier transform infrared laboratory measurements of HFC-13a and compared the spectra with the two HITRAN datasets and with the data from the Pacific Northwest National Laboratory (PNNL). The experimental methods and data analysis techniques are examined and typical sources of errors are discussed. The integrated intensities of the main bands are compared to other literature values. It was found that the integrated absorption cross-section values in the highest range – around 1 in the 570–1500 cm 1 spectral band – show the most con13:8 10 17 cm:molecule sistency between authors. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Hydrofluorocarbon HFC-143a Cross-section Temperature-dependence Integrated band intensity Laboratory measurements
1. Introduction HFC-143a (1,1,1-trifluoroethane) is used as a substitute for CFC-12, HCFC-22 and R-502 in refrigeration. Because this molecule is not an ozone-depleting substance, its emission is not controlled by the Montreal protocol and its amendments. However, its atmospheric lifetime of 47.1 year [1] coupled with its strong absorbance in the midinfrared spectral region makes HFC-143a a substantial greenhouse gas with a radiative forcing of 1.9 mWm 2 in 2011 [2]. This molecule is currently the third most abundant HFC in the atmosphere with a global average of 12.04 ppt in 2011 increasing by 6.39 ppt since 2005 [2]. Previous studies have given HFC-143a a radiative efficiency ranging from 0.13 to 0.22 Wm 2ppb 1 [3]. The Fifth Assessment Report (AR5) of the IPCC has chosen a mean value of 0.159 Wm 2ppb 1. The uncertainty on the radiative efficiency results from the inconsistency between
n
Corresponding author. Tel.: þ1 902 867 2392; fax: þ1 902 867 2414. E-mail address: [email protected] (K. Le Bris).
http://dx.doi.org/10.1016/j.jqsrt.2014.09.005 0022-4073/& 2014 Elsevier Ltd. All rights reserved.
the integrated absorption cross-section spectra available in the literature. The HITRAN 2012 database [4] contains, at this time, two sets of cross-section spectra for HFC-143a – one from Smith et al. [5] in the main directory and the other one from Di Lonardo and Masciarelli [6] in the supplemental directory. The integrated band intensities vary significantly between the two sets. The discrepancy is particularly obvious near the Q-branch of the 1281.2 cm 1 band, an important region for atmospheric remote sensing. To resolve these disparities, we recorded high-resolution absorption cross-section spectra of a pure vapour of HFC-143a at the University of Toronto and compared our data with the HITRAN datasets and with the broadened dataset from the Pacific Northwest National Laboratory (PNNL) [7]. 2. Dataset summary Smith et al. [5] presented pure vapour infrared crosssections at six different temperatures (203, 213, 233, 253, 273, and 297 K) and 37.5-, 150-, and 750.1-Torr air-broadened cross-section spectra at 297 K. The data are separated in
14
K. Le Bris, L. Graham / Journal of Quantitative Spectroscopy & Radiative Transfer 151 (2015) 13–17
three bands (580.00–630.00, 749.99–1049.90, and 1099.90– 1499.90 cm 1) in the HITRAN database. Di Lonardo and Masciarelli [6] dataset contains pure vapour, 150-Torr and 600-Torr air-broadened spectra at six different temperatures (203, 213, 233, 253, 273, and 293 K). The spectral acquisition was divided into three regions 790–870 cm 1, 920–1030 cm 1 and 1120–1470 cm 1. Each region was recorded at different pressures to account for the diversity of band strengths. However, the data appears in the HITRAN database as a single continuous spectral band from 694.249 to 1504.192 cm 1. The division lines between the spectral regions, apparent by a change in the signal-to-noise ratio, are visible at 885 cm 1 and 1050 cm 1. The PNNL dataset [7] is composed of N2-broadened spectra for a total pressure of 760 Torr at three different temperatures (277, 298 and 323 K) over the 600–6500 cm 1 spectral range. Our data for a pure vapour were recorded at 263, 273 and 283 K between 550 and 3500 cm 