Title of Presentation:
Primary (Corresponding) Author: James Abshire / Haris Riris
Organization of Primary Author: Goddard Space Flight Center
Co-Authors:
Abstract:
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.