Improving the Retrieval of XCO2 from Total Carbon Column Observing Network Solar Spectra

A quadratic speed-dependent Voigt (qSDV) line shape with line mixing (LM) was implemented into the forward model of the spectral fitting algorithm GFIT in order to fit high-resolution solar absorption spectra. Total columns of CO2 were retrieved from the solar spectra by fitting the 30013<-00001 and 30012<-00001 bands of CO2 (centered at 6220 cm-1 and 6340 cm-1 respectively) using the qSDV+LM line shape. Total columns of O2 were also retrieved from the solar spectra by fitting the a1Δg←Χ3Σg- band (centered at 7885 cm-1) using the qSDV line shape, with spectroscopic parameters obtained from cavity ring-down spectra of the O2 a1Δg←Χ3Σg- band using the qSDV line shape. The column-averaged dry-air mole fraction of CO2 (XCO2) was calculated from these total columns of CO2 and O2 for a year of measurements from four Total Carbon Column Observing Network (TCCON) sites. These XCO2 measurements were compared to XCO2 retrieved using a Voigt line shape. We show that when the qSDV+LM line shape is used to retrieve CO2 and the qSDV is used to retrieve O2, there is a 6-fold decrease in the airmass dependence of XCO2. Network-wide TCCON XCO2 measurements were compared to aircraft XCO2 measurements, showing that XCO2 retrieved with the qSDV+LM is more accurate than that obtained using the Voigt line shape. The bias between TCCON and the integrated aircraft profile XCO2 measurements was found to be 0.9897±0.0005 and 1.0041±0.0005 with a Voigt and qSDV+LM respectively.