At the 12^th WMO/IAEA Meeting of Experts on Carbon Dioxide Concentration and Related Tracers Measurement Techniques, it was agreed that NOAA would assume the role of the WMO-GAW Central Calibration Laboratory (CCL) for methane. The NOAA04 scale was designated as the official calibration scale, and consists of 16 gravimetrically prepared primary standards covering the nominal range of 300 to 2,600 nmol mol^-1. This makes it suitable for use in calibrating standards for the measurement of air extracted from ice cores and contemporary measurements from GAW sites. This new scale results in CH4 mole fractions that are a factor of 1.0124 greater than the previous scale (now designated CMDL83) (Dlugokencky et al., 2005). The CCL will transfer the CH4 scale to calibrated CH4-in-air standards with an uncertainty of <1 nmol mol^-1. All laboratories participating in the GAW programme must calibrate measurements to relative the WMO CH4-in-air mole fraction scale and report them to the WMO World Data Centre for Greenhouse Gases (WDCGG) in Japan [WMO, 2007]. Against this background, JMA decided to adopt the new scale (NOAA04 scale) in 2006, and recalculated the mole fractions of the methane reference gases it circulates for intercomparison.

Five methane-in-air standard gases (ex-primary standard gases) in 9.4-liter aluminum alloy cylinders were prepared gravimetrically by Japan's Nippon Sanso Corporation in 1999 using a method developed by Prof. Takakiyo Nakazawa and his coworkers at Tohoku University in Sendai, Japan [Aoki et al., 1992]. Five methane-in-air primary standard gases (ex-secondary standard gases) in 48-liter aluminum alloy cylinders were prepared volumetrically by Nippon Sanso Corporation in 1999. JMA adopted the JMA scale based on the gravimetric method for methane measurement in 2000.

Due to a significant drift (click to see the details) identified in the ex-primary standards by self-calibration, in 2005 we upgraded the ex-secondary standards (which have been more stable since they were first prepared) to the primary standards. The primary standard gases were calibrated on the NOAA04 scale in NOAA/ESRL in 2006. The history of the standard gases maintained at JMA is shown in Figure 1.

Figure 1. History of the standard gases maintained at JMA

The differences between the NOAA04 and JMA scales for the primary standard gases maintained at JMA are shown in Table 1 and Figure 2. We have recalculated and revised the mole fraction values of all standard gases for the NOAA04 scale.

Figure 2. Difference between the NOAA04 and JMA scales for the primary standard gases maintained at JMA.