Diurnal variability of atmospheric O₂, CO₂ and their exchange ratio above a boreal forest in southern Finland

Abstract. The exchange ratio (ER) between atmospheric O₂ and CO₂ is a useful tracer on global and local scales to better understand the carbon budget. The variability of ER (in mol O₂ per mol CO₂) between terrestrial ecosystems is not well-known, and there is no consensus on how to derive the ER signal to represent an ecosystem, as there are different approaches available, either based on concentration (ER_atmos) or flux measurements (ER_forest). In this study we measured atmospheric O₂ and CO₂ concentrations at two heights above the boreal forest in Hyytiälä, Finland. Such measurements of O₂ are unique and enable us to potentially identify which forest carbon loss and production mechanisms dominate over various hours of the day. We found that the ER_atmos signal at 23 m is not representative for the forest exchange alone but is also influenced by other factors, including for example entrainment of air masses with different thermodynamic and atmospheric composition characteristics in the atmospheric boundary layer. To derive ER_forest we infer O₂ fluxes using multiple theoretical and observation-based micro-meteorological formulations to determine the most suitable approach. Our resulting ER_forest shows a distinct difference in behaviour between daytime (0.92 ± 0.17 mol/mol) and nighttime (1.03 ± 0.05 mol/mol). These insights demonstrate the diurnal variability of different ER signals above a boreal forest and we also confirmed that the signals of ER_atmos and ER_forest can not be used interchangeably. Therefore, we recommend measurements on multiple vertical levels to derive O₂ and CO₂ fluxes for the ER_forest signal, instead of a single level time series of the concentrations for the ER_atmos signal. We show that ER_forest can be further split into specific signals for respiration (1.03 ± 0.05 mol/mol) and photosynthesis (0.96 ± 0.12 mol/mol). This estimation allows us to separate the Net Ecosystem Exchange (NEE) into Gross Primary Production (GPP) and Total Ecosystem Respiration (TER), giving comparable results to the more commonly used eddy covariance approach. Our study shows the potential of using atmospheric O₂ as an alternative method to gain new insights on the different CO₂ signals that contribute to the forest carbon budget.