Title: Pulsed Lidar for Measurement of CO2 Concentrations for the ASCENDS Mission - Progress
Primary Author: Abshire, James
Organization: NASA- Goddard, Solar System Exploration Division
Co-Author(s): Haris Riris, S. Randy Kawa, Xiaoli Sun, Mark A. Stephen, NASA GSFC; Graham Allan, Sigma Space Inc.; Jianping Mao, Clark Weaver, GEST Program, UMBC
Abstract: We have been developing a laser-based sounding technique for the remote measurement of the tropospheric CO2 concentrations from orbit for NASAís ASCENDS mission. The missionís goals are to provide measurements of tropospheric CO2 abundance with global-coverage, a few hundred km spatial and monthly temporal resolution are needed to better understand emissions and the processes that regulate CO2 storage by the land and oceans. For the IIP, we are developing and demonstrating the lidar techniques and instrument technology that will permit measurements of the CO2 column abundance in the lower troposphere from aircraft. Our final goal is to develop a space instrument and mission approach for active measurements of the CO2 mixing ratio at the 1-2 ppmv level. We use a pulsed lidar technique, which is much less sensitive to errors from cloud and atmospheric scattering and to noise from solar background. It allows continuous measurements of CO2 mixing ratio in the lower troposphere during day and night.
Our approach is to use the 1570nm CO2 band and a two-wavelength laser absorption spectrometer, which continuously measures at nadir from a circular polar orbit. It directs the narrow co-aligned laser beams from the instrument's lasers toward nadir, and measures the energy of the laser echoes reflected from land and water surfaces. It uses several tunable fiber laser transmitters, which allowing measurement of the extinction from a single selected CO2 absorption line in the 1570 nm band and from a line pair in the Oxygen A-band near 765 nm. These regions have temperature insensitive absorption lines are free from interference from other gases. The lasers use tunable diode seed lasers followed by fiber amplifiers, and have MHz spectral widths. During the measurement the lasers are tuned on- and off the selected lines at kHz rates. The receiver uses a 1.5-m diameter telescope and photon counting detectors and measures the background light and energies of the laser echoes from the surface. The extinction and column densities for the CO2 and O2 gases are estimated from the ratio of the on and off line echo pulse energies via the integrated path differential absorption (IPDA) technique. Our technique exploits the atmospheric pressure broadening of the lines to weight the measurement sensitivity to the atmospheric column below 5 km.
The tunable narrow linewidth lasers and sensitive photon counting detectors enable high spectral resolution and precision. Laser altimetry and backscatter profiles are also measured to determine the path length and measurements made to cloud tops and through scattering layers. Pulsed laser signals, time gated receiver are used to isolate the surface laser echo signals and to exclude photons scattered from clouds and aerosols. We have successfully demonstrated many key aspects of the approach and lidar technology in the laboratory. The CO2 column measurements have been demonstrated in field tests, and in airborne flights during 2008 and 2009. These included measurements from 3- 13 km altitudes over a variety of surfaces and through thin and broken clouds. More details will be given in the presentation.