Impact of Error in Atmospheric State on Column CO2 Retrievals from a Laser CO2 Sounder

NASA Goddard is developing an integrated-path, differential absorption (IPDA) lidar approach to measure global atmospheric column CO2 concentrations from space as a candidate for NASA's Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission. This pulsed laser approach uses a step-locked laser diode source and a high-efficiency detector to measure atmospheric column CO2 absorption at multiple wavelengths across a CO2 line centered at 1572.335 nm with minimum temperature sensitivity. Atmospheric states from a global numerical forecast and data assimilation model are used as ancillary data to produce the best retrievals of column-averaged CO2 mixing ratio with regards to dry air. Retrieval error, both bias and random error, depends on uncertainties of atmospheric states for atmospheric radiative transfer calculations that are then used to fit measured CO2 absorption line shape for retrievals. Temperature data uncertainty, for example, can modify air density as well as absorption line intensity and line shape, which could cause significant error in radiative transfer calculations and then in column CO2mixing ratio retrievals. Uncertainty in atmospheric pressure and water vapor could also further increase retrieval error. We use atmospheric temperature profiles from Atmospheric InfraRed Sounder retrievals and the European Center for Medium range Weather Forecasting Model to assess temperature impact on spaceborne measurement of ASCENDS using our Goddard IPDA approach. We find the temperature differences produce a small impact on optical depth measurements on our CO2 line. Uncertainty in the atmospheric surface pressure could cause greater impact, implying a requirement for accurate dry air column density information in addition to laser ranging capability. We use data from the 2014 and 2016 ASCENDS airborne science campaigns to evaluate the atmospheric impact on our column CO2 concentration retrievals using the Goddard GEOS-5 meteorological data along aircraft tracks, aircraft flight data, and radiosondes during spiral down segments. We will show some preliminary results of this investigation and discuss the measurement requirements related to ancillary data, water vapor information, dry air density and its relationship with surface pressure and photon pathlength.