Sensitivity Studies for Space-based Measurement of Atmospheric Total Column Carbon Dioxide using Reflected Sunlight

摘要

A series of sensitivity studies is carried out to explore the feasibility of space-based global carbon dioxide (C02) measurements for global and regional carbon cycle studies. The detection method uses absorption of reflected sunlight in the C02 vibration-rotation band at 1.58 micron. The sensitivities of the detected radiances are calculated using the line-by-line model (LBLRTM), implemented with the DISORT (Discrete Ordinates Radiative Transfer) model to include atmospheric scattering in this band. The results indicate that (1) the small changes in C02 near the Earth's surface are detectable in this C02 band provided adequate sensor signal-to-noise ratio and spectral resolution are achievable; (2) the radiance signal or sensitivity to C02 change near the surface is not significantly diminished even in the presence of aerosols andor thin cirrus clouds in the atmosphere; (3) the modification of sunlight path length by scattering of aerosols and cirrus clouds could lead to large systematic errors in the retrieval; therefore, ancillary aerosol/cirrus cloud data are important to reduce retrieval errors; (4) C02 retrieval requires good knowledge of the atmospheric temperature profile, e.g. approximately 1K RMS error in layer temperature; (5) the atmospheric path length, over which the C02 absorption occurs, must be known in order to correctly interpret horizontal gradients of C02 from the total column C02 measurement.