Eddy covariance measurements of air-sea CO₂ flux in summer 2025 at Helgoland, and associated water-side measurements

Between 1 June and 13 September 2025, eddy covariance (EC) and associated water-side measurements were conducted on the northern shore of Helgoland, a small island in the southern North Sea. Observations of air-sea CO2 exchange were made with an EC system built around a closed-path infrared gas analyzer (Li-Cor 7200R/S) and sonic anemometer (Gill Windmaster), both of which responding at 10 Hz and mounted ca 4.5 m above the mean water level. All wind speed measurements were scaled to a reference height of 10 m (U10) assuming an exponential profile. Additional meteorological measurements from the station include net radiation (Kipp & Zonen NR Lite2), photosynthetically active radiation (Li-190R), and temperature and relative humidity. Water-side measurements were provided by an in-situ CO2 sensor (4H-Jena, HydroC-CO2) and CTD (Sea and Sun Technology) installed at the MARGATE site operated by AWI (Walcher et al., 2013), approximately 800 m NW of the EC station, at an average water dept..., , # Eddy covariance measurements of air-sea CO~2~ flux in summer 2025 at Helgoland, and associated water-side measurements Dataset DOI: [10.5061/dryad.bvq83bkqx](https://doi.org/10.5061/dryad.bvq83bkqx) ## Description of the data and file structure ### Files and variables #### File: helgoland_gtv.csv **Description:** Merged 30-minute time series combining eddy covariance CO₂ flux measurements from the Margate platform at Helgoland (North Sea) with co-located oceanographic, meteorological, and atmospheric data. Used to derive and evaluate gas transfer velocity (GTV) parameterisations for air–sea CO₂ exchange. Data span June–September 2025. **Missing values:** Labeled as `NA` throughout. ##### Variables | Variable | Description | Units ..., ,