Measuring atmospheric carbon dioxide from space
Satellite measurements of atmosphere CO2 concentrations are a rapidly evolving area of scientific research which can help reduce the uncertainties in the global carbon cycle fluxes and identify regional surface sources and sinks. One of the emerging CO2 measurement techniques is a relatively new retrieval algorithm called Weighting Function Modified Differential Optical Absorption Spectroscopy (WFM-DOAS) (Buchwitz et al., 2000). This algorithm is designed to measure the total columns of CO2 (and other greenhouse gases) through the application to spectral measurements in the near infrared (NIR), made by the SCIAMACHY instrument on-board ESA's ENVISAT satellite. The WFM-DOAS technique is based on fitting the logarithm of a model reference spectrum and its derivatives to the logarithm of the ratio of a measured nadir radiance and solar irradiance spectrum. In this thesis, a detailed error assessment of this technique has been conducted and it has been found necessary to include suitable a priori information within the retrieval in order to minimize the errors on the retrieved CO2 columns. Hence, a more flexible implementation of the retrieval technique, called Full Spectral Initiation (FSI) WFM-DOAS, has been developed which generates a reference spectrum for each individual SCIAMACHY observation using the estimated properties of the atmosphere and surface at the time of the measurement.