Preprints
https://doi.org/10.5194/egusphere-2023-2833
https://doi.org/10.5194/egusphere-2023-2833
14 Dec 2023
 | 14 Dec 2023

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

Kim A. P. Faassen, Jordi Vilà-Guerau de Arellano, Raquel González-Armas, Bert G. Heusinkveld, Ivan Mammarella, Wouter Peters, and Ingrid T. Luijkx

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 (ERforest) 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 (ERatmos) 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 ERatmos signal to the ERforest signal and to understand the biophysical meaning of the ERatmos 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 ERatmos and ERforest to investigate the behavior of ERatmos and ERforest during different environmental conditions. We show that the ERatmos 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 ERatmos 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 ERatmos 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 ERatmos signal obtained from single height measurements rarely directly represents ERforest 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.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
Kim A. P. Faassen, Jordi Vilà-Guerau de Arellano, Raquel González-Armas, Bert G. Heusinkveld, Ivan Mammarella, Wouter Peters, and Ingrid T. Luijkx

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2833', Anonymous Referee #1, 28 Jan 2024
    • AC1: 'Reply on RC1', Kim Faassen, 29 Apr 2024
  • RC2: 'Comment on egusphere-2023-2833', Anonymous Referee #2, 10 Mar 2024
    • AC2: 'Reply on RC2', Kim Faassen, 29 Apr 2024
  • EC1: 'Comment on egusphere-2023-2833', Paul Stoy, 25 Mar 2024
    • AC3: 'Reply on EC1', Kim Faassen, 29 Apr 2024

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2833', Anonymous Referee #1, 28 Jan 2024
    • AC1: 'Reply on RC1', Kim Faassen, 29 Apr 2024
  • RC2: 'Comment on egusphere-2023-2833', Anonymous Referee #2, 10 Mar 2024
    • AC2: 'Reply on RC2', Kim Faassen, 29 Apr 2024
  • EC1: 'Comment on egusphere-2023-2833', Paul Stoy, 25 Mar 2024
    • AC3: 'Reply on EC1', Kim Faassen, 29 Apr 2024
Kim A. P. Faassen, Jordi Vilà-Guerau de Arellano, Raquel González-Armas, Bert G. Heusinkveld, Ivan Mammarella, Wouter Peters, and Ingrid T. Luijkx

Data sets

Atmospheric measurements results archive, Hyytiälä Ingrid Luijkx and Kim Faassen https://doi.org/10.18160/SJ3J-PD38

Model code and software

CLASS model, explanation and model code Jordi Vilà-Guerau de Arellano, Chiel van Heerwaarden, Bart van Stratum, and Kees van den Dries https://classmodel.github.io/

Kim A. P. Faassen, Jordi Vilà-Guerau de Arellano, Raquel González-Armas, Bert G. Heusinkveld, Ivan Mammarella, Wouter Peters, and Ingrid T. Luijkx

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Short summary
The ratio between atmospheric O2 and CO2 can be used to characterise the carbon balance of the surface. By combining a model and observations from the Hyytiala 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 to always use multiple heights of O2 and CO2 measurements to study surface CO2 exchange.