1. The experimental setup was described in a previous article [8]. A survey of the molecule spectrum is presented in Fig. 1. HCFC-143a has five separated main bands in the 570-1500 cm 1 spectral region. The assignment of the bands was done using Nivellini et al. [9] results. The bands corresponding to the harmonic frequencies ν9 and ν3 are overlapping and are therefore analysed as a single band in the 1100–1350 cm 1 range. The same applies to the band corresponding to the harmonic frequency ν2 and the combination ν4 þ ν5 in the 1350–1480 cm 1 range. Because of our larger spectral range, we were also able to observe a very weak band, centered around 2975 cm 1, corresponding to the ν1 vibrational frequency and another band, centered around 3034 cm 1, corresponding to the ν7 vibrational frequency. 3. Experimental setup and method All the datasets had been acquired using a Fourier transform spectrometer with a globar source, a KBr beamsplitter and a liquid-nitrogen cooled mercury–cadmium– telluride (MCT) detector. Pressures were read through a Baratron gauge. Table 1 summarizes the main characteristics for each dataset. Acquisitions were performed using samples of stated purity equal or better than 99%. Samples were purified by several freeze-pump-thaw cycles and injected inside the
cell either as pure vapours or were previously mixed with dry air or nitrogen. The advantage of air- or N2-broadened vapour spectra is that they give spectral shapes similar to those found in atmospheric measurements. Pure vapours offer a better precision on pressure, and thus a better integrated intensity accuracy. The cross-section spectra, however, need to be processed through a pseudo-line method [10] before being used for atmospheric retrieval. Resolution plays an important role in resolving the fine structures in a pure vapour. Time is often the main constraint because higher resolutions require more co-added scans to keep a suitable signal-to-noise ratio. To determine if there are no resolution-induced spectral effects, our pure vapour spectral acquisitions have been performed at a resolution slightly higher than the 0.03 cm 1 used by references [5,6]. Mixed vapours have broader spectral bands and therefore a lower resolution can be used without losing information. Molecules containing C–F bonds have high absorption strengths in the mid-IR. For pure vapours, the values of the cross-section in the Q-branches are often in the 10 18 to 10 17 cm2 molecule 1 order of magnitude. A relatively short cell length is therefore suitable to prevent nonlinearity on the strongest bands while allowing pressures large enough for good precision readout. This point is particularly important for pure vapour spectral acquisitions where the cross-sections in the Q-branches can dramatically increase as the temperature drops. The accuracy of the cross-section depends strongly on the precision of the full-scale and zero photometric values. Variations of the source intensity during acquisitions are one of the most common sources of errors. The simplest way to improve accuracy is to take baseline spectra before and after each sample acquisition. To further reduce the full-scale error on our data, we also adjusted, by a polynomial regression, the baseline to several zero-absorption spectral bands on the sample spectrum. Imprecision on the zero of the photodetector is often responsible for non-linear effects on the strong bands. In our experiment, the zero was obtained by subtracting the residual baseline offset observed outside the limit of the detector and on saturated absorption bands. All groups used a second aperture at the beam focal point to prevent most of the secondary mid-IR radiation from reaching the detector. Errors on the 100% and 0% line location had been accounted for in the error calculation of Di Lonardo and Masciarelli [6]. Smith et al. [5] recorded HFC-143a spectra
Fig. 1. Survey spectrum of a pure vapour of HFC-143a at 283 K in the mid-IR at a resolution of 0.02 cm 1.
