Using atmospheric O2 to disentangle the natural and anthropogenic CO2 signals

Atmospheric oxygen (O₂) allows to separate the natural and anthropogenic components in the atmospheric CO₂ signal, thereby providing additional constraints on these processes in the global carbon cycle. This is enabled through the ratio of O₂ and CO₂ in carbon cycle processes: the Exchange Ratio (ER). This ER signal has distinct values for combustion of different fossil fuel types, as well as between photosynthesis and respiration processes. Using these ER signals, we aim to further explore the potential of using atmospheric O₂ observations in CO₂ emission verification. For that, we are developing a global scale data assimilation system that can, next to CO₂, assimilate O₂ observations. This is our new multi-tracer implementation, specifically aimed at decadal and annual timescales: the CarbonTracker Europe Long Window system. Additionally, we implemented O₂ and the O₂/CO₂ exchange ratios into the Simple Biosphere model (SiB4) to further understand the influence of biosphere exchange on using Atmospheric Potential Oxygen (APO) as a tracer for fossil fuel emissions. We will present the results from this biosphere O₂ and CO₂ modelling to get a first theoretical assessment of the variability of the biosphere O₂ and CO₂ ER signals, both over space (related to the plant functional types) and time (related to seasonal patterns). These biosphere model results, are subsequently used in our first attempt of atmospheric inverse estimates of CO₂ fluxes using O₂ as a tracer. Finally, we will show our progress towards understanding the implications of the variability in the ERs for photosynthesis and respiration on APO calculations, as well as their influence on fossil fuel estimates using atmospheric O₂.