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.
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