Measurement and Modeling of Air-Broadened Methane Absorption in the MERLIN Spectral Region at Low Temperatures

The ability to precisely model methane absorption in the R(6) manifold of the 2ν₃ band at atmospheric pressure and temperature conditions is a key technical requirement of the German-French, Methane Remote Sensing Lidar (MERLIN) space mission. To this end, 27 high-resolution and high-signal-to-noise-ratio absorption spectra of air-broadened ¹²CH₄ were recorded using a variable-temperature frequency-stabilized cavity ring-down spectroscopy apparatus. The measurement conditions corresponded to sample temperature, pressure, and methane molar fraction values spanning 220–290 K, 4–110 kPa, and 4–7 μmol/mol, respectively. The measured spectra were fit using the sum of isolated Hartmann-Tran profiles with the addition of line mixing. For each line within the manifold, all spectroscopic parameters at room temperature were fixed to our previously obtained values (Delahaye et al., 2016, https://doi.org/10.1002/2015JD024524) and only the temperature dependences of the model parameters were adjusted. The results show that the fitted model agrees with the measured methane absorption to better than 0.3% for the entire R(6) manifold spectral region and to within 0.1% at the online position of the MERLIN for all considered pressure and temperature conditions. A first comparison with ground-based atmospheric measurement was also made showing significant improvement with respect to existing spectroscopic modeling of methane absorption.