Impact of the variability of O₂ and CO₂ biosphere exchange on modeled O₂-based fossil fuel CO₂ emission estimates

Abstract. Atmospheric oxygen (O₂) can be used to partition components of the carbon budget at global and regional scales. These methods generally assume a constant net biospheric O₂:CO₂ exchange ratio (ER_net), although the exchange ratios of the gross fluxes that form ER_net, assimilation (ER_a) and respiration (ER_r), vary across ecosystems and time, suggesting ER_net may also vary spatially and temporally. To investigate this, we implemented O₂ fluxes in the Simple Biosphere Model (SiB4) by assigning plant functional type- and carbon pool-specific ER_a and ER_r values based on limited observations. This produces the first global O₂ biosphere flux dataset with spatially and temporally varying fluxes (0.5° × 0.5°, 3-hourly). Results show seasonal and spatial variability in ER_net, ranging from about 1.1 in winter to 0.8 in summer, with regional summer values differing by more than 0.1 across Europe. Temporal variability is primarily driven by changes in the relative contributions of photosynthesis and respiration, whereas spatial variability reflects differences in dominant plant functional types. The modeled ER_net range exceeds the currently recognized terrestrial O₂:CO₂ variability and may effect O₂-based carbon partitioning methods. Neglecting ER_net variability can bias regional estimates of fossil fuel CO₂ signals derived from atmospheric potential oxygen. At a strongly biosphere-influenced location, a modeled O₂-derived fossil fuel CO₂ signal of 1.5 ppm is overestimated by nearly 12 ppm when ER_net variability is ignored. These results highlight the importance of accounting for biospheric contributions in O₂ partitioning methods.