This study assesses the potential of 2 to 10&thinsp;km resolution imagery of CO<sub>2</sub> concentrations retrieved from the Short Wave Infra Red measurements of a space borne passive spectrometer for monitoring the spatially integrated emissions from the Paris area. Such imagery could be provided by missions similar to CarbonSat, which was studied as a candidate Earth Explorer 8 mission by the European Space Agency (ESA). This assessment is based on Observing System Simulation Experiments (OSSEs) with an atmospheric inversion approach at city scale. The inversion system solves for hourly city CO<sub>2</sub> emissions and natural fluxes, or for these fluxes per main anthropogenic sector or ecosystem, during the 6 hours before a given satellite overpass. These 6&thinsp;hours correspond to the period during which emissions produce CO<sub>2</sub> plumes that can be identified on the image from this overpass. The statistical framework of the inversion accounts for the existence of some prior knowledge about the hourly emissions from an inventory based on energy use and carbon fuel consumption statistics. The link between the hourly or sectorial emissions and the vertically-integrated column of CO<sub>2</sub> observed by the satellite is simulated using a coupled flux and atmospheric transport model. This coupled model is built with the information on the spatial and temporal distribution of emissions from the emission inventory produced by the local air-quality agency (Airparif) and a 2&thinsp;km horizontal resolution atmospheric transport model. Tests are conducted for different realistic simulations of the spatial coverage, resolution, precision and accuracy of the imagery from sun-synchronous polar-orbing missions, corresponding to the specifications of CarbonSat and Sentinel-5 or extrapolated from these specifications. First, OSSEs are conducted with a rather optimistic configuration in which the inversion system is perfectly informed about the statistics of the limited number of error sources. These OSSEs indicate that the image resolution has to be finer than 4&thinsp;km to decrease the uncertainty in the 6-hour mean emissions by more than 50&thinsp;%. More complex experiments assess the impact of more realistic error estimates that current inversion methods do not properly account for, in particular the systematic measurement errors with spatially correlated patterns. These experiments highlight the difficulty to improve current knowledge on CO<sub>2</sub> emissions for urban areas like Paris with CO<sub>2</sub> observations from satellites, and call for more technological innovations in the remote sensing of vertically integrated columns of CO<sub>2</sub> and in the inversion systems that exploit it.
Broquet, G., Bron, F. M., Renault, E., Buchwitz, M., Reuter, M., Bovensmann, H., … Ciais, P. (2018). The potential of satellite spectro-imagery for monitoring CO2 emissions from large cities. Atmospheric Measurement Techniques, 11(2), 681–708. https://doi.org/10.5194/amt-11-681-2018