Carbon dioxide determination in atmosphere by non dispersive infrared spectroscopy: A possible approach towards the comparability with seawater CO 2 measurement results

The rising levels of carbon dioxide in the atmosphere are responsible for fundamental changes occurring in seawater carbonate chemistry. The partial pressure of oceanic CO2 (pCO2) is one of the four measurable parameters defining the marine carbon system. For this reason, the pressing need of assuring metrological traceability of pCO2 measurement results has been recognized and pointed out by the oceanographic community. In order to achieve this fundamental goal, the lack of suitable reference materials has been identified as one of the most limiting factors. At INRIM several activities are carried out to establish the metrological traceability for carbon dioxide measurement results. The present paper describes two primary methods, the gravimetry and the dynamic dilution, used for the preparation of gaseous reference standards for composition which are fundamental to calibrate sensors and analytical instrumentation for carbon dioxide determination in atmosphere. Suitable procedures for the calibration and use of Non Dispersive Infrared Analysers (NDIR) are also presented and discussed. At present, feasibility studies are ongoing at INRIM to extend the use of these metrologically traceable mixtures to the calibration of sensors used for pCO2 determination in seawater. An extensive work was carried out on a non-dispersive infrared analyser employed in air monitoring, to assess its robustness and stability, which are the major starting points to set up a calibration procedure to obtain comparable results in the atmospheric and marine compartments.