Impact of Atmospheric State Uncertainties on Retrieved XCO2 Columns from Laser Differential Absorption Spectrometer Measurements and the Effect of O2-Derived Surface Pressure on XCO2 Retrieval Accuracy

An increasing interest and need exist in determining the distribution of greenhouse gases (GHG)s for a diverse set of conditions and environments that range from localized industrial sites, to those at regional, continental and global scales.  In order to achieve these objectives a growing number of both passive and active aircraft and space-based instruments have been designed and deployed to assess the global distribution of greenhouse gases including carbon dioxide (CO₂) and, more importantly, the associated time varying CO₂ fluxes.  While the active sensing approaches have significant benefits in their ability to provide high quality measurements in non-sunlit conditions, and are potentially less susceptible to effects of clouds and aerosols, they still face technological challenges that must be addressed in order to meet their stringent measurement goals and objectives.  Additionally, a number of atmospheric state and spectroscopy issues that impact end-to-end performance and overall measurement accuracy must be characterized and accounted for. One such error source is the uncertainty in the observed atmospheric state and its impact on the spectroscopic variability due to pressure, temperature, moisture, and the associated altitude-dependent spectral weighting functions.

This work extends prior assessment of the impact of uncertainties in atmospheric state knowledge on retrievals of carbon dioxide column amounts (XCO₂) from laser differential absorption spectrometer (LAS) measurements to include additional absorption wavelengths in the 1.57 and 2.05 µm regions.  In addition, we assess the impact of using surface pressure derived from LAS-based O₂ measurements in XCO₂ retrievals as a means of reducing retrieval uncertainties.  LAS estimates of XCO₂ columns that are derived from integrated-path differential absorption observations require measured or prior knowledge of atmospheric state parameters that include temperature, moisture and pressure along the viewing path.  In the case of global space-based monitoring systems, it is often difficult if not impossible to provide collocated in situ measurements of atmospheric state parameters for all observations, so retrievals often rely on collocated remote sensed data or values derived from Numerical Weather Prediction (NWP) models to describe the atmospheric state.  A radiative-transfer-based simulation framework, combined with representative global upper-air observations and matched NWP profiles, was used to assess the impact of model differences on estimates of column CO₂ and O₂ concentrations. These analyses focus on characterizing these errors for LAS measurements of CO₂ in the 1.57 and 2.05 µm regions and of O₂ in the 0.76 and 1.26 µm regions. The results provide a comprehensive set of signal-to-noise metrics that characterize the errors in retrieved XCO₂ values associated with uncertainties in the atmospheric state, and provide a method for selecting optimal differential absorption line pairs to minimize the impact of these noise terms. 

Furthermore, this analysis is extended to explore the feasibility of using surface pressure derived from LAS-based O₂ measurements in lieu of modeled surface pressure as a potential means of reducing uncertainty in XCO₂ retrievals.  In theory, since XCO₂ retrievals are dependent on surface pressure, deriving surface pressure from LAS O₂ measurements that are collocated in time and space with the CO₂ LAS measurements would thus eliminate uncertainty in retrieved XCO2 associated with modeled surface pressure.  However, uncertainty due to temperature and moisture must still be accounted for in the O₂-based retrievals of surface pressure.  Again using the aforementioned radiative-transfer-based simulation framework, we attempt to characterize and quantify potential improvements in XCO₂ retrieval error associated with O₂-derived surface pressure for a set of CO₂ and O₂ absorption line combinations.