CO_2 retrieval algorithm for the Infrared Atmospheric SounderInterferometer: the potential of retrieving the vertical profileof carbon dioxide from its hot or laser bands in the 800-1200cm~（-1） atmospheric window

摘要

In a series of recent papers we have developed and tested a full physical retrieval scheme for the InfraredAtmospheric Sounder Interferometer or IASI, which is capable of simultaneously retrieving surface temperature,emissivity and atmospheric proles of temperature, H_2O, O_3, HDO, CO_2, N2O, CO, CH_4, SO_2, HNO_3, NH_3,OCS, CF_4 atmospheric proles. Until now, the performance of this scheme has concerned the column amountof gas species. In this paper, we will assess the capability of the methodology to retrieve the vertical proleof CO_2. In this respect, the eectiveness of the method mostly relies on the degree of freedom (dof ) of theretrieval. In principle, for an unconstrained Least Square approach, dof achieves its larger value and is equalto the number of layers, NL which is used to represent the vertical prole. Using Optimal Estimation we havedof NL and the actual value is determined by the background covariance matrix. In this study, we use a novelapproach, which directly allows us to represent the CO2 prole with orthogonal eigenvectors or vertical modesthat explain which ne-scale structures we can resolve with our retrieval. The number of vertical modes we caneffectively resolve depends on the IASI information content for CO_2. To check the capability of the scheme toretrieve the CO_2 profile we have considered a retrieval exercise for a target area close to Mauna Loa and CapeKumukahi validation stations in the Hawaii region. Five years of IASI data have been collected on such an areaand processed according to the above methodology. It will be shown that IASI is capable of retrieving at most2-3 information pieces or vertical modes for the CO_2 vertical profile, which yield an accurate estimation of theCO_2 total amount and the correct smooth shape expected for a long-lived species such as CO_2 far from intensepollution sources.