The ratio between atmospheric O₂ and CO₂ can be used to characterize the carbon balance at the surface. By combining a model and observations from the Hyytiälä forest (Finland), we show that using atmospheric O₂ and CO₂ measurements from a single height provides a weak constraint on the surface CO₂ exchange because large-scale processes such as entrainment confound this signal. We therefore recommend always using multiple heights of O₂ and CO₂ measurements to study surface CO₂ exchange.