Midtropospheric CO₂ concentration retrieval from AIRS observations in the tropics

Midtropospheric carbon dioxide (CO₂) concentration is retrieved in the tropics [20S:20N], over sea, at night, for the period April to October 2003 from the Atmospheric Infrared Sounder (AIRS) observations. The method relies on a non-linear regression inference scheme using neural networks. A rough estimate of the mean precision of the method is about 2.5 ppmv (0.7%). The retrieved seasonal cycle and its latitudinal dependence agree well with aircraft CO₂ in situ measurements made at the same altitude range. Maps produced on a monthly basis at a resolution of 15°x 15°, although not yet fully understood, show good agreement with known characteristics of CO ₂ distribution reflecting both atmospheric transport and surface fluxes (fossil fuel emissions, biomass burning, air-surface gas exchanges).