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 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 a maximum drift of < 1 ppb per 24-hour period.

LI-7820 recorded timestamped measurements (>1800 measures were recorded in each fjord) were subsequently averaged (integrated) over 30 min to make data analysis feasible (Table 1). N2O measurements were then adjusted by pushing back the data by 20 minutes to account for headspace equilibration response time and for the time it takes to the water to travel from the sea to the exchanger itself.

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 circulated at 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 the 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−).

Dissolved N2O in water [partial pressure (pN2O) and N2O molar fraction (nmol L−1)] and N2O saturation ratio were calculated from the measured N2O headspace partial pressure (showerhead gas equilibrator) knowing temperature and salinity-dependent solubility (Weiss & Price, 1980). The N2O air-water gradient (pwater – pair) was calculated from N2O concentrations (ppb), using current annual estimates of gas concentrations in the atmosphere from NOAA (National Oceanic and Atmospheric), with respect to the sampling period. These saturation percentages were based on in situ temperature and salinity records, and adjusted to the N2O solubility parameterization (Weiss & Price, 1980).

Nitrous oxide gas fluxes (f) at the water-air interface were calculated using the formula:

f=k α (p_Water- p_Air)

where k is the gas transfer velocity calculated from the wind-based empirical model of (Wanninkhof, 2014) using in-situ daily averaged wind speed measurements. The gas transfer velocity was converted from k600 (i.e., k for gas with a Schmidt number of 600) to k at in-situ temperature and salinity using the equations of Wanninkhof (2014). Where α is the solubility coefficient calculated from temperature and salinity using the equations of Weiss and Price (1980). Whereas. pwater and pair are the partial pressures of the gas in water and air respectively. N2O fluxes at the sea-air interface are expressed in µmol N2O m−2 day−1, and values are reported as Average ± SD.