Atmospheric and Laser Spectral Influences on the Column CO2 Measurements at 1.57 μm

The 2007 National Research Council Decadal Survey recommended implementation of the Active Sensing of CO2 Emission over Nights, Days, and Seasons (ASCENDS) mission to address uncertainties in the knowledge of the sources and sinks of atmospheric carbon dioxide. Consequently, ASCENDS has become a part of NASA plans to monitor atmospheric carbon dioxide form space using active remote sensing. One such technique under evaluation by NASA Langley Research Center is the integrated path differential absorption (IPDA) lidar operating near 1571 nm. High precision and accuracy, of about 0.1% each, are required for the carbon dioxide mixing ratio measurements in order to improve our understanding of the gas sources and sinks. Therefore, applying the IPDA technique requires an evaluation of the influence of systematic errors and biases resulting from uncertainties in various atmospheric conditions. These conditions include atmospheric temperature, pressure and relative humidity. Furthermore, the impacts of these conditions on the spectral characteristics of the carbon dioxide absorption, transmitted laser source and other interfering atmospheric species, such as absorbing molecules and scattering aerosols have to be investigated. In this paper systematic error sources are evaluated for measurements of atmospheric carbon dioxide using the IPDA technique. The study is focused on the carbon dioxide R24 line at 1571.111903 nm wavelength. This line has been selected for operation by the airborne Multi-functional Fiber Laser Lidar (MFLL) system that is a precursor to a space-based IPDA system. For modeling the systematic effects, 2008 HITRAN database was used, as well as most recent updates, to obtain the spectral lines parameters. The Voigt profile was applied to model the absorption spectra of carbon dioxide, as well as those of the interfering species consisting of water vapor, carbon monoxide, nitrous oxide, oxygen, ozone, acetylene and methane. For all species, absorption lines within the spectral range of 1570.95 nm to 1571.30 nm were included in the calculation. The 1986 AFGL mid-latitude summer atmospheric model parameters were applied covering 0 to 80 km altitude. The calculations were performed at the carbon dioxide line-center (on-line), +3 pm and +10 pm side-lines, and +50 pm and -50 pm off-lines. Influences of pressure (and pressure shift) and temperature sensitivities of the absorption cross-section on column carbon dioxide measurements are presented. Influences of laser line-width, spectral purity and spectral stability for operation at the on-line and side-line positions are also discussed.