7.3.3 Calibration for atmospheric N₂O observation

   JMA has an N₂O calibration system to maintain N₂O standard gases in two ranks (Figures 7.3.3.1 and 7.3.3.2). A set of 5 cylinders of primary standard gases are used as the first rank gases, which form the basis of JMA's N₂O observation, in concentrations ranging from about 290 to 335 ppb. The primary standard gases are a mixture of N₂O and natural air packed in a 48-L cylinder made of aluminium alloy causing little drift. The concentrations of the primary standard gases were determined accurately against the WMO reference standard gases (Hall et al., 2007) at NOAA in January 2007. The standard scale is called NOAA 2006, by which JMA's observation assures international traceability.

   Concentrations of working standard gases, the second rank gases used at observation stations, are determined based on that of the primary standard gases using the N₂O calibration system, after which the calibrated working standard gases are sent to stations. After their use in observation at each station, the gases are sent back to the JMA headquarters to check for drift, and the concentration is confirmed against that of the primary standard gases (refer to section 7.2.4).

   The N₂O calibration system is a GC model with ECD detector (Shimadzu GC-2014). The repeatability of the N₂O calibration system does not exceed 3 ppb, and is obtained by introducing standard gases 10 times, then taking the standard deviation for the mean peak area.