Line profile study and intensity depletion of the R-branch manifolds in the 2ν3 band of 12CH4 in dry air

• High S/N spectra of 12 CH 4 in air are recorded for the R-branch of the 2ν 3 band • A cavity ring down spectrometer referenced to an optical frequency comb is used • Line parameters are retrieved with reduced uncertainty for the R0 to R10 manifolds • Intensity depletion coefficients are obtained for the first time for CH4 in air • This new dataset would be valuable to improve the spectroscopic databases Methane is the second most important anthropogenic greenhouse gas. The 2ν 3 band of 12 CH 4 is widely used for the global mapping of methane concentrations by satellite instruments requiring highly accurate spectroscopic parameters for the rotational manifolds broadened by air. From a more fundamental standpoint, it is also important to study the dependence of the line-shape parameters with the rotational quantum number. The present work is devoted to an accurate study of the R(0) to R(10) manifolds of this 2ν 3 band located between 6015 and 6115 cm -1 , at room temperature. For that, a mixture of methane in dry air at pressures between 6.67 and 100 kPa is flushed into a high finesse cavity of a cavity ring down spectrometer linked to an optical frequency comb referenced to a GPS-disciplined Rb oscillator. A RF tunable narrow-line comb-disciplined laser source is coupled into the optical cavity thanks to the implementation of the comb-coherence transfer technique using an electro-optic modulator. A multi-spectrum fit procedure allows retrieving the line-shape parameters for each component of the studied manifolds with reduced uncertainties. In order to achieve fit residuals at the noise level (∼2 × 10 -11 cm -1 ), Hartmann-Tran profiles (HTP) are adopted and the line-mixing effect is taken into account. The effect of experimental intensity depletion with density, ∼0.4 % amagat -1 , is determined for the first time in methane and shows almost no rotational dependence. The obtained dataset could be valuable to improve the spectroscopic databases like HITRAN which are inputs of radiative transfer models used to invert atmospheric spectra.