Regional-Scale, Sector-Specific Evaluation of Global CO₂ Inversion Models Using Aircraft Data From the ACT-America Project

We use 148 airborne vertical profiles of CO₂ for frontal cases from the summer 2016 Atmospheric Carbon and Transport—America (ACT-America) campaign to evaluate the skill of 10 global CO₂ in situ inversion models from the version 7 Orbiting Carbon Observatory–2 (OCO-2) Model Intercomparison Project (MIP). Model errors (model posterior—observed CO₂ dry air mole fractions) were categorized by region (Mid-Atlantic, Midwest, and South), frontal sector (warm or cold), and transport model (predominantly Tracer Model 5 (TM5) and Goddard Earth Observing System—Chemistry [GEOS-Chem]). All inversions assimilated the same CO₂ observations. Overall, the median inversion profiles reproduce the general structures of the observations (enhanced/depleted low-level CO₂ in warm/cold sectors), but 1) they underestimate the magnitude of the warm/cold sector mole fraction difference, and 2) the spread among individual inversions can be quite large (>5 ppm). Uniquely in the Mid-Atlantic, inversion biases segregated according to atmospheric transport model, where TM5 inversions biases were −3 to −4 ppm in warm sectors, while those of GEOS-Chem were +2 to +3 ppm in cold sectors. The large spread among the mean posterior CO₂ profiles is not explained by the different atmospheric transport models. These results show that the inversion systems themselves are the dominant cause of this spread, and that the aircraft campaign data are clearly able to identify these large biases. Future controlled experiments should identify which inversions best reproduce mid-latitude CO₂ mole fractions, and how inversion system components are linked to system performance.