The impacts of fossil fuel emission uncertainties and accounting for 3-D chemical CO2production on inverse natural carbon flux estimates from satellite andin situdata

Atmospheric carbon dioxide (CO2) inversions for estimating natural carbon fluxes typically do not allow for adjustment of fossil fuel CO(2)emissions, despite significant uncertainties in emission inventories and inadequacies in the specification of international bunker emissions in inversions. Also, most inversions place CO(2)release from fossil fuel combustion and biospheric sources entirely at the surface. However, a non-negligible portion of the emissions actually occurs in the form of reduced carbon species, which are eventually oxidized to CO(2)downwind. Omission of this 'chemical pump' can result in a significant redistribution of the inferred total carbon fluxes among regions. We assess the impacts of different prescriptions of fossil fuel emissions and accounting for the chemical pump on flux estimation, with a novel aspect of conducting both satellite CO(2)observation-based and surfacein situ-based inversions. We apply 3-D carbon monoxide (CO) loss rates archived from a state-of-the-art GEOS chemistry and climate model simulation in a forward transport model run to simulate the distribution of CO(2)originating from oxidation of carbon species. We also subtract amounts from the prior surface CO(2)fluxes that are actually emitted in the form of fossil and biospheric CO, methane, and non-methane volatile organic compounds (VOCs). We find that the posterior large-scale fluxes are generally insensitive to the finer-scale spatial differences between the ODIAC and CDIAC fossil fuel CO(2)gridded datasets and assumptions about international bunker emissions. However, accounting for 3-D chemical CO(2)production and the surface correction shifts the global carbon sink, e.g. from land to ocean and from the tropics to the north, with a magnitude and even direction that depend on assumptions about the surface correction. A GOSAT satellite-based inversion is more sensitive to the chemical pump than one usingin situobservations, exhibiting substantial flux impacts of 0.28, 0.53, and -0.47 Pg C yr(-1)over tropical land, global land, and oceans, due to differences in the horizontal and vertical sampling of the two observation types. Overall, the biases from neglecting the chemical pump appear to be minor relative to the flux estimate uncertainties and the differences between thein situand GOSAT inversions, but their relative importance will grow in the future as observational coverage further increases and satellite retrieval biases decrease.