Three-dimensional modeling of the O2(1Δ) dayglow and implications for ozone in the middle atmosphere.

Future space missions dedicated to measuring CO₂ on a global scale can make advantageous use of the O₂ band at 1.27 µm to retrieve the air column. The 1.27 µm band is close to the CO₂ absorption bands at 1.6 and 2.0 µm, which allows a better transfer of the aerosol properties than with the usual O₂ band at 0.76 µm. However, the 1.27 µm band is polluted by the spontaneous dayglow of the excited state O₂(¹Δ), which must be removed from the observed signal.

We investigate here our quantitative understanding of the O₂(¹Δ) dayglow with a chemistry-transport model. We show that the previously reported -13% deficit in O₂(¹∆) dayglow calculated with the same model is essentially due a -20 to -30% ozone deficit between 45-60 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 O₂(¹Δ) deficits in the lower mesosphere. Once integrated vertically to simulate a nadir measurement, the deficit in modeled O₂(¹Δ) brightness is reduced to -4±3%. This illustrates the need for accurate mesospheric temperatures for a priori estimations of the O₂(¹Δ) brightness in algorithms using the 1.27 µm band.