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

Abstract

In this article we describe a high‐precision laboratory measurement targeting the R(6) manifold of the 2ν3 band of 12CH4. 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.

MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane

“…The geophysical auxiliary parameters (surface pressure, temperature profiles and water vapour profiles) are provided by (NWP) centres. Absorption cross sections of methane and other "interfering gases" for the desired temperature and pressure ranges are derived from line-by-line calculations using updated spectroscopic databases in combination with numerical models for the calculation of the integrated weighting function [26]. This method can also be applied to infer additional information on partial CH 4 columns above cloudy scenes by taking the signal reflected (scattered) by cloud particles at cloud tops [66].…”

“…Further, the accurate retrieval of these small changes is limited by the knowledge of spectroscopic parameters and of other atmospheric species which may have overlapping absorptions in the same spectral region. Recent progress in the methane spectroscopy in the near-infrared should reduce uncertainty in retrievals (e.g., [26]). CH 4 fluxes emitted from the surface result in small changes in the spatial distributions of the above CH 4 dry column mole fraction (e.g., typically a few ppb, and up to a few tens of ppb, on a day-to-day basis and at a typical model spatial resolution of 200 × 200 km, (see Science Plan of MEthane Remote sensing Lidar missioN (MERLIN) [27] with most of the variation happening in the boundary layer.…”

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

“…Note that the speed dependence of the line mixing parameter was also neglected. In the fitting procedure, the room temperature parameters of our HT + line mixing (LM) model were fixed to those obtained in Delahaye et al (), and only the temperature dependences of Γ 0 , Δ 0 , and ζ were adjusted. To simplify the fitting procedure, in the first step, only spectra measured at the lowest temperature (i.e., 223 K, see Table ) were considered in the fit.…”

The challenges of measuring methane from space with a lidar

“…Our preliminary radiative transfer calculations show that both lines (Er:YAG at 1645.55 nm and Er:YGG at 1650.96 nm) have similar temperature sensitivity and are well suited for space born CH4 measurements. Recent high accuracy spectroscopic measurements indicate that line mixing effects in the Er:YAG 1645.55 nm line 57 should also be taken into account. We expect similar effects to be present for the Er:YGG line at 1650.96 nm.…”