The HITRAN2024 methane update

Abstract

Spectroscopic parameters of methane from many different studies were gathered to improve the HITRAN database towards its 2024 version. After a validation process using high-resolution FTS and CRDS spectra, about 80,000 lines of the four most abundant isotopologues were replaced from the dyad to the triacontad regions. These changes amount to 51,000 transition wavenumbers, 18,000 line intensities, 33,000 pressure-broadening half-widths, and 3300 assignments. 44,000 new lines were added with 16,000 old lines removed, extending the database from 12,000 cm⁻¹ up to 14,000 cm⁻¹, and covering some gaps. A greater focus was brought on the pentad, octad, and tetradecad regions, targeted by several remote sensing instruments. In these regions, comparisons of spectral fits from multiple line lists were performed, taking only the parameters that provide best fit for each line. In the ${\nu }_3$ band, in addition to replacing the previous values, speed-independent pressure broadening parameters of ¹²CH₄ were gathered and used to fit Padé-approximant functions. These functions then replaced any outdated experimental data in ${\nu }_3$, missing data in the new lines, as well as the values that were determined to be outside their physical boundaries. The CH₃D broadening parameters were replaced in the same manner, for missing and low or high values, using a semi-empirical formula instead.