Methane standard gases for atmospheric measurements at the MRI and JMA and intercomparison experiments

A calibration system of standard gases for atmospheric methane measurements using a gas chromatograph equipped with a flame ionization detector was developed at the Meteorological Research Institute (MRI) in 1993 and later at the Japan Meteorological Agency (JMA) in 2000. Calibration results for the two systems exhibited good agreement. Although the primary standard gases of the MRI and JMA were prepared independently using the gravimetric method, their methane standard scales were quite similar, (a difference of less than 1 ppb). A comparison of gravimetric standards prepared by several Japanese gas companies showed a methane scale difference within ∼5 ppb relative to the primary standards of the MRI. These gravimetric-scale differences were primarily caused by methane impurities in the diluent air used for standard gas preparation. Long-term storage experiments indicated that the methane contents of standard gases were stable in high-pressure cylinders for 8-9 years. No significant drift of methane was found in any of the standard gases in the MRI; however, to evaluate the stability of the JMA standards, data collected over a longer period of time is required. We evaluated the differences between the methane scales of the MRI and other laboratories based on the previous intercomparison experiments. The differences found in the standard scales used by different laboratories were largely due to the diverse origins of the primary standard gases.