Scale and stability of methane standard gas in JMA and comparison with MRI standard gas

Abstract

We examined the standard gas scales and the stability of methane (CH₄) standard gases that have been used for atmospheric measurements at the Japan Meteorological Agency (JMA) since 2000. Calibration of the JMA standards at the National Oceanic and Atmospheric Administration (NOAA) using the NOAA04 gravimetric scale, which is the accepted World Meteorological Organization (WMO) CH₄ mole fraction scale, showed that CH₄ mole fractions in the NOAA04 scale differ by +1.3 to −4.5 nmol mol⁻¹ from those in the gravimetric scale in use at JMA. We established a linear relationship between the differences, which can be used for conversion between the two scales. Stability tests showed significant drift of −1 nmol mol⁻¹ yr⁻¹ for the mole fractions of two of the standard gases tested; all other standards were shown to be stable. Experiments comparing the results obtained for standards used at JMA and the Meteorological Research Institute (MRI) between 2000 and 2014 verified the conversion to the WMO scale and the drift correction used. The Inter-Comparison Experiments for Greenhouse Gases Observation (iceGGO) program in Japan can provide a useful means of validating the MRI/JMA CH₄ scale and comparing it with other gravimetric scales used in Japan.