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

Published May 06, 2026 on Dryad. https://doi.org/10.5061/dryad.bvq83bkqx

Data files

May 06, 2026 version files 4.23 MB

helgoland_gtv.csv

4.20 MB
README.md

29.22 KB

Abstract

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 depth of 10m. Due to a period of biological fouling in late September to early October, spurious salinity values were trimmed by removing values with salinity less than 28 or values where salinity changed by more than 0.5 in one hour. To reduce the risk of biofouling-induced data loss, the CO₂ sensor was exchanged 3 times over the course of the deployment, and between exchanges, the sensors were also inspected and cleaned by divers from AWI at approximately 2-week intervals. The CO₂ sensor completed a self-zeroing routine once per day, and the data during this period were discarded. Air-side CO₂ concentrations were provided by the ICOS site located ca 800m west of the EC station (ICOS ID: HEL; Lat: 54.1804; Lon: 7.8833; measurement height: 110 m).

Water currents were measured with a bottom-mounted upward-facing TRDI Workhorse 1200 kHz. The ADCP collected one ensemble of 50 pings every minute with 1 m bins, with coordinates transformed with ADCPs sensor data, following on-site compass calibration. Raw binary data were filtered with flags for currents exceeding reasonable local maximum values (1.2 m/s), bins with low amplitude (50 counts) and correlation (64 counts), high error velocities, and bins beyond the surface side lobe interference range during post-processing. All remaining ADCP current data were considered good.

