Contributions of argon, nitrogen, and oxygen to air broadening in the oxygen A-band

Abstract

The oxygen (O₂) A-band is used to determine the airmass in ground- and space-based remote sensing measurements because O₂ is well-mixed in the Earth’s atmosphere and its column-integrated amount fraction on a dry-gas basis is nearly constant. Because biases in the retrieved airmass propagate to measurements of target species, low-uncertainty spectroscopic parameters are essential for increasingly precise and accurate greenhouse gas measurements. However, laboratory measurements and atmospheric retrievals of this O₂ band typically neglect the line-shape effects caused by collisions with argon (Ar), which comprises 0.934 % by volume of the Earth’s atmosphere. To quantify the contribution of Ar to O₂ A-band air-broadening line-shape parameters, we measured pressure broadening and shifting parameters for ten high J lines in the P-branch of this band. These data were acquired in the laboratory over a range of pressures and nitrogen (N₂), O₂, and Ar amount fractions using cavity ring-down spectroscopy. Respective line-shape parameters for these collisional partners were determined with a multi-spectrum fitting algorithm. These results were combined with literature data to provide an empirical model for the rotational dependencies of the broadening and shifting parameters by each collisional partner. Incorporating these results into analyses of atmospheric column-integrated solar absorption spectra in the O₂ A-band shows that the neglect of Ar can lead to a small but potentially relevant systematic bias in surface pressure retrievals and a slight increase in the fit residuals of atmospheric spectra.