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CO2 profile retrieval from near-infrared spectra

The Total Carbon Column Observing Network (TCCON) is composed of high-resolution ground-based Fourier transform infrared (FTIR) spectrometers that record solar absorption spectra. Column-averaged dry-air mole fractions of CO2 (XCO2), are retrieved from the recorded spectra and used to validate satellite observations of XCO2 as well as study the carbon cycle. Even though TCCON XCO2 observations are precise, they lack information about the vertical distribution of CO2 in the atmosphere, which is of interest for the validation of satellite measurements and model simulations.

GFIT is the current retrieval algorithm of TCCON, it does not change the shape of the prior trace gas profiles. GFIT2 is an algorithm that allows the profile shape to vary during the retrieval process. The algorithm thus has more freedom to fit the observed spectra, but is also more sensitive to uncertainties in the forward model calculations such as spectroscopic errors and instrument misalignment, for example.

To improve CO2 profile retrievals, it was necessary to improve the forward model of GFIT2. To exploit different sources of vertical information, CO2 profiles were retrieved from spectral windows of varying opacity. Retrievals were performed using the true CO2 profile as a priori, where in-situ Aircore CO2 profiles were used as the truth. In that case, the scale factors retrieved by GFIT2 would ideally be equal to one at each grid level, but the retrieved profiles present oscillations caused by remaining errors in the forward model. A method to correct those oscillations and combine the profiles retrieved from the different windows is being tested.