The NIST Atmospheric Methane Gas Standard Scale

The National Institute of Standards and Technology (NIST) has entered into a collaboration with the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado on dveloping SI-traceable standards for greenhouse gas monitoring. The Earth System Research Laboratory (ESRL) of NOAA has been designated by the World Meteorological Organization to provide SI-traceable gases to laboratories participating in the WMO climate science community Global Atmosphere Watch program. NOAA has agreed to demonstrate comparability to other National Metrology Institutes under international an existing international agreement (BIPM MRA). NIST and NOAA have begun a series of bilateral comparisons to demonstrate comparability. The Gas Metrology Group at NIST has developed a new suite of atmospheric methane Primary Standard Gas Mixtures (PSMs) that range in amount-of-substance fraction (dry-air mole fraction) from 1700 to 2050 nmol/mol (ppb) with ± 0.03% (0.6 nmol/mol) relative uncertainties (68 % confidence interval). Six levels of standards were made to support NIST programs plus the atmospheric level of 1700-2050 nmol/mol with 30 cylinders total. Each intermediate level was compared to existing NIST PSMs at completion for consistency. The 6th level was also compared to an existing suite of PSMs prepared in 1998. A ratio of the methane in each PSM to a control mixture was calculated by dividing the methane peak area response measured by gas chromatography of a PSM by that of the control. The ratio data and gravimetric mole fractions were plotted to a general least squares program called GenLine (1). The results of this regression are shown in Table 1. The average residual difference between the predicted values and the gravimetric values of the old and new PSMs was 0.08 nmol/mol. NOAA/ESRL has analyzed one of the new PSMs from the NIST atmospheric level suite and has shown good agreement. NOAA has filled 30 cylinders with real air from their Niwot Ridge, Colorado, site and determined the methane mole fractions based on their calibration scale. NIST is in the process of analyzing these real air cylinder mixtures and will certify them for methane, as well as carbon dioxide and nitrous oxide. Methane data from both NOAA and NIST will be discussed. These real air cylinder mixtures will be certified as NIST SRM 1720 Northern Hemisphere Air. (1) Milton MJT, et.al. Metrologia 2006;43:S291-S298.Table 1. GenLine regression of PSM data. a PSMs from 1998 Suite