HCFC-141b (CH3CCl2F) PSEUDO-LINELIST G. C. Toon INTRODUCTION. A HCFC-141b pseudo-linelist was derived at JPL in July 2010, based on on laboratory spectra described by Clerbaux et al., (1993). The absorption cross-sections were downloaded from the HITRAN website Unfortunately, all the negative values had been set to zero, biasing the absorption in weakly absorbing regions. The absorption coefficients were converted into transmittance spectra before any fitting was performed by multiplying them by the density and the cell length. In transmittance space, the noise should be whiter than in absorption space, preventing undue weighting to regions where the features are nearly saturated. LAB MEASUREMENTS The assumed measurement conditions are tabulated below. Each measurement used the same cell of 5 cm length. Each spectrum covers a region between 600 and 6500 cm-1 with a resolution of 0.1125 cm-1 and a spectral point spacing of 0.0603 cm-1. Spectrum nu_start nu_end npts temp P_tot P_gas cell_len resn "CH3CCl2F.253" 709.9783 1470.0017 88280 253.0 2.5 2.5 0.05 0.03 "CH3CCl2F.270" 709.9783 1470.0017 88280 270.0 2.5 2.5 0.05 0.03 "CH3CCl2F.287" 709.9783 1470.0017 88280 287.0 2.5 2.5 0.05 0.03 Temp - Temperature in K P_tot - total pressure in torr P_gas - HCFC-141b partial pressure in torr Norton-Beer strong apodization (N2) was assumed during the analysis. The Clerbaux paper was not explicit about the pressures that apply to each spectrum. It simply said 1-4 Torr. So an average value of 2.5 Torr was assumed. The resolution was cited as 0.03 cm-1 apodized. The OPDmax was not given, so I assumed 0.9/0.03 = 30 cm. DESCRIPTION. First, the cross-sections were converted back into transmittance spectra from knowledge of the cell length and gas concentrations. The resulting laboratory transmittance spectra were then simultaneously fitted (using the GFIT algorithm) by iteratively adjusting the strengths and ground-state energies of the pseudo-lines. Due to the resolution of the laboratory spectra of 0.1125 cm-1 a pseudo-line spacing of 0.05 cm-1 was considered to be appropriate. Fitting was performed in the frequency regions around 900 cm-1 (where the nu_4 band is located), around 1050 cm-1 (where the nu_7 band is located), and around 1450 cm-1 (for the nu_3, nu_6, and nu_7+nu_8 bands). These regions include the two bands with the strongest absorption features. A zero level offset of 0.2% has been assumed throughout, based on fits to spectra in which the absorption feature at 1463 cm-1 was saturated. The result of the fitting process is a continuous pseudo-linelist containing 15601 lines between 870 and 1650 cm-1. CALCULATION OF S, E", and ABHW. At each line frequency, an effective strength and ground-state energy was derived by simultaneous non-linear least squares fitting to the 29 spectra. Furthermore, the ABHW was calculated from the ground-state energy using the formula ABHW = 0.04 * (E" + 2000)/(E" + 1000), giving a ABHW of 0.08 cm-1/atm for E"->0 and a ABHW of 0.04 cm-1/atm for E"->oo. This formulation seemed to be the most approriate to fit the feature at 1042 cm-1, which is the narrowest feature in the considered frequency region. These widths are smaller than those measured by Drouin (2003) in the microwave region, but we found that using larger widths produced significantly poorer fits to the sharp spectral features. As part of the fitting, the strengths and ground-state energies were both constrained to be positive. PARTITION FUNCTION. The rotational partition function for HCFC-141b was assumed to be (296/T)^2.0. The vibrational partition function was calculated in the way it had been done for the ATMOS experiment, as described e. g. by Norton and Rinsland (1991). The following vibrational frequencies and degeneracies were assumed: freq. | 2954 2267 1385 920 3009 1448 1041 362 deg. | 1 1 1 1 2 2 2 2 These were guesses based on ACCURACY. To estimate how well the pseudo-linelist represents the PNNL spectra, test retrievals were performed in which the laboratory spectra were fitted using the pseudo-linelist. The retrieved scale factors for the HCFC-141b abundances in the different spectra are tabulated below. Scale factors retrieved in freq. region # 900 cm-1 1050 cm-1 1450 cm-1 ------------------------------------------------- 1 1.0073 0.9984 1.0065 26 0.9918 0.9891 1.000 27 0.9993 0.9928 1.0035 28 1.0087 1.0009 1.0119 29 0.9958 0.9940 1.0013 ----------------------------------------------- mean 1.00072 0.99403 1.00158 stddev 0.00655 0.00588 0.00655 The pseudolines correctly represent the PNNL spectra to within 0.7% of the given HCFC-141b amount in all bands analyzed. The main exception to this is spectrum 12 which appears to contain ~2% less HCFC-141b than advertised. ACKNOWLEDGMENTS. We would like to thank Steven Sharpe and Curtis Rinsland for providing HCFC-141b cross-sections prior to publication, and Aaron Goldman and Isabelle Kleiner for valuable discussions about the HCFC-141b partition function. REFERENCES. Drouin, B. J., Temperature dependent rotational transition lineshape parameters for O3, O2, SO2, HCFC-141b and CO, International Symposium on Molecular Spectroscopy, Columbus, OH, USA, June 16-20, 2003. Norton, R. H. and C. P. Rinsland, ATMOS data processing and science analysis methods, Appl. Opt., 30, 389-400, 1991. Rinsland, C. P., S. W. Sharpe, and R. L. Sams, Temperature-dependent infrared absorption cross-sections of methyl cyanide (acetonitrile), JQSRT, 96, 271-280, 2005.