Dataset DOI: 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
`datetime`	Timestamp, 30-minute intervals, UTC	POSIXct (UTC)
`pCO2`	Seawater partial pressure of CO₂ measured by HydroC sensor; despiked	µatm
`sensor`	Identifier for the HydroC pCO₂ sensor unit	character
`air_co2_uatm`	Atmospheric CO₂ mole fraction from the ICOS Helgoland station, linearly interpolated to 30-min grid for rows where pCO₂ is not NA	µatm
`air_ch4_ppb`	Atmospheric CH₄ mole fraction from the ICOS Helgoland station	ppb
`sal`	Practical salinity from AWI CTD, linearly interpolated; sudden drops >0.5 PSU removed before interpolation	PSU (PSS-78)
`sst`	Sea surface temperature from AWI CTD, linearly interpolated	°C
`depth`	Water depth, CTD	m
`DO`	Dissolved oxygen from AWI CTD	(units depend on CTD source file; likely µmol L⁻¹ or % saturation)
`DOY`	Day of year, rounded to integer	day
`daytime`	Flag from EddyPro: 1 = daytime, 0 = night-time	binary (0/1)
`co2_flux`	Turbulent CO₂ flux from eddy covariance (EddyPro full-output); positive = upward (sea-to-air)	µmol m⁻² s⁻¹
`qc_co2_flux`	EddyPro quality flag for CO₂ flux; 0 = highest quality, 1 = moderate, 2 = low quality	integer (0–2)
`LE`	Latent heat flux from eddy covariance	W m⁻²
`qc_LE`	EddyPro quality flag for LE; same scale as `qc_co2_flux`	integer (0–2)
`H`	Sensible heat flux from eddy covariance	W m⁻²
`qc_H`	EddyPro quality flag for H; same scale as `qc_co2_flux`	integer (0–2)
`sonic_temperature`	Sonic (virtual) temperature measured by the ultrasonic anemometer	K
`air_temperature`	Air temperature from EddyPro biomet file	K
`u.`	Friction velocity (u*) from eddy covariance	m s⁻¹
`TKE`	Turbulent kinetic energy from eddy covariance	m² s⁻²
`L`	Obukhov length, indicator of atmospheric stability; negative = unstable, positive = stable	m
`wind_speed`	Mean horizontal wind speed at measurement height (~4 m) from sonic anemometer	m s⁻¹
`max_wind_speed`	Maximum wind speed within the averaging period	m s⁻¹
`wind_dir`	Wind direction at measurement height	degrees (0–360°, meteorological)
`RH`	Relative humidity from EddyPro output	%
`X.z.d..L`	Dimensionless stability parameter (z−d)/L, where z is measurement height, d is displacement height, L is Obukhov length; used as QC filter (threshold > −0.6)	dimensionless
`co2_var`	Variance of the CO₂ signal within the 30-min averaging window; used as QC filter (threshold <0.3)	(µmol mol⁻¹)²
`v_var`	Variance of the lateral (v) wind component; used as input to flux footprint model	m² s⁻²
`bowen_ratio`	Bowen ratio = H / LE	dimensionless
`x_peak`	Along-wind distance to the peak of the flux footprint	m
`x_90.`	Along-wind distance enclosing 90% of the flux footprint contribution	m
`PPFD_1_1_1`	Photosynthetic photon flux density from biomet station	µmol m⁻² s⁻¹
`RH_1_1_1`	Relative humidity from biomet station (sensor position 1,1,1)	%
`RN_1_1_1`	Net radiation from biomet station	W m⁻²
`co2_mole_fraction`	Ambient CO₂ mole fraction measured by the LI-7200 analyser	µmol mol⁻¹ (ppm)
`TA_1_1_1`	Air temperature from biomet station (sensor position 1,1,1)	°C or K (from EddyPro biomet)
`avg_signal_strength_7200_mean`	Mean signal strength of the LI-7200 closed-path gas analyser over the averaging period; used to assess optical path contamination	% (0–100)
`delta_signal_strength_7200_mean`	Change in LI-7200 signal strength over the averaging period; indicator of optical path soiling trend	%
`ICOS_wind`	Wind speed at 110 m from ICOS Helgoland tall-tower station (not height-adjusted)	m s⁻¹
`ICOS_u10`	ICOS tall-tower wind speed scaled from 110 m to 10 m using a logarithmic wind profile (`airsea::wind.scale.base`); linearly interpolated to 30-min grid	m s⁻¹
`WD`	Wind direction from the ICOS Helgoland station	degrees
`AT`	Air temperature from the ICOS Helgoland station; fill value −999.99 replaced with NA	°C
`precip`	Hourly precipitation from DWD station Helgoland (station ID 02115); source fill value −999 replaced with NA	mm h⁻¹
`adcp_vel`	Water column average current speed from ADCP (Margate_23789), averaged to 30-min	m s⁻¹
`adcp_dir`	Water column average current direction from ADCP, averaged to 30-min	degrees
`adcp_press`	Pressure recorded by the ADCP transducer, averaged to 30-min	dbar
`adcp_temp`	Temperature recorded by the ADCP transducer, averaged to 30-min	°C
`Avg amplitude incl beyond surf`	ADCP acoustic backscatter amplitude averaged over all depth bins including those beyond the surface, averaged to 30-min	counts (dB scale, instrument-specific)
