A strategy for the measurement of CO₂ distribution in the stratosphere

Abstract. In this study we introduce a new strategy for the measurement of CO₂ distribution in the stratosphere. The proposed experiment is based on an orbiting limb sounder that measures the atmospheric emission within both the thermal infrared (TIR) and far-infrared (FIR) regions. The idea is to exploit the contribution of the pure rotational transitions of molecular oxygen in the FIR to determine the atmospheric fields of temperature and pressure that are necessary to retrieve the distribution of CO₂ from its rovibrational transitions in the TIR. The instrument envisaged to test the new strategy is a Fourier transform spectrometer with two output ports hosting a FIR detector devoted to measuring the O₂ transitions and a TIR detector devoted to measure the CO₂ transitions. Instrumental and observational parameters of the proposed experiment have been defined by exploiting the heritage of both previous studies and operational limb sounders. The performance of the experiment has been assessed with two-dimensional (2-D) retrievals on simulated observations along a full orbit. For this purpose, optimal spectral intervals have been defined using a validated selection algorithm. Both precision and spatial resolution of the obtained CO₂ distributions have been taken into account in the results–evaluation process. We show that the O₂ spectral features significantly contribute to the performance of CO₂ retrievals and that the proposed experiment can determine 2-D distributions of the CO₂ volume mixing ratio with precisions of the order of 1 ppmv in the 10–50 km altitude range. The error budget, estimated for the test case of an ideal instrument and neglecting the spectroscopic errors, indicates that, in the 10–50 km altitude range, the total error of the CO₂ fields is set by the random component. This is also the case at higher altitudes, provided the retrieval system is able to model the non-local thermal equilibrium conditions of the atmosphere. The best performance is obtained at altitudes between 20 and 50 km, where the vertical resolution ranges from 3 to 5 km, and the horizontal resolution is of the order of 300–350 km depending on latitude.