CO$_2$ collision-induced line parameters for the $ν_3$ band of $^{12}$CH$_4$ measured using a hard-collision speed-dependent line shape and the relaxation matrix formalism

TL;DR

This study quantifies CO$_2$-induced collisional line parameters for the ν$_3$ band of $^{12}$CH$_4$ near 3.3 μm at $296.5$ K using a hard-collision speed-dependent line-shape and two line-mixing approaches: Rosenkranz first order and the relaxation-matrix formalism. Eleven high-resolution Fourier transform spectra, including pure CH$_4$ and CH$_4$–CO$_2$ mixtures up to ~803 hPa, are analyzed with multispectrum nonlinear least squares, calibrating the wavenumber scale and extracting CO$_2$ broadening, broadening speed-dependence, Dicke narrowing, and CO$_2$ shift coefficients; HITRAN parameters provide line centers and intensities. The relaxation-matrix model improves residuals for congested P/R manifolds, but challenges remain in the low-$J$ region of the Q-branch, suggesting the value of a hybrid modeling approach and further data. Notably, this work reports CO$_2$ shift coefficients for the ν$_3$ band for the first time and sets a comprehensive set of CO$_2$-perturbed parameters at near-atmospheric pressures that can inform planetary-atmosphere radiative-transfer models and CIA studies in CO$_2$-rich environments.

Abstract

Ten high resolution Fourier transform spectra of the pentad region near 3.3 μm of methane diluted in carbon dioxide at total pressures up to 800 hPa have been recorded at 296.5(5) K. Including a high resolution spectrum of pure methane at low pressure, these spectra have been analyzed using multi-spectrum fitting techniques. The methane lines were modeled using hard-collision speed-dependent line profiles and line mixing was included in the strongest absorption regions, considering the first order Rosenkranz approximation and the relaxation matrix formalism. CO$_2$ broadening and shift coefficients have been measured, together with the speed dependence of broadening. Results obtained using the two line mixing models are intercompared and compared with previous work.

Figures (15)

• Figure 1: Least squares fit of the R(0, $A_1$, 1, 3) line of the $\nu_3$ band of $^{12}$CH$_4$ (spectrum S1 in Table \ref{['tab:exp_details']}, 296.5 K and 19.7 cm) with the ILS determined using N$_2$O (blue trace) and modified relying on the R(0, $A_1$, 1, 3) and R(1, $F_1$, 1, 10) lines observed in spectrum S1 (red trace).
• Figure 2: Results of the fit of the R(0, $A_1$, 1, 3) (left) and R(1, $F_1$, 1, 10) (right) lines of the $\nu_3$ band of $^{12}$CH$_4$ perturbed by CO$_2$ using 4 different theoretical line shape models ("qSD" stands for "quadratic speed dependent"). Four of the 11 transmittance spectra measured are presented in the top row (296.5 K and 19.7 cm; the pressures and mole fractions are provided in Table \ref{['tab:exp_details']}). The best-fit residuals obtained for the 4 spectra of the two lines are presented in the lower 4 rows, the color of each trace being associated with a specific line shape model.
• Figure 3: Transmittance spectra of the P branch of the $\nu _3$ band of methane (top panel) and corresponding residuals obtained for the fits F1, F2, F3 and F4 (from top to bottom).
• Figure 4: Transmittance spectra of the Q branch of the $\nu _3$ band of methane (top panel) and corresponding residuals obtained for fit F4.
• Figure 5: Transmittance spectra of the R branch of the $\nu _3$ band of methane (top panel) and corresponding residuals obtained for the fits F1 (middle panel) and F4 (bottom panel).