The NOAA nitrous oxide standard scale for atmospheric observations

A new nitrous oxide (N2O) calibration scale has been developed for atmospheric observations. The NOAA‐2006 N2O scale is based on gravimetrically prepared compressed gas standards. This scale supercedes the NOAA‐2000 scale, which was accepted by the community of experts within the World Meteorological Organization Global Atmosphere Watch program (GAW) as the GAW reference standard. The new scale is defined by thirteen “daughter” standards with dry air mole fractions ranging from 261–371 parts‐per‐billion (nmol mol−1, ppb). These were derived from four part‐per‐million (mmol mol−1, ppm) level “parents”. Standards were evaluated using gas chromatography with electron capture detection. The daughter standards are internally consistent with a standard deviation of residuals of 0.33 ppb, and there is essentially no detectable difference among “cousins” (standards prepared from different parents). The NOAA‐2006 scale is 0.19 ppb lower than the NOAA‐2000 scale at 320 ppb. The global mean N2O mixing ratio (dry air mole fraction), calculated from in situ observations at five monitoring sites was 318.46 ppb in 2004 on the new scale. The NOAA‐2006 scale compares well with other scales based on comparisons of compressed gas standards. The NOAA‐2006 scale is, on average, 0.23% higher than that defined by NIST Standard Reference Materials 2608 and 2609, and an average of 0.01% lower than the Scripps Institution of Oceanography SIO‐98 scale over the range 298–319 ppb.