Atmospheric N2O is observed using a GC system equipped with a non-radiative electron capture detector (ECD) up to February 2004. Since March 2004, however, a radiative ECD has been used in order to improve observation precision. Figure 188.8.131.52 shows a view of the atmospheric N2O observation system.
Air samples are continuously introduced into the observation room from the same intake used for the CFC observation at a height about 7 m above ground level. The observation system feeds an air sample taken from the intake into a 3 ml (2.5 ml from February 2008) sample tube once every hour and introduces it into the analyzer. The composition to be observed (i.e., N2O) is separated by introducing the sample into a column (300 cm of PorapakTM Q 80-100 mesh and 100 cm of PorapakTM Q 80-100 mesh). Four working standard gases in concentrations of ambient levels are used. The concentrations are determined and drifts are monitored for these working standard gases by calibration against JMA's standard gases with JMA's GC-based N2O calibration system before and after use. The standard gases are measured for 1 hour, 15 minutes per gas, and then air samples are measured for 7 hours, once per hour, forming a cycle of 8 hours to be repeated. Concentrations of air samples are determined based on the quadratic calibration curve calculated from the known concentrations and outputs of the working standard gases. A target gas in a concentration of ambient air is measured once for each calibration curve to monitor long-term variations in the working standard gases as well as anomalies in the observation system.
The selection procedure for BG data is the same as that of atmospheric CFCs. The prescribed threshold is 10 ppb.