Abstract
The accuracy of atmospheric trace gas retrievals depends directly on the accuracy of the molecular absorption model used within the retrieval algorithm. For remote sensing of well-mixed gases, such as carbon dioxide (CO2), where the atmospheric variability is small compared to the background, the quality of the molecular absorption model is key. Recent updates to oxygen (O2) absorption coefficients (ABSCO) for the 0.76 µm A-band and the water vapor (H2O) continuum model within the 1.6 µm and 2.06 µm CO2 bands used within the Orbiting Carbon Observatory (OCO-2 and OCO-3) algorithm are described here. Updates in the O2 A-band involve the inclusion of new laboratory measurements within multispectrum fits to improve relative consistency between O2 line shapes and collision-induced absorption (CIA). The H2O continuum model has been updated to MT_CKD v3.2, which has benefited from information from a range of laboratory studies relative to the model utilized in the previous ABSCO version. Impacts of these spectroscopy updates have been evaluated against ground-based atmospheric spectra from the Total Carbon Column Observing Network (TCCON) and within the framework of the OCO-2 algorithm, using OCO-2 soundings covering a range of atmospheric and surface conditions. The updated absorption coefficients (ABSCO version 5.1) are found to offer improved fitting residuals and reduced biases in retrieved surface pressure relative to the previous version (ABSCO v5.0) used within B8 and B9 of the OCO-2 retrieval algorithm and have been adopted for the OCO B10 Level 2 algorithm.
Highlights
In experiments utilized for the analysis for absorption coefficients (ABSCO) v5.1, we have investigated the temperature dependence of the O2 A-band using a cavity ringdown spectroscopy (CRDS) technique, with a cell developed at the National Institute of Standards and Technology (NIST) initially for studying the temperature dependence of CO2 line shapes in the 1.6 μm region
While inspecting the range of empirical line-mixing scaling factors and the effect on collision-induced absorption (CIA), as well as the impact of these adjustments on Total Carbon Column Observing Network (TCCON) retrievals, it was observed that the CIA has a strong impact on retrieved products in the region of the resonant P-branch lines where the intensity is dropping, i.e. at the intensity inflection points
While making the adjustments to empirical line mixing and collision induced absorption, the behavior or the CIA was checked at the low pressure limit, and the effect of adjusting the intensity cutoff for calculation of resonant lines was confirmed to have no effect on laboratory residuals
Summary
Orbiting Carbon Observatory missions (OCO-2 and OCO-3) [8,12], TanSat [54] and GeoCARB [30,35] rely on measurements of the absorption of reflected sunlight by oxygen (O2) and CO2. From these measurements, estimates of the column-averaged CO2 dry air mole fraction, (XCO2), can be retrieved e.g. O’Dell et al [31]. The CO2 concentration fields can be used within atmospheric inversion frameworks to infer sources and sinks of CO2 Results of these atmospheric inversions, which are dependent on small differences in large numbers, can be strongly impacted by systematic errors in the retrieval of XCO2. The accuracy of the molecular absorption model used within the retrievals is a key consideration for the accuracy of the retrieved XCO2 products
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Journal of Quantitative Spectroscopy and Radiative Transfer
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
