Preprints
https://doi.org/10.5194/os-2020-115
https://doi.org/10.5194/os-2020-115

  23 Dec 2020

23 Dec 2020

Review status: a revised version of this preprint is currently under review for the journal OS.

Diurnal cycle of the CO2 system in the coastal region of the Baltic Sea

Martti Honkanen1, Jens Daniel Müller2,4, Jukka Seppälä3, Gregor Rehder4, Sami Kielosto1,3, Pasi Ylöstalo3, Timo Mäkelä5, Juha Hatakka5, and Lauri Laakso1,6 Martti Honkanen et al.
  • 1Meteorological and Marine Research Programme, Finnish Meteorological Institute, Finland
  • 2Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland
  • 3Marine Ecology Research Laboratory, Finnish Environment Institute, Finland
  • 4Department of Marine Chemistry, Leibniz Institute for Baltic Sea Research, Warnemunde, Germany
  • 5Climate Research Programme, Finnish Meteorological Institute, Finland
  • 6School of Physical and Chemical Sciences, North-West University, Potchefstroom Campus, South Africa

Abstract. The direction and magnitude of carbon dioxide exchange between the atmosphere and the sea is regulated by their difference in partial pressure of carbon dioxide (pCO2). Typically, observations of pCO2 are carried out by using research vessels and voluntary observing ships which cannot easily detect the diurnal cycle of pCO2 at a given location. This study evaluates the magnitude and driving processes of the diurnal cycle of pCO2 in a coastal region of the Baltic Sea during the different seasons.We present pCO2 data from July 2018–June 2019 carried out in the vicinity of the island of Utö in the Archipelago Sea and quantify the relevant physical, biological and chemical processes affecting pCO2. The highest monthly median diurnal pCO2 peak-to-peak amplitude (31 μatm) was observed in August. This high diurnal variation was found to be related predominantly to biological processes. The biological transformations of carbon generated a sinusoidal diurnal pCO2 variation, with a maximum in the morning and a minimum in the afternoon. Compared to the biological carbon transformations, the effect of air sea exchange of carbon dioxide and the effect of temperature changes on pCO2 are smaller, with their monthly median peak-to-peak amplitudes were up to 12 and 5 μatm, respectively. Single diurnal peak-to-peak amplitudes can be significantly larger (up to 500 μatm), during upwelling. If the net exchange of carbon dioxide between the sea and atmosphere on our study site and sampling period is calculated based on a data set that consists of only one measurement per day, the error in the budget depends on the sampling time and can be up to ±12 %.

Martti Honkanen et al.

 
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Martti Honkanen et al.

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Dataset for Diurnal cycle of the CO2 system in the coastal region of the Baltic Sea (Submitted 2020/11) Honkanen, Martti, Müller, Jens Daniel, Seppälä, Jukka, Rehder, Gregor, Kielosto, Sami, Ylöstalo, Pasi, Mäkelä, Timo, Hatakka, Juha, and Laakso, Lauri https://doi.org/10.5281/zenodo.4292384

Martti Honkanen et al.

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Short summary
The exchange of carbon dioxide (CO2) between the sea and the atmosphere is regulated by the gradient of CO2 partial pressure (pCO2) between the sea and the air. The daily variation of the seawater pCO2 recorded at the fixed station Utö in the Baltic Sea was found to be mainly biologically driven. Calculation of the annual net exchange of CO2 between the sea and atmosphere based on daily measurements of pCO2 carried out using the same sampling time every day could introduce a bias of up to a 12 %.