Improved definition of prior uncertainties in CO ₂ and CO fossil fuel fluxes and the impact on a multi-species inversion with GEOS-Chem (v12.5)

Monitoring, Reporting and Verification (MRV) frameworks for greenhouse gas (GHG) emissions are being developed by countries across the world to keep track of progress towards national emission reduction targets. Data assimilation plays an important role in MRVs, combining different sources of information to get the best possible estimate of fossil fuel emissions and as a consequence better estimates for fluxes from the natural biosphere. Robust estimates for fossil fuel emissions rely on accurate estimates of uncertainties corresponding to the different pieces of information. We describe prior uncertainties in CO ₂ and CO fossil fuel fluxes, with special attention paid to spatial error correlations and the covariance structure between CO ₂ and CO. This represents the first time that the prior uncertainties in CO ₂ and the important co-emitted trace gas CO are defined consistently, including error correlations, which allows us to make use of the synergy between the two trace gases to better constrain CO ₂ fossil fuel fluxes. The CO:CO ₂ error correlations differ per sector, depending on the diversity of sub-processes occurring within a sector, and also show a large range in values between pixels for the same sector. For example, for other stationary combustion the pixel correlation values range from 0.1 to 1.0, whereas for road transport the correlation is mostly larger than 0.6. We illustrate the added value of our prior uncertainty definition using closed-loop numerical experiments over mainland Europe and the UK, which isolate the influence of using error correlations between CO ₂ and CO and the influence of prescribing more detailed information about prior emission uncertainties. We find that using our realistic prior uncertainty definition helps our data assimilation system to differentiate more easily between CO ₂ fluxes from biogenic and fossil fuel sources. Using the improved prior emission uncertainties we find fewer geographic regions with significant changes from the prior than using the default prior uncertainties, but they almost consistently move closer to the prescribed true values, in contrast to the default prior uncertainties. We also find that using CO provides additional information on CO ₂ fossil fuel fluxes, but only if the CO:CO ₂ error covariance structure is defined realistically. Using the default prior uncertainties, the CO ₂ fossil fuel fluxes move farther away from the truth for many geographical regions. With the default uncertainties the maximum deviation of fossil fuel CO ₂ from the prescribed truth is about 7 % in both the prior and posterior result. With the advanced uncertainties this is reduced to 3 % in the posterior.