The ν1, ν4 and 3 ν6 bands of methyl chloride in the 3.4-μm region: Line positions and intensities

Abstract

Methyl chloride (CH 3Cl) is one of the most abundant chlorine-containing molecules in the atmosphere. For this reason a recent update was performed in HITRAN in the 640–2600 cm −1 region based on the line parameters generated in Nikitin et al. [Nikitin A, Champion JP, Bürger H. J Mol Spectrosc 2005;230:174–84] with the intensities scaled to existing experimental data. CH 3Cl has a rather strong signature around 3000 cm −1 which was used recently by the Atmospheric Chemistry Experiment (ACE) satellite mission to produce the first study of the global distribution of methyl chloride in the upper troposphere and stratosphere. However, it was mentioned that the CH 3Cl line positions and intensities spectroscopic parameters are of very low quality in this spectral region in the public access HITRAN or GEISA databases. We present a complete update of the line positions and line intensities for the ν 1, ν 4, 3 ν 6 bands of CH 3 35Cl and CH 3 37Cl in the 3.4 μm region. For this task, Fourier transform spectra have been recorded at high resolution at the Laboratoire de Dynamique, Interactions et Réactivité in France. Measurements of line positions and line intensities have been retrieved for both isotopologues 12CH 3 35Cl and 12CH 3 37Cl in the ν 1, ν 4, 3 ν 6 bands. The theoretical model accounts for the interactions coupling the ( ν 1=1; ℓ=0), ( ν 4=1; ℓ=±1) and ( ν 6=3; ℓ=±1) energy levels, together with additional resonances involving several dark states.