Equilibrator-based measurements of dissolved nitrous oxide in the surface ocean using an integrated cavity output laser absorption spectrometer

Abstract. Dissolved nitrous oxide (N₂O) concentrations are usually determined by gas chromatography (GC). Here we present laboratory tests and initial field measurements using a novel setup comprising a commercially available laser-based analyser for N₂O, carbon monoxide and water vapour coupled to a glass-bed equilibrator. This approach is less labour-intensive and provides higher temporal and spatial resolution than the conventional GC technique. The standard deviation of continuous equilibrator or atmospheric air measurements was 0.2 nmol mol⁻¹ (averaged over 5 min). The short-term repeatability for reference gas measurements within 1 h of each other was 0.2 nmol mol⁻¹ or better. Another indicator of the long-term stability of the analyser is the standard deviation of the calibrated N₂O mole fraction in marine air, which was between 0.5 and 0.7 nmol mol⁻¹. The equilibrator measurements were compared with purge-and-trap gas chromatography–mass spectrometry (GC-MS) analyses of N₂O concentrations in discrete samples from the Southern Ocean and showed agreement to within the 2% measurement uncertainty of the GC-MS method. The equilibrator response time to concentration changes in water was from 142 to 203 s, depending on the headspace flow rate. The system was tested at sea during a north-to-south transect of the Atlantic Ocean. While the subtropical gyres were slightly undersaturated, the equatorial region was a source of nitrous oxide to the atmosphere, confirming previous findings (Forster et al., 2009). The ability to measure at high temporal and spatial resolution revealed submesoscale variability in dissolved N₂O concentrations. Mean sea-to-air fluxes in the tropical and subtropical Atlantic ranged between −1.6 and 0.11 μmol m⁻² d⁻¹ and confirm that the subtropical Atlantic is not an important source region for N₂O to the atmosphere, compared to global average fluxes of 0.6–2.4 μmol m⁻² d⁻¹. The system can be easily modified for autonomous operation on voluntary observing ships (VOS). Future work should include an interlaboratory comparison exercise with other methods of dissolved N₂O analyses.