Atmospheric O₂ and CO₂ measurements at a single height provide weak constraint on surface carbon exchange

Abstract. The ratios of atmospheric tracers are often used to interpret the local CO₂ budget, where measurements at a single height are assumed to represent local flux signatures. Alternatively, these signatures can be derived from direct flux measurements or using fluxes derived from measurements at multiple heights. In this study, we contrast interpretation of surface CO₂ exchange from tracer ratio measurements at a single height versus measurements at multiple heights.

Specifically, we analyse the ratio between atmospheric O₂ and CO₂ (exchange ratio, ER) above a forest canopy. We consider two alternative approaches: the exchange ratio of the forest (ER_forest) obtained from the ratio of the surface fluxes of O₂ and CO₂, derived from their vertical gradients measured at multiple heights, and the exchange ratio of the atmosphere (ER_atmos) obtained from changes in the O₂ and CO₂ mole fractions over time measured at a single measurement height. We investigate the diurnal cycle of both ER signals, with the goal to relate the ER_atmos signal to the ER_forest signal and to understand the biophysical meaning of the ER_atmos signal. We combined CO₂ and O₂ measurements from Hyytiälä, Finland during spring and summer of 2018 and 2019 with a conceptual land-atmosphere model and a theoretical relationship between ER_atmos and ER_forest to investigate the behavior of ER_atmos and ER_forest during different environmental conditions. We show that the ER_atmos signal rarely directly represents the forest exchange, mainly because it is influenced by entrainment of air from the free troposphere into the atmospheric boundary layer. The influence of these larger scale signals leads to very high ER_atmos values (even larger than 2), especially in the early morning transition. These high values do not directly represent carbon cycle processes, but are rather a mixture of different signals. We show that the resulting ER_atmos signal is not the average of the contributing processes, but rather an indication of the influence of large scale processes such as entrainment or advection. Our findings show that these processes are furthermore influenced by climate conditions, such as the 2018 heatwave, through their dependence on soil moisture and temperature.

We conclude that the ER_atmos signal obtained from single height measurements rarely directly represents ER_forest and therefore only provides a weak constraint on local scale surface CO₂ exchange, because large scale processes confound the signal. Single height measurements therefore always require careful selection of the time of day and should be combined with atmospheric modelling to yield a meaningful representation of forest carbon exchange. More generally, we recommend to always measure at multiple heights when using multi-tracer measurements to study surface CO₂ exchange.