Dataset: Methane concentrations at Station ALOHA

The following describes the methodology for both this (N2O) dataset as well as the CH4 dataset since the analysis was done together.  N2O results can be found on the page: Nitrous oxide concentrations at Station ALOHA

Calibration of the analytical system was conducted using gaseous standards purchased from Scott-Marin (CH4: 20.15 ± 1% ppmv; N2O: 4.81 ± 2% ppmv in a balance of N2) and NOAA (CH4: 1965.32 ppbv; N2O: 357.56 ppbv in a balance of air).  From March 2016 onwards, the calibration for CH4 and N2O was compared against reference standards prepared by John Bullister at NOAA PMEL on behalf of SCOR Working Group #143.  In all instances, standards were injected prior to the purge and trap set-up and therefore passed through the purge chamber and gas drying apparatus.  A linear curve was applied to the CH4 calibration values and a polynomial curve was fitted to the N2O calibration values.  The precision of CH4 measurements for surface seawater with concentrations of 2.57 ± 0.07 (SD) nmol kg-1 (n=14), as calculated by the coefficient of variation was 3%.  The accuracy of CH4 measurements was evaluated by analyzing filtered (0.2 μm) seawater samples that had been equilibrated with atmospheric air at a range of set temperatures between 19–27°C which were maintained using a water-bath.  The measured values agreed to within 2.4 ± 0.9% of predicted values.  The precision of N2O measurements in surface seawater, with concentrations of 6.47 ± 0.14 (SD) nmol kg-1 (n=14), as calculated by the coefficient of variation was 2%.  Using the same air-equilibrated seawater set-up, as described for CH4, the accuracy of N2O measurements was 2.6 ± 1.9% of predicted values.

Instruments: 

For quantifying the dissolved gases, the analytical column (30 m x 0.32 mm GS-CarbonPLOT capillary column; J&W Scientific) was housed within a gas chromatograph (GC) Agilent 7890A equipped with a flame ionization detector (FID) and an electron capture detector (ECD).  The carrier flow was alternated from the FID to the ECD using a Dean’s switch® (Agilent Technologies) which allowed the quantification of both CH4 and N2O from a single sample.

Reference:

Wilson, S. T., Ferrón, S., & Karl, D. M. (2017). Interannual variability of methane and nitrous oxide in the North Pacific Subtropical Gyre. Geophysical Research Letters, 44, 9885–9892. https://doi.org/10.1002/2017GL074458