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. Combining measurements of atmospheric CO 2 and itsradiocarbon ( 14 CO 2) fraction and transport modeling in atmosphericinversions offers a way to derive improved estimates of CO 2 emitted fromfossil fuel (FFCO 2) . In this study, we solve for the monthly FFCO 2 emission budgets at regional scale (i.e., the size of a medium-sized countryin Europe) and investigate the performance of different observation networksand sampling strategies across Europe. The inversion system is built on theLMDZv4 global transport model at 3.75 ∘ ×  2.5 ∘ resolution. We conduct Observing System Simulation Experiments (OSSEs) and usetwo types of diagnostics to assess the potential of the observation andinverse modeling frameworks. The first one relies on the theoreticalcomputation of the uncertainty in the estimate of emissions from theinversion, known as “posterior uncertainty”, and on the uncertaintyreduction compared to the uncertainty in the inventories of these emissions,which are used as a prior knowledge by the inversion (called “prioruncertainty”). The second one is based on comparisons of prior and posteriorestimates of the emission to synthetic “true” emissions when these trueemissions are used beforehand to generate the synthetic fossil fuel CO 2 mixing ratio measurements that are assimilated in the inversion. With17 stations currently measuring 14 CO 2 across Europe using 2-weekintegrated sampling, the uncertainty reduction for monthly FFCO 2 emissions in a country where the network is rather dense like Germany, islarger than 30 %. With the 43  14 CO 2 measurement stationsplanned in Europe, the uncertainty reduction for monthly FFCO 2 emissionsis increased for the UK, France, Italy, eastern Europe and the Balkans, dependingon the configuration of prior uncertainty. Further increasing the number ofstations or the sampling frequency improves the uncertainty reduction (up to40 to 70 %) in high emitting regions, but the performance of theinversion remains limited over low-emitting regions, even assuming a denseobservation network covering the whole of Europe. This study also shows thatboth the theoretical uncertainty reduction (and resulting posterioruncertainty) from the inversion and the posterior estimate of emissionsitself, for a given prior and “true” estimate of the emissions, are highlysensitive to the choice between two configurations of the prior uncertaintyderived from the general estimate by inventory compilers or computations onexisting inventories. In particular, when the configuration of the prioruncertainty statistics in the inversion system does not match the differencebetween these prior and true estimates, the posterior estimate of emissionsdeviates significantly from the truth. This highlights the difficulty offiltering the targeted signal in the model–data misfit for this specificinversion framework, the need to strongly rely on the prior uncertaintycharacterization for this and, consequently, the need for improved estimatesof the uncertainties in current emission inventories for real applicationswith actual data. We apply the posterior uncertainty in annual emissions tothe problem of detecting a trend of FFCO 2 , showing that increasing themonitoring period (e.g., more than 20 years) is more efficient than reducinguncertainty in annual emissions by adding stations. The coarse spatialresolution of the atmospheric transport model used in this OSSE (typical ofmodels used for global inversions of natural CO 2 fluxes) leads to largerepresentation errors (related to the inability of the transport model tocapture the spatial variability of the actual fluxes and mixing ratios atsubgrid scales), which is a key limitation of our OSSE setup to improve theaccuracy of the monitoring of FFCO 2 emissions in European regions. Usinga high-resolution transport model should improve the potential to retrieveFFCO 2 emissions, and this needs to be investigated.

atmospheric chemistry and physics

Potential of European &amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; observation network to estimate the fossil fuel CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; emissions via atmospheric inversions