Conversion of NOAA atmospheric dry air CH₄ mole fractions to a gravimetrically prepared standard scale

Sixteen mixtures of methane (CH 4 ) in dry air were prepared using a gravimetric technique to define a CH 4 standard gas scale covering the nominal range 300–2600 nmol mol −1 . It is designed to be suitable for measurements of methane in air ranging from those extracted from glacial ice to contemporary background atmospheric conditions. All standards were prepared in passivated, 5.9 L high‐pressure aluminum cylinders. Methane dry air mole fractions were determined by gas chromatography with flame ionization detection, where the repeatability of the measurement is typically better than 0.1% (≤1.5 nmol mol −1 ) for ambient CH 4 levels. Once a correction was made for 5 nmol mol −1 CH 4 in the diluent air, the scale was used to verify the linearity of our analytical system over the nominal range 300–2600 nmol mol −1 . The gravimetrically prepared standards were analyzed against CH 4 in air standards that define the Climate Monitoring and Diagnostics Laboratory (CMDL) CMDL83 CH 4 in air scale, showing that CH 4 mole fractions in the new scale are a factor of (1.0124 ± 0.0007) greater than those expressed in the CMDL83 scale. All CMDL measurements of atmospheric CH 4 have been adjusted to this new scale, which has also been accepted as the World Meteorological Organization (WMO) CH 4 standard scale; all laboratories participating in the WMO Global Atmosphere Watch program should report atmospheric CH 4 measurements to the world data center on this scale.