Application and assessment of a membrane-based pCO2 sensor under field and laboratory conditions

Zong-Pei Jiang, David J. Hydes, Sue E. Hartman, Mark C. Hartman, Jon M. Campbell, Bruce D. Johnson, Bryan Schofield, Daniela Turk, Douglas Wallace, William J. Burt, Helmuth Thomas, Cathy Cosca, Richard Feely

Limnol. Oceanogr. Methods 12:264-280 (2014) | DOI: 10.4319/lom.2014.12.264

ABSTRACT: The principle, application, and assessment of the membrane-based ProOceanus CO2-Pro sensor for partial pressure of CO2 (pCO2) are presented. The performance of the sensor is evaluated extensively under field and laboratory conditions by comparing the sensor outputs with direct measurements from calibrated pCO2 measuring systems and the thermodynamic carbonate calculation of pCO2 from discrete samples. Under stable laboratory condition, the sensor agreed with a calibrated water-air equilibrator system at –3.0 ± 4.4 μatm during a 2-month intercomparison experiment. When applied in field deployments, the larger differences between measurements and the calculated pCO2 references (6.4 ± 12.3 μatm on a ship of opportunity and 8.7 ± 14.1 μatm on a mooring) are related not only to sensor error, but also to the uncertainties of the references and the comparison process, as well as changes in the working environments of the sensor. When corrected against references, the overall uncertainties of the sensor results are largely determined by those of the pCO2 references (± 2 and ± 8 μatm for direct measurements and calculated pCO2, respectively). Our study suggests accuracy of the sensor can be affected by temperature fluctuations of the detector optical cell and calibration error. These problems have been addressed in more recent models of the instrument through improving detector temperature control and through using more accurate standard gases. Another interesting result in our laboratory test is the unexpected change in alkalinity which results in significant underestimation in the pCO2 calculation as compared to the direct measurement (up to 90 μatm).