Accurate Intensities and Line Parameters for the 2v3-Band of Methane Based on Coordinated Fourier-Transform and Cavity Ring-Down Spectroscopy Measurements from three Independent Laboratories

Abstract

A quantitative understanding of methane (CH4) spectra in the 6050 cm-1 - 6250 cm-1 region is essential for atmospheric remote-sensing applications (e.g., TCCON, GOSAT-2, MERLIN) based on both solar- and laser-based absorption methods. This region corresponds to the 2v3 vibrational band, consisting of manifolds and multiplets with two or more transitions exhibiting strong spectral congestion, causing up to 10% uncertainties in HITRAN data, limiting methane retrieval precision. Here we report the combined results of a two-part study of the R(0) through R(10) manifolds of this methane vibrational band, Reed et al.1 , and Yin et al.2 , involving independent Fourier-transform spectroscopy (FTS) and cavity ring-down spectroscopy (CRDS) measurements made in three laboratories over a wide range of pressure. In Part I, measurements were made at relatively low pressures by DLR (using FTS) and NIST (with CRDS), while in Part II, N2-broadened methane spectra were acquired at USTC using CRDS. The lines were nearly Doppler-broadened in Part I, with collisional line-shape parameters and line-mixing informed by the Part II study entering only as small correction. For these measurements, the zero-pressure line positions were constrained to the high-accuracy (kHz-level) values reported by Votava et al.3 - thus largely eliminating correlations between blended lines retrieved from multispectrum fits. The ratio of the summed manifold intensities was 0.6 permille with a standard deviation of 1.4 permille. In Part II, spectra were modeled using the speed-dependent Voigt profile, with first-order line mixing and using the line intensities determined in Part I, for atmospheric applications, air-broadened values for the line-shape parameters were determined using scaling rules. The resulting line-shape parameters had uncertainties of less than 1 %, which when combined with the line intensities from Part I, represents a significant improvement over the previous HITRAN2020 data. These results serve as an important benchmark for theoretical calculations and future measurements, thereby improving methane spectral line lists.