Title of Presentation: Laser Sounder for Measurement of CO2 Concentrations in the Troposphere for the ASCENDS Mission - Progress

Primary (Corresponding) Author: James Abshire / Haris Riris

Organization of Primary Author: Goddard Space Flight Center

Co-Authors: Haris Riris, S. Randy Kawa, Graham Allan, Xiaoli Sun, Michael A. Krainak, Jianping Mao, Mark A. Stephen, Emily Wilson

Abstract: Measurements of tropospheric CO2 abundance with global-coverage, a few degree spatial and monthly temporal resolutions are needed to better understand processes controlling CO2 fluxes with atmosphere. With the support of the ESTO IIP program, we have been developing a laser-based sounding technique for measurements of the tropospheric CO2 concentrations from orbit for the ASCENDS Earth Science mission. Our initial goal is to demonstrate the technique and technologies for ground-based and aircraft measurements. Our final goal is to develop a space instrument and mission approach for laser measurements of the CO2 mixing ratio at the 1-2 ppmv level. Our pulsed technique allows measurements of CO2 mixing ratio though thin clouds from orbit over land and ocean surfaces during day and night.

Our approach is to use the 1570nm CO2 band and a 3-channel pulsed laser absorption spectrometer, which continuously measures at nadir from a near polar circular 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 and 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 pulse energies.

Our technique exploits the atmospheric pressure broadening of the lines to weight the measurements to the column below 5 km. This maximizes sensitivity to CO2 in the boundary layer, where variations caused by surface sources and sinks are largest. Laser altimetry and backscatter profiles are also measured, to determine the path length and measurements made to cloud tops and through aerosol 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. Nonetheless, the optical absorption change due to a change of a few ppm CO2 is small, 600:1 are needed to estimate CO2 mixing ratio at the 1-2 ppm level.

We have calculated characteristics of the technique, and have demonstrated key aspects of the laser, detector and receiver approaches in the laboratory. We have also measured CO2 and O2 in an absorption cells, and made a series CO2 measurements over outdoor path lengths from 200m to 2.2 km. We will describe more details in the presentation.