Sensitivity analysis of oxygen absorption lines in the 1.26-1.27 micron spectral band

In the Decadal Survey prepared by the National Research Council (Reference: Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond), the ASCENDS mission (Active Sensing of CO2 Emissions over Nights, Days and Seasons), requires simultaneous laser remote sensing of CO2 and O2 in order to convert CO2 atmospheric concentrations to mixing ratios. As the mission is envisioned, the CO2 mixing ratio needs to be measured to a precision of 0.5 percent of background or better (slightly less than 2 ppm) at 100-km horizontal length scale overland and at 200-km scale over open oceans. While the O2 measurement could be made at 0.765 µm (the oxygen A band), the absorption cross section is substantially higher and the scattering is lower in the 1.26-1.27 µm wavelength band, and as such it is anticipated that better accuracies could be accomplished. Hence, NASA Langley Research Center is developing oxygen lidar technology in the 1.26-1.27 micron band for surface pressure measurements. One or more wavelengths for differential absorption lidar operation have to be carefully chosen to eliminate ambient influences on them. The model optical depth calculation is very sensitive to knowledge of the transmitted wavelengths and to the choice of Voigt input parameters. Uncertainties in atmospheric profiles of temperature, pressure and relative humidity can cause ~0.5 % errors in model optical depths. In order to select candidate wavelengths in the 1.26 micron spectral band, wavelength uncertainties due to temperature and pressure have to be determined. Uncertainties at line center and offset wavelengths have to be known precisely to reduce uncertainties in oxygen concentration measurements from airborne and space based platforms. In this paper, based on HITRAN database and absorption line measurements, we evaluate systematic relative errors and their sources of pressure shift and atmospheric temperature influences for selected O2 lines suitable for making high precision measurements.