K. Le Bris, L. Graham / Journal of Quantitative Spectroscopy & Radiative Transfer 151 (2015) 13–17
15
Table 1 Main characteristics of the laboratory dataset of Smith et al. [5], Di Lonardo and Masciarelli [6], the PNNL [7] and our data. Experimental setup
Ref. [5]
Ref. [6]
PNNL
This work
Sample Range (cm 1) Resolution (cm 1) Type Cell length (cm) Spectrometer Purity Average error
Pure and air-broad. at 297 K 560–1900 0.03 Averaged 5.12 Bruker IFS 120HR 99.3% 5%
Pure and air-broad. 694.2–1504.2 0.03 Averaged 10.50 Bomem DA8 99% 2.68%
N2-broad. 500–6500 0.112 Composite 19.96 Bruker IFS 66v/S 99% o 3% ð2sÞ
Pure 550–3500 0.02 Composite 2.93 Bomem DA8 99% o 5% ð2sÞ
Table 2 Wavenumber shifts of Smith et al. [5] and Di Lonardo and Masciarelli [6] spectra relatively to our experimental data for pure vapours and the PNNL data for broadened vapours. Reference
Band (cm 1)
Red shift (cm 1) Pure vapour
Broadened vapour
[5]
580.00–630.00 749.99–1049.90 1099.90–1499.90
0.01 0.03 0.09
0.00 0.07 0.09
[6]
694.25–885.00 885.00–1050.00 1050.00–1504.19
0.02 0.02 0.03
0.02 0.02 0.04
with the globar on and off to remove secondary source artifacts. 4. Data processing The cross-sections from Smith et al. [5] and Di Lonardo and Masciarelli [6] come from averaged measurements. Series of co-added scans are taken in a closely related range of pressures, which are all chosen to give a good signal-tonoise ratio without obvious saturation. The final crosssection at each wavenumber is obtained by averaging the data. While this technique is efficient for small spectral windows where absorption bands can be of the same intensity range, it becomes troublesome for spectral windows where weak and strong absorption bands co-exist. Small spectral features in low-pressure acquisitions can be buried in the background noise while high-pressure acquisitions often trigger non-linear effects on the strongest bands. An alternative technique consists in acquiring several spectra in a large range of pressures for each given temperature. The optical depth χ is then calculated as a function of the pressure for each wavenumber ν [7]. To prevent nonlinear effects in an optically thick medium while keeping a good signal-to-noise ratio at every wavenumber, the data corresponding to optically thick χ ðνÞ 4 χ max or optically thin χ ðνÞ o χ min conditions are eliminated. χmax and χmin values are empirically determined. A linear behavior is obtained for strong absorption bands from the lowpressure measurements while weak absorption features are represented by the high-pressure measurements. For broadened vapours, a linear fit of the optical depth versus the pressure gives the cross-section at each wavenumber [7]. When a pure vapour is used, the spectral lines
Fig. 2. Wavenumber bias for (top) a pure vapour at 273 K and (bottom) a broadened vapour at 297 K (293 K for [6]). The sharpest lines from our work come from the higher resolution and the removal of self-broadening effects. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
are sharper and self-broadening effects may be visible. In this case, the optical depth does not increase linearly with the pressure. However, it was observed [11] that the crosssection can still be retrieved through a quadratic regression. The resulting composite spectra are at a theoretical zeroTorr limit removed from self-broadening effects. Those spectra present good signal-to-noise ratios everywhere and no saturation effects. The inconvenience of this technique is obviously the time of acquisitions. All our data as well as the PNNL dataset are composite cross-section spectra. 5. Dataset comparison 5.1. Wavenumber shift The wavenumber accuracy of our instrument had been tested prior to the acquisitions by comparing several lines of CO2 at high resolution with the HITRAN database. We therefore used our data as reference for pure vapour spectra. For broadened vapours, the reference was the PNNL database wavenumbers. The results are reported in Table 2. Red shifts appear in both [5] and [6] with Smith
16
K. Le Bris, L. Graham / Journal of Quantitative Spectroscopy & Radiative Transfer 151 (2015) 13–17
et al. (1998) data having the largest relative difference (see Fig. 2). Because the biases remain constant within each experimental spectral window, they may be due to rounding errors in the HITRAN files. In the rest of the article, [5,6] data are handled with their corrected wavenumbers. 5.2. Integrated intensity The ν5 band is at the limit of the MCT detector and the ν4 band is of very low intensity. Therefore, uncertainties due to noise do not allow a fair comparison between the datasets in this spectral range. The next three bands are of higher intensities. The comparison of their integrated intensities is presented in Fig. 3. The maximum deviation of the integrated intensities of Smith et al. [5] data with temperature are always well within their error estimate. This is not the case for the values of Di Lonardo and Masciarelli [6], which present variations between 8 and 10% as the temperature decreases. Ideally, integrated band intensities should be independent of resolution and pressure broadening effects. A comparison of the integrated intensities of Smith et al. [5]
Fig. 4. Absorption cross-section spectra of Smith et al. [5] and Di Lonardo and Masciarelli [6] around the ν3 Q-branch. Non-linearity behaviours are visible on the 600-Torr broadened vapour spectrum at 253 K and all Ref. [6] spectra at 203 K. The artificially enlarged Q-branches of Di Lonardo and Masciarelli [6] spectra explain partially the raise of their integrated crosssections observed in Fig. 3.
