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With the stable increase of carbon dioxide (CO2) concentrations, space based measurements of CO2
concentration in lower atmosphere by reflected sunlight in near infrared band has become a hot
research topic at present. Recently, the instruments sensitive to total CO2 column data in near-surface
have become available through the SCIAMACHY instrument on ENVISAT and TANSO-FTS on
GOSAT. The developing hyper spectral CO2 detector in China carried by TANSAT will be launched
in late 2015. Hyper spectral CO2 detector is designed to provide global measurements of CO2 in
lower troposphere. It employs high resolution spectra of reflected sunlight taken simultaneously in
near-infrared CO2 (1.61μm and 2.06μm) and O2 (0.76μm) bands.
Associating climate change with the observation requirements of carbon sources and sinks, the
feasibility of making CO2 column concentration measurements with high-resolution and
high-precision is studied by high resolution atmosphere radiation transfer model. The effects of key
specifications of the hyper spectral CO2 detector such as spectral resolution, sampling ratio and
sign-to-noise ratio (SNR) on CO2 detection are analyzed combining the scientific requirements of
CO2 measurements of China.
The typical characteristics of hyper spectral CO2 detector on TANSAT are grating spectrometer
and array-based detector. To achieve the column averaged atmospheric CO2 dry air mole fraction
(XCO2) precision requirements of 1×10-6-4×10-6, hyper spectral CO2 detector should provide high
resolution at first to resolve CO2 absorption lines from continuous spectra of reflected sunlight.
Compared to a variety of simulated spectral resolutions, the spectral resolution of hyper spectral CO2
detector on TANSAT can resolve CO2 spectral features and maintain the moderate radiance
sensitivity. Since small size array detector-based instruments may suffer from undersampling of the
spectra, the influences of spectral undersampling to CO2 absorption spectra are studied, the results
indicate that sampling ratio should exceed 2 pixels/FWHM to ensure the accuracy of CO2 spectrum.
Signal-to-noise ratio is one of the most important parameters of hyper spectral CO2 detectors to
ensure the reliability of CO2 signal. SNR requirements of CO2 detector to different detection
precisions are explored based on the radiance sensitivity factors. The results show that it is difficult
to achieve the SNR to detect 1×10-6-4×10-6 CO2 concentration change in the boundary layer by solar
shortwave infrared passive remote sensing, limited by the instrument development at present.
However, the instrument SNR to detect 1% change in the CO2 column concentration is attainable.
The results of this study are not only conductive to universal applications and guides on developing
grating spectrometer, but also helpful to have a better understanding of the complexity of CO2
retrieval.
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