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The Greenhouse gases Observing Satellite (GOSAT) is a Japanese satellite that is intended to observe CO2 concentration
from space and to contribute to advancement of research of the source/sink estimation of CO2. The GOSAT main sensor
is a Fourier Transform Spectrometer (FTS) named "TANSO-FTS", which covers a wide terrestrial radiation spectrum
including CO2 absorption bands at 1.6 μm, 2.0 μm, and 15 μm. The former two bands are used to estimate columnar
concentration of CO2; the latter is used to retrieve the vertical profile of CO2 in the upper atmosphere above about 700
hPa pressure level. In addition, another installed on the satellite is an imaging sensor that will be used to detect clouds
and aerosols: Cloud and Aerosol Imager (CAT). The Center for Climate System Research (CCSR) has contracted with
the Japan Aerospace Exploration Agency (JAXA) to develop an algorithm to retrieve CO2 concentration profiles from
data measured by the thermal infrared (TIR) band of the TANSO-FTS sensor. We adopt the maximum a posteriori
method (MAP) to retrieve the vertical profile of atmospheric parameters from thermal infrared spectra. Key techniques
for retrieving CO2 concentrations are 1) reduction of temperature estimation error through channel selection, 2)
optimization of the initial guess for CO2 profile based on the output from a chemical transport model (CTM), and 3)
usage of data from the 1.6 μm band of TANSO-FTS as an additional constraint in retrieval of vertical profiles of CO2.
Although thermal infrared spectrum data have poor vertical resolving power for CO2 concentration in the lower
atmosphere, particularly in the boundary layer, we expect that CO2 amount in the lower atmosphere can be deduced by
substituting the upper level concentration from the columnar concentration estimated from the 1 .6 μm band data.
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