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 %.
Diurnal cycle of the CO<sub>2</sub> system in the coastal region of the Baltic Sea