Jean‐Loup Bertaux

Future space missions dedicated to measuring CO 2 on a global scale can make advantageous use of the O 2 band at 1.27 μm to retrieve the air column. The 1.27 μm band is close to the CO 2 absorption bands at 1.6 and 2.0 μm, which allows a better transfer of the aerosol properties than with the usual O 2 band at 0.76 μm. However, the 1.27 μm band is polluted by the spontaneous dayglow of the excited state O 2 ( 1 ∆), which must be removed from the observed signal. We investigate here our quantitative understanding of the O 2 ( 1 ∆) dayglow with a chemistry‐transport model. We show that the previously reported −13% deficit in O 2 ( 1 ∆) dayglow calculated with the same model is essentially due a −20% to −30% ozone deficit between 45 and 60 km. We find that this ozone deficit is due to excessively high temperatures (+15 K) of the meteorological analyses used to drive the model in the mesosphere. The use of lower analyzed temperatures (ERA5), in better agreement with the observations, slows down the hydrogen‐catalyzed and Chapman ozone loss cycles. This effect leads to an almost total elimination of the ozone and O 2 ( 1 ∆) deficits in the lower mesosphere. Once integrated vertically to simulate a nadir measurement, the deficit in modeled O 2 ( 1 ∆) brightness is reduced to −4.2 ± 2.8%. This illustrates the need for accurate mesospheric temperatures for a priori estimations of the O 2 ( 1 ∆) brightness in algorithms using the 1.27 μm band.