Precise methane absorption measurements in the 1.64 μm spectral region for the MERLIN mission. Issue 12 (18th June 2016)

• Record Type: Journal Article
• Authors: Delahaye, T.; Maxwell, S. E.; Reed, Z. D.; Lin, H.; Hodges, J. T.; Sung, K.; Devi, V. M.; Warneke, T.; Spietz, P.; Tran, H.
• Is Part Of: Journal of geophysical research. Volume 121:Issue 12(2016)
• Page Start: 7360
• Page End: 7370
• Publication Date: 2016-06-18
• Subjects: MERLIN -- methane -- LIDAR -- absorption -- line shape -- CRDS; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1002/2016JD025024
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: In this article we describe a high‐precision laboratory measurement targeting the R(6) manifold of the 2ν3 band of 12 CH4 . High‐fidelity modeling of this absorption spectrum for atmospheric temperature and pressure conditions 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 ratio 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 µmol mol −1 and 12 µmol 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 online 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. Key Points: We present a high‐precision measurement of methane absorption in the 1.64 μm spectral region for the MERLIN mission High‐resolution and high signal‐to‐noise absorption spectra of methane in air were recorded using a frequency‐stabilized CRDS apparatus Obtained data and model allow for the calculation of CH4 absorptions with a precision of better than 0.1% at the online position of MERLIN