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

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.