Comment on amt-2022-48

Abstract. Various satellite imagers of the vertically integrated column of carbon dioxide (XCO₂) are under development to enhance the capabilities for the monitoring of the fossil fuel (FF) CO₂ emissions. XCO₂ images can be used to detect plumes from cities and large industrial plants, and to quantify the corresponding emissions using atmospheric inversions techniques. However, this potential and the ability to catch the signal from more diffuse FF CO₂ sources can be hampered by the mix between these FF signals and a background signal from other types of CO₂ surface fluxes, and in particular of biogenic CO₂ fluxes. The deployment of dense ground-based air-sampling networks for CO₂ and radiocarbon (¹⁴CO₂) could complement the spaceborne imagery by supporting the separation between the fossil fuel and biogenic or biofuel (BF) CO₂ signals. We evaluate this potential complementarity with a high resolution analytical inversion system focused on Northern France, Western Germany, Belgium, Luxembourg and a part of the Netherlands, and with pseudo-data experiments. The inversion system controls the FF and BF emissions from the large urban areas and plants, in addition to regional budgets of more diffuse emissions or of biogenic fluxes (NEE, Net Ecosystem Exchange), at an hourly scale over a whole day. The system assimilates pseudo data from a single track of a 300-km swath XCO₂ imager at 2 km resolution and from surface ground-based CO₂ and/or ¹⁴CO₂ networks. It represents the diversity of ¹⁴CO₂ sources and sinks and not just the dilution of radiocarbon-free FF CO₂ emissions. The uncertainty in the resulting FF CO₂ emissions at local (urban area/ plant) to regional scales is directly derived and used to assess the potential of the different combinations of observation systems. The assimilation of satellite observations yield estimates of the morning regional emissions with an uncertainty down to 10 % (1 sigma) in the satellite field of view, from an assumed uncertainty of 15 % in the prior estimates. However, it does not provide direct information about emissions outside the satellite field of view and neither about afternoon or nighttime emissions. The co-assimilation of ¹⁴CO₂ and CO₂ data lead to a further reduction of the uncertainty in the estimates of FF emissions. However, this further reduction is significant only in administrative regions with three or more ¹⁴CO₂ and CO₂ sampling sites. The uncertainty in the estimates of 1-day emission in North Rhine-Westphalia, a region with three sampling sites, decreases from 8 to 6.6 % when assimilating the in situ ¹⁴CO₂ and CO₂ data in addition to the satellite data. Furthermore, this new decrease appears to be larger when the ground stations are close to large FF emission areas, providing an additional direct constraint for the estimate of these sources rather than supporting the characterization of the background signal from the NEE and its separation from that of the FF emissions.