Atmospheric CO2 Column Measurements from Laser Absorption Spectrometry Lidar Systems

Advanced knowledge in atmospheric CO2 is critical in predicting the Earth's future climate. Large uncertainties in the prediction persist due to limited observations; thus, the National Research Council (NRC) recommended the Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) as a mid-term, Tier II, mission to NASA in its 2007 Decadal Survey of Earth Science and Applications from Space. As part of the preparation for the ASCENDS mission, NASA Langley Research Center (LaRC) and ITT Exelis have been collaborating in development and demonstration of the Intensity-Modulated Continuous-Wave (IM-CW) approach for measuring column CO2 from space. Airborne prototype Laser Absorption Spectrometers such as the Multi-Functional Fiber Laser Lidar (MFLL) and ASCENDS CarbonHawk Experiment Simulator (ACES) have been developed and tested to obtain precise atmospheric CO2 measurements. These systems simultaneously measure atmospheric CO2 and O2 columns in the 1.57-micron and 1.26-micron regions, respectively, to retrieve dry air column-averaged CO2 mixing ratios.

These LAS instruments, especially MFLL, have been operated onboard various NASA aircraft over the past several years for high-precision column CO2 measurements over both land and ocean surfaces under a variety of atmospheric conditions. IM-CW Online and Offline laser signals are transmitted and the lidar returns of these signals are detected and measured using a matched filter. The Integrated Path Differential Absorption (IPDA) approach is used to determine column CO2 and O2 number densities and ranges from aircraft to surfaces or cloud/aerosol layers. In situ atmospheric CO2, moisture, pressure, and temperature measurements are obtained from aircraft spirals and used as CO2 truth in the evaluation of the accuracy of the remote lidar measurements. Our lidar remote sensing observations from NASA flight campaigns have demonstrated insensitivity to variations in surface reflectivity and compared well with CO2 column amounts derived from in situ observations. Range-encoded IM schemes used by these instruments have provided the capability to minimize the influence of intermediate backscatterers such as thin clouds on the column CO2 measurements and to determine the range to the surface and backscatterers. In this presentation, the atmospheric CO2 column measurements from previous flight campaigns will be discussed. Analysis shows that the lidar CO2 column measurements over desert and vegetated surfaces agree with those calculated from in-situ measurements of atmospheric meteorological and CO2 profiles to within an average of 0.17% or ~0.65 ppmv. A measurement precision of 0.08% or ~0.3 ppmv for a 10-s average over these surfaces has also been achieved. Generally, airborne flight campaigns have demonstrated that the CO2 measurements of the current IM-CW LAS systems meet the accuracy and precision requirements of the ASCENDS mission. Furthermore, analyses of space CO2 measurements shows that the current IM-CW LAS technology and approach will enable the ASCENDS mission to achieve its science goals.