New Strategy for the measurement of the CO2 distribution in Stratosphere and Upper Troposphere

In the Thermal InfraRed region, the spectral features generated by vibro-rotational transitions of CO2 have been exploited to retrieve pressure (P) and temperature (T) distributions by space-borne experiments that investigate the atmospheric composition. Besides its intrinsic importance, the knowledge of P and T is necessary in the data analysis of the experiment to retrieve the distribution of any target molecule from its spectral features. The P, T retrieval process is based on the assumption that the CO2 atmospheric distribution is known. Therefore the role assigned to CO2 in the data analysis prevents its spectral features to be used for the retrieval of its distribution. The accuracy of the CO2 distribution obtained by simultaneously retrieving it with P and T did not meet the required accuracy (> 10%). In this study we propose a new strategy for the measurement of the CO2 distribution using a passive orbiting limb sounder. The idea is to exploit the pure rotational transitions of molecular oxygen in the far-infrared region for the retrieval of P and T. As these transitions originate from a magnetic dipole moment their line strength is very low. Nevertheless, due to the huge abundance of oxygen and to the large optical path of limb sounding observation geometries, they are among the most prominent features of the far infrared atmospheric spectrum. The experiment considered for this study exploits a FT spectrometer with two output ports hosting respectively a far-infrared detector devoted to measure oxygen, and a mid-infrared detector devoted to measure CO2 in the 700 cm-1 region. In this study we consider different spectral resolutions of the spectrometer and different options for the far-infrared detectors (that are a crucial aspect of the experiment due to the challenge of cooling in a space experiment). We use retrievals on simulated observations to assess the performance of the different options considered, in terms of precision of the retrieved CO2 distributions. Since the oxygen transitions occur at frequencies where the Plank function is rather weak at atmospheric temperatures, we had to verify whether their spectral features contain enough information to determine temperature parameters. Preliminary tests indicate satisfactory precision for the P and T distributions retrieved from 70 km down to the tropopause.