AIRS channel selection for CO₂ and other trace-gas retrievals

New high-resolution infrared sounders, such as the National Aeronautics and Space Administration's Aqua satellite Atmospheric Infrared Sounder (AIRS) or the European Meteorological Satellite systems. MetOp Infrared Atmospheric Sounding Instrument (IASI), are expected to improve our capability to monitor atmospheric carbon dioxide (CO₂) and other trace-gas concentrations from space. As they present thousands of channels, the first problem arising is the selection of a set of channels presenting the best properties to retrieve these concentrations. A new method, the Optimal Sensitivity Profile (OSP) method, based on the study of the sensitivities of AIRS channels to variations in the vertical of the different atmospheric components, is proposed. It is then compared with two 'classical' methods based on the information content and the degrees of freedom for signal of the AIRS channels regarding CO₂. Applying the OSP method to a set of 82 representative atmospheric situations obtained from the 2311 situations of the Thermodynamic Initial Guess Retrieval (TIGR) dataset, divided into three air masses (tropical, temperate and polar), and using simulated AIRS observations, a global set of 43 channels, well covering the whole atmospheric column, is selected for CO₂ retrieval. The OSP method is finally used to select channels for retrieval of atmospheric nitrous oxide (N₂O), carbon monoxide (CO) and methane (CH₄) concentrations.