Anthropogenic CO2 monitoring satellite mission: the need for multi-angle polarimetric observations

Scattering due to aerosols and cirrus has long been identified as one of main sources of uncertainties in retrieving XCO₂ from solar backscattered radiation. In this work, we investigate the added value of multi-angle polarimeter (MAP) measurements in the context of Copernicus candidate mission for anthropogenic CO₂ monitoring (CO2M). To this end, we compare aerosol-induced XCO₂ errors from standard retrievals using spectrometer only (without MAP) with those from retrievals using both MAP and spectrometer. MAP measures radiance and degree of linear polarization (DLP) simultaneously at multiple wavelengths and at multiple viewing angles; these observations are expected to provide information about aerosols that is useful for improving XCO₂ accuracy. Using an ensemble of 500 synthetic scenes over land, we show that the standard XCO₂ retrieval approach that makes no use of MAP observations returns XCO₂ errors with an overall bias of 1.04 ppm, and a spread (equivalent to standard deviation for a normal distribution) of 2.07 ppm. The latter is far higher than the required XCO₂ accuracy (0.5 ppm) and precision (0.7 ppm) of the CO2M mission. Moreover, these XCO₂ errors exhibit a significantly larger bias and scatter at high aerosol optical depth, high aerosol altitude, and low solar zenith angle, which suggest a worse performance in retrieving XCO₂ from polluted areas where CO₂ and aerosols are co-emitted. Given the CO2M mission requirements, we proceed to derive MAP instrument specifications in terms of measurement uncertainties, number of viewing angles, and the wavelength range. Two different MAP instrument concepts are considered in this requirement analysis. We find that for either concept, MAP measurement uncertainties on radiance and degree of linear polarization should be no more than 3% and 0.003, respectively. Adopting the derived MAP requirements, a retrieval exercise on the 500 synthetic scenes using both MAP and spectrometer measurements delivers XCO₂ errors with an overall bias of -0.09 ppm and a spread of 0.57 ppm, indicating compliance with the CO2M mission requirements. For the test ensemble, we find effectively no dependence of the XCO₂ errors on aerosol optical depth, altitude of the aerosol layer, and solar zenith angle. These results represent a significant improvement in the retrieved XCO₂ accuracy compared to the standard retrieval approach, which may lead to a higher data yield, better global coverage, and a more comprehensive determination of CO₂ sinks and sources. As such, this outcome underscores the contribution of, and therefore the need for, a MAP instrument onboard the CO2M mission.