Abstract. Dissolved nitrous oxide (N 2 O) 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 N 2 O, carbon monoxide and water vapour coupled to a glass-bed equilibrator. This approach is less labourintensive 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 shortterm 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 N 2 O 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 N 2 O 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)

Dissolved hydrogen measurements were made at high resolution in surface waters along a tropical north Atlantic transect between Guadeloupe and Cape Verde in 2015 (Meteor 116). Parallel water samples acquired to assess the relative abundance of the nifH gene from several types of diazotrophs, indicated that Trichodesmium and UCYN‐A were dominant in this region. We show that a high degree of correlation exists between the hydrogen saturations and UCYN‐A nifH abundance, and a weak correlation with Trichodesmium. The findings suggest that nitrogen fixation by UCYN‐A is a major contributor to hydrogen supersaturations in this region of the ocean. The ratio of hydrogen released to nitrogen fixed has not been determined for this symbiont, but the indications are that it may be high in comparison with the small number of diazotrophs for which the ratio has been measured in laboratory cultures. We speculate that this would be consistent with the diazotroph being an exosymbiont on its haptophyte host. Our high resolution measurements of hydrogen concentrations are capable of illustrating the time and space scales of inferred activity of diazotrophs in near real‐time in a way that cannot be achieved by biological sampling and rate measurements requiring incubations with 15N2. Direct measurement of high resolution spatial variability would be relatively challenging through collection and analysis of biological samples by qPCR, and extremely challenging by 15N‐uptake techniques, neither of which methods yields real‐time data. Nonetheless, determination of fixation rates still firmly depends on the established procedure of incubations in the presence of 15N2.

A laser-based analyser for nitrous oxide, carbon monoxide and water vapour was coupled to an equilibrator for continuous high-resolution dissolved gas measurements in the surface ocean. Results for nitrous oxide measurements from laboratory tests and field deployments are presented here. Short-term precision for 10 s-average N₂O mole fractions at an acquisition rate of 1 Hz was better than 0.2 nmol mol⁻¹ for standard gases and equilibrator measurements. The same precision was achieved for replicate standard gas analyses within 1 h of each other. The accuracy of the equilibrator measurements was verified by comparison with purge-and-trap GC-MS measurements of N₂O concentrations in discrete samples from the Southern Ocean and showed agreement to within the 2% measurement uncertainty of the GC-MS method. Measured atmospheric N₂O mole fractions agreed with AGAGE values to within 0.4%. The equilibrator response time to concentration changes in water was 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. The ability to measure at high temporal and spatial resolution revealed sub-mesoscale variability in dissolved N₂O concentrations. The magnitude of the observed saturation is in agreement with published data. Mean sea-to-air fluxes in the tropical and subtropical Atlantic ranged between −1.6 and 0.11 μmol m⁻²d⁻¹ and confirm that the subtropical Atlantic is not an important source region for N₂O to the atmosphere, compared to average global fluxes of 0.6 to 2.4 μmol m⁻²d⁻¹. The system can be easily modified for autonomous operation on voluntary observing ships (VOS). Further work should include an interlaboratory comparison exercise with other methods of dissolved N₂O analyses

The Southern Ocean is an important region for global nitrous oxide (N2O) cycling. The contribution of different source and sink mechanisms is, however, not very well constrained due to a scarcity of seawater data from the area. Here we present high-resolution surface N2O measurements from the Atlantic sector of the Southern Ocean, taking advantage of a relatively new underway setup allowing for collection of data during transit across mesoscale features such as frontal systems and eddies. Covering a range of different environments and biogeochemical settings, N2O saturations and sea-to-air fluxes were highly variable: Saturations ranged from 96.5% at the sea ice edge in the Weddell Sea to 126.1% across the Polar Frontal Zone during transit to South Georgia. Negative sea-to-air fluxes (N2O uptake) of up to −1.3 µmol m−2 d−1 were observed in the Subantarctic Zone and highest positive fluxes (N2O emission) of 14.5 µmol m−2 d−1 in Stromness Bay, coastal South Georgia. Although N2O saturations were high in areas of high productivity, no correlation between saturations and chlorophyll a (as a proxy for productivity) was observed. Nevertheless, there is a clear effect of islands and shallow bathymetry on N2O production as inferred from supersaturations.