Empirical analysis of aerosol and thin cloud optical depth effects on CO2 retrievals from GOSAT

Abstract:

Ground-based sunphotometer observations of aerosol and cloud optical properties at AEROCAN / AERONET sites co-located with TCCON (Total Carbon Column Observing Network) high resolution Fourier Transform Spectrometers (FTS) were used to investigate the aerosol and cloud influence on column-averaged dry-air mole fraction of carbon dioxide (XCO₂) retrieved from the TANSO-FTS (Thermal And Near-infrared Sensor for carbon Observation - FTS) of GOSAT (Greenhouse gases Observing SATellite). This instrument employs high resolution spectra measured in the Short-Wavelength InfraRed (SWIR) band to retrieve XCO₂estimates.

GOSAT XCO₂ retrievals are nominally corrected for the contaminating backscatter influence of aerosols and thin clouds. However if the satellite-retrieved aerosol and thin cloud optical depths applied to the CO₂ correction is biased then the correction and the retrieved CO₂ values will be biased. We employed independent ground based estimates of both cloud screened and non cloud screened AOD (aerosol optical depth) in the CO₂ SWIR channel and compared this with the GOSAT SWIR-channel OD retrievals to see if that bias was related to variations in the (generally negative) CO₂ bias (ΔXCO₂= XCO₂(GOSAT) - XCO₂(TCCON)). Results are presented for a number of TCCON validation sites.