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The 2ν_3 band of CH_4 revisited with line mixing: Consequences for spectroscopy and atmospheric retrievals at 1.67 μm

Tran, H. and Hartmann, J.-M. and Toon, G. and Brown, L. R. and Frankenberg, C. and Warneke, T. and Spietz, P. and Hase, F. (2010) The 2ν_3 band of CH_4 revisited with line mixing: Consequences for spectroscopy and atmospheric retrievals at 1.67 μm. Journal of Quantitative Spectroscopy and Radiative Transfer, 111 (10). pp. 1344-1356. ISSN 0022-4073. http://resolver.caltech.edu/CaltechAUTHORS:20150508-100951039

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Abstract

This paper presents a study of absorption in N_2-broadened P and R manifolds of the 2ν_3 band of CH_4 near 6000 cm^(−1) using high resolution laboratory and atmospheric spectra. This region is of prime importance for the retrieval of methane abundances in the Earth's atmosphere using ground-based or space-borne spectrometers. Recent laboratory investigations have been devoted to the methane spectroscopic parameters in this band, motivated by their previous poor knowledge and their increasing use by remote sensing experiments. In the absence of a better model, previous studies have used Voigt line shapes and thus purposely neglected line mixing (LM). In this paper, we first present direct comparisons between measured laboratory spectra and the results of a model which accounts for LM without adjusting any of the spectroscopic parameters. A good agreement is obtained and the results show that LM does have a significant influence on the shapes of P and R manifolds. Hence, most previously observed discrepancies were not due to improper broadening and shifting coefficients but to the neglect of this effect. This also confirms that widths and shifts derived in recent 2ν_3 band studies neglecting LM are “effective” and lack physical meaning, as suggested in a previous work [17] (Frankenberg et al., 2008). In a second step, the conclusions from the laboratory data are tested using ground-based atmospheric solar absorption spectra. The fit residuals obtained confirm the quality of the proposed model and evidence the impact of line mixing on CH_4 atmospheric spectra. The present results also confirm that laboratory and atmospheric spectra can alternatively be accurately modeled neglecting LM and using ad hoc broadening and shifting parameters. Conclusions of this exercise can be drawn from two perspectives. From the point of view of spectroscopy and understanding of processes, accurate line parameters will not be deduced from fits of laboratory measurements unless line-mixing effects are included in the spectral-shape model. In the meantime, and from the point of view of atmospheric retrievals, neglecting LM with suitable effective line parameters is convenient and accurate (within current retrieval uncertainties). Note that this is only true if this approach is not used for total pressures significantly above 1 atm (e.g. Jupiter).


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/j.jqsrt.2010.02.015DOIArticle
http://www.sciencedirect.com/science/article/pii/S0022407310000865PublisherArticle
ORCID:
AuthorORCID
Frankenberg, C.0000-0002-0546-5857
Additional Information:© 2010 Elsevier Ltd. Received 26 January 2010; Received in revised form 25 February 2010; Accepted 25 February 2010. The authors thank A. Nikitin for communicating his CH_4 line list. We thank Paul Wennberg (TCCON PI) for the use of the Park Falls spectra. Part of the research described in this paper was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The authors from Bremen acknowledge financial support from the European Union projects HYMN and GEOMON.
Funders:
Funding AgencyGrant Number
NASA/JPL/CaltechUNSPECIFIED
European Union Project HYMNUNSPECIFIED
European Union Project GEOMONUNSPECIFIED
Subject Keywords:Methane; Line-mixing; Line-broadening; Ground-based spectra
Record Number:CaltechAUTHORS:20150508-100951039
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20150508-100951039
Official Citation:H. Tran, J.-M. Hartmann, G. Toon, L.R. Brown, C. Frankenberg, T. Warneke, P. Spietz, F. Hase, The 2ν3 band of CH4 revisited with line mixing: Consequences for spectroscopy and atmospheric retrievals at 1.67 μm, Journal of Quantitative Spectroscopy and Radiative Transfer, Volume 111, Issue 10, July 2010, Pages 1344-1356, ISSN 0022-4073, http://dx.doi.org/10.1016/j.jqsrt.2010.02.015. (http://www.sciencedirect.com/science/article/pii/S0022407310000865)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:57373
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:08 May 2015 17:19
Last Modified:08 May 2015 17:19

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