Measuring atmospheric carbon dioxide—the calibration challenge

Abstract

The measurement of CO₂ in the atmosphere presents a significant metrology and quality assurance challenge. While global trends can be well determined with just a few sampling sites, the plethora of natural processes involved in exchange of CO₂ with the atmosphere makes the identification of those most effective in regulating the long–term atmospheric levels elusive. To unambiguously link particular processes with significant global trends requires continuous monitoring of small spatial and temporal differences in the atmospheric mixing ratio of CO₂ (and related tracers) over the major global CO₂–exchanging regions. Such differences are often comparable in magnitude to the precision of conventional non–dispersive infrared or gas chromatograph analysers, and much smaller than the uncertainty in the link to a primary standard. In general, laboratories cannot currently merge data at high precision and thus achieve adequate global coverage. We describe an improvement in precision (and operating cost) of the conventional infrared analyser technique. Apart from immediate biogeochemical applications, the new system has demonstrated outstanding diagnostic capabilities and revealed a number of unsuspected sources of bias affecting conventional measurement and calibration methods. In addressing these biases, opportunities are created to improve the link between CO₂ measurement and fundamental constants, and to improve the propagation of CO₂ standards to field measurement systems.