Retrieving XCO2 from the CO2M mission: Joint use of Multi-Angle Polarimeter and spectrometer measurements

In the support of the Copernicus Anthropogenic Carbon Dioxide Monitoring (CO2M) mission, Space Research Organization Netherlands (SRON) developed the Remote sensing of Trace gas and Aerosol Product (RemoTAP) algorithm. RemoTAP is the only algorithm with the capability to simultaneously retrieve trace gases and aerosol using measurements from the Multi-Angle Polarimeter (MAP) and CO2I Instrument. At the same time, it has the capability to perform the retrieval of trace gas from only CO2I measurements.

This study evaluates the performance of RemoTAP for combined CO2I-MAP and CO2I-only retrievals, respectively. We base our evaluation on synthetic CO2M measurements simulated for realistic atmospheric (aerosol, cirrus), surface, geometry conditions. CO2I-MAP retrieval method can reduce the regional bias in column-averaged dry-air mole fraction of CO2 (XCO2) by a factor of 3 for both land and ocean. It shows that only by the inclusion of MAP measurements, the large aerosol-induced biases can be mitigated, resulting in the retrievals that meet the mission requirement (precision <0.7 ppm and bias <0.5 ppm).

To further improve the accuracy of trace-gas retrievals, we develop a Neural Network (NN) approach to provide an accurate first guess of aerosol and surface parameters. We also develop a bias correction and quality filtering using an NN approach.