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In atmospheric remote sensing of methane, a dominant source of error is the uncertainty in spectroscopic parameters. For example, in the retrieval algorithms described by Batchelor et al. (2009), the CH₄ error due to line intensities is estimated at 16.5% (dominating the total error of 16.8%), versus 10.5% for O₃, 3.8% for N₂O, and only 1.6% for HCl.

The article documenting the 2012 HITRAN spectroscopic database update (Rothman et al., 2013) states:

High-resolution spectroscopy of methane and the generation of accurate line lists, especially as one moves up the polyad ladder to shorter and shorter wavelength (necessary for many applications) is extremely challenging.

The 2012 edition includes a very large update on methane measurements, as detailed by Brown et al. (2013), who describe how the number of HITRAN methane lines has increased from 11,803 in the 1982 edition, via 251,440 in the 2004 edition, to 468,013 in the 2012 edition. However, neither Rothman et al. (2013) nor Brown et al. (2013) describe why the determination of methane spectroscopic parameters is so difficult.

Why are methane spectroscopic parameters harder to determine (and therefore more poorly known) compared to other common Earth atmospheric trace gases?


  • Batchelor, Rebecca L., Kimberly Strong, Rodica Lindenmaier, Richard L. Mittermeier, Hans Fast, James R. Drummond, and Pierre F. Fogal. "A New Bruker IFS 125HR FTIR Spectrometer for the Polar Environment Atmospheric Research Laboratory at Eureka, Nunavut, Canada: Measurements and Comparison with the Existing Bomem DA8 Spectrometer." Journal of Atmospheric & Oceanic Technology 26, no. 7 (2009).
  • L.S. Rothman, I.E. Gordon, Y. Babikov, A. Barbe, D. Chris Benner, P.F. Bernath, M. Birk, L. Bizzocchi, V. Boudon, L.R. Brown, A. Campargue, K. Chance, E.A. Cohen, L.H. Coudert, V.M. Devi, B.J. Drouin, A. Fayt, J.-M. Flaud, R.R. Gamache, J.J. Harrison, J.-M. Hartmann, C. Hill, J.T. Hodges, D. Jacquemart, A. Jolly, J. Lamouroux, R.J. Le Roy, G. Li, D.A. Long, O.M. Lyulin, C.J. Mackie, S.T. Massie, S. Mikhailenko, H.S.P. Müller, O.V. Naumenko, A.V. Nikitin, J. Orphal, V. Perevalov, A. Perrin, E.R. Polovtseva, C. Richard, M.A.H. Smith, E. Starikova, K. Sung, S. Tashkun, J. Tennyson, G.C. Toon, Vl.G. Tyuterev, G. Wagner, The HITRAN2012 molecular spectroscopic database, Journal of Quantitative Spectroscopy and Radiative Transfer, Volume 130, November 2013, Pages 4-50, ISSN 0022-4073, http://dx.doi.org/10.1016/j.jqsrt.2013.07.002. (http://www.sciencedirect.com/science/article/pii/S0022407313002859)
  • L.R. Brown, K. Sung, D.C. Benner, V.M. Devi, V. Boudon, T. Gabard, C. Wenger, A. Campargue, O. Leshchishina, S. Kassi, D. Mondelain, L. Wang, L. Daumont, L. Régalia, M. Rey, X. Thomas, Vl. G. Tyuterev, O.M. Lyulin, A.V. Nikitin, H.M. Niederer, S. Albert, S. Bauerecker, M. Quack, J.J. O’Brien, I.E. Gordon, L.S. Rothman, H. Sasada, A. Coustenis, M.A.H. Smith, T. Carrington Jr., X.-G. Wang, A.W. Mantz, P.T. Spickler, Methane line parameters in the HITRAN2012 database, Journal of Quantitative Spectroscopy and Radiative Transfer, Volume 130, November 2013, Pages 201-219, ISSN 0022-4073, http://dx.doi.org/10.1016/j.jqsrt.2013.06.020. (http://www.sciencedirect.com/science/article/pii/S0022407313002744)
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A good explanation is found at the University of Bourgogne site MODELING THE ABSORPTION SPECTRUM OF METHANE.

Although it is a small and simple molecule, its rovibrational spectroscopy is very complicated. This is mainly due to the high symmetry of this tetrahedral system (which leads to the existence of many degeneracies) and to its intricated vibrational structure.

Vibrational and Rotational-vibrational energy level diagrams are available at the site.

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