FY 1999

On obtaining high-precision measurements of oceanic pCO₂ using infrared analyzers

Murphy, P.P., R.A. Feely, and R. Wanninkhof

Mar. Chem., 62(1–2), 103–115, doi: 10.1016/S0304-4203(98)00030-9 (1998)

Assessments of ocean carbon uptake using the air-sea disequilibrium of CO₂ require very high quality measurements of pCO₂ in the atmosphere and in surface seawater. These measurements are often collected and analyzed using infrared detectors. Laboratory data are presented here which suggest that errors of the order of several parts per million in xCO₂ can result if the analyzer temperature and pressure are not carefully matched during calibration and sampling. Field data were examined to address questions about the importance of measuring analysis temperature and pressure under more extreme conditions, sample averaging, and calibration frequency. The results indicate that calibration frequency can be minimized without significant compromises in data quality if the analyzer temperatures and pressures are suitably monitored and/or controlled. Daily calibrations gave results to within 0.4 ppm of the results obtained by hourly calibration when the temperature of the analyzer was controlled to ±0.2°C and the voltages were corrected for pressure differences between calibration and sampling.