Line mixing and collision-induced absorption by oxygen in the A band: Laboratory measurements, model, and tools for atmospheric spectra computations

[ 1] The absorption by oxygen in the region of the O-2 A band near 760 nm has been measured in the laboratory under various conditions of pressure ( 20 - 200 atm) and temperature (200 - 300 K) for both pure O-2 and O-2-N-2 mixtures. In order to calculate the contribution of the "allowed'' A band transitions, Lorentzian profiles and a model accounting for line-mixing (LM) effects using the energy corrected sudden (ECS) approximation have been used. The differences between computed spectra and measured values enable extraction of the collision-induced absorption (CIA) contribution. It is shown that neglecting line mixing overestimates absorption in the wings and underestimates absorption at the P and R branch peaks, whereas the CIA extracted by the line-mixing approach shows the "smooth'' profile expected. Applying this approach to our spectra enables determination of the CIA and allowed contributions for both O-2-O-2 and O-2-N-2 collisions versus temperature and pressure. The resulting model and data are then used to build a database and some software suitable for the calculation of oxygen ( in air) atmospheric absorption and for easy inclusion in radiative transfer codes ( available upon request). These tools are then applied to a theoretical study of the influences of both line-mixing and collision-induced processes on atmospheric photon path escape factors and on cloud-top altitude retrievals. It is shown that LM and CIA make significant contributions and explain a large part of the discrepancies between measured and calculated atmospheric absorption observed recently.