Measurements of N2- and O2-broadening and shifting parameters of methane spectral lines in the 5550–6236 cm−1 region

doi:10.1016/j.jqsrt.2009.02.012

Elsevier

Journal of Quantitative Spectroscopy and Radiative Transfer

Volume 110, Issues 9–10, June–July 2009, Pages 654-668

Journal of Quantitative Spectroscopy and Radiative Transfer

https://doi.org/10.1016/j.jqsrt.2009年02月01日2 Get rights and content

Abstract

The absorption spectra of mixtures of methane (CH₄) with N₂ and O₂ at different partial pressures of both CH₄ and buffer gases for three temperatures 240, 267, and 296 K have been recorded using the Bruker IFS 125 HR FTIR spectrometer in the 5550–6236 cm⁻¹ region. The multispectrum fitting procedure has been applied to these spectra to recover the spectral line parameters. The main goal of this procedure was the determination of the N₂- and O₂-broadening and shifting coefficients and the exponents of their temperature dependences. These parameters have been derived for 452 assigned lines with good values of the signal to noise ratio. The rotational dependence of the mean values of these parameters is discussed. The temperature dependence exponents were observed for both N₂ and O₂ buffer gases.

Introduction

The absorption band of methane (CH₄) at 1.67 μm is planned to be utilized to retrieve column abundances of CH₄ by means of the Greenhouse gases Observing SATellite (GOSAT) [1] which was successfully launched in 2009. 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 correct cloud and aerosol extinction and optical properties interference. Column abundances and/or profiles of concentration of carbon dioxide and CH₄ are estimated globally, except the thick cloudy region detected by TANSO-CAI, from spectra obtained by TANSO-FTS.

It is well known that the set of the CH₄ spectral line parameters contained in the HITRAN2004 database [2] for the above mentioned region is not complete. There are a lot of lines which are not assigned up to now. The existing line positions and line intensities in this database for this region need to be refined. In the case of the air-broadening coefficients their mean empirical values for each given angular momentum quantum number J published in Ref. [3] were used for this region in HITRAN2004. But these mean values are based on the measurements in the low frequency region. The exponents of temperature dependence of line width were fixed to 0.75 and the air pressure induced shift equal to −0.008 cm⁻¹ atm⁻¹ was used [4] for all lines of this region contained in HITRAN2004.

This spectroscopic information causes the bias in retrieval analysis of spectroscopic atmospheric remote sensing. For example, the difference between the Kitt Peak column-averaged CH₄ volume mixing ratio and Mauna Loa flask samples is about 4% [5]. It may come from the uncertainties in the absolute CH₄ line intensities, broadening, and shifting parameters. Indeed, the measurements performed in Refs. [6], [7] have given the values for the N₂-shifting parameter of the R(3) manifold of the 2ν₃ (F₂) band equal to −0.00996 and −0.0112 cm⁻¹ atm⁻¹, respectively. It leads to a larger value of the air-shifting parameter than the value adopted for the HITRAN2004 database. The exponents of the temperature dependence of the N₂-broadening parameter published in Refs. [8], [9], [10] for several P and R manifolds of the 2ν₃ (F₂) band are considerably larger than that adopted in the HITRAN2004 database for the air-broadening coefficients in the case of the lines in 1.67 μm region. Very recently the N₂-broadening coefficients for the lines of 2ν₃ band have been studied by Fourier transform spectroscopy [11]. Using the N₂-broadening parameter obtained in Ref. [11] rescaled with 0.985 to give an estimate of air broadening and also an exponent of temperature dependence of broadening equal to 0.85 for all lines and default values for the shifts ranging from −0.008 to −0.011 cm⁻¹ atm⁻¹, the authors of this reference have managed to reduce the residuals of the fit of the ground-based direct solar Fourier transform infrared measurements by about a factor 3–4.

Based on the above discussion with the aim to support the GOSAT mission we have undertaken the systematic studies of the spectral line parameters of the CH₄ molecule in the 5550–6236 cm⁻¹ region. We plan to publish the obtained results in several papers. This paper deals with the N₂- and O₂-broadening and shifting of the CH₄ spectral lines.

