This data set contains the partial pressure of carbon dioxide (CO2) present in seawater together with temperature, salinity and other necessary auxiliary information measured on a voluntary observing ship, M/S Silja Serenade between April 2018 and April 2020. During this period, M/S Silja Serenade operated daily on a route Helsinki-Mariehamn-Stockholm in the Baltic Sea. During the ice season of 2018-2019, the flow-through system was set offline and no measurements were taken. The measurement setup of seawater CO2 (SuperCO2, Sunburst Sensors) consists of two showerhead equilibrators where the CO2 is equilibrated between the continuous water stream and the overlaying gas which is directed to an infrared gas analyzer (LI-840A, LI-COR), which measures the molar fraction of CO2. The measured molar fraction of CO2 has been corrected for the sensor drift by using four reference gases, which are measured every four hours. The calculation of the partial pressure of CO2 is based on these drift corrected molar fractions. The flow-through system is part of the Alg@line network of Finnish Environment Institute and the pCO2 measurement is governed by Finnish Meteorological Institute. This measurement has been supported by the BONUS INTEGRAL project, the JERICO research infrastructure and Finnish Marine Research Infrastructure and the SEASINK Finnish Academy project. The metadata (see dataset comments) contains information about the data quality control and assurance, including an information about the flagging system used.

PI: Lauri LaaksoMeasurement: Sami KielostoData: Martti HonkanenData of Apr 2018 - Apr 2020 are given in half minute median values in monthly tab separated data files. During ice time, no measurements were taken.