Rapid estimation of column averaged CO2 concentration using a correlation algorithm
Abstract
Measurement of XCO2, the column averaged mole fraction of CO2, using reflected sunlight in the near infrared bands of CO2 is fraught, because photons scattered in the atmosphere either can decrease or increase the mean path length compared with the direct path from the Sun to the surface to the satellite. A very simple algorithm to compensate for the errors introduced by scattering is presented. The algorithm is based on the observation that the apparent optical path differences in selected pairs of channels in the weak CO2 band at 1.6 μm and the O2 A-band at 0.76 μm are tightly correlated for large ensembles of scattering atmospheres. The number of tightly correlated pairs of channels is many hundreds for the bands measured by NASA's Orbiting Carbon Observatory (OCO). The physical reasons for the correlation are that the mean values of the distributions of photon paths for the members of each pair of channels are comparable, and that the extinction profiles vary similarly with height. For atmospheres with modest scattering optical thickness (less than 0.3), the slope and the intercept of the linear correlation for any pair depends weakly on the surface reflectance, the surface pressure and the viewing geometry. Through numerical simulations the slope and intercept may be parameterized simply in terms of these variables. Thereafter, the task of retrieving XCO2 from measured spectra may be reduced to linear interpolation in precomputed tables of slopes and intercepts. Results with simulated data for NASA's OCO satellite are presented, and random errors and biases are investigated. Although OCO did not reach orbit because the fairing did not open to release the satellite, the method is applicable to any instrument that operates using similar principles (such as GOSAT and the replacement satellite OCO-2).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.A51C0110P
- Keywords:
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- 0394 ATMOSPHERIC COMPOSITION AND STRUCTURE / Instruments and techniques