GOSAT-2009 methane spectral line list in the 5550–6236 cm−1 range

doi:10.1016/j.jqsrt.2010.05.010

Elsevier

Journal of Quantitative Spectroscopy and Radiative Transfer

Volume 111, Issue 15, October 2010, Pages 2211-2224

Journal of Quantitative Spectroscopy and Radiative Transfer

https://doi.org/10.1016/j.jqsrt.2010年05月01日0 Get rights and content

Abstract

A methane spectral line list for the 5550–6236 cm⁻¹ range with the intensity cut off ×ばつ10⁻²⁶ cm/molecule at 296 K is presented. The line list is based on new extensive measurements of methane spectral line parameters performed at different temperatures and pressures of methane and buffer gases N₂, O₂ and air. This spectral line list is prepared in HITRAN-2008 format and contains the following spectral line parameters of about 11,000 lines: position, intensity, energy for lower state (where possible), air-broadening and air-shifting coefficients, exponent of temperature dependence of air-broadening coefficient and self-broadening coefficient.

Introduction

Methane (CH₄), like water and carbon dioxide, is an important greenhouse gas. Nowadays many groups in the world are performing continuous monitoring of several greenhouse gases including methane in the Earth’s atmosphere. For these purposes in January 2009 the Japanese Greenhouse gases Observing SATellite (GOSAT) was launched. The main aim of this mission is to measure the column amounts and profiles of the concentration of carbon dioxide and methane over the globe from space [1]. Two types of sensors are installed on GOSAT. The Thermal And Near infrared Sensor for carbon Observation Fourier Transform Spectrometer (TANSO-FTS) detects the solar light reflected on the Earth’s surface as well as the thermal radiance emitted from the surface and the atmosphere. The TANSO Cloud and Aerosol Imager (TANSO-CAI) is a radiometer to detect optically thick clouds and correct the effects of optically thin clouds and aerosol. Details of GOSAT sensors have already been described in Ref. [2]. The first, preliminary result obtained with GOSAT TANSO-FTS SWIR was reported in Ref. [3].

The absorption band of CH₄ at 1.67 μm is chosen to retrieve the methane column. But, as we discussed in our previous paper [4], the set of CH₄ spectral line parameters contained in the HITRAN-2004 database [5] for this region was not complete and some of the parameters, in particular, the broadening and shifting parameters were not good enough because they were extrapolated from the measurements performed for lower frequency regions. For this reason we have undertaken systematic studies of the spectral line parameters of the methane molecule in the 5550–6236 cm⁻¹ range. Based on these studies, in 2008 the GOSAT methane spectral line list was prepared but has not been published. In this paper we publish a slightly modified version of this spectral line list. It should be emphasized that since that time, new measurements of line positions, line intensities and N₂ broadening coefficients have been performed in the region under consideration [6], [7], [8]. Some of these new results [4], [6], [7] were used in generating the methane line list for the HITRAN-2008 database [9].

Section snippets

Experiment

The spectra were recorded using Bruker IFS 120 HR and 125 HR high resolution Fourier transform spectrometers at Toray Research Center Inc. The spectrometers were equipped with a CaF₂ beam splitter, InSb detector and tungsten lamp as the light source. An optical band-path filter covering the 5500–6200 cm⁻¹ region was used to improve the signal to noise ratios (S/N) of the spectra. The unapodized spectral resolution was 0.005 cm⁻¹ and no numerical apodization was applied (Boxcar function). This

Spectra assignment

The assignment of the spectral line parameters is very important for the different kinds of applications. First of all the assignment gives the energy of a lower state that is used in the equation for the temperature dependence of the line intensity. The assignment can also be useful for modeling line shape parameters. To get the total assignment it is necessary to solve the inverse spectroscopic problem using an effective Hamiltonian model. The upper vibrational states of the cold bands

Retrieval of the spectral line parameters

The spectral line parameters were retrieved using the multispectrum fitting procedure [15] in which a nonlinear least-squares method is applied simultaneously to the spectra recorded under various experimental conditions. This procedure was further developed and realized in a home-made computer code. This code determines line parameters by adjustment of the synthetic spectra to the observed ones. The adjustable parameters include position, intensity, self-broadening and shifting and buffer gas

