Determining the sinks and sources of CO2 is an important open question in climate science. Since O2 and CO2 are linked in several processes, O2 can act as a tracer for different CO2 fluxes on global and local scale. For the terrestrial CO2 sink, O2 and CO2 are linked with the processes assimilation and respiration. These fluxes are coupled through their exchange ratio (ER). The exchange ratio indicates how much O2 is exchanged for CO2. This number can be used on local scale to better understand: the behaviour of a specific ecosystem, determine if there are sources of fossil fuel combustion and to separate the respiration and photosynthesis fluxes. I will start my PhD by modelling and analysing the behaviour of the ER in the atmospheric boundary layer on local scale in above a boreal forest. I hope to gain insight into how well O2 performs as a tracer for CO2, which I eventually can use on a global scale.