An exhaustive list of the references dealing with the broadening and shifting of the CH₄ spectral lines can be found in Ref. [12].

Section snippets

Experimental details

The spectra were recorded using a Bruker IFS 125HR high-resolution Fourier transform spectrometer equipped with a CaF₂ beam splitter, InSb detector, and tungsten halogen lamp as the light source. An optical band-pass filter covering the 5600–6200 cm⁻¹ region was used to improve signal to noise ratios (S/N). The spectrometer resolution was 0.005 cm⁻¹ and no numerical apodization was applied (Boxcar function). This resolution is specified as 0.9/L (L: maximum optical pass difference).

The

Retrieval of the spectral line parameters

The spectral line parameters were retrieved using the multispectrum fitting procedure [13] in which a nonlinear least-squares method is applied simultaneously to 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 parameters, and

Broadening of the CH₄ lines by pressures of N₂ and O₂

The coefficients of the CH₄ line broadening by the pressures of buffer gases N₂ and O₂ derived by the above described method for all three temperatures 296, 267, and 240 K are given in Supplementary Materials. A selection of these coefficients is presented in Table 2. We have plotted these coefficients for each temperature versus rotational quantum number |m| (

| m | = J

for P- and Q-branches, and

| m | = J + 1

for R-branch). The respective plots are presented in Fig. 2 and in Fig. 3 for the N₂-broadening

Shifting of the CH₄ lines by pressures of N₂ and O₂

In this section we present the coefficients of the CH₄ line shifting by pressures of buffer gases N₂ and O₂ derived together with other spectral line parameters by the above described method. These coefficients for three temperatures 296, 267, and 240 K are given in Supplementary Materials. A selection of these coefficients is presented in Table 2. We have plotted the shifting coefficients for each temperature versus rotational quantum number m (

m = - J, J

and J+1 for P-, Q- and R-branches,

Temperature dependence of the broadening and shifting coefficients

To describe the temperature dependence of the broadening and shifting coefficients we used the following equations:

γ (T) = γ (T_{0}) {(\frac{T_{0}}{T})}^{n},

δ (T) = δ (T_{0}) {(\frac{T_{0}}{T})}^{η},

where γ(T) and δ(T) are broadening and shifting coefficients at temperature T,

γ (T_{0})

and

δ (T_{0})

are broadening and shifting coefficients at reference temperature T₀, n and η are the exponents of the temperature dependences of the broadening and shifting coefficients.

As has been discussed in Section 3, the broadening and shifting coefficients have

Discussion and conclusion

Line parameters of 452 methane transitions have been obtained in the 5550–6236 cm⁻¹ region. These parameters include line position, N₂-, O₂-, and air-broadening and shifting coefficients together with the exponents of their temperature dependences. It is interesting to compare these parameters to those contained in HITRAN2004 database [2]. In Fig. 11 we present the ratios of air-broadening coefficients derived in this paper to those from HITRAN2004 versus |m|. These ratios show that the majority

Acknowledgments

This work was supported in part by the NIES GOSAT project. The authors thank L.R. Brown for the critically reading the manuscript and for the useful suggestions.

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Measurements of N2- and O2-broadening and shifting parameters of methane spectral lines in the 5550–6236 cm−1 region

Abstract

Introduction

Section snippets

Experimental details

Retrieval of the spectral line parameters

Broadening of the CH4 lines by pressures of N2 and O2

Shifting of the CH4 lines by pressures of N2 and O2

Temperature dependence of the broadening and shifting coefficients

Discussion and conclusion

Acknowledgments

JQSRT

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Measurements of N₂- and O₂-broadening and shifting parameters of methane spectral lines in the 5550–6236 cm⁻¹ region

Broadening of the CH₄ lines by pressures of N₂ and O₂

Shifting of the CH₄ lines by pressures of N₂ and O₂