CO2 Spectroscopy as a challenge for Space-based Laser measurements of CO2

NASA Goddard Space Flight Center is developing an approach for the instrument for the ASCENDS Mission. The objective is to provide global CO2 measurement in the lower troposphere with an measurement precision of less than 1%. The focus of the measurement is sampling the lower tropospheric CO2 column. The laser technique uses the 1570 nm CO2 absorption band and samples one strong absorption line near 1572 nm at several wavelengths. 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. The influence of and potential errors caused by atmospheric scattering can be greatly reduced by this method. All candidate laser techniques for ASCENDS require high-accuracy CO2 spectroscopic data. Recent work has improved the spectroscopic database of CO2 line parameters for the state-of-the-art atmospheric remote sensing applications in both thermal infrared and near infrared bands. One of the updates added to HITRAN database in 2007 was CO2 line coupling from 442 cm-1 to 9648 cm-1. This line coupling enhances CO2 absorption in the near infrared line at 1572 nm by nearly 30% in terms of 2-way transmittance from ground to space. This paper will report the line-by-line radiative transfer calculations using the 2007 HITRAN. It will evaluate the impact of this update on the optimal absorption line and laser wavelengths selection for CO2 vertical weighting functions and retrieval errors for ASCENDS mission approaches using a line in the 1570 nm band. The results to date indicate that the uncertainty in the existing spectroscopic data could be a major error source of CO2 remote sensing. It appears that further improvement of CO2 spectral database is needed to achieve high-precision CO2 remote sensing for carbon budget and climate change studies.