On the potential of the 2041-2047 nm spectral region for remote sensing of atmospheric CO2 isotopologues

Pressing open questions about the carbon cycle can be addressed with precise measurements of the three most abundant CO2 isotopologues (OCO)-O-16-C-12-O-16, (OCO)-O-16-C-13-O-16, and (OCO)-O-16-C-12-O-18. Such measurements can, e.g., help to further constrain oceanic and biospheric net fluxes or to differentiate between the gross biospheric fluxes photosynthesis and respiration. The 2041-2047 nm (about 4885-4900 cm(-1)) spectral region contains separated absorption lines of the three most abundant CO2 isotopologues. Their spectral properties make this spectral region well suited for the use of a light path proxy method for the retrieval of delta C-13 and delta O-18 (the ratio of heavier to lighter isotopologues relative to a standard). An optimal estimation based light path proxy retrieval for delta C-13 and delta O-18 has been set up, applicable to GOSAT (Greenhouse gases Observing Satellite) and ground-based FTS (Fourier transform spectrometer) measurements. Initial results show that it is possible to retrieve delta C-13 and delta O-18 from ground-based FTS instruments with a precision of 0.6-1.6 parts per thousand and from GOSAT with a precision of about 30 parts per thousand. Comparison of the achievable precision with the expected atmospheric signals shows that ground-based FTS remote sensing measurements have the potential to gain valuable information on delta C-13 and delta O-18 if averaging a sufficient number of measurements. It seems unlikely that this applies also to GOSAT because of the lower precision and a conceptual larger sensitivity to scattering related errors in satellite viewing geometry. (C) 2012 Elsevier Ltd. All rights reserved.