Reevaluating the Use of O2 a1 Δg Band in Spaceborne Remote Sensing of...

Sun, K., I. E. Gordon, C. E. Sioris, X. Liu, K. Chance, and S. C. Wofsy (2018), Reevaluating the Use of O2 a1 Δg Band in Spaceborne Remote Sensing of Greenhouse Gases, Geophys. Res. Lett., 45, 5779-5787, doi:10.1029/2018GL077823.

Although the O2 a1 Δg band has long been used in ground-based greenhouse gas remote sensing to constrain the light path, it is challenging for nadir spaceborne sensors due to strong mesosphere/ stratosphere airglow. Spectroscopic simulations using upper state populations successfully reconstruct the airglow spectra with excellent agreement with SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY limb observations (residual root-mean-square <0.7%). The accurate knowledge of airglow spectrum enables retrieval of O2 (a1 Δg ) number density, volume emission rate, and temperature. For nadir spaceborne observations, the a1 Δg airglow will lead to a negative bias of ∼10% to O2 column, if not considered. However, when properly included, the airglow spectral feature can be adequately separated from O2 absorption (mean bias <0.1%) at the spectral resolution of modern spaceborne spectrometers. Plain Language Summary Carbon dioxide (CO2 ) and methane are two of the most important anthropogenic greenhouse gases. Highly accurate global-scale spaceborne measurements using backscattered sunlight are needed to adequately quantify the sources and sinks of CO2 and methane, which are still poorly understood. To achieve the required accuracy, atmospheric oxygen needs to be measured simultaneously to constrain the path length of the backscattered sunlight through the atmosphere. The best oxygen band for this purpose, the a1 Δg , or singlet Delta, band at 1.27 μm, has long been deemed unusable in spaceborne measurements, due to the intense and uncertain airglow emitted from the upper atmosphere at the same wavelength. This study characterizes the spectral shape and spatial distribution of airglow using recent satellite observations and concludes that the contamination of airglow can be adequately separated from the backscattered sunlight without significant loss of precision and accuracy. The use of the singlet Delta band will substantially simplify the design of spaceborne greenhouse gas instruments, reduce the cost, and potentially make more accurate greenhouse gas retrievals. As such, we call on the greenhouse gas remote sensing community to reconsider the oxygen singlet Delta band in future missions.

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