Three-Dimensional Modeling of the O₂(¹) Dayglow: Dependence on Ozone and Temperatures

Future space missions dedicated to measuring CO2 on a global scale can make advantageous use of the O-2 band at 1.27 mu m to retrieve the air column. The 1.27 mu m band is close to the CO2 absorption bands at 1.6 and 2.0 mu m, which allows a better transfer of the aerosol properties than with the usual O-2 band at 0.76 mu m. However, the 1.27 mu m band is polluted by the spontaneous dayglow of the excited state O-2 ((1)triangle), which must be removed from the observed signal. We investigate here our quantitative understanding of the O-2((1)triangle) dayglow with a chemistry-transport model. We show that the previously reported -13% deficit in O-2((1)triangle) 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)triangle) deficits in the lower mesosphere. Once integrated vertically to simulate a nadir measurement, the deficit in modeled O-2((1)triangle) 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)triangle) brightness in algorithms using the 1.27 mu m band. Plain Language Summary Future space missions dedicated to measuring CO2 in the atmosphere can make advantageous use of the O-2 absorption band at 1.27 mu m. Indeed, the 1.27 mu m band is close to the wavelengths where CO2 absorbs the solar radiation, which allows more precise calculations. However, the 1.27 mu m band is polluted by the spontaneous infrared emission of O-2 in its excited state called O-2((1)triangle), which occurs in the upper atmosphere and must be removed from the observed signal. We investigate here our understanding of the O-2((1)triangle) emission with a chemistry-transport model. When compared to observations, there is a -13% deficit in O-2((1)triangle) in our model. We find that this deficit is due to excessively high temperatures (+15 K) used in the calculations. The use of temperatures more in line with the observations slows down the ozone-destroying chemical cycles, which leads to an almost total elimination of the O-2((1)triangle) deficit. Once integrated vertically to simulate a satellite measurement, the deficit in modeled O-2((1)triangle) brightness is reduced to -4.2 +/- 2.8%. This illustrates the need for accurate temperatures in the middle atmosphere for a reliable prediction of the O-2((1)triangle) emission occurring in the 1.27 mu m band.