S.: Measuring atmospheric CO2 from space using Full Spectral Initiation (FSI

Atmospheric CO2 concentrations, retrieved from spectral measurements made in the near infrared (NIR) by the SCIAMACHY instrument, using a new retrieval algorithm called Full Spectral Initiation Weighting Function Modified Differential Optical Absorption Spectroscopy (FSI WFM-DOAS), are compared to ground based Fourier Transform Infrared (FTIR) data and to the output from a global chemistry-transport model. Comparisons to CO2 measurements made by the ground based FTIR spectrometer at Egbert, Canada, reveal a negative bias in the FSI WFM-DOAS columns of approximately-4.0%, though this offset appears to be decreasing with time. Similar comparisons to the TM3 chemistry transport model show that that the temporal behaviour of the seasonal cycle is captured well but that its amplitude is over estimated. From these comparisons, the overall precision and bias of the CO2 columns retrieved by the FSI algorithm are estimated to be close to 1.0 % and <4.0 % respectively