Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81 K: (I) The region of the 2ν3 band at 1.66 μm
Résumé
In a recent contribution, (Gao B, Kassi S, Campargue A. Empirical low energy values for methane transitions in the 5852–6181 cm−1 region by absorption spectroscopy at 81 K. J Mol Spectrosc 2009;253:55–63.), the low energy values of methane transitions between 1.71 and 1.62 μm were derived from the variation of the line intensities between 296 and 81 K. The line intensities at 81 K were retrieved from the high resolution absorption spectrum of methane recorded at liquid nitrogen temperature by direct absorption spectroscopy using a cryogenic cell and a series of distributed feed back (DFB) diode lasers. For the line intensities at 296 K, the values provided by the HITRAN database were used. As a consequence of the relatively high intensity cut off (4×10−24 cm/molecule) of the HITRAN line list in the considered region, the lower energy values were derived for only 845 of the 2187 transitions measured at 81 K. In the present work, our line list was extended by the retrieval of many weak line intensities leading to a set of 3251 transitions. The minimum value of the measured line intensities (at 81 K) is on the order of 10−26 cm/molecule. In relation with the project “Greenhouse Gases Observing Satellite” (GOSAT), a much more complete line list for CH4 at 296 K has become available (intensity cut off of 4×10−26 cm/molecule). By applying the two temperature method to our line intensities at 81 K and GOSAT intensities at 296 K, the lower energy values of 2297 transitions could be derived. These transitions represent 99.1% and 90.8% of the total absorbance in the region, at 81 and 296 K respectively. This line list provided as Supplementary Material allows then accounting for the temperature dependence of CH4 absorption below 300 K. The investigated spectral range is dominated by the 2ν3 band near 6005 cm−1 which is of particular interest for atmospheric retrievals. The factor 2 narrowing of the Doppler linewidth from room temperature down to 81 K has allowed the resolution of a number of 2ν3 multiplets and improving the line intensity retrievals. A detailed comparison with GOSAT and HITRAN line lists has revealed a number of possible improvements.