Line positions and line intensities

In order to determine line positions and line intensities for the strongest lines, those with the intensities larger than 10⁻²³ cm/molecule, we used spectra 12–16 (see Table 1) recorded with the 8.75 cm path length absorption cell. These lines include the congested J-manifolds of the 2ν₃(F₂) band. For weak lines with intensities smaller than 10⁻²³ cm/molecule, we used spectra 3–5 (see Table 1) recorded with the 20.099 m path length absorption cell. Because of a rather low value of signal to noise

Shifting and broadening by N₂ and O₂

Two methods were used to obtain the air broadening and shifting coefficients for the GOSAT spectral line list. In the case of the congested J-manifolds of 2ν₃(F₂) band these coefficients were retrieved from the air broadening experiments. The results are presented in Section 8. For other lines, these parameters were deduced from results of studying the broadening and shifting by N₂ and O₂ at three temperatures 296, 267 and 240 K (spectra 6–11, 23–28 and 32–37 in Table 1). These results were

Self-broadening parameters

The GOSAT spectral line list like the HITRAN database contains the self-broadening coefficients. To retrieve the self-broadening and -shifting of the methane spectral lines in the 5550–6236 cm⁻¹ region at different temperatures we used the spectra 1–5, 12–16, 20–22, 29–31 and 38–47. The results of this study will be published in the separate paper [19]. The self-broadening coefficients were derived for 406 assigned lines with good values of S/N. Using the values obtained for the self-broadening

Air-broadening and shifting parameters for 2ν₃(F₂) band

The recorded absorption spectra in the region of the 2ν₃(F₂) band of CH₄ consist of substantially spaced line manifolds forming R and P branches and of partially overlapped line manifolds in the Q-branch. In order to retrieve spectroscopic parameters of an individual line it is necessary to consider the total shape of the manifold as the lines of a manifold overlap. At broadening pressures up to 1 atm, the individual line shape transforms from a Gaussian profile at lower pressures due to Doppler

GOSAT-2009 methane line list

A list of the methane spectral line parameters has been generated in the region 5550–6236 cm⁻¹ with an intensity cut off ×ばつ10⁻²⁶ cm/molecule at 296 K. It is presented as Supplementary Material. This list is prepared in HITRAN-2008 format and contains 10,917 lines. Only 2918 lines are completely assigned so far [13]. In addition 1576 lines have partial assignment of the lower state (J value). For these lines we give the energy of the lower state. The values of the line positions and line

Conclusion

In this paper we present a methane spectral line list for the 5550–6236 cm⁻¹ region. It is based on new extensive measurements of the methane spectral line parameters performed by authors of this paper. This line list prepared in HITRAN-2008 format is aimed to support the Japanese GOSAT mission.

It is necessary to emphasize that this spectral line list needs to be further developed. Only a third of the lines contained in it is completely assigned and only one half of the lines have an assigned

Acknowledgments

This work is supported in part by the NIES GOSAT project. The authors are very grateful to the reviewers, especially, to Alain Campargue; their criticism and suggestions considerably improved the quality of the paper. The authors thank Jonathan Tennyson for the careful reading of the manuscript and improvement in its English.

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GOSAT-2009 methane spectral line list in the 5550–6236 cm−1 range

Abstract

Introduction

Section snippets

Experiment

Spectra assignment

Retrieval of the spectral line parameters

Line positions and line intensities

Shifting and broadening by N2 and O2

Self-broadening parameters

Air-broadening and shifting parameters for 2ν3(F2) band

GOSAT-2009 methane line list

Conclusion

Acknowledgments

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J Mol Spectrosc

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J Mol Spectrosc

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GOSAT-2009 methane spectral line list in the 5550–6236 cm⁻¹ range

Shifting and broadening by N₂ and O₂

Air-broadening and shifting parameters for 2ν₃(F₂) band