OXYCO2: A New Experiment for the Measurement of the CO2 Distribution in Stratosphere and in the Upper Troposphere

We present the results of a study for a new strategy for the measurement of the CO2 distribution in the stratosphere. The proposed experiment (Carlotti et al. 2016) is based on an orbiting limb sounder that measures the atmospheric emission in the spectral regions of both the Thermal InfraRed (TIR) from 685 to 930 cm-1 and Far-InfraRed (FIR) from 80 to 180 cm-1. 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 independently the distribution of CO2 from its rovibrational transitions in the TIR. The instrument considered to test the new strategy is a Fourier transform spectrometer with two output ports hosting a FIR detector devoted to measure 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 previous studies and operational limb sounders. The performance of the experiment has been assessed with two-dimensional retrievals on simulated observations along a full orbit. For the purpose, optimal spectral intervals have been defined using a validated selection algorithm. Both precision and spatial resolution of the obtained CO2 distribution have been taken into account in the assessment of the performance of the proposed experiment. We have seen that the O2 spectral features significantly contribute to the performance of the CO2 retrieval, and that the proposed experiment can determine the two-dimensional distribution of the CO2 volume mixing ratio with a precision 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 spectroscopy errors, indicates that, in the 10-50 km altitude range, the total error of the CO2 fields is dominated by the random component. This could also be 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 km to 5 km and the horizontal resolution is of the order of 300-350 km depending on latitude. The same experiment could be used to observe the distribution of several greenhouse gases spectroscopically active in the observed spectral region.