7.3.2 Calibration for atmospheric CH₄ observation

   JMA has a CH₄ calibration system to maintain CH₄ standard gases in three ranks (Figures 7.3.2.1 and 7.3.2.2). The primary standard gases form the basis of JMA's CH₄ observation as the first rank gases and consist of 5 cylinders. The primary standard gases are a mixture of CH₄ 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 at NOAA in October 2006 (Dlugokencky et al., 2005). The standard scale is called NOAA 04 scale, by which JMA's observation assures international traceability.

   For second rank standard gases, 5 cylinders are used over the same concentration range as that for primary gases: from 1,600 to 2,100 ppb. Prolonged use of primary standard gases is made possible by introducing secondary standard gases to reduce the amount of primary standard gases used. The secondary gases are packed in 48 L aluminium cylinders, with concentrations of the secondary standard gases being determined based on the concentrations of the primary standard gases using the CH₄ calibration system.

   Concentrations of working standard gases, the third rank gases used at observation stations and observation vessels, are determined based on the concentrations of the secondary standard gases using the CH₄ calibration system, after which the calibrated working standard gases are sent out to the stations. After use in observation at each station, the gases are sent back to JMA to check for drift, and the concentration is confirmed against that of the secondary standard gases (refer to section 7.2.2). As of 2009, CH₄ observation at the stations is currently transitioning from NDIR method to GC method (Yonagunijima: switched in January 2008; Ryori: switched in January 2009; Minamitorishima: scheduled to switch in January 2010), which involves changing from a set of 2 working standard gases used in the NDIR observation system to a set of 4 working standard gases used for the GC observation system.

   The CH₄ calibration system is a GC model with FID detector (Shimadzu GC-14BPF). System details can be found in http://gaw.kishou.go.jp/wcc/ch4/system.html and Matsueda et al. (2004). The repeatability of the CH₄ calibration system does not exceed 0.07, which is obtained by introducing standard gases 6 times, then dividing the standard deviation for the mean peak area by the mean area (coefficient of variation).