SI Traceable Laboratory Determinations of A-Band Line Parameters for O2 Isotopologues With Remote Sensing Implications

The oxygen A-band has been used extensively in the retrievals of remote sensing measurements since the oxygen mole fraction is accurately known and time invariant. To ensure the integrity and long-term continuity of such retrievals, these measurements need to be grounded in high-quality intrinsic molecular data with well-defined traceability to the International System of Units (SI). Unfortunately, the presence of overlapping absorption bands of low natural abundance isotopologues (especially 16O18O and 16O17O) can introduce systematic errors in these retrievals unless they are precisely known and accounted for. In this study, the P-branch of the oxygen A-band was investigated for the 16O18O, 16O17O, 17O18O, and 18O18O isotopologues. We used frequency-stabilized cavity ring-down spectroscopy, a new high-accuracy and absolute spectroscopic method based on direct observations of time and frequency, to determine intensities, pressure-broadened widths, collisional narrowing coefficients, and positions. Absolute positions were measured relative to the hyperfine lines of 39K, resulting in an uncertainty of less than 10 MHz (0.0003 cm-1). Good agreement (~ 2 %) was shown between our reported intensities for 16O18O and those currently available in spectroscopic databases. However, a systematic, J-dependent, deviation is seen between our reported intensities for 16O17O and those of earlier studies, which utilized long pathlength Fourier-transform spectrometers. In some cases the values differ by as much as 10%. In contrast our reported line positions for 16O17O are in good agreement with currently accepted values (within 0.002 cm- 1), but show significant deviations for 16O18O (up to 0.3 cm-1). For long-time continuity it is crucial that remote sensing measurements are tied to well to known molecular constants which are connected to the SI. Therefore, we report molecular constants for the relevant excited states which were calculated based upon our observations. These constants are in close agreement with current database values.