Abstract
We developed an automated GC/FID analysis system to determine methane mixing ratios of standard gases with a mean precision of ±0.12%. The GC/FID analysis of five gravimetric standards showed that their methane scale was stable and internally consistent. However, the diluent air for these standards had a small amount of methane, which was estimated to be 35.5 ppb. Thus, our MRI/GRL scale was corrected for methane impurity in the diluent air.
The methane mixing ratio of dry natural air in an aluminum cylinder prepared for the intercalibration experiment was determined to be 1.7545±0.0015 ppm on the basis of the MRI/GRL scale. This mixing ratio was higher by 23.0 ppb than that assigned by NOAA/CMDL. This difference indicated that the offset of 23.0 ppb between NOAA/CMDL and MRI/GRL scales will need to be taken into account when methane data from both laboratories are compared.
