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

Grefe, I. and Kaiser, J. (2014) Equilibrator-based measurements of dissolved nitrous oxide in the surface ocean using an integrated cavity output laser absorption spectrometer. Ocean Science Journal, 10. 501–512. ISSN 1738-5261

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.