Further studies of the comparability of baseline atmospheric carbon dioxide measurements

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Abstract
The paper describes an international intercomparison of the major baseline atmospheric carbon dioxide monitoring stations. Six compressed gas tanks, three containing CO2/N2 mixtures and three CO2/air mixtures, were used to check the precision of the intercalibration system and determine the magnitude of the carrier gas error at each station. For each of the CO2/N2 mixtures, the mean concentration for all stations differed by less than 0.3 ppmv from that measured at any one station. In most cases the agreement was better than 0.1 ppmv. Stations using NDIR analysers with parallel detector cells indicated CO2 concentrations in the CO2/air mixtures 3 to 4 ppmv below the average for all stations, while those using series type detector instruments indicated 1 1/2 to 4 ppmv above average. Significant differences between stations using the same detector configurations are suggested to be due to differences in analyser model, length of sample cell and ambient pressure. However, where carrier gas effects for analysers had been measured, they did not fully explain the differences in indicated concentration observed during this study. Problems of non-linearity in the calibration scale and in the analysers are also shown as possible causes of error up to 0.3 ppmv. Consequently, reported mean atmospheric CO2 concentration differences between stations of up to 1 or 2 ppmv cannot be interpreted as evidence for large-scale horizontal gradients in the atmosphere. Methods for improving the comparability of data are discussed with reference to recommendations given by the WMO meeting of “experts on CO2 monitoring” held at La Jolla in March 1975. DOI: 10.1111/j.2153-3490.1977.tb00721.x

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