Improved constraints on northern extratropical CO2 2 fluxes obtained by combining surface-based and 3 space-based atmospheric CO2 measurements

Top-down estimates of CO2 fluxes are typically constrained by either surface-based or 39 space-based CO2 observations. Both of these measurement types have spatial and tem40 poral gaps in observational coverage that can lead to differences in inferred fluxes. As41 similating both surface-based and space-based measurements concurrently in a flux in42 version framework improves observational coverage and reduces sampling related arti43 facts. This study examines the consistency of flux constraints provided by these differ44 ent observations and the potential to combine them by performing a series of six-year 45 (2010–2015) CO2 flux inversions. Flux inversions are performed assimilating surface-based 46 measurements from the in situ and flask network, measurements from the Total Carbon 47 Column Observing Network (TCCON), and space-based measurements from the Green48 house Gases Observing Satellite (GOSAT), or all three datasets combined. Combining 49 the datasets results in more precise flux estimates for sub-continental regions relative to 50 any of the datasets alone. Combining the datasets also improves the accuracy of the pos51 terior fluxes, based on reduced root-mean-square differences between posterior-flux-simulated 52 CO2 and aircraft-based CO2 over midlatitude regions (0.33–0.56 ppm) in comparison to 53 GOSAT (0.37–0.61 ppm), TCCON (0.50–0.68 ppm), or in situ and flask measurements 54 (0.46–0.56 ppm) alone. These results suggest that surface-based and GOSAT measure55 ments give complementary constraints on CO2 fluxes in the northern extratropics and 56 can be combined in flux inversions to improve constraints on regional fluxes. This stands 57 in contrast with many earlier attempts to combine these datasets and suggests that im58 provements in the NASA Atmospheric CO2 Observations from Space (ACOS) retrieval 59 algorithm have significantly improved the consistency of space-based and surface-based 60 flux constraints.