shows that their values also remain consistent with our higher resolution composite data and the lower resolution N2-broadened composite data of the PNNL. The integrated intensities of Di Lonardo and Masciarelli [6], on the other hand, are systematically lower than the other groups with variations as large as 15.5% for the 920–1020 cm 1 and 1100–1350 cm 1 bands. Only in a few occurrences in the 1100–1350 cm 1 band do the reference [6] integrated intensities reach the values of the other authors. However, a close look shows that this apparent similarity is only due to important non-linear effects on the ν3 Q-branch of Di Lonardo and Masciarelli [6] cross-sections (see Fig. 4). Non-linear shapes appear at 203 K and 233 K for a pure vapour, 203 K, 213 K, and 273 K for a 150-Torr broadened vapour and 203, 253 and 273 K for a 600-Torr broadened vapour. Table 3 presents the averaged band integrated intensities of the four mentioned datasets. They are compared with the literature values of Sihra et al. [12] at 296 K, Highwood and Shine [13] at 253 K, Pinnock et al. [14] at 296 K, Oliff and Fisher [15] and Gehring [16] at “room temperature”. The total integrated intensity values in the highest range (this work, references [5,7,16]) are very consistent with each other with a maximum percentage difference of less than 2%. The values of the total integrated intensities in the lowest range, however, spread between 12.01 and 1 13:09 10 17 cm:molecule . 6. Conclusion
Fig. 3. Comparison of the integrated intensities of the three strongest bands of HFC-143a as a function of temperature. For clarity, only Smith et al. [5] pure vapour and Di Lonardo and Masciarelli [6] uncertainty ranges have been inserted.
We presented three high-resolution composite crosssection spectra of a pure vapour of HFC-143a in the 263–283 K temperature range. The two datasets in the HITRAN database have been evaluated in light of those new spectra and the PNNL data. The integrated absorption
K. Le Bris, L. Graham / Journal of Quantitative Spectroscopy & Radiative Transfer 151 (2015) 13–17
17
Table 3 Comparison of the integrated intensities of HFC-143a bands. Bold text indicates a value averaged over the available spectra. Reference
This work PNNL Sihra et al. Di Lonardo and Masciarelli Smith et al. Highwood and Shine Pinnock et al. Oliff and Fisher Gehring
Integrated cross section ( 10 17 cm.molecule 1) ν5
ν4
ν10
ν9 , ν3
ν2, ν4 þ ν5
Totala
0.39 0.37
0.06 0.08
1.69 1.67
10.11 9.91
1.60 1.55
0.37
0.07 0.09
1.48 1.69
9.12c 10.1
1.52 1.62
0.31 0.35
0.08 0.16
1.65 1.75
9.38 9.90
1.43 1.59
13.85 13.58 12.71 (13.07)b 12.19 (12.55)b 13.87 12.01 12.30 (12.66)b 12.84 13.72
a The two weak bands in the 3000 cm 1 area observed by the PNNL and by us, have not been included in the total integrated intensity. For information, 1 1 for the ν1 bands and around 1:12 10 18 cm:molecule for the ν7 bands we found an integrated intensity lower than 3 10 20 cm:molecule 1 (2:80 10 20 and 1:18 10 18 cm:molecule respectively for the PNNL database). 1 b The values in parenthesis represent the total integrated cross-section with the addition of an average value of 0:36 10 17 cm:molecule for the missing ν5 band. c The bands with nonlinear effects have been removed from the calculation.
cross-sections of the datasets have also been compared to other available literature values. Our data are very consistent with the values of Smith et al. [5] and the PNNL database. For lower temperatures, the pure vapour spectra of Smith et al. [5] remain the first choice until a more complete dataset becomes available. The red shifts should be corrected prior use. Di Lonardo and Masciarelli data [6] include broadened vapour crosssection spectra in a range of temperature down to 203 K. However, because of the lower values of the integrated intensities relative to recent other sources and the presence of strong non-linearity effects on the ν3 Q-branch at 1281.2 cm 1, the data should be used with caution. It also appears from this study that the radiative efficiency of HFC-143a should be revised upward due to the higher value of the total integrated cross-sections. Acknowledgements This work was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). We thank Prof. Kimberly Strong for the use of the Bomem DA8 Fourier transform spectrometer at the University of Toronto. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j. jqsrt.2014.09.005.
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