Reevaluating the Use of O₂ a¹Δ_g Band in Spaceborne Remote Sensing of Greenhouse Gases

Although the O₂a¹Δ_g 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(a¹Δ_g) number density, volume emission rate, and temperature. For nadir spaceborne observations, the a¹Δ_g 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.