Errors in atmospheric CO2 concentration measurements arising from the use of reference gas mixtures different in composition to the sample air

Comparisons have been made between the URAS 2, UNOR 2 and Grubb Parson's SB2 non-dispersive infra-red gas analysers, involving the measurement of CO₂ concentration in gas mixtures containing N2 and O2 in varying proportions. The results imply that the CO₂ concentration in air when using CO₂/N2 mixtures as references, is under-estimated by 3.7 and 2.3 ppmv for the URAS and Grubb Parson's instruments respectively and over-estimated by 4.5 ppmv by the UNOR. The measurement of CO₂ concentration in air is not affected if CO₂/air mixtures are used as references. The indicated CO₂ concentration of air relative to a CO₂/N₂ reference mixture is found to depend on ambient pressure for both the UNOR and URAS analysers. Thus aircraft or high altitude in situ measurements need an additional correction, to be comparable with measurements made relative to the same CO₂/N2 reference mixtures at a standard (near sea-level) pressure of 1 000 mb. The results are similar to those obtained by Bischof (1973) and have important implications on the interpretation of carbon dioxide concentration measurements made at monitoring stations.