Impact of Errors in Atmospheric State on XCO2 Retrievals from a Laser CO2 Sounder

NASA Goddard is developing an integrated-path, differential absorption 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 very high spectral resolution and high sensitivity to atmospheric CO₂ changes. Atmospheric states from a global data assimilation and forecast model are used as ancillary data to produce the best retrievals of column-averaged CO₂ mixing ratio with regards to dry air (XCO₂). Retrieval error, both bias and random error, depends on uncertainties in atmospheric state for radiative transfer calculations that are used to fit measured CO₂ absorption line shape for XCO₂ 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 XCO₂ retrievals. Uncertainty in atmospheric pressure and water vapor also further increase retrieval error.

We use data from the 2017 ASCENDS DC-8 airborne science campaign to quantify the atmospheric impact on XCO₂ retrievals using meteorological data from the Goddard GEOS 'forward processing' system interpolated to aircraft ground tracks, aircraft flight data, and radiosondes during spiral down segments. We also use atmospheric states from re-analysis systems, MERRA, MERRA-2, and ECMWF ERA-Interim to study the impact of differences in atmospheric state on simulated XCO₂ retrievals from space as a part of ASCENDS Observing System Simulation Experiments. We will present 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. Given sufficiently accurate meteorological data, the laser CO₂ Sounder will provide high-precision, low-biased, and full global and seasonal sampling atmospheric CO₂ measurements that will contribute substantially to quantifying surface fluxes and advancing carbon cycle sciences.