CH4 Spectroscopy at 2.3 µm: analysis of ambient and low temperature pure CH4 measurements

Kurzfassung

The TROPOspheric Monitoring Instrument (TROPOMI) aboard the European Space Agency's Copernicus Sentinel-5 Precursor satellite, to be launched this year, requires high-accuracy spectral reference data for CH4 in the 2.3 µm region [1]. Retrieval simulations have been performed to specify spectroscopic requirements showing the need for line intensity measurements down to 1e-25 cm-1/(molecule cm-2). Pure CH4 measurements targeting line intensities have been carried out with a Bruker IFS 125HR Fourier transform spectrometer and a coolable multireflection cell. Eight ambient temperature spectra with CH4 pressures between 0.4 and 10 mbar and absorption paths between 15 and 168 m and three 193 K spectra with pressures between 1 and 5 mbar and absorption paths between 33 and 168 m were recorded at Doppler limited resolution. Baseline for the analysis was a line list from Vladimir Tyuterev [2] merged with HITRAN2012 [3]. The eleven spectra were analysed with a multispectrum fitting software developed at DLR [4] using the Voigt profile. The combination of low and ambient temperature measurements allowed fitting/validating lower state energies. It turned out that 2000 new lines had to be added and about 1600 lines were dropped from the initial list due to wrong position or lower state energy. The remaining 3900 fitted Tyuterev/Hitran2012 lines showed an averaged absolute line position difference of 0.0014 cm-1 and line intensity difference of 21%. Due to numerous blended lines and misassigned/missing lines with weak intensities the spectra, especially those with larger column amounts, could not be fitted down to the noise level. Therefore, line parameter based uncertainties are not meaningful. An alternative concept called “residual absorption cross sections” was introduced. The impact of the new results on simulated retrievals with TROPOMI together with the application of residual absorption cross sections will be discussed. References [1] J.P. Veefkind et al, Remote Sensing of Environment 120, 70 (2012) [2] V. Tyuterev, private communication [3] L.S. Rothman et al, JQSRT 130, 4 (2013) [4] J. Loos et al, 13th HITRAN database conference, 2014 (doi: 10.5281/zenodo.11156)