Effect of HITRAN database improvement on retrievals of atmospheric carbon dioxide from reflected sunlight spectra in the 1.61-µm spectral window

A large number of experimental and theoretical investigations of carbon dioxide (CO2) spectra have been conducted since the most recent update of the High-Resolution Transmission Molecular Absorption (HITRAN) database. To maintain optimal parameters, the HITRAN 2004 CO2 line list has been completely replaced by HITRAN 2008 data in the near-infrared region from 4300 cm−1 to 7000 cm−1. To examine the effect of this change on the retrieval of CO2 vertical column data from reflected sunlight spectra in the 1.61-µm spectral window, synthetic measurements for a given atmospheric state and instrument setup were generated and compared using radiative transfer model with the line-transition parameters from the HITRAN 2004 and 2008 databases. Simulated retrievals were then performed based on the optimal estimation retrieval theory. The results show that large systematic errors in atmospheric CO2 column retrievals were induced by the differences in the HITRAN laboratory line parameters in the 1.61-µm region. The retrieved CO2 columns were underestimated by >10 ppm using the HITRAN 2004 data, and improvements resulting from the use of the improved HITRAN database were more pronounced at a higher spectral resolution.