Comparison of line-by-line and band models of near-IR methane absorption applied to outer planet atmospheres

Abstract

Recent improvements in high spectral resolution measurements of methane absorption at wavenumbers between 4800cm−1 and 7919cm−1 have greatly increased the number of lines with known lower state energies, the number of weak lines, and the number of lines observed at low temperatures (Campargue, A., Wang, L., Kassi, S., Mašát, M., Votava, O. [2010]. J. Quant. Spectrosc. Radiat. Trans. 111, 1141–1151; Campargue, A., Wang, L., Liu, A.W., Hu, S.M., Kassi, S. [2010]. Chem. Phys. 373, 203–210; Mondelain, D., Kassi, S., Wang, L.C. [2011]. Phys. Chem. Chem. Phys. 13, 7985–7996; Nikitin, A.V. et al. [2011a]. J. Mol. Spectrosc. 268, 93–106; Nikitin, A.V. et al. [2010]. J. Quant. Spectrosc. Radiat. Trans. 111, 2211–2224; Wang, L., Kassi, S., Campargue, A. [2010]. J. Quant. Spectrosc. Radiat. Trans. 111, 1130–1140; Wang, L., Kassi, S., Liu, A.W., Hu, S.M., Campargue, A. [2011]. J. Quant. Spectrosc. Radiat. Trans. 112, 937–951), making it possible to fit near-IR spectra of Titan using line-by-line calculations instead of band models (Bailey, J., Ahlsved, L., Meadows, V.S. [2011]. Icarus 213, 218–232; de Bergh, C. et al. [2011]. Planet. Space Sci. doi:10.1016/j.pss.2011.05.003). Using these new results, we compiled an improved line list relative that used by Bailey et al. by updating several spectral regions with either calculated or more recently measured line parameters, revising lower state energy estimates for lines lacking them, and adding room temperature lines to make the list applicable over a wider range of temperatures. We compared current band models with line-by-line calculations using this new line list, both to assess the behavior of band models, and to identify remaining issues with line-by-line calculations when applied to outer planet atmospheres and over a wider range of wavelengths. Comparisons were made for a selection of uniform paths representing outer planet conditions and for representative non-uniform paths within the atmospheres of Uranus, Saturn, and Jupiter, as well as comparisons with 77K lab measurements of McKellar (McKellar, A.R.W. [1989]. Can. J. Phys. 67, 1027–1035). At room temperatures and pressures band models and new line-by-line calculations generally agree within 1.6–3% RMS between 1800cm−1 and 7919cm−1, but disagree more significantly near 3200–3500cm−1 and in the region where CH3D line data are missing between 5200cm−1 and 5600cm−1, and also at band edges near 3250cm−1 and 5600cm−1, where far wing line shapes may need improvement. For intermediate temperatures and methane paths, the Irwin et al. (Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Bowles, N., Calcutt, S.B., Remedios, J.J. [2006]. Icarus 181, 309–319) band model agrees best with the line-by-line calculations at wavenumbers less than 5000cm−1. At low temperatures and long path lengths the band models diverge more seriously, with that of Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2010]. Icarus 205, 309–319) providing the best agreement with line-by-line calculations. Model spectra computed from the band and line-by-line models were also compared with a Keck/NIRC2 H-band spectrum of Uranus (Sromovsky, L.A., Fry, P.M. [2008]. Icarus 193, 252–266), which could be fit well with either of the two band models, but the main aerosol layer required an optical depth five times smaller using the Irwin et al. band model than for either line-by-line calculations or the Karkoschka and Tomasko band model. By far the best fit to the Uranus H-band spectrum was obtained using line-by-line absorption calculations with a far wing line shape intermediate between that of Hartmann et al. (Hartmann, J.-M., Boulet, C., Brodbeck, C., van Thanh, N., Fouchet, T., Drossart, P. [2002]. J. Quant. Spectrosc. Radiat. Trans. 72, 117–122) and that of de Bergh et al. (de Bergh, C. et al. [2011]. Planet. Space Sci. doi:10.1016/j.pss.2011.05.003).