A local- to national-scale inverse modeling system to assess the potential of spaceborne CO2 measurements for the monitoring of anthropogenic emissions
Authors: Santaren Diego , Broquet Grégoire , Bréon François-Marie , Chevallier Frédéric , Siméoni Denis , Zheng Bo , Ciais Philippe 
Published: January 2021

Abstract. This work presents a flux inversion system which assesses the potential of new satellite imagery measurements of atmospheric CO2 for monitoring anthropogenic emissions at scales ranging from local intense point sources to regional and national scales. Such imagery measurements
will be provided by the future Copernicus Anthropogenic Carbon Dioxide
Monitoring Mission (CO2M). While the modeling framework retains the complexity
of previous studies focused on individual and large cities, this system
encompasses a wide range of sources to extend the scope of the analysis.
This atmospheric inversion system uses a zoomed configuration of the CHIMERE
regional transport model which covers most of western Europe with a
2 km resolution grid over northern France, western Germany and Benelux. For
each day of March and May 2016, over the 6 h before a given satellite
overpass, the inversion separately controls the hourly budgets of
anthropogenic emissions in this area from ∼ 300 cities, power
plants and regions. The inversion also controls hourly regional budgets of
the natural fluxes. This enables the analysis of results at the local to
regional scales for a wide range of sources in terms of emission budget and
spatial extent while accounting for the uncertainties associated with natural
fluxes and the overlapping of plumes from different sources. The potential
of satellite data for monitoring CO2 fluxes is quantified with posterior
uncertainties or uncertainty reductions (URs) from prior inventory-based
statistical knowledge. A first analysis focuses on the hourly to 6 h budgets of the emissions of
the Paris urban area and on the sensitivity of the results to different
characteristics of the images of vertically integrated CO2 (XCO2)
corresponding to the spaceborne instrument: the pixel spatial resolution,
the precision of the XCO2 retrievals per pixel and the swath width.
This sensitivity analysis provides a correspondence between these parameters
and thresholds on the targeted precisions of emission estimates. However,
the results indicate a large sensitivity to the wind speed and to the
prior flux uncertainties. The analysis is then extended to the large ensemble of
point sources, cities and regions in the study domain, with a focus on the
inversion system's ability to separately monitor neighboring sources whose
atmospheric signatures overlap and are also mixed with those produced by
natural fluxes. Results highlight the strong dependence of uncertainty
reductions on the emission budgets, on the wind speed and on whether the
focus is on point or area sources. With the system hypothesis that the
atmospheric transport is perfectly known, the results indicate that the
atmospheric signal overlap is not a critical issue. All of the tests are
conducted considering clear-sky conditions, and the limitations from cloud
cover are ignored. Furthermore, in these tests, the inversion system is
perfectly informed about the statistical properties of the various sources
of errors that are accounted for, and systematic errors in the XCO2
retrievals are ignored; thus, the scores of URs are assumed to be
optimistic. For the emissions within the 6 h before a satellite
overpass, URs of more than 50 % can only be achieved for power plants and
cities whose annual emissions are more than
∼ 2 MtC yr−1. For regional budgets encompassing more diffuse emissions, this
threshold increases up to ∼ 10 MtC yr−1. The results therefore suggest an imbalance in the monitoring capabilities of the
satellite XCO2 spectro-imagery towards high and dense sources.