The gas chromatographic methods, NDIR, and some TDLS techniques require calibration against samples with known gas amounts. As far as we are aware,
2 GLOBAL CO DISTRIBUTIONS 31
there is no national or commercial laboratory that provides certified CO reference gases at levels found in the background atmosphere. Groups measuring CO must therefore dilute high concentration certified gases to atmospheric levels or obtain standards from other laboratories. Laboratory intercom pari sons of the reference gases used by various researchers have shown large differences between groups (up to 25 to 50%).31 23 CO standards may also be subject to drift over time. Efforts have been made since the early 1990s to compare CO measurements in both the laboratory and the held,14""1 However, differences between groups still exist, and the integration of data sets requires some prior understanding of how the measurements compare.
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