Data from: Global Fjords Are Minor Sources Of Nitrous Oxide To The Atmosphere

The study sites included six different fjords located in an area spanning from 56.6˚N to 66.6˚N and from −39.0˚W to 13.7˚E.

All data were collected throughout five different cruises from April to July 2023, with instruments installed on R/V Skagerak (University of Gothenburg). N2O dissolved in the surface water was measured continuously via Cavity Enhanced Absorption Spectroscopy using a LI-7820 N2O/H2O trace gas analyzer (LI-COR Biosciences). The LI-7820 produced high-precision N2O (ppb) measurement data every second, response time of 0 to 330 ppb ≤ 2 seconds and maximum drift of < 1 ppb per 24-hour period.

 Nitrous oxide measurements (>1800 in each fjord) were integrated over 30 min and adjusted to account for gas exchange equilibration over the closed loop and for the ~10 min time lag (delayed response time of ~5 min from the exchanger to the gas detector and additional lag of ~5 min from sea to exchanger).The ship was equipped with a -4H-FerryBox (JENA Engeneering GmbH, Germany), an automatic flow-through system with various sensors measuring hydrographic and biochemical parameters such as salinity, temperature, dissolved oxygen (O2), pH, chlorophyll, and turbidity (-4H-JENA engineering GmbH, n.d.). All potentiometric FerryBox pH (EGA150, Meinsberg) measurements on the NBS scale were corrected by a constant offset (ΔpH = -0.451 ±0.016) based on the concurrent spectrophotometric pH measurements (Müller et al., 2018) during the Greenlandic and Icelandic cruises. Water was pumped through a subsurface-inlet and circulates with a speed of 1 m s−1 in the system. Bubbles and particles are removed by a debubbling unit (Ferry- Box Task Team, n.d.). Windspeed (m s−1) measurements were recorded with an onboard sonic anemometer (Airmar PB200) mounted approximately 19 meters above the water line and then logarithmically corrected to 10 meters above water line.

Additionally, water samples for dissolved nitrates and nitrites (NOx-) and ammonium (NH4+) concentrations were collected along the survey transects using Niskin-rosette bottles from a depth of ~3 m. Dissolved nutrient samples were collected by filtering sample water through cellulose acetate filters (0.45 μm) into pre-rinsed 12 mL polypropylene vials before immediate freezing until laboratory analysis. Nutrient samples for dissolved NH4+, NO3− and NO2− were filtered through pre-rinsed cellulose acetate filters (0.45 μm, Sartorius) and frozen at −20 °C until segmented flow analysis (QuAAtro, XY-3 Sampler, Seal Analytical 2015; detection limits and precisions 0.2 μM and 7% for NH4+, 0.05 μM and 7% for NO3− and 0.02 μM and 7% for NO2−).

N2O saturation values were calculated as the ratio between concentrations of dissolved N2O in seawater and the corresponding computed concentrations in the atmosphere (Walter et al., 2004). Diffusive sea-air fluxes (f) were measured according to the formula: " f=(p_water- p_air )  α k " where p_water is the partial pressure of N2O in the surface water layers calculated after correcting for the water vapor partial pressure in the equilibrated headspace and local atmospheric pressure. p_air is the partial pressure of N2O in air. For pair the global values from NOAA were used (Lan, 2024). Both partial pressures are expressed in natm. α is the solubility coefficient that was calculated from temperature and salinity using equations of Weiss and Price (1980). k is the gas transfer velocity calculated from the wind-based empirical model by (Wanninkhof, 2014). The k values were calculated according to the formula: "k = 0.251 U^2  〖(Sc/660)〗^(-0.5)" where U is the wind speed. We used in situ daily averaged wind speed measurements. Sc is the Schmidt number, which is water kinematic viscosity divided by the molecular diffusion coefficient of N2O (Wanninkhof, 2014). We used 4H–FerryBox temperature observations from the same water line as for the continuous N2O measurements. N2O fluxes at the sea-air interface are expressed in µg N2O m−2 day−1.

Refeferences:

Lan, X., Thoning, K.W., Dlugokencky, E.J.:. (2024). Trends in globally-averaged CH4, N2O, and SF6 determined from NOAA Global Monitoring Laboratory measurements. Version 2024-10 https://doi.org/ https://doi.org/10.15138/P8XG-AA10

Müller, J. D., Schneider, B., Aßmann, S., & Rehder, G. (2018). Spectrophotometric pH measurements in the presence of dissolved organic matter and hydrogen sulfide. Limnology and Oceanography: Methods, 16(2), 68-82.

Wanninkhof, R. (2014). Relationship between wind speed and gas exchange over the ocean revisited. Limnology and Oceanography: Methods, 12(6), 351-362.

Walter, S., Bange, H. W., & Wallace, D. W. (2004). Nitrous oxide in the surface layer of the tropical North Atlantic Ocean along a west to east transect. Geophysical Research Letters, 31(23).

Weiss, R., & Price, B. (1980). Nitrous oxide solubility in water and seawater. Marine chemistry, 8(4), 347-359.