Comb-anchored, Cavity Ring-down Spectroscopy Of The 1.27 μm Band Of O₂

New measurements of the $a^{1}\Delta_{\mathrm g} \leftarrow X^{3}\Sigma_{\mathrm g}^{-}$ band of oxygen (\chem{^{16}O_2}) at 1.27 $\mu$m will be presented, improving on a previous study [Mendonca et al., Atmos. Meas. Tech. 12, 35-50 (2019)]. Spectra were acquired by frequency-stabilized cavity ring-down spectroscopy over a 160 \wn wave number range (7792 \wn to 7952 \wn). The frequency axis was anchored to a Cs-clock-referenced optical frequency comb through a heterodyne beat note between the comb and the probe laser at about 20 points across the wave number range. The probe laser was phase locked to the frequency comb prior to measuring the beat note frequency in order to improve the accuracy, yielding a 10-Hz uncertainty in the ring-down cavity free spectral range and a 50-kHz absolute frequency uncertainty for all mode orders. Six air-broadened spectra were recorded, at pressures ranging from 3.3 kPa to 100 kPa. They were analyzed with custom multi-spectrum fitting software based on the HAPI python library, using the speed-dependent Nelkin-Ghatak profile. The resulting line shape parameters reveal important discrepancies with the HITRAN2016 values. These results will also be compared to those reported in recent studies by the Grenoble group [Konefał et al., JQSRT 241, 106653 (2020) and Tran et al., JQSRT 240, 106673 (2020)]

The HITRAN2020 molecular spectroscopic database

The HITRAN database is a compilation of molecular spectroscopic parameters. It was established in the early 1970s and is used by various computer codes to predict and simulate the transmission and emission of light in gaseous media (with an emphasis on terrestrial and planetary atmospheres). The HITRAN compilation is composed of five major components: the line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, experimental infrared absorption cross-sections (for molecules where it is not yet feasible for representation in a line-by-line form), collision-induced absorption data, aerosol indices of refraction, and general tables (including partition sums) that apply globally to the data. This paper describes the contents of the 2020 quadrennial edition of HITRAN. The HITRAN2020 edition takes advantage of recent experimental and theoretical data that were meticulously validated, in particular, against laboratory and atmospheric spectra. The new edition replaces the previous HITRAN edition of 2016 (including its updates during the intervening years).All five components of HITRAN have undergone major updates. In particular, the extent of the updates in the HITRAN2020 edition range from updating a few lines of specific molecules to complete replacements of the lists, and also the introduction of additional isotopologues and new (to HITRAN) molecules: SO, CH$_3$F, GeH$_4$, CS$_2$, CH$_3$I and NF. Many new vibrational bands were added, extending the spectral coverage and completeness of the line lists. Also, the accuracy of the parameters for major atmospheric absorbers has been increased substantially, often featuring sub-percent uncertainties. Broadening parameters associated with the ambient pressure of water vapor were introduced to HITRAN for the first time and are now available for several molecules.The HITRAN2020 edition continues to take advantage of the relational structure and efficient interface available at www.hitran.org and the HITRAN Application Programming Interface (HAPI). The functionality of both tools has been extended for the new edition