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Precise methane absorption measurements in the 1.64 micron spectral region for the MERLIN mission

Published

Author(s)

Joseph T. Hodges, Stephen E. Maxwell, Zachary D. Reed, Hong Lin, Keeyon Sung, Malathy Devi, Thomas Warneke, Peter Spietz, Ha Tran, Thibault Delahaye

Abstract

In this article we describe a high-precision laboratory targeting the R(6) manifold of the 2ν3 band of 12CH4. Accurate physical models of this absorption spectrum will be required by the Franco-German, Methane Remote Sensing LIDAR (MERLIN) space mission for retrievals of atmospheric methane. The analysis uses the Hartmann-Tran profile for modeling line shape and also includes line-mixing effects. To this end, six high-resolution and high signal-to-noise absorption spectra of air-broadened methane were recorded using a frequency-stabilized cavity ring-down spectroscopy apparatus. Sample conditions corresponded to room temperature and spanned total sample pressures of 40 hPa -1013 hPa with methane molar fractions between 1 mol mol-1 and 12 mol mol-1. All spectroscopic model parameters were simultaneously adjusted in a multispectrum nonlinear least-squares fit to the six measured spectra. Comparison of the fitted model to the measured spectra reveals the ability to calculate the room-temperature, methane absorption coefficient to better than 0.1% at the on-line position of the MERLIN mission. This is the first time that such fidelity has been reached in modeling methane absorption in the investigated spectral region, fulfilling the accuracy requirements of the MERLIN mission. We also found excellent agreement when comparing the present results with measurements obtained over different pressure conditions and using other laboratory techniques. Finally, we also evaluated the impact of these new spectral parameters on atmospheric transmissions spectra calculations.
Citation
Journal of Geophysical Research-Atmospheres

Keywords

methane, remote sensing, MERLIN, atmosphere, greenhouse gas, spectroscopy

Citation

Hodges, J. , Maxwell, S. , Reed, Z. , Lin, H. , Sung, K. , Devi, M. , Warneke, T. , Spietz, P. , Tran, H. and Delahaye, T. (2016), Precise methane absorption measurements in the 1.64 micron spectral region for the MERLIN mission, Journal of Geophysical Research-Atmospheres, [online], https://doi.org/10.1002/2016JD025024 (Accessed December 4, 2021)
Created June 2, 2016, Updated November 10, 2018