`Avg amplitude surf to surf-2`	ADCP acoustic backscatter amplitude averaged over depth bins from the surface down to surface−2 bins (near-surface layer), averaged to 30-min	counts (dB scale, instrument-specific)
`adcp_backscatter`	Renamed from `Avg amplitude surf-2 to surf-4`; ADCP acoustic backscatter in the subsurface layer (surf−2 to surf−4 bins), averaged to 30-min	counts (dB scale, instrument-specific)
`wave_ht_m`	Significant wave height derived from the CTD pressure sensor, averaged to 30-min	m
`pCO2_NT`	Non-thermal component of seawater pCO₂, after Takahashi et al. (2002): pCO₂ × exp(0.0423 × (T̄ − SST)); isolates biological/mixing effects on pCO₂ variability	µatm
`pCO2_T`	Thermal component of seawater pCO₂, after Takahashi et al. (2002): p̄CO₂ × exp(0.0423 × (SST − T̄)); isolates temperature-driven pCO₂ variability	µatm
`dU10_dt`	Rate of change of wind speed over the preceding `wind_time_lag` (=3) half-hour intervals: (U₁₀(t) − U₁₀(t−3)) / 3	m s⁻¹ per 30-min interval
`wind_trend`	Categorical wind history based on `dU10_dt`: "Rising (>0.5)", "Rising (>0)", "Falling (<−0.5)", or "Falling (<0)"	character
`fetch`	Wind fetch in the direction of `wind_dir`, looked up from a pre-computed angular fetch table (`helgoland_fetch.csv`) for Helgoland	m
`dTw_dt`	Rate of change of SST, computed as centred finite difference; used in water heat storage calculation	°C s⁻¹
`S_w`	Rate of change of water-column heat storage: ρ × Cp × depth × dTw/dt, where ρ is seawater density (gsw) and Cp = 4186 J kg⁻¹ K⁻¹	W m⁻²
`qeff`	Effective air–sea heat flux: Q_eff = RN − H − LE − S_w; used to compute buoyancy flux	W m⁻²
`B`	Buoyancy flux at the water surface: B = (g × α × Q_eff) / (ρ × Cp_sw), where g = 9.8 m s⁻², α is thermal expansion coefficient (gsw), ρ is density (gsw), Cp_sw is specific enthalpy (gsw)	m² s⁻³
`w.w`	Waterside convective velocity scale: (−B × depth)^(1/3); real-valued only when B < 0 (convective conditions)	m s⁻¹
`delta_T`	Air–sea temperature difference: SST − (air_temperature − 273.15); note air_temperature is in K from EddyPro	°C
`sc`	Schmidt number of CO₂ at in-situ SST and salinity, computed with `airsea::Sch("CO2", sst, sal, method = "JS")` (Johnson & Salter formulation)	dimensionless
`delta_pCO2_uatm`	Air–sea CO₂ partial pressure difference: pCO₂_water − air_co2_uatm; positive = supersaturated (outgassing)	µatm
`delta_pCO2_atm`	Air–sea CO₂ partial pressure difference in atm: delta_pCO2_uatm / 10⁶	atm
`ko`	CO₂ solubility (Bunsen coefficient) at in-situ SST and salinity via `airsea::GasSolubility("CO2", sst, sal)` × 1000	mol m⁻³ atm⁻¹
`FCO2`	Measured EC CO₂ flux converted to SI units: co2_flux / 10⁶	mol m⁻² s⁻¹
`k`	Gas transfer velocity computed from EC flux: FCO2 / (delta_pCO2_atm × ko), converted to cm hr⁻¹	cm hr⁻¹
`k660`	Gas transfer velocity normalised to Schmidt number 660 (CO₂ at 20°C in seawater): k × (Sc/660)^(−0.5)	cm hr⁻¹
`u10`	Wind speed at 10 m height, scaled from measurement height (4 m) using `airsea::wind.scale.base(wind_speed, 4)`	m s⁻¹
`k660_RC01`	k₆₆₀ parameterisation after Raymond & Cole (2001): 1.91 × exp(0.35 × u10) × (600/660)^(−0.5)	cm hr⁻¹
`k660_N00`	k₆₆₀ parameterisation after Nightingale et al. (2000): (0.222 × u10² + 0.333 × u10) × (600/660)^(−0.5)	cm hr⁻¹
`k660_DH23`	k₆₆₀ parameterisation after Dobashi & Ho (2023): 0.143 × u10² × (600/660)^(−0.5)	cm hr⁻¹
`k660_PY21`	k₆₆₀ parameterisation after Prytherch & Yelland (2021): 0.179 × u10²	cm hr⁻¹
`k660_M01`	k₆₆₀ parameterisation after McGillis et al. (2001): 3.3 + 0.026 × u10³	cm hr⁻¹
`k660_M14`	k₆₆₀ parameterisation after Mørk et al. (2014): 0.3 × u10^{2.5 × (600/660)}(−0.5)	cm hr⁻¹
`k660_W14`	k₆₆₀ parameterisation after Wanninkhof (2014): 0.251 × u10² (already referenced to Sc = 660)	cm hr⁻¹
`delta_k660`	Difference between measured k₆₆₀ and Nightingale et al. (2000) parameterisation: k660 − k660_N00	cm hr⁻¹
`qc`	Final quality control flag: 1 = passes all filters (qc_co2_flux == 0, u10 ≥ 1 m s⁻¹, co2_var < 0.3, (z−d)/L > −0.6, \|delta_pCO2_uatm\| > 50 µatm, k > 0, \|co2_flux\| < 2 µmol m⁻² s⁻¹); 0 = fails one or more filters	binary (0/1)
`wind_bin_cat`	Wind speed bin as a factor with interval labels (2 m s⁻¹ wide bins from 0 to max u10 + 2)	factor (e.g., "[0,2)")
`wind_bin`	Wind speed bin midpoint value (numeric); bin upper edge used as label (e.g., 2, 4, 6, …)	m s⁻¹
`sector`	Wind sector classification based on `wind_dir`: "margate" (255°–345°, open sea fetch toward the Margate platform), "channel" (≤135° or >345°), "Land" (all other directions)	character

Code/software

All data processing was completed in R Studio

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