Sensitivity Studies for a Space-based CO2 Laser Sounder

Jianping Mao, NASA GSFC/Wyle, jianping.mao@nasa.gov (Presenting)
Stephan R. Kawa, NASA GSFC, stephan.r.kawa@nasa.gov
James B. Abshire, NASA GSFC, james.b.abshire@nasa.gov
Haris Riris, NASA GSFC, haris.riris@nasa.gov

NASA is developing a space-based CO₂ Laser Sounder at Goddard Space Flight Center that is aimed at providing global CO₂ measurement in the troposphere with an ultimate measurement precision goal of less than 1%. The laser technique uses a CO₂ absorption band in the near infrared and has a number of laser wavelengths across one strong absorption line centered at 1.572 µm. The focus of the laser measurement is lower atmosphere CO₂. The lasers are pulsed and the surface return signal can be well separated from that returned by the atmosphere using time gating in the receiver. Atmospheric scattering effects on the returned signals can be greatly reduced by this method.



In this paper, we report our line-by-line radiative transfer calculation results for the selection of laser frequencies used in this active technique, including the optimal selection of absorption line and laser frequencies at which the sensitivity to atmospheric temperatures is minimal and response to lower atmospheric CO₂ is maximal. Other effects on this measurement, e.g., water vapor as the most variable atmospheric composition, will be also presented. In addition, the simultaneous measurement of surface pressure is fundamentally required in order to appropriately estimate the change of CO₂ absorption corresponding to pressure fluctuation, to compute the CO₂ mixing ratio relative to dry air, and to separate actual CO₂ surface flux from variations in atmospheric density. The measurement technique for surface pressure will be similar to that for CO₂ but uses the O₂ A-band near 0.768 µm. Radiative transfer calculations for the surface pressure measurement will be also reported. Other information (e.g., temperature and water vapor estimates) required to go from differential absorption measurements to final CO₂ concentration retrievals will be assessed and discussed.

NASA Carbon Cycle & Ecosystems Active Awards Represented by this Poster:

• Award: OTHER