Global observations of the carbon budget, 2, CO₂ column from differential absorption of reflected sunlight in the 1.61 μm band of CO₂

This paper investigates the impact of uncertainty in atmospheric composition and state upon the feasibility of measuring the CO 2 column from spectral analysis of sunlight reflected to space in the 1.61 μm absorption band of CO 2 . In principle, measurements of clear sky radiance at two frequencies, one where CO 2 absorbs strongly and the other weakly, allow the difference between the optical thicknesses of the atmosphere at the two frequencies to be determined precisely. That difference, denoted by L , is a linear functional of the CO 2 density profile, which depends strongly on the CO 2 amount and only weakly upon its vertical distribution, thus suggesting that the CO 2 column may be estimated from L . A simple model for the radiance reflected to space is used to estimate the magnitude of the error in the CO 2 column inferred from L when the atmosphere contains thin cloud and aerosol. It emerges that measurements in two channels in the 1.61 μm CO 2 absorption band are too sensitive to cloud and aerosol to allow the CO 2 column to be inferred with precision better than a few percent in the presence of thin cloud and aerosol. However, simultaneous measurements of optical thickness in the nearby 1.27 μm absorption band of O 2 are tightly correlated with those for CO 2 , even in the presence of aerosol and thin cirrus, and therefore may allow the CO 2 column to be determined relative to the O 2 column, provided that the latter is known independently from surface pressure. The correlation between O 2 and CO 2 optical thicknesses depends upon the mean scattering height, but this quantity may be estimated with sufficient accuracy from radiances measured in the O 2 band. A prototype algorithm is developed to estimate the CO 2 column from data in two CO 2 channels and three O 2 channels. The algorithm is used to estimate the probable bias and standard error of measurements of CO 2 column from space under conditions where the optical thicknesses of aerosol and cirrus may be as large as 0.2 and 0.1, respectively, and where the temperature profile is known to within ±1 K. The simulations suggest that the error in the estimated CO 2 column caused by these sources is approximately 0.5%. This conclusion is interpreted cautiously because the analysis assumes inter alia that the spectroscopic properties of both CO 2 and O 2 are known accurately, that the surface reflectance and the scattering properties of aerosol and cirrus vary predictably between 1.27 μm and 1.61 μm, and that difficult technical issues associated with high spectral resolution measurements can be resolved. Nevertheless, the importance of global measurements of CO 2 is such that the method warrants further investigation.