Speed-Dependent Broadening and Line-Mixing in CH4 Perturbed by Air near 1.64 μm

Aug 25, 2026

Uploaded by Margaret

Climate change research highlights methane as a key anthropogenic greenhouse gas, motivating the MERLIN satellite mission for global atmospheric methane retrieval with high precision and spatial resolution. Reliable spaceborne spectroscopic monitoring near 1.64 μm requires absorption line shapes beyond the standard Voigt profile, since Voigt-based predictions can be inadequate for methane. Modeling focuses on air-broadened CH4 line parameters and includes line-mixing, Dicke narrowing, and speed-dependent collisional broadening and shifting. Parameters are simultaneously fitted to cavity ring-down spectroscopy over 5–100 kPa, supporting later analysis of TCCON observations.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages1

Reading Time3 min

AccessFree to download

Modeling of Methane Line Shapes for MERLIN

Limits of the Voigt Profile

Included Collisional Effects

Experimental Fitting and Applications

SPEED-DEPENDENT BROADENING AND LINE-MIXING IN CH4 PERTURBED BY AIR NEAR 1.64 􀀖mFOR THE FRENCH/GERMAN CLIMATE MISSION MERLIN THIBAULT DELAHAYE, THI NGOC HA TRAN, CNRS et Universite´s Paris Est et Paris Diderot, Laboratoire Interuniversitaire des Syst mesAtmosphe´riques (LISA), Cre´teil, France; ZACHARY REED, STEPHEN E MAXWELL, JOSEPH HODGES, Chemical Sciences Division, National Institute of Standards and Tech nology, Gaithersburg, MD, USA. Climate change is one of the greatest challenges presently facing mankind, and methane is one of the most powerful anthropogenic greenhouse gases. For a better understanding of future climate trends, a satellite dedicated to the measurements of atmospheric methane is under joint development by the French and German space research centers (CNES and DLR) . The so-called MERLIN mission (Methane Remote Sensing Lidar Mission, 2019) aims at providing global information on atmospheric methane concentration with a relative uncertainty less than 2% and with a spatial resolution of 50 kma. Such spectroscopic monitoring of gases in the atmosphere of the Earth, requires a precise description of absorption lines shapes that goes beyond the usual Voigt pro􀀂le (VP) . In the case of methane, the differences between the measured pro􀀂les and those given by the VP can be very important b , making the VP completely incompatible with the reliable detection of sources and sinks from space. In this work, we present the 􀀂rst results on the modeling of methane lines broadened by air in the 1.64 􀀖m region and the associated spectroscopic parameters, taking into account various collisional effects between molecules that are neglected by the VP: collisional interference between the lines (line-mixing), collision-induced velocity changes (Dicke narrowing effect) and speed dependence of the collisional broadening and shifting. These results were obtained by simultaneously 􀀂tting the model parameters to high sensitivity and high-resolution cavity ring-down spectroscopy (CRDS) spectra recorded at the National Institute of Standards and Technology (NIST) over a wide pressure range (5 to 100 kPa) . These spectroscopic data and the associated model to calculate the spectrum absorption coef􀀂cient will be then used to analyze ground-based atmospheric spectra at the TCCON facility in Park Falls, Wisconsin. aC. Kiemle, M. Quatrevalet, G. Ehret et al., Atmos. Meas. Tech. 4 (2011) bH. Tran, J.-M. Hartmann, G. Toon et al., Journal of Quant. Spectrosc. Radia. Trans. 111 (2010)

FAQs

Why is the Voigt profile insufficient for methane near 1.64 μm?

Observed methane line shapes can differ substantially from Voigt predictions, making the Voigt profile incompatible with dependable detection of atmospheric sources and sinks from space.

Which physical effects are included in the methane line-shape model?

The model accounts for line-mixing (collisional interference), Dicke narrowing from collision-induced velocity changes, and speed dependence of collisional broadening and shifting.

How were the model parameters obtained?

Model parameters were simultaneously fitted to high-sensitivity, high-resolution cavity ring-down spectroscopy spectra measured at NIST across a pressure range of 5 to 100 kPa.

Aug 25, 2026

Uploaded by Margaret