Atmospheric CO2 Column Measurements from Laser Absorption Spectrometry Lidar Systems
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.