Channel selection using information content analysis A case study of CO2 retrieval from near infrared measurements

A major challenge in retrieving CO2 concentrations from thermal infrared remote sensing comes from the fact that measurements in the 4.3 and 15mm absorption bands (AIRS or TES) are sensitive to both temperature and CO2 variations. This complicates the selection of absorption channels with maximum CO2 concentration information content. In contrast, retrievals using near infrared (NIR) CO2 absorption bands are relatively insensitive to temperature and are most sensitive to changes of CO2 near the surface, where the sources and sinks are located. The Orbiting Carbon Observatory (OCO) was built to measure reflected sunlight in three NIR spectral regions (the 0.76mm O2 A-band and two CO2 bands at 1.61 and 2.06mm). In an effort to significantly increase the speed of accurate CO2 retrieval algorithms for OCO, we performed an information content analysis to identify the 20 best channels from each CO2 spectral region to use in OCO retrievals. Retrievals using these 40 channels provide as much as 75% of the total CO2 information content compared to retrievals using all 1016 channels in each spectral region. The CO2 retrievals using our selected channels have a precision better than 0.1ppm. This technique can be applied to the retrieval of other geophysical variables (e.g., temperature or CH4), or modified for other instruments, such as AIRS or TES. & 2010 Elsevier Ltd. All rights reserved.