Preparation and Validation of Fully Synthetic Standard Gas Mixtures with Atmospheric Isotopic Composition for Global CO^sub 2^ and CH^sub 4^ Monitoring

We report the preparation and validation of the first fully synthetic gaseous reference standards of CO^sub 2^ and CH^sub 4^ in a whole air matrix with an isotopic distribution matching that is in the ambient atmosphere. The mixtures are accurately representative of the ambient atmosphere and were prepared gravimetrically. The isotopic distribution of the CO^sub 2^ was matched to the abundance in the ambient atmosphere by blending ^sup 12^C-enriched CO^sub 2^ with ^sup 13^C-enriched CO^sub 2^ in order to avoid measurement biases introduced by measurement instrumentation detecting only certain isotopologues. The reference standards developed here have been compared with standards developed by the National Institute of Standards and Technology and standards from the WMO scale. They demonstrate excellent comparability. [PUBLICATION ABSTRACT]