18 Jul 2022
18 Jul 2022
Status: a revised version of this preprint is currently under review for the journal ACP.

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

Kim A. P. Faassen1, Linh N. T. Nguyen2, Eadin R. Broekema2, Bert A. M. Kers2, Ivan Mammarella3, Timo Vesala3,4, Penelopy A. Pickers5, Andrew C. Manning5, Jordi Vilà-Guerau de Arellano1,6, Harro A. J. Meijer2, Wouter Peters1,2, and Ingrid T. Luijkx1 Kim A. P. Faassen et al.
  • 1Meteorology and Air Quality, Wageningen University and Research, Wageningen, the Netherlands
  • 2Centre for Isotope Research (CIO), Energy and Sustainability Research Institute Groningen, University of Groningen, Groningen, the Netherlands
  • 3Institute for Atmospheric and Earth System Research (INAR) / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
  • 4INAR/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, Helsinki, Finland
  • 5Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, United Kingdom
  • 6Atmospheric Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany

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 (ERatmos) or flux measurements (ERforest). 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 ERatmos 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 ERforest we infer O2 fluxes using multiple theoretical and observation-based micro-meteorological formulations to determine the most suitable approach. Our resulting ERforest 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 ERatmos and ERforest can not be used interchangeably. Therefore, we recommend measurements on multiple vertical levels to derive O2 and CO2 fluxes for the ERforest signal, instead of a single level time series of the concentrations for the ERatmos signal. We show that ERforest 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.

Kim A. P. Faassen et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-504', Anonymous Referee #1, 28 Aug 2022
    • AC1: 'Reply on RC1', Kim Faassen, 18 Nov 2022
  • RC2: 'Comment on acp-2022-504', Anonymous Referee #2, 06 Sep 2022
    • AC2: 'Reply on RC2', Kim Faassen, 18 Nov 2022
  • RC3: 'Comment on acp-2022-504', Anonymous Referee #3, 22 Sep 2022
    • AC3: 'Reply on RC3', Kim Faassen, 18 Nov 2022

Kim A. P. Faassen et al.

Data sets

Atmospheric O2 and CO2 measurements at Hyytiälä, Finland Ingrid T. Luijkx, Kim A. P. Faassen

Kim A. P. Faassen et al.


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Short summary
The Exchange Ratio (ER) between atmospheric O2 and CO2 provides a useful tracer for separately estimating photosynthesis and respiration processes in the forest carbon balance. This is highly relevant to better understand the expected biosphere sink which will determine future atmospheric CO2 levels. We therefore measured O2, CO2 and their ER above a boreal forest in Finland and investigated their diurnal behaviour for a representative day and we show the most suitable way